Climate change is the biggest of all the environmental threats that we face, and the magnitude of this threat is only gradually dawning on humanity. It is, in my view, the very toughest of public policy problems that humanity on a whole has ever faced. Let me mention why. First it’s global, many problems are local, local communities can solve them. So problems are national, this one involves the entire world. The problem goes to the core of the economy. Our economy runs on energy. And energy in the form of fossil fuels is both the driver of the world economy, and the essence of the climate change problem. Because when we burn fossil fuels, we emit carbon dioxide into the atmosphere. And that is the major reason why humanity is changing the climate. Not the only one, but the major one. Third, this is a dire problem. The consequences and the stakes are absolutely huge. Sometimes problems are difficult, but they don’t matter all that much. This one matters a lot. Next it’s slow moving. We’re just not good enough to see it day by day, year by year, what’s really happening to the planet. The scientists are telling us, watch out, you’re getting to the cliff, you’re hitting the thresholds, you’re crossing the boundaries of safety, but life goes on. And because this is a problem that stretches out over decades the politicians, the businesses and all of us don’t necessarily feel lit and know it and realize the dangers of it. Because it’s slowly, slowly creeping up on us. Not slow by the chronology of the planet because events are changing very fast by that standard but a little bit slow by news events and that’s part of what confuses us. It’s a complicated problem because it involves many parts of the world economy and involves every part of the world both in the impacts of climate change on humanity, but also on the causes of climate change. Because every part of the world is contributing to the problem, though some places, like the United States, on a per person basis are contributing hugely relative to other places, which are mainly victims, especially the poorest parts of the world that still don’t use very much fossil fuel energy. This is a multi-generational problem. A lot of the impact will be felt by people not even born yet, they’re obviously not voting. They’re not able to raise their voice, and yet the impacts on them will be absolutely huge. And finally, this is an issue on which there are very powerful lobbies. None more powerful than big oil. The oil industry is a rich industry. Massive flows of revenues and profits.And that that can be used for media, that can be used to confuse the public to defend the status quo, to fund politicians that are going to continue to look after the interests of the fossil fuel industry even when that interest may be so dangerous for the planet. So we have a lot of vested politics in this issue as well. Well in order to see really what needs to be done and how it might be done, we have to start with the science. And the science I think it’s important to state it’s not some new fangled, invention of, of the latest, scientific publication. The basics of how greenhouse gases warm the planet and what humanity might do, therefore, to the Earth’s average temperature as a result of the increase of greenhouse gases has been known for more than 100 years. You’d never know that from climate skeptics and the climate change deniers, but the fact of the matter is that the first serious, absolutely brilliant scientific study of what would happen if the amount of carbon dioxide, CO2, in the atmosphere were to double, how temperature would, would change was published in 1896 by Gustavus Arrhenius, an absolutely brilliant Nobel Laureate chemist in Sweden who worked out by paper and pencil, and did it just right. What would happen if CO2 were to double? He got the answer right, that the temperature on the planet would rise by a few degrees centigrade. He didn’t get the timescale right, because he said that while human use of coal and oil and other fossil fuels would eventually double carbon dioxide, he thought that would take hundreds and hundreds of years, maybe 700 or 800 years. But in fact, because of the geometric growth of the world economy, a process that looked like it might take hundreds and hundreds of years could happen within roughly 150 years from the time that Eranius wrote, by the middle of the 21st century, by 2050 or so. That’s a very frightening prospect. Why is it frightening? Well, the basic reason is shown in this schematic diagram of the greenhouse gas effect. What it shows is that sunshine coming from the sun reaches the planet as ultraviolet radiation. A little bit of it is reflected by the clouds. Most of it reaches the earth. A little bit of that is reflected by the ice and other land surfaces on the planet but most of it is absorbed by the earth. The Earth warms and as a result of it’s warming, the Earth radiates infrared radiation back into space. The Earth warms enough so that the outgoing infrared radiation equals the incoming ultraviolet radiation. That’s a thermal balance, or thermal equilibrium. Now you put a level of greenhouse gasses into the atmosphere. Carbon dioxide, methane, nitrous oxide and others. Those molecules have a particular property that they absorb some of the infrared radiation and trap it and warm the planet more than the planet would be warmed if those greenhouse gases didn’t exist. Indeed, if we had no greenhouse gasses the temperature of Earth would be something like the temperature of the moon. A lot colder and unable to support life as we know it. It’s because of this envelope of greenhouse gasses that Earth is indeed warmer. So far, so good. But now if humanity increases the amount of the greenhouse gases in the atmosphere, then we create problems. Because life on the planet, and humanity, and our civilization, have evolved in a certain kind of climate, but now we’re changing the climate by adding more molecules of greenhouse gasses in the atmosphere, warming the climate and pushing the thermal equilibrium to higher temperatures that threaten us in a variety of ways. There are several chemical compounds or molecules that have this property that they trap infrared radiation. Carbon Dioxide is the most important of these; nitrous oxide, methane, and some industrial chemicals. The total warming effect sometimes called the total radiative forcing of the greenhouse gases, is the sum of the effects of each of these greenhouse gases. The total radiative forcing caused by human activity that is caused by the emissions of CO2, methane, nitrous oxide and these industrial chemicals leads to the overall warming effect and of that, carbon dioxide accounts for about 77% or about three quarters of the total effect. Methane is the second most important of the human induced greenhouse gases. And nitrous oxide is the third and the sum of the radiative forcings of those three types of greenhouse gases account for about 99% of the total greenhouse effect. In magnitude, we’ve been emitting more and more of those gases over time. The measurement of the total emissions is in what’s called giga-tons, meaning billions of tons of carbon dioxide equivalent. We ask how much warming is caused by the sum of all of these greenhouse gases, and we ask what would be the equivalent amount of just carbon dioxide be. And that’s called CO2 e, or carbon dioxide equivalent tons of emission each year. And to give you the round numbers we’re emitting right now on the order of about 55 billion tons of carbon dioxide equivalent per year. The carbon dioxide part of that total is about 35 billion tons of carbon dioxide. Most of that is coming from burning coal, oil and gas a much smaller part of that, around 10% is coming from cutting down trees. Cutting down the rainforest and there by releasing carbon dioxide from the trees back into the atmosphere. Well, how big is this effect? It’s big enough to cause us really huge dangers. About 50 years ago, a very far-sighted scientist put some monitors a, on a mountaintop in Hawaii, and he started measuring the amount of carbon dioxide in the atmosphere. And thanks to those measurements from 1958 till now, we have annual and actually even within the year fluctuations of carbon dioxide levels from then until now. And this is known among earth scientists as the Keeling Curve because it was Dr. Keeling who put up the monitor, and what the Keeling Curve shows is that the amount of carbon dioxide in the atmosphere has been rising significantly. Measured as parts per million of carbon dioxide, the number of CO2 molecules per million of total molecules in the air, most of which is nitrogen part of which is oxygen, and then just a bit of carbon dioxide. But that little bit is enough to change the whole climate. Starting back in 1958 when that machine first went to, up on the top of Mauna Loa in Hawaii, the carbon dioxide was 320 molecules for every one million molecules in the air, or we say 320 parts per million. By now, CO2 has reached 400 parts per million. Before James Watt came with his brilliant steam engine, the atmosphere contained about 280 parts per million. In the geologic history of the last three million years, the CO2 varied roughly between 150 and 300 parts per million. Then came humanity, and we’ve been burning so much oil, gas, and coal, that we’ve now sent the CO2 levels absolutely soaring. And reaching the 400 parts per million level in the spring of 2013. A concentration of carbon dioxide not seen on the planet for three million years. In other words, human activity is pushing the planet into a climate zone completely unknown in human history, and unknown in the Earth’s recent history. Some of the great scientists, many of, of my colleagues I’m honored to say, like Professor James Hanson, are able to use various techniques to look at the long history of carbon dioxide and temperatures on the planet Earth. And the picture you’re looking at now is a, a, a kind of manuscript of the Earth’s history showing a long record over the past 400,000 years of fluctuations in carbon dioxide and in temperature on the planet. Now these fluctuations in carbon dioxide were not caused by humanity using a steam engine obviously, this is hundreds of thousands of years ago. These were natural fluctuations of carbon dioxide driven by natural processes. By volcanoes, by the re-absorption of carbon dioxide into the oceans. And by changes of the Earth’s orbital cycle on a periodicity of tens of thousands of years. But what the paleoclimate record shows is that when CO2 is high, temperatures are high. And this is the basic greenhouse effect. Raise the CO2 in the atmosphere, expect a warmer planet. This has been true throughout history. And it is true now. We have been in a process of raising the carbon dioxide level and the other greenhouse gases. Nitrous oxide, methane and the industrial chemicals that the, floor carbons and so forth, the HFCs and the others. And by doing so we have led to a warming of the planet within modern history. If you look to the temperature at the start of the industrial revolution until now. The Earth has warmed by about 8 10ths of one degree centigrade. Because there’s more greenhouse gas in the atmosphere. And the Earth hasn’t yet finished its warming in response to the greenhouse gas increases that humanity has caused. Even if we were to put no more greenhouse gases into the atmosphere, but just to stay with what’s there right now, Earth would continue to warm by perhaps another 0.6 of one degree centigrade. Because the oceans large bath tubs take a long time to warm up in response to the greenhouse gases. So we’ve seen perhaps half or a little bit more than half of the warming that will come from the carbon dioxide and other greenhouse gases already put in the air. But we’re not done, we’ve been accelerating the amount of greenhouse gases that we’re putting into the air, we’re increasing the amount of methane, increasing the amount of nitrous oxide. The fact of the matter is, that even though in more than 20 years since the world’s governments said, we have an urgent challenge of heading off the human induced greenhouse gases in order to prevent a huge dangerous change of the Earth’s climate. 20 years on, we’ve still not turned the needle of slowing down our emissions, much less stopping them. The rate of emissions has been going up year to year as the world economy has been increasing in scale. And with the growth of China, we’ve seen an enormous increase of greenhouse gas emissions in recent years. Because China by itself by virtue of its huge size and by virtue of its use of coal as its primary energy source has become the world’s largest emitter of carbon dioxide. We’re on a dangerous path. Let’s now understand, what are the dangers of this rise of temperature. And of other changes of the climate system. What does it mean for us? Why should we worry?