The Basics of Climate Change Science V

Recent History of Climate Change

We’ve been looking at climate science, at the Greenhouse Effect, at greenhouse gases, at other drivers of climate change–inter-annual, decadal, even we’ve talked briefly about the long term geological changes caused by changes in the earth’s orbital. We’re living in real time right now where we’re already experiencing significant climate change and the human-induced component of that is our preeminent worry. We know that the greenhouse gas effect is powerful; it is already present. It is responsible for raising the earth’s temperature compared to the preindustrial level by almost one degree Celsius or almost 1.8 degrees Fahrenheit and we know that we’re on a trajectory of great danger.

We’re going to talk in future lectures in detail about impacts and about why we care about the two degree centigrade limit agreed in Copenhagen and Cancun as a reflection of or a definition in need of avoiding dangerous interference in the climate system.

I want to mention briefly some of the things that we’re currently observing, and I want to start with the quite remarkable set of maps produced by my quite wonderful colleague, Professor James Hansen. You’ll recall Dr. Hansen testifying in 1998 to the U.S. Congress giving the Congress the first authoritative warning “This is real; it’s coming”, and Dr. Hansen called it correctly–he’s a brilliant global climate scientist, who has been studying every aspect of the underlying physics–the radiative forcing, the earth’s dynamics with remarkable perspicacity for decades.


He’s been telling us we’re already in the midst of deep change, and a quite remarkable way that he has demonstrated this is shown by this set of nine maps. If you focus in on the map in the upper left hand corner, you can see through the blotches of color a world map. This particular world map is for the year 1955. What Professor Hansen has done is to take the average temperature in each part of the world, each pixel on the map, each location on the map and calculated the average for the years 1951 to 1980, and a kind of bell-shaped curve of temperatures in that place during that time interval.

Well, you know what a bell-shaped curve is or a normal or Gaussian curve. It shows the probability of falling above or below the average temperature line, and we say that in the tails of that bell-shaped curve those are when you have big deviations from the average outcome either on the plus side that something is much larger than average or on the other tail of the distribution when something is much less than average. So, say that for the years 1951 to 1980, you have a bell-shaped curve of temperature in a particular place, but a few years are really warm and a few years are really cold–those are the outliers; those would be at the tails of the normal distribution.

Now, events that are two standard deviations above the average or two standard deviations below the average are at the outer tails, and the plus/minus two standard deviations in total accounts for about 5% of the occurrences. Three standard deviations out is an absolutely extraordinary case that happens only a couple of times every thousand occurrences.

So, Professor Hansen said, “Suppose we know the normal distribution of temperatures in each part of the world based on what we observed from the period 1951 to 1980 and suppose we define a really extreme event as a three-standard-deviation outlier–super hot or super cold.” Well during the period where you calculate that, 1951 to 1980, that wouldn’t happen very much by construction that would only happen a couple times out of a thousand, but Professor Hansen wanted to ask the question “are such extreme events, real outliers, occurring with more frequency?”

So he made maps that showed when different parts of the world had extreme temperature events either extreme hot, which is shown in dark maroon here, or extreme cold, which is shown in purple on the map, and if you look back at 1955, there are a couple tiny spots just by random that happen to have extreme heat waves.

You can see around Hudson Bay in northern Canada or maybe just a part of northern Spain had a real heat wave during the summer months of June, July, August of 1955, but the map as a whole doesn’t show anything very extraordinary. Most of the temperatures lie well within plus or minus two standard deviations of the normal temperature for that particular place.

Professor Hansen has made this map for every year. If we go to the central column at the top row; that’s for the year 1965. I don’t see any dark maroon here. There isn’t any single part of the planet in the year 1965 that for the Northern Hemisphere summer months had an extreme heat wave. Well, that’s natural. These extremes are very very rare by definition because he constructed the definition of extreme to happen very rarely during the period 1951 to 1980. 1975, well again, focus in extreme northern Canada pretty hot, unusual; the rest of the world, nothing too much special. Now, move the calendar forward to our time. Professor Hansen is making a point: what used to be extreme is becoming normal.252

This is really the shock for the planet. Look at 2006. All of a sudden the map looks nothing like the 1955 or 1965 or 1975 map; there’s red all over the place; in the Indian ocean, in North Africa, in the Northeast of the US and Canada. Red blotches all over the map! They weren’t there before. What he’s saying is that in 2006 by the standards of the historical period of 1951 to 1980, suddenly we’re finding ourselves in the tail of the distribution on the hot end of what used to be the distribution in which such events would be very rare, so stay with the bell-shaped curve of 1951 to 1980, stay with the same definition of extreme heat, all of a sudden it’s happening in many parts of the world. 2007: red, red, red blotches! 2009: oh my word!

Looks like half the world’s experiencing extraordinary three-standard-deviation heat waves because that was an absolute again period of this increase in hot temperatures and what we’re seeing is the core message; what used to be extraordinary is becoming normal not normal pleasant, normal unpleasant. What used to be an extreme heat wave that happened maybe one time in a thousand or two times in a thousand is now happening fifty times in a thousand, 5% of the time, or a hundred times in a thousand, 10% of the time. The extremes are the new normal. We shouldn’t take that as normal.

We should take that as an alarming reality because of what is shown in the next graph. These are data produced by the insurance companies. They need to know what happens with disasters because they pay out when disasters occur. They have to price their contracts, and what they are recording is a massive increase of disasters and especially not so much the disasters we don’t cause, although maybe some of them are better measured or there are more people living in places, say, hit by earthquakes, but the disasters that can be changed in frequency and intensity by global, anthropogenic climate change–the so called hydro-meteorological disasters: the extreme storms, the extreme tropical cyclones like typhoon Haiyan or superstorm Sandy or the massive droughts afflicting much of the world today in Brazil, California, parts of Australia, parts of India, parts of Pakistan, or the massive flooding.

These are the kinds that the insurance companies are noting because they’re making payouts and they are realizing that we’re having a huge rise of hydro-meteorological disasters. Well, we have looked earlier and Emmanuel Guerin in the next lecture is going to look in detail at some of the impacts, I just want to reflect on 2014–our year, our current period. If you happen to be a tennis-lover and you were watching the Australian Open at the beginning of the year, you were watching tennis players playing under nearly impossible conditions–massive heat wave as shown in this map for January 2014. Australia was hit by one of these extraordinary heat waves that have become the new normal.253

Here’s a different map for the spring of this year. California experiencing a state-wide drought and a very significant part of California experiencing the most intense and remarkable drought  leading to normal water reservoirs used for irrigation and drinking water dropping to absolutely frightening proportions. And as I’ve shown earlier, Brazil the same, Indonesia the same.

When I was in Istanbul earlier this year, the reports of major drought around the Istanbul area. A water reservoir shown here with the water basically gone and incidentally, hit by, Istanbul hit by massive flooding a couple of months later. Not unexpected or paradoxical. Droughts followed by floods, precipitation patterns characterized by extreme storm events with very heavy downfalls when the downfalls occur and in Bosnia, Herzegovina, and in Serbia in the spring, it was shocking. I arrived in Serbia just after these floods. These were not one-in-a-century floods; these were basically characterized as one-in-five-hundred-years or one-in-the-millennium flooding.254

They’d never seen the extent of this kind of flooding and tremendous property loss and many people lost their lives. In my work as special adviser to Secretary General Ban Ki-Moon on the Millennium Development Goals, I work in a lot of the poorest places in the world and not by coincidence many of those are dryland environments; places where rainfall is so marginal that getting a crop in particular year is always a risk; it is always a bit of a gamble–will the rains fail? These drylands stretch across the Sahill of West Africa to the horn of Africa–Ethiopia, Sudan, northern Uganda, Kenya, Somalia; they stretch across the Red Sea into the Arabian Peninsula into Yemen, Oman, Saudi Arabia; they stretch into western Asia–Iraq, Iran–and into central Asia–Afghanistan and neighboring countries. And that whole swath of thousands and thousands of kilometers and so many millions of people living in vulnerability face the ever-present risk that the rainy season is going to fail.

They’re already dry, and climate change for many of these places is already a clear and present danger and a taker of lives when these droughts and ensuing famines occur. Now, one of the things that I also see in my UN capacity is that not only do these droughts and crop failures lead to huge suffering, but they are themselves the tinderbox that can lead to conflict.

When people are hungry, when people are desperate, demigods, dangerous people absolutely can sway unemployed, desperate, hungry young men or just force them into paramilitaries and the consequences now shown statistically with great care that when the rains fail and poor regions, especially in Sub-Saharan Africa, conflicts are likely to follow. One of the most horrific conflicts on the whole planet now is in Syria. Syria is part of the Mediterranean environment, which the climate models tell us is already a drying region that is likely to get much dryer in the future. This is by virtue of the Mediterranean location in the world climate system. Warm, dry air is now descending on the Mediterranean decreasing the amount of rainfall that is coming.

Well, all through the Mediterranean Basin there has been a drop of rainfall over the last twenty years, but in Syria, there was nearly ten years of continuous drought and over time, one year of drought may be manageable, two years of drought-very hard to get a crop, three years of drought-the soils lack all moisture and the crops start to fail, populations start to move, farm families can’t survive, people abandon their homesteads and in very complex, not well-governed places when people move, frictions explode, frustrations, hunger, and of course desperation also take off. I remember in Syria in 2007-2008-2009 the United Nations was putting out alarms. We’ve got major drought, major population movements, major food insecurity.

Nothing much was done because the world doesn’t respond to these kinds of warnings nor did the government, an authoritarian government capable of great brutality and lack of responsiveness and also being overwhelmed by chronic, nearly decade-long drought. What was the consequence? Well, drought wasn’t THE cause of Syria’s conflict, but one can say that drought and food scarcity, soaring food prices, hungry people, displaced populations were part of the complex mix of bad governance, frustration, corruption that meant in 2011 when the so-called “Arab Spring” erupted in Tunisia and Egypt, protests began massively in Syria.

The Assad regime cracked down on those protestors, a military insurgency broke away from the Syrian Army, and then as one thing leads to another, suddenly there was support coming from outside powers—the United States, Turkey, Saudi Arabia supporting the insurgency. There was Russia and Iran supporting the Assad regime and suddenly drought, famine, food insecurity, and the other factors erupted into violence; violence into war; war into proxy war; and soon enough by now more than 160,000 people dead and disaster and still a completely unresolved and indeed spreading war. I mention this because climate change is gonna have many impacts and it’s going to have the kinds of impacts that Emmanuel will be talking about in the coming lectures, but we should understand that human capacity, we have it to adjust sometimes, sometimes we’re able to counteract these kinds of events, sometimes we’re able to use our intelligence and our smarts to get ahead of the curve, sometimes we are just dumber than dumb.

The visceral emotions, the hate, the fear, the arms take over and what begins as an ecological crisis turns into a full-fledged human disaster, war, threat of ever-widening proportions. The point is climate change is going to have massive impacts on the planet, and we have to learn to think ahead and think cooperatively if this is not to get the best of us. Well, in this course we are not going to be examining in detail how we can do that, how we can use our intelligence, our technology, our know-how, and the global tools of cooperation in the UN Framework Convention on Climate Change to get ahead of the curve.

By the end of this course, I hope we have the tools so that when we enter the next semester as delegates to the global, online negotiations, we’ll be able to find the path forward and help to illuminate the ways that we don’t end in this kind of disaster but rather find a way to cooperation, to peace, and to security for everybody.

The Basics of Climate Change Science IV

Other Drivers of Climate Change

We’ve been looking at the greenhouse effect, at the role of particular greenhouse gases, at the relentless rise of carbon dioxide. I want to say a few words at least in this chapter about the year to year and decade to decade rise of temperature that has accompanied the rising greenhouse gases.

The main point is that the greenhouse gas concentrations especially the rising carbon dioxide have been the main drivers of the overall rise of Earth’s temperature during especially the past century and in recent decades, but year-to-year fluctuations of temperature even decade to decade fluctuations of temperature have even more causes and more complexity then a simple relationship between greenhouse gas concentrations and I the resultant annual temperature. There are many reasons for that but most importantly the greenhouse gases are not by themselves me only factors that affect the Earth’s temperatures.241

There are many complicated both natural and other human-caused factors that for affect the evolution of Earth’s temperature and there is a basic point which is worth the bearing in mind right at the start. When the greenhouse gas concentration rises, say carbon dioxide is emitted into the atmosphere, that changes the equilibrium energy balance. The greenhouse effect means that Earth will have to be warmer in order to equate the incoming solar radiation with the outgoing infared radiation of Earth but in a short period of time there’s going to be heat trapped before Earth warms to its new equilibrium level and in fact that process of Earth warming to its new balance, new equilibrium is itself complicated because the land will tend to warm faster than the oceans and so the overall rise of Earth’s temperature will take time.

Just as it takes time to heat a big tub of water ,it takes a lot of time to heat the oceans. The Earth has to absorb a lot of and net energy to bring the oceans up to the new equilibrium temperature. That means that even with the high concentrations of greenhouse gases that we have today and the warming that has accompanied them, even if those greenhouse gas concentrations were to remain unchanged from now on and stay at the current level, there would still be a tendency for the Earth gradually to be warming to catch up with this greenhouse effect because the oceans have not yet caught up with the changes of energy balance caused by the higher levels of greenhouse gas concentrations.242

Now if we refer back to the graph by NASA by the Goddard Institute for Space Studies that we looked at earlier showing the change of Earth’s temperature over the period 1880 til recently taking as the baseline 1951 to 1980 we see that on  average over a long period of time the temperature has been increasing but we also note importantly that there have been periods where greenhouse gases were rising but the temperature did not rise very much.

A notable period is during the 1940s through the 1960s when there wasn’t so much increase in global temperature and if you look at the more recent period from around 1999 or the year 2000 till now, there has been some warming perhaps but it’s been slower then the period say from the mid-1970s up to the end of the 20th century up to say 1998. So certain periods have a fairly steep slow on this graph, from the seventies through the nineties, other periods have a fairly flat slope, say from the forties to the early seventies, or from 2000 up until now, not necessarily 0 but lower then the rapid increases of temperature.243

What can account for these kinds of changes year to year or decade to decade? That’s really the topic but I want to mention briefly. We know that in addition to the greenhouse gases being drivers of the energy system other states of nature and other human activities play a role. Aerosols are a big driver. When pollution increases on a global scale,  f it’s the kind of white aerosols or sulphate pollution that would tend to mask some of the warming that would be caused by the greenhouse gas emissions.

That’s at least one hypothesis for the relative lack of temperature increase from the forties to the seventies. That was a period mass industrialization and massive air pollution. After that a lot of the sulfate pollution was cleaned up as smokestacks were cleaned, the emissions from smokestacks were reduced of pollutants by smokestack scrubbers, for example, to stop sulfur oxide emissions.

244So one thought is that it’s the aerosol pollutants that are part of the story either slowing the greenhouse effect or masking it or partly offsetting it. Or it’s the brown and organic aerosols, the black aerosols, the soot, or the burning of organics that could lead to a warming effect that would accelerate.

But of course there are also natural aerosols when Mount Pinatubo in the Philippines erupted in 1991 that had a global scale cooling effect by the massive amount of sulfates that were put into the stratosphere by that mega volcanic eruption and so scientists track the volcanic eruptions year to year and course on a decadal scale. That has an effect on the precise time pattern of the warming. Another hypothesized effect comes from the roughly 11-year cycle of solar irradiance which is shown in this graph. Sometimes it’s called in shorthand the sunspot cycle because sun spots are  correlated with the intensity of solar radiation.246

This is a natural cycle not human-induced in any way. You can see that the range of fluctuation is not so large, it’s about .25 watts per meter squared when the total solar flux is about 1,360 watts per meter squared and the amount incident on earth at the top of the atmosphere is 1/4 of that so the changes of solar irradiance are not insignificant but they’re not large. Many climate skeptics say well the Sun is getting warmer that’s why but you can measure the solar radiance with tremendous precision. It has a very small effect but it does not have a first-order large-scale effect. It certainly does not account for the long-term rise of temperatures nor does it explain much about the decade to decade fluctuations.247

Now there is another natural cycle that is more important not fully elucidated but clearly quite important in the interannual fluctuation of Earth’s temperature and that’s the state of the oceans and and especially the state of the world’s mega ocean the Pacific ocean. The Pacific Ocean has natural oscillations we are most familiar with something called ENSO, El Niño Southern Oscillation. This is a change in the state of the Pacific Ocean especially in relative pressures wind patterns and ocean surface temperatures across from the western Pacific in the Indonesian archipelago to the eastern Pacific which is off the west coast of South America.

We know what ENSO or especially El Niño, most familiar, is the warm water that appears off the coast of Peru often around Christmastime by and so it was named El Niño for the birth of Jesus or the Christmastime season and what it means is a phenomenon that occurs on natural intervals of a few years variation in which you have warm water from the western Pacific crossing the Pacific to show up on the coast of South America.

When it does that leads to rainfall in what is 248otherwise a very dry part of South America. It also causes a massive decrease of the normally prevailing easterly winds that blow the ocean water from the east to the west and pile up the warm water of the Pacific around Indonesia and so there’s a complicated atmosphere-ocean coupling in which the trade winds of the east, the easterlies, diminish significantly, the warm water sloshes from west to east, changing all of the tropics in temperature patterns and in precipitation patterns but from our point of view something quite important.

When the warm water of the El Niño arrives, global temperatures tend to rise. Not hugely but enough to really make a difference so when we have a big El Niño event as we did in 1998 that’s a blowout for global temperature. Many scientists think that 2014 perhaps 2015 could be a big El Niño year in which case right now our season could turn out to have a global boost of temperature coming from the current El Niño. Well there’s another oscillation in the pacific less clear hypothesized perhaps of a longer scale, called PDO Pacific decadal oscillation which may be related to the El Niño state or its counterpart La Niña when the warm water really piles up in the West and it’s very cold with cold upwelling water off the west coast of South America.249

This PDO or Pacific decadal oscillation is a more gradual change of the Pacific ocean state that also seems to be related to the frequency of El Niños or the opposite La Niñas and thereby to changes of global temperature. So long story short during the 1980s and 1990s there were lots of El Niños and that tended to boost the warming of the global temperature. The big one was 1998, and 1998 was really a blowout year, a very, very hot year. We’ve since surpassed it according to some measurements in some of the hottest years more recently, but 1998 really stands out as a very, very warm year because of the El Niño and perhaps that high-frequency of El Niños during those couple of decades associated with warmer global temperatures was related to the state of the Pacific in this decadal oscillation.

Now since 1998 we’ve tended to be more towards La Niña episodes, very cold water off of South America, perhaps related to a change in the longer-term, decades-long oscillation, and that perhaps is responsible for a slowing down of the year to year warming since the late 1990s. This is all hypothesis except to say that what seems to be the case in trying to project year to year or even decade to decade changes we need to understand the greenhouse gas emissions, we need to understand the aerosols both human caused and natural caused by volcanoes, and we need to understand the state of the oceans. When the state of the oceans are in a particular phase  he world tends to be warmer on average, global temperatures are high. When the oceans, the Pacific in particular are in another phase, the warming tends to be reduced.249a

All of this means that we don’t yet have a precise year to year model that can absolutely predict how the Earth’s temperature will evolve on an annual basis or over a period of a few years but the science does help us to understand certainly the major drivers of the longer-term change and gives overwhelming evidence as the Intergovernmental Panel on Climate Change continues to emphasize that it is human induced causes and especially the greenhouse gas emissions and among  those the energy-related carbon dioxide emissions that are the fundamental long-term drivers on the decade to century scale of the global warming that Earth has experienced and that we are likely to see continuing in the years and decades to come until we face up to the challenge posed to us in the framework convention to stabilize the greenhouse gas concentrations at safe  levels.249b

249e 249d 249c

In the next section, in the final chapter of this lecture, we’ll look briefly at some of the recent implications of climate change.


The Basics of Climate Change Science III

The Relentless Rise of CO2

We’ve been talking about the role of particular greenhouse gases what their radiative forcing is what their contribution to total greenhouse climate change is where they come from, and of course we’re going to aim to understand what we can do to control their emission, but we’ve seen that carbon dioxide is really at the center of the story.

It’s about three-quarters of the total anthropogenic greenhouse effect the vast majority about six-sevenths of the carbon dioxide emitted by humanity comes from energy and industrial processes, mainly burning of coal oil, and, gas. About one seventh of the total comes from land use change, mainly deforestation.

Our focus is going to be heavily on the energy sector at the energy sector in a way obviously in a way sad to say is so much at the center of the world economy and fossil fuels are so much at the center of the world’s energy system that finding a way to reduce dramatically the carbon dioxide emissions is not a simple matter It is the central focus of our challenge both in this course and of course in a global agreement.

In this chapter I want to talk about briefly the relentless rise of carbon dioxide emissions. There has just been no stopping it, not even any slowing it.231

This graph shows CO2 emissions from fossil fuel use in the course of the 20th century and up to 2008 in this particular figure, and you can see that this has been the pretty steep ascent, around a 15x increase of CO2 emissions from 1900 till today, from around 2.5 billion tons or it’s showing here as 2500 million tons but I’ll say 2.5 billion tonnes shown in this graph to today around 35 billion tons of carbon dioxide. Now part of that increase has come from the fact that the world population has increased rather dramatically from 1900  until today, from a bit over one and a half billion back in 1900 to today, 7.2 billion people so we’ve had more than a 4x increase the world’s population.

But of course the use of energy per person and the use of fossil fuels as the basis of that energy per person has also skyrocketed. Today and it’s good to keep in mind with 35 million tons of carbon dioxide being emitted with about seven billion people it’s about 5 tons of carbon dioxide per person on the planet. Now that relentless rise of emissions we know, and Charles Keeling whom we’ve already met earlier was the key scientist instrumental in bringing this fact to our attention, has led to the rise in carbon dioxide concentrations in the atmosphere Again we see the seasonality be upward and downward squiggles: CO2 high in the Northern hemisphere springtime just after the fall and winter has returned carbon dioxide into the air and CO2 low each year as carbon dioxide is reabsorbed by vegetation on the planet.

But those those annual cycles are dwarfed by the long-term upward ascent of carbon dioxide. In this graph, we’re just reaching 390 parts per million but I have to tell you sadly as of 2014 in the spring of this year we have reached 400 parts per million month by month for the first time on the planet in about 3 million years and so we have already changed the carbon dioxide concentration over three million years, a longer time period by far than our species Homo Sapiens has existed. We’re carrying the climate in other words to a new configuration unlike anything that humanity has ever experienced and the Keeling curve just reminds us of that fact fact.

Now why has CO2 continued to rise so relentlessly, both in emissions levels and the part that stays in the atmosphere raising the CO2 concentration? It’s because fossil fuels are such a central part of the world economy. In my course the Age of Sustainable Development I really do sing peons of praise to the steam engine as perhaps the single most transformative technological breakthrough in modern history by being able to tap fossil fuel energy via oil, coal, and gas put down in geologic time over tens of millions of years, hundreds of millions of years, and use that what was solar energy and biological form and now is fossil fuel energy we’ve been able to make a modern economy which depends on vast throughputs of energy for our quality of life and yet it is of course that fact, the centrality of fossil fuels, since the steam engine till today, that has made carbon dioxide such a relentless source of increasing greenhouse effect over the past century.232

Now in our time the single most dramatic change in the world economy absolutely second to none is China’s rise to economic pre-eminence as a macro economist I have to love it. Watching an economy go from poverty where the poverty rate was well over half of the entire population to a country, a modern economy where poverty has come down to much less than 10 percent, life expectancy has soared, conveniences of life have increased dramatically, its a wonderful thing from a development experience.

I think can be claimed as the most successful economic development experience in history. It’s in a very short period of time really after 1978 when Deng Xiaoping came to power till now, perhaps a 35 time increase in the size of China’s economy since that time. But it has had very significant side effects and downsides Within China the downsides are massive pollution, massive air and water pollution that has accompanied this very rapid economic development and industrialization. The air is noxious, the waters poisonous, years of life expectancy have been lost to this massive pollution.

The Chinese the leadership and certainly the people know it because they breathe the air every day. But there is a global effect as well, an effect that has global meaning that it also affects China very dramatically and that is that the Chinese have become the largest user of energy in the world, not per person mind you, but given the fact that China is the most populous country in the world with 1.3 billion people, China is the largest user fossil fuels and now by far the largest emitter of greenhouse gases in the world. So dramatic in fact that I think the Chinese government clearly recognizes as does the whole world that China’s leadership in finding a way forward together with the United States, with the European Union, with other parts of the world, on reducing greenhouse gases is now a preeminent aspect of any real solution in this climate challenge. So in the graph that you’re looking at now we see two quite dramatic curves.233

One in China’s total energy use, that’s shown in red. It’s measured in this case in tons of oil equivalent. You take the energy in natural gas, you take the energy in coal and so forth converted to an equivalent number of tons of oil add them up–that’s the totally energy expressed in units of oil and you can see comparing that to the blue curve, the dramatic rise of real gross domestic product measured in the trillions of dollars that China’s economy has boomed and alongside it not unexpectedly has boomed energy use. You can seen that since the blue curve has increased by more than the red curve that energy per dollar of GDP has actually gone down.

The energy intensity of the Chinese economy is falling slightly, but the rise at the Chinese economy is so dramatic that total energy use has increased significantly. And if you look at the next picture you see that China has soared to the top of the world leagues in admissions of carbon dioxide. This is showing total billions of tons of CO2 omitted per year and around 2007 or so China overtook the United States which was long in the lead as the largest emitting economy of the world China became the number one emitting economy. Its a side effect if you will of its stupendous economic rise.

Now why are the emissions so large? For two big reasons: one is of course that the Chinese economy is growing so sharply and the second is that China’s energy is heavily based on fossil fuels and in particular based on the single fossil fue, coal, which emits the most carbon dioxide per unit of energy so coal is the most important driver of China’s emissions and what you can see in this pie chart which looks at total energy consumption of China that China’s overwhelmingly a fossil-fueled driven economy.234

Most are by the way so China is not alone in that but China extraordinarily depends on coal resources for about 70 percent of its total primary energy use, oil another nineteen percent, and natural gas about 4 percent more on top of that. China has major hydro-electric power like get Three Gorges Dam, which is so famous, hat’s about 6 percent of its total primary energy, and about 1 percent is nuclear energy with an intention to have a much larger proportion of energy coming from nuclear power in the future because nuclear power is not carbon emitting.

It’s got many other issues which we’ll discuss but it is a zero-carbon energy emitter and therefore part of China’s trajectory to reduce its CO2 emissions. Now if we look at the global emissions by country in two different interesting ways we see the primacy of China and the United States in this story. On the left hand pie chart we have the world’s emission, they’re 9.6 gigatons or that is billion tons of carbon. Remember to multiply by 3.667 and that would keep you roughly 35 billion tonnes of carbon dioxide, a little bit less back in 2012 but about that. And China as you can see is that that big red wedge, roughly a quarter of the world’s emissions, the United States about 15 percent of the world’s emissions as shown here.

The two combined? Well you’re talking about forty percent of the whole world’s emissions. Look to the chart on the right and you see a little bit different story: the US has been emitting CO2 for so long, it’s been the world’s third-largest economy for so long that it has the lion’s share of the cumulative emissions from the start of the industrial era, roughly the middle of the 18th century. So if we look not at the flow of the emissions but at the accumulation of the emissions in history, there the United States is number one. Well what should we look at?235

Both The flow of emissions tells us how we’re evolving now, the historical emissions tell us how did we get this increased level of CO2 in the atmosphere to begin with since it’s been rising relentlessly over the last 100 years and there the United States plays the biggest role. And not surprisingly the United States looks to China and says you’re such a big emitter you have to cut emissions and China looks to the United States and says well where have you been for the last century thank you, you’re responsible for the accumulation of emissions, you take the lead. What we know and what we’re going to be analyzing with considerable care and detail and what we will be negotiating in the next semester of this course when we’re all global delegates to the global online negotiation is how to take into account historical responsibility, current to flow missions, how to do this in a balanced, fair, and effective manner.236

Now china would also be keen to emphasise this picture. Yes China is the biggest emitting country in the world but after all China’s about one-fifth the world’s population Given that, it’s not surprising that it is a large emitter but per person per person in the United States remains way in the lead and this is a chart which shows for 2012 the emissions per person gain measured as tons per carbon. If you want to change it to carbon dioxide multiply by 3.667 or 44/12 and you can see that the United States is about four-and-a-half tons per person of carbon or about sixteen times up carbon dioxide per person. China’s about two tons per person or roughly speaking a little over 7 tons per capita.237

What about the world as a whole? Well that’s the dotted line, the global mean, and we remember that’s about five tons per person because it’s roughly $35 billion tons of co2 emissions divided by seven billion people on the planet, that’s five tons per capita. So who are the big emitters per person? It’s the richer countries and it’s the heavy energy producing countries: the United States Canada, Australia, Russia, those are the high emissions per capita.238

The big absolute emitters have big populations like China, and China’s now above the global mean, it’s about the global average, because it has been so successful in economic development that it is now using more energy on average and heavily coal-based energy and that is why together with its absolute size it has become the number one emitting country in the world.

What we will look at next time is again putting some of this in context with the most recent debates on climate change out, what’s happening year to year, decade to decade, to the change in Earth’s temperature? That’s the topic for the next chapter.

The Basics of Climate Change Science II

The Greenhouse Gases and Feedbacks

I am going to talk in this chapter in more detail about the individual greenhouse gases and some of the other forcings or human-induced as well as natural-induced changes of earth’s climate.221

It’s been recognized and, as I discussed in the previous chapter, that several different compounds–carbon dioxide being the most important, but several other compounds all three atoms or more because of this infrared trapping property play a role in the Greenhouse Effect, and when the UN Framework Convention on Climate Change was introduced and then the Kyoto Protocol, the focus was on the so-called anthropogenic, well-mixed greenhouse gases.

These are greenhouse gases that are caused by human activity, so not water vapor which is a huge greenhouse gas but not directly caused by human activity, and a kind of gas that mixes in the atmosphere. that’s extremely important also to understand when carbon dioxide is emitted from a power plant in China or in New York State or in South Africa or in Indonesia within about a month the carbon dioxide that is emitted from any of those particular sites is pretty much uniformly distributed in the whole world. In other words, the atmosphere is well mixed.

The carbon dioxide that I’m inhaling right now or exhaling isn’t going to be specific to New York City where I’m speaking right now; it’s going to have an effect on the global carbon dioxide concentration in a uniform way. We could add as a footnote that if each place on the world emitted its own greenhouse gases that have stayed there over their own heads and their own responsibility in their own local climate, we’d probably reach a resolution of this crisis a lot more quickly because a place that was experiencing warming or climate disruption would say look what we’re doing to ourselves, but since when we emit carbon dioxide or methane or nitrous oxide has effects globally, we don’t pay so much attention to our local actions causing global effects spilling over all over the world. Well, the Kyoto Protocol identified these well-mixed anthropogenic greenhouse gases and the focus has been on the major 6 which are shown in this table.221

Carbon dioxide, the most important of all, and it will be our focus through most of this course because it’s so important, it is so long-lasting in its effect, it is so deeply part of the world economy because the core of these anthropogenic emissions are from our use of fossil fuels for use of modern energy that it is appropriate to call the whole challenge decarbonization, but you can see there are other very important greenhouse gases as well.

Methane or CH4 is emitted by human activity in the atmosphere in many many ways that we’ll discuss these briefly: in agriculture, in landfills, in escaped methane from natural gas pipelines as gases piped to our cities, and into kitchens in many parts of the world.

Nitrous oxide is another greenhouse gas that comes from ways that we burn fossil fuel that comes from fertilizer use of nitrogen-based fertilizers that comes from other industrial processes, and then there are these long complicated so-called “F gases” fluorine-based gases: the hydrofluorocarbons or HFCS the perfluorocarbons or PFCs and sulphur hexafluoride or SF6. These are also very potent greenhouse gases. Fortunately, they’re used in such limited amount that they’re still quite small part of the overall anthropogenic process.

Now, in order to aggregate across the six anthropogenic greenhouse gases, we have to ask the question how powerful are they in their greenhouse effect, how much of that infrared radiation emitted from the earth does each molecule of these 6 gases absorb and by comparing that absorption of infrared radiation by these different gases we can give a weighting to the different gases in terms of their overall greenhouse effect.

Everything is scored relative to carbon dioxide, and so we want to understand the greenhouse effect of each gas relative to carbon dioxide so carbon dioxide is given a warming potential of one that you can see in the middle column top row of this table.

Then, each of the other gases molecule for molecule has even a more powerful warming effect than a molecule of CO2; methane 23 times molecule for molecule taken on a hundred-year timescale more greenhouse warming than carbon dioxide. Nitrous oxide, you see in the table, 296 times more powerful.

Perfluorocarbons 5,500 times more powerful, so why is CO2 so important because there’s so much human emission of carbon dioxide compared to the others that even adjusting for the per-molecule warming potential taking the number of molecules that humanity is emitting of each kind of greenhouse gas multiplying it by the warming potential per molecule, CO2 comes out way ahead and that you can see in the final column this is for the year 2000 and of course it keeps changing as the relative proportions of emissions change but in the year 2000, carbon dioxide accounted for about 77 percent of the human-induced greenhouse effect of missions that year.

Methane number two 14 percent, nitrous oxide number three and the sum total of the fluorine-based gases under 2 percent, so that’s the summation; that’s why decarbonization is number one three-quarters of the total effect but there’s another factor that’s also extremely important to keep in mind, and that is that when we put these molecules into the atmosphere through human activity whether it’s burning coal, oil, and gas or whether it is farming in a way that emits methane from a rice paddy or from livestock that are being grown for  meat production whatever it is that molecule in the atmosphere has a certain lifetime expectancy of being in the atmosphere before it is reabsorbed onto the earth or into the oceans.


Part of the carbon dioxide that is emitted each year is immediately absorbed by the ocean, part goes into the photosynthesis on the planet and is stored biologically rather than staying in the atmosphere, but the fact of the matter is that when carbon dioxide molecules enter the atmosphere at least a lot of that input of CO2 into the atmosphere is gonna stay there for a long time.

In this chart, it says five to two hundred years but the fact of the matter is a certain fraction of that carbon dioxide is gonna stay in the atmosphere for thousands of years maybe 20 percent in total of the co2 in the atmosphere will stay for hundreds or thousands of years–what’s called the residence time in the atmosphere will be very very long what we’re doing to change the atmospheric composition is not going to be reversed very quickly.

On the other hand when methane is put into the atmosphere the residence time for methane is very short through  chemical and physical processes the residence time methane in the atmosphere is much much smaller than for carbon dioxide, which means that if we control the methane emissions, the methane that we have historically emitted into the atmosphere isn’t gonna stay there for decades or centuries like carbon dioxide it’s going to fall much more quickly so when we think about the role of each of these greenhouse gases, we need to think about the molecule for molecule warming potential, the total amount of the molecules that human activity is putting into the atmosphere and the residence time of those molecules in the atmosphere we need all three of those dimensions to shape an appropriate response.223

It’s even worse because of course the greenhouse gases are not the only changes caused by humanity that affect the climate and on top of the human-induced effects there are multiple feedbacks and multiple natural forcings as well. No one said this is easy  and no one said that it’s an absolute simple matter so when we look at the total net radiative effect of all these greenhouse gases and other chemicals that humanity’s putting into the atmosphere and other effects  of humanity on the climate system were led to a rather complicated chart of the kind that you are looking at right now.

Now what this is is an attempt to add up across not only the greenhouse gases but other kinds of factors that changed the net radiative balance of the planet the watts per meter squared of net radiation that determines the earth’s eventual balance in temperature. At the top of this chart are the long-lived, well-mixed anthropogenic greenhouse gases, and the biggest bar in red that means warming, net radiative warming, of the planet is CO2. That’s what is our main focus.

Then, as you see in the next bar not as important as CO2 roughly 1/4 of the total CO2 effect are the other greenhouse gases: methane, nitrous oxide and the fluorine-based gases or here called Halocarbons. Then there is an ozone effect which is a kind of feedback because many of our chemical processes change the composition of ozone in the troposphere–that’s the lower atmosphere– and in the stratosphere–that’s the upper atmosphere.

On balance as ozone increases for example in the troposphere through  various chemical effects and warming itself you get another bit of radiative forcing to warm the planet that’s the third red bar that you see not as important as  carbon dioxide or the other greenhouse gases but still a major warming effect. Well, you can go down category by category. Let me draw your attention to surface albedo that remember is the reflectance of the earth’s surface.If you have forests, they absorb a lot of sunlight; clear the forest, you get more reflection of the sunlight not absorption.

The sunshine comes in, goes straight out to space and doesn’t get absorbed as much as if you have forest cover so as humanity changes the surface of the earth: ice, forests, cities and so forth we changed the reflectance or the albedo of the earth. The net effect of that is mixed. Land use change has been having a net cooling or negative radiance effect Some kinds of surface changes such as pollution which makes the ice darker because soot falls on the ice means that what normally would be reflected is absorbed by the ice  so you see a little bit here called black carbon on snow meaning that even the snow was not reflecting because it’s a it has this pollution on it and it absorbs more of the radiation.

Then comes a major category called aerosols. Aerosols are a set of small particles also partly driven by nature such as a volcanic eruption which huge amounts and of sulfates into the stratosphere or by human activity when we burn coal and coal has sulfur pollutants and we put sulfates into the atmosphere and these sulfates are tiny little particles which we call aerosols. They are often disastrous for you in health when you have those huge smog attacks in Beijing Beijing or in Indian cities in recent years. That’s aerosol pollution. Now, there are many many kinds of aerosols, unfortunately. There are so called white aerosols like sulfates, there are black aerosols like soot, there are organic aerosols that come from burning certain kinds of chemical compounds and certain kinds of biomass. They each have their effects.

The sulfates, the white aerosols, tend to dim the sunshine and they have a net cooling effect even though they’re very polluting There are even some very bad ideas called geoengineering ideas of putting sulfates deliberately into the air to dim the sunlight as a kind of remedy for our greenhouse gas emissions. Bad idea we’lll come back to it but in any event it’s out there.


Other kinds of these aerosols especially the soot tend to warm the atmosphere so we have to add up all these effects and then there are the direct effects of dimming the sunshine and what are called the indirect effects or the cloud albedo effects. When we put aerosols into the atmosphere, they change the cloud formation they sometimes provide the nucleus for clouds to form those clouds then reflect incoming solar radiation and thereby have a cooling effect on the planet so aerosoles not only diffuse sunlight and dim sunshine and cool the planet that way but they also change the cloud cover and can have an indirect cooling effect as well.

Well you see in this that the total aerosol effect of anthropogenic aerosols that’s human-caused aerosols is deemed to be cooling on balance but there are certain kinds of  aerosols that are definitely warming kinds of aerosols. Well you can add up all of these and get the total anthropogenic effect that is the effect of the net energy balance caused by human activity but of course that’s not the only thing going on even on a relatively short scale we have changes though modest in the amount of incoming solar radiation. The sun has a natural cycle of more radiation or less; it’s a very small margin but it does have a very small effect on earth’s temperature.

We have long long cycles of changes of solar radiation that come from changes in the Earth’s orbit over tens of thousands of years that’s what gives us the long fluctuations of up and down of CO2 that we saw in an earlier lecture that are also part of the ice ages of the interglacial periods of the Pleistocene epoch and those are very long changes of dynamics so we have the human caused greenhouse gases, we have changes of the earth’s reflectance, we have pollutants that through aerosol effects both direct and  indirect indirectly change the climate, we have changes in solar radiation in the short term solar cycles, and we have long-term changes that depend on earth’s orbit.

It’s a complicated story. it is the role of climate science to parse these various effects, to use advanced physics, both in theory and through many different kinds of observation, to create a graph like the one that you’re looking at that is able to add up across all these different factors to ask: what is the human effect? Now if we turn to the next graph, we can get overtime two crucial facts: first that the total amount of emissions is rising that we know because the world economy is growing, more fossil fuels are being used, more nitrous oxide and methane a fluorine-based gases are being used and emitted into the atmosphere.

We can also allocate the total net greenhouse effect across these different gases by using and CO2  equivalents multiplied by the number of molecules of each kind of greenhouse gas put into the atmosphere year by year. If you look all the way to the right hand side of this rising curve you can see the allocation of the total greenhouse effect according to key categories.224

The big base in beige at the bottom is the carbon dioxide emitted through fuels and through other industrial processes there are a few industrial processes like cement manufacturing that emit CO2 not by burning fossil fuels but by other kinds of material transformation.

Then the next bar darker maroon color is the carbon dioxide that comes from deforestation and other land-use change and this is also notable but a lesser contribution of carbon dioxide emissions than the energy use.

Then the next major category of the greenhouse effect is methane emissions. Methane emissions come from many different kinds of industrial activities come from so-called fugitive gas that’s being released by drilling for gas and oil or being piped in pipelines. Methane is released by ruminant animals such as cows from the anaerobic digestion in their multiple stomachs. Methane is released from rice paddies again by anaerobic respiration of bacteria in the flooded rice paddies so humanity introduces a lot of methane. Methane next to carbon dioxide is the second most important of the greenhouse effects an then the next bar up in light blue is the nitrous oxide, the N2O.

That is again a side effect of combustion processes, of industrial activities, of chemical changes to nitrogen based fertilizers and other agricultural activities and then the small amount of about one   to two percent of the total greenhouse effect are the fluorine based industrial chemicals used as refrigerants and for other industrial processes.

225Take the total picture CO2 from energy & industry sixty-five percent add in the CO2 from land-use change that’s another 11 percent we’re up to 76 percent 3/4 of the total greenhouse effect. Add in methane and you’re at ninety-two percent of the total add in the nitrous oxide and basically you’re at about ninety eight to ninety-nine percent of the total.

Our focus in most of these lectures will be on carbon dioxide mainly on carbon dioxide from energy and industry but any real agreement that is meaningful next year in Paris at COP 21 is going to have to pay attention to all of the anthropogenic greenhouse gases. If we turn to the next graph, we get yet a different way to view this issue and that is by asking what sectors are the source of emissions of these various greenhouse gases and so we add up across all the greenhouse gases and ask what’s responsible for this and of the total emissions we can then allocate them through direct actions in various sectors and indirectly from electricity generation that is then used by these various end use activities so what are the end use activities that are shown here if you go around the circle starting at the top in green is something called AFOLU which is agriculture, forestry, and other land-use, thank you, and that is ll the emissions that come from the land use sector including from agricultural activity.

Now agriculture emits not only carbon dioxide through energy used in agriculture but emits methane from the bellies of our livestock and our ruminants and from rice paddies it emits nitrous oxide from chemical changes to urea and other nitrogen-based fertilizers it turns out that this AFOLU  sector agriculture, forestry, meaning deforestation, and other land-use change is the single biggest sector of all in terms of anthropogenic greenhouse gas emissions if you go around counterclockwise this time you have the dark blue of the building sector and that means greenhouse gases released within buildings. What is that? Well, the direct emissions are from our boilers and furnaces and our stoves and gas, natural gas, cooking and so forth.

The next in red is the transport sector. Under the hood, what do almost all vehicles have? Internal combustion engines; a few now have batteries that are running electric motors but most of our vehicles until now over the last century have been internal combustion engine burning diesel or gasoline or other petroleum-based fuels and a few of these internal combustion engines burn biofuels as well but that transport sector which includes not only automobiles and trucks but also rail and shipping, ocean shipping and aviation, is a very substantial part of total emissions about 14 percent of the total greenhouse gases.

The industrial sector obviously a major emitter, major user of energy, a major transformer  of materials, such as turning calcium carbonate to calcium oxide on the way to cement production thereby releasing carbon dioxide into the atmosphere. Iron and steel, pulp and paper petrochemicals–they’re all major emitters of carbon dioxide. It’s why the heavy industrial economies are in a way structurally major emitters of greenhouse gases. Now all of those shown around the circle until we get to the right hand side are direct emissions of carbon dioxide.226

What’s called here the indirect CO2 emissions are the emissions that come from generating electricity at a power plant, meaning perhaps by burning coal or by burning natural gas and then that electricity is used in one of these other sectors. It can be used in industry, it can be used a little bit in transport like electric vehicles, it could be used of course in buildings to heat, cool, and ventilate buildings and so this is another piece of the action it’s the power sector you see the power sector is a big deal.

It is a major source of emissions decarbonizing the electricity generation will be one of the keys to decarbonizing the world economy and we see that the industry in the building sector are the two big users of electricity and therefore they are indirect, this is an indirect source of the emissions now I just love the next diagram I want you to go cross-eyed looking at it.

It’s a beautiful, artistic schematic of almost the same thing but is a way to track where these emissions come from on the right hand side are the greenhouse gas emissions with that big brown section being the CO2 and then just below it, the “F gases” and just below that the methane and then at the bottom the nitrous oxide and then if you go all the way to the right hand the left hand side of this diagram you ask what sectors are the sectors that are emitting these greenhouse gases found on the right hand side so it’s another way to go from a basic sector allocation to the greenhouse gas emissions and let’s just take an example.227

Start at the top on the left hand side you have the transport sector and the transport sector is responsible for in this characterization of about a quarter of the total greenhouse gas emissions. Go to the middle and ask what kind of transport? Well, roads are most of it but then also air, rail, shipping, and other transport are also there and what kind of greenhouse gases does the transport sector emit? Almost entirely carbon dioxide the transport sector doesn’t emit methane; it doesn’t emit nitrous oxide much; it doesn’t emit any of the other fluorine-based gases and so in the end it’s a carbon dioxide emitter. The next big block is the energy sector for producing electricity and heat or for direct fuel combustion for boilers and furnaces and buildings or for combustion for industrial processes such as iron and steel production or petrochemical production.

If you go towards the bottom you have a Purple Line which is agriculture. What does agriculture do? Well agriculture is responsible for changes of nitrous oxide emissions for instance through fertilizer use. Agriculture includes livestock raising and the livestock as I’ve noted emit methane through digestive fermentation processes and the like.

This is a very detailed I think rather ingenious rendering of the complexity of where all of the greenhouse gases come from. In essence to get greenhouse gas concentrations under control, we are going to need to move logically and systematically across this graph.

What to do with transport what to do with agriculture what to do with the energy sector That’s the topic of lectures to come and if we finally just look at the spaghetti of the energy sector alone and in just one country we can see the remarkable complexity of different sources of energy and different uses of energy and so in this final graphical rendering made by Lawrence Livermore National Laboratory in the United States for the US energy system we are primary energy sources on the left hand side of this table at the base is petroleum you can see carrying along it goes mainly into the transport sector.

Then comes coal and if you track that along some goes to industry a lot goes to electricity generation and the like. We have many forms of primary energy. They are transformed partly into electricity for end use, partly through direct use up the primary energy in buildings or in automobiles or in industrial processes and then in the end, it is the carbon use in particular as opposed to alternatives like solar, nuclear, hydro, wind, and so forth that contribute to the greenhouse gas emissions.

A chart like this is extremely important because two-thirds of the total radiative forcing of anthropogenic greenhouse gases is carbon dioxide from the energy sector and therefore reforming the energy sector so that there’s more reliance on low or zero carbon sources and more energy efficiency will be absolutely central to our ability to stabilize carbon dioxide in the atmosphere in the next section we’re gonna talk about carbon dioxide more because it’s the relentless increase of human-induced carbon dioxide that is really at the core of the drama.


The Basics of Climate Change Science I

The Earth’s Energy Balance

We will be talking about the basics of climate science. We need to build on these basics in order to understand our choices about deep decarbonisation and other actions related to other greenhouse gases in order to understand why, how, when, at what pace we should be reducing emissions of greenhouse gases in order to stabilize the concentration of greenhouse gases at safe levels. What’s safe? What’s the relationship between the greenhouse gases and climate?

That’s the purpose of this lecture: to give us an introduction to this very rich, very sophisticated a hundred ninety years of science, and in the first chapter I’m going to talk about the Earth’s energy balance.

This goes back as we’ve already noted about a hundred ninety years to Joseph Fourier who first realized that the Earth’s average temperature would be determined by a kind of balance or equilibrium between the incoming energy of solar radiation and the outgoing energy that the earth radiates back to space.211

That’s when the Earth’s temperature is at a level such that the incoming radiation and the outgoing radiation are in balance. That we have an equilibrium, a place of stationary temperature for the Earth and understanding how greenhouse gases affect that balance has been the core of climate science since Fourier’s very creative understanding of this process since the 1820. This diagram that you’re looking at shows in very simple, schematic terms this global energy balance.

At the center of the graph is the center of the whole issue and that is that the Earth receives radiation from the sun, that the radiation from the sun warms the planet. If we look at the amount of radiation at the top of the Earth’s atmosphere that’s determined by the distance of Earth from the Sun and from the sun’s energy in radiative flux and we can measure that just as we do with our lightbulbs in watts: that’s a unit of power and watts per area or watts per meter squared is the standard unit that scientists use to measure the incoming solar radiation and what you can see from this diagram is that on average given the solar constant output we have an average amount of about 341 watts per meter squared at the top up the atmosphere.

Now as this incoming sunlight comes in, that’s mainly how we experience this electromagnetic radiation is sunlight, that is radiation at a certain frequency much of which is within the visible range, part of that is immediately reflected, the part that you see towards the left hand side bouncing off clouds back up into space Another part of the incoming solar radiation is reflected by the surface of the Earth.

When the sunshine comes in and hits an ice sheet, say the Greenland ice sheet, or hits sea ice floating in the North Atlantic and and the sunshine just radiates, is reflected in re-radiates back out into space that is reflected on the left hand side. But of course a certain amount of the radiation doesn’t bounce off the clouds back into space and doesn’t bounce off of the earth’s surface but is absorbed by Earth and warms the planet. And the basic idea is that any body, including the body of planet Earth, when it has a certain temperature itself radiates energy.212

This is a basic fact of physics This is a basic fact of physics and the basic study of it is called the study of blackbody radiation and in fact the Earth absorbs radiation from the sun. You see that from the center to the left of the diagram but then it radiates energy back into space, and one of the most interesting and basic facts of all of this is that the incoming radiation is  in the form of visible light or ultraviolent radiation, UV radiation, and that is relatively short wavelength, high-frequency radiation, and the radiation that the earth itself causes by it being a warm body is a bit longer wavelength called infrared radiation. So the incoming arrows are visible sunshine for example and the outgoing radiation on the righthand side of the diagram is infrared radiation.

That’s gonna play a very very key role in our understanding of climate and the greenhouse gas affect because the basic idea is that the greenhouse gases (carbon dioxide, methane, nitrous oxide, some industrial chemicals) are basically transparent to the incoming solar radiation, they allow it to come in, but they are not quite so transparent to the outgoing infrared radiation from Earth itself, the longer wavelength. In fact, they absorb that infrared radiation in part, and it’s that absorption of the Earth’s own infrared radiation that traps energy that otherwise would go out to space. It traps energy and creates a kind a blanket or the greenhouse effect if you will that makes Earth warmer than it otherwise would be.213

You see that in this simple diagram by the fact that we have surface radiation aiming back towards space and then you see that some of that circles back after it hits the greenhouse gases in terms of what’s in this diagram call the back radiation towards the plane. Now in balance or in equilibrium the Earth’s temperature is determined such that the arrows coming in equal the arrows going out, and if we were to have no greenhouse gas in the atmosphere if we were a a planet without an atmosphere, if we were like the moon, then there would be only radiation going back out in space, none of that back radiation going from the greenhouse gases back to earth, and the balance would be reached at a relatively low temperature of the planet.

Indeed, the temperature would be roughly 33 degrees Celsius lower than it actually is on the planet. The actual temperature of Earth on average is about 14 degrees Celsius. If we didn’t have the greenhouse gas cover we would be roughly 18 degrees minus, negative 18 degrees Celsius, instead of the actual 14 degrees Celsius that we have, and that is the difference of having a greenhouse effect that traps some of the outgoing infrared radiation and not having an atmosphere with that greenhouse effect that would just allow the radiation to go back into space directly. Now this diagram’s filled with all sorts of complications  and this is why be underlying science of the greenhouse effect has many challenges.214

How much of the incoming radiation actually reflects back to space? That depends on cloud cover, that depends on the surface of the earth, how much is ice for example, how reflective is the Earth’s surface, what’s called the albedo of the Earth. If ice melts then what used to be reflected back into space of the solar radiation now gets absorbed and you get a kind of feedback effect where a warming up the planet melts the ice, reduces the reflectance of the incoming radiation, increases the absorption of the incoming radiation, and further warms the planet. And many other dynamic effects are present here meaning that your simple simplest calculations can’t quite do the job telling us precisely how an added level love greenhouse gas is going to change the radiative  balance and thereby change the equilibrium temperature but this simple illustration is very very helpful in explaining the basic greenhouse effect.

Now to move one step more deeply, it’s important to understand this specificity of what makes a greenhouse gas and that is shown by what’s called a radiation spectrum of both the incoming radiation and some of the absorption of that spectrum. Light comes to Earth from the Sun or electromagnetic radiation comes to Earth across different wavelengths and so we go from very very short wavelengths on the left handside of the spectrum to very long wavelengths on the righthand side from ultraviolet towards the left hand side of the electromagnetic spectrum towards infrared and long wavelengths on the righthand side of the spectrum, and the amount of energy that is in the solar radiation is shown essentially by this spectrum of the sunlight and it’s that dark line which shows the radiation spectrum.

How much of irradiance, how many watts per meter squared is coming at each wavelength of solar radiation. You can see that the peak of that radiation is in what’s called the visible range of the electromagnetic spectrum. Visible because that’s what we see that’s the light we see. We don’t perceive ultraviolet or infrared, that’s outside of the visible range for human beings, not for some animals but for us, and so most of the energy, most of the watts per meter squared of the incoming solar radiation is in the visible range.215

Not so for the outgoing radiation from the planet, and this is a part of physics that comes from that theory of blackbody radiation called the Stefan-Boltzmann equation. It basically says that a very hot object like the sun will have more irradiance at the high frequency or low-wavelenth end of the spectrum whereas a cooler body like the Earth will have more radiation at the long-wavelength or infared part the spectrum.

So since Earth is a lot cooler than the Sun, we radiate at the righthand side of the spectrum. Now why does that matter? It matters because certain compounds, these are the greenhouse gases, absorb infrared radiation. That’s part of their chemistry, part of their quantum physics. The compounds that absorb radiation all have more than two atoms so O2 or N2, oxygen as it is in the atmosphere or nitrogen, dinitrogen, as it is in the atmosphere, is not a greenhouse gas.

To be a greenhouse gas you need to be 3 atoms or more. That allows the atoms to jiggle in particular ways and to absorb the infrared radiation So CO2 was three atoms, 1 carbon, 2 oxygen atoms. Nitrous oxide, N2O, methane which is 5 atoms, carbon and 4 hydrogen atoms all have configurations in their bonding that allows them to or makes them absorb infrared radiation and by absorbing the infrared radiation, they absorb the energy that otherwise would radiate to space.

They warm the planet and so one can see shown in this diagram, the so-called absorption bands of carbon dioxide and water. They’re to the right hand side of this figure. What does that mean?

They absorb longer wavelength electromagnetic radiation, the kind that Earth radiates, they don’t absorb the kind of radiation coming. The long and the short of it, they are transparent to the visible sunlight that we see when we go out on a sunny day but they absorb the infrared that we don’t see that the Earth is re-radiating as a warm body at an average of 18 degrees Centigrade and it is precisely the absorption of the infrared radiation that keeps the planet at the average of about fourteen degrees Celsius rather than the minus 18 degrees Celsius that would prevail if we didn’t have the greenhouse effect. And what we know is what Arrhenius told us back in 1896 and that is that as we increase the concentration of carbon dioxide or methane or nitrous oxide or other greenhouse gases in the atmosphere we’re going to get a warming.

We can measure the temperature, you have to do it very carefully in weather stations all over the world, my colleagues at the Goddard Institute of Space Studies, NASA’s leading scientific outfit for measuring the Earth’s mean temperature and one of the major enterprises in the whole world for this has produced very very careful data on changes of Earth’s temperature that’s illustrated by this graph.216

Now in this particular graph, the 0 line is the Earth’s average temperature for the years 1951 to 1980 and what you can see is that by our period by the years after 2010 or so, we’re at about .6 of 1 degree Celsius or about one degree Fahrenheit warmer than the average of 1951 to 1980, and you can see from this upward slope that the Earth is warming warming. It’s not warming every year, there’s a lot of variability. In fact there are even episodes, look at graph from around 1940 to around 1980, where there wasn’t a lot of warming and that raises a lot of questions CO2 and other greenhouse gases were rising but the temperature wasn’t rising all that much so that poses a question of what else is happening but the general direction is unmistakable and it instead of taking an average of 1951 to 1980, we took the average temperature before the whole industrial revolution started we would see that we’re close to a one degree Celsius increase of temperature now, about .9 of one degree Centigrade and this is of course the upward slope that is so frightening because it’s already disrupting the planet and it will cause a lot more disruption. Now let’s turn in more detail to the specific greenhouse gases that are responsible for this human-induced change. That will be the topic of the next lecture.