We’re on a dangerous path. Humanity is emitting greenhouse gases, changing the climate, and threatening ourselves and other species and future generations in many, many very dangerous ways. We need to respond to this challenge. We use two terms to reflect two different ways of responding. Both of which are important. One term, mitigation, means to reduce what we’re doing that’s causing the climate to change. So we want to mitigate climate change by reducing the anthropogenic or human-caused emissions of greenhouse gases. The other term that we use is adaptation. That means learning to live with climate change as best as possible. To protect our cities from storms surges to protect our crops from high temperatures or flooding or droughts by helping to improve the quality of the seeds to have traits like drought resistance or heat tolerance or submergence tolerance in the seeds by advanced breeding of, of seed varieties. There’s a limit to how much we can adapt, because if the changes are so dramatic that sea levels rise several meters that the food supply is profoundly threatened by high temperatures and mega droughts and loss of soil moisture. We’re not going to be able to keep up with that under any circumstance. At the same time, it’s important that we do adapt, because climate change is happening, and it’s going to continue to happen even if we are very successful at mitigation. There is inertia in the warming as I’ve noted. So we are going to experience more warming in the years and decades ahead, simply as the oceans catch up with the warming already implied by the greenhouse gases we’ve put into the air. But we should expect that if we continue on business as usual, we will never be able to keep up with the extent and the dangers that we will be causing ourselves. Mitigation, therefore, is an enormous priority. How to mitigate? Well, for that, we need a careful diagnosis. What is it that we’re doing? And since we understand that we have several specific ways that were con, contributing to greenhouse gas concentrations, we have to take measures to head off those increases. Since about three-fourths of the radiative forcing is carbon dioxide, our highest priority is to reduce the increase of carbon dioxide in the atmosphere. And since most of the CO2 increase comes from our use of fossil fuels, that’s the number one item on the mitigation agenda. The second way that greenhouse gases are increasing through carbon dioxide is land use change. And so, a second item on our carbon dioxide list of actions is to head off the deforestation that’s causing the emission of CO2 by land use change. Then, next on our list comes methane. Methane is admitted in a variety of ways. We have to address each of those. Then comes nitrous oxide. For each of these human-induced emissions of greenhouse gases, we have to have a look what’s feasible, how much would it cost, what are our alternatives? What is the most cost effective way to reduce substantially the human impact on the climate system? Now, the right place to start is the with the, the carbon dioxide. And the way that scientists have usefully posed the question is, what would we have to do to carbon dioxide emissions, mainly from fossil fuels, but also from land use, in order to keep the total increase of the Earth’s temperature at two degrees centigrade or below? The answer is, that since we’ve already increased the temperature by almost one degree, we would need to dramatically reduce CO2 emissions in the coming decades. So, one recent scientific study of what this would require is shown in the graph that you’re looking at now. What you see here is a very busy picture because there are lots of possible trajectories of emissions. On the horizontal axis are the years to 2050. On the vertical axis are the tons of carbon dioxide put into the atmosphere, measured actually as tons of carbon rather than tons of carbon dioxide. Now in all of this mix of pathways, take a look at two that are most important. First, look at the red path. This is the business as usual trajectory. It says that if we continue as we’re going, emissions of carbon dioxide will be higher and higher every year. Why? Because the world economy is growing. As it grows, it’s using more energy on business as usual, that energy’s going to be fossil fuel, and the CO2 emissions will rise. What trajectory of CO2 is needed to avoid two degrees centigrade increase? That’s shown in the blue trajectory. It says while CO2 emissions per year have been rising, now they have to start falling and falling sharply. We want the world economy to grow because we want poor countries to be catching up with the richer countries in living standards. But at the same time, we want the total emissions of carbon dioxide to be falling sharply. And to put it in very general terms, what’s shown here is that, while the world economy might increase threefold by the middle of the century, the total emissions should probably fall by at least half, if not more. That means that emissions per dollar of output need to decline by a factor of six or even more. We use the term decarbonization to mean reducing the amount of carbon dioxide per unit of output, per dollar of gross world product. We need a deep decarbonization of the world economy. But since most of the carbon dioxide comes from fossil fuel, we need a sharp reduction in the use of fossil fuel, at least in the way that we use fossil fuel now. One fascinating, quite wonderful study, recently done, asked the question, so how could California, a major part of the U.S. economy reduce its emissions by 80% by the year 2050. How could California, in effect, decarbonize the California state economy? And the answer is really important for us because it’s the general principles of mitigation. Three categories of action. First, energy efficiency, get more output per unit of energy input. How can we do that? Through more efficient appliances, for example.Through light bulbs whether it’s compact fluorescent light bulbs or light emitting diode light bulbs, as opposed to our more than century old incandescent bulbs.These are examples of new technologies that allow us to get more for less. More output per unit of energy. The second thing we need to do is to shift our energy supply, our primary source of energy. Instead of using coal, oil, and gas, all of which emit CO2, we need to use wind, or solar power, or other low carbon technologies. The third category of action is to take parts of the economy where we now use fossil fuels, other than the electricity sector, convert those other sectors to electricity, and then run those new electrified sectors of the economy on electricity produced by low-carbon energy sources. So what do I mean? Well, take the case of automobiles with internal combustion engines. We know that those automobiles, instead of running on internal combustion of petroleum, can run on electricity. In fact, they can do a lot of really clever and smart things for a very safe and comfortable ride as electric vehicles. And those electric vehicles need to be charged. You plug them into a power source. And if the power source itself is a low-carbon energy producing the electricity, for instance, wind farms that are used to charge the automobiles, then we have a dramatic reduction of carbon emissions coming from automobile use. And so, by electrifying the fleet of automobiles and charging that electric fleet on a clean energy grid, we could have a dramatic reduction as well. So the point is a three part process: energy efficiency, low-carbon energy sources running our electricity grid, and electrification of parts of our economy. Automobiles or home heating or industrial processes, shifting from local uses of fossil fuels in the gas tank or in the factory or in the boiler at home, and shifting that to electricity that is on a grid of clean electricity. Well, what’s an example of clean electricity? It’s electricity that comes from fo, photovoltaics, for example. Or so called concentrated solar thermal. Both are technologies that take solar energy and convert the solar energy to electricity. Photovoltaics does it directly by the solar energy hitting certain kinds of materials that then release electrons and create an electron flow, the photovoltaic effect. First escribed by the way it, it’s basic science by Albert Einstein in 1905. Or, by taking the solar energy reflecting it in mirrors, and heating water to a boiling point, and then using the steam to turn a steam turbine, and thereby producing electricity. Either way, you get electricity without fossil fuel and without carbon dioxide. And of course, we have many potential ways to produce large amounts of electricity. It could be solar power. It could be wind turbines, which have come down so much in cost that in many parts of the world, the windy places, they’re already cost competitive with fossil fuel generation. It could be geothermal energy where along tectonic zones where you can tap into the heat within the Earth, within the mantel. It’s possible to generate a lot of heat, which is used to boil water, turn steam turbine, and produce electricity. As is done, for instance in Iceland or now, increasingly in the Rift Valley of East Africa. Or more controversially, you can produce electricity with nuclear power. Electricity produced in a nuclear power plant does not create carbon dioxide emissions. It creates other risks fissile material that could potentially be used for weapons, or the risks of terror, or the risks of, of meltdowns. Or disaster such as hit Fukushima in Japan during their recent earthquake. Or Chernobyl, because of a mis-management of the power plant in Ukraine, back in the mid 1980s. But nuclear power already supplies a significant part of the electricity in many parts of the world, and it is a, essentially, a zero carbon electricity supply. And so, if we can move to a clean electricity grid, not dependent on fossil fuels, then we can also move to what you see here, an electric vehicle. There are many, many new models coming. Batteries are being improved. Technology is being improved. Electric vehicles happily can be a lot smarter than internal combustion engines, because a car running on electricity can also run on a lot of smart systems as well. Rather than the mechanical power, it’s electric motors linked to sensors, and even to self driving vehicles as of we know are being very actively explored and, and pursued by companies such as Google. And so, the chances of better transport and cleaner transport better home heating and ventilation through smart buildings and through smarter electrification, rather than using furnaces and boilers, make it possible to envision a very, very steep decline of CO2 emissions. When the California study added it all up, they found the way to reach that bold 80% target is, as illustrated in this figure. The baseline emissions are the line at the top, showing that CO2 emissions are on the rise in California because of growth of the economy. The preferred mitigation trajectory is the downward sloping line at the bottom of the curve. And the gap is explained by all of these ways of reducing CO2 emissions. The light blue zone shows the reductions of emissions coming from energy efficiency. The dark blue zone shows the reduction of emissions coming from de-carbonizing the energy sources used to produce electricity. Then you see that electrification in the yellow zone is moving to electric vehicles. There are also other smaller categories of turning to biofuels, for example. What is a biofuel? It’s of course using somehow a natural biologic process to produce a fuel that’s a substitute for fossil fuel. And so, there are many, many ways to mitigate, but the broad categories through energy efficiency, a clean electricity grid, and electrification have been shown by many studies to be the pillars of any successful strategy. Here’s the good news, all of these technologies are within reach. All of them have potential ability for a large scale. There are solutions, in other words, that are within reach. And we’re going to look at some of those solutions shortly, at what can be done in different parts of the world to make a very large correction in this dangerous path that we’ve been on. Now, there’s one more idea around. I’m not an advocate at all. Many people are very worried about it. Though, a few scientists are very excited. It is the Band-Aid approach shown here with the planet, with the, the Band-Aid on top of it. It’s called geo-engineering. The idea is, well, maybe we won’t really stop the carbon emissions. But we can find other things to do to compensate for the effect. One idea, it’s an actual idea, though I find it very frightening, is that while the carbon dioxide’s warming the planet, we would put up particles into the air. Sulfate aerosol particles that would dim the sunshine and actually cool the planet to offset what we’re doing with the carbon dioxide. Feed the air with one poison and the idea would be, well, let’s add another poison to kind of compensate. The problem is it doesn’t really compensate. It most likely adds some significant dangers. It doesn’t stop the increase of CO2. It doesn’t stop the acidification of the oceans. It doesn’t stop the changing patterns of precipitation. Indeed, it would have highly, highly uncertain and potentially very, very dangerous effects. But I think that the Band-Aid approach is what it is. If the bleed is so dangerous that it threatens life itself and well being on the planet, a band-aid is probably not an adequate response. And in this case, I think we should look for what’s within reach, what our technologies are telling us. We could deeply reduce carbon dioxide emissions by taking similar steps on methane and nitrous oxide and fertilizer use and landfills, and in other areas, turn that curve of greenhouse gas emissions sharply lower. Help keep the planet below the two degree centigrade rise. It’s possible, but time is running out. So let’s look at how we can accelerate progress.