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.

 

Pedro Pereira Leite

Researcher and professor. He had his PhD. on museology in 2011, with the title “Muss-amb-ike Homeland: The commitment on musicological process”, that was published in 2011. In 2012 he finishes a Post-PhD Research on "Biographical Glances: The intersubjectivity poetry on museology, at Lusófona University (Lisbon). Presently he is working in his Post PhD. Research about: “Global Heritages" with the aims to build a network on local cognizance and memory manager has a tool to build the will of action in 3 different communities, linked by past communed heritages.” He works at CES. He participates on different Research network, presented papers in national and international conferences, and had published books on research subjects.

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