To Anonymous— I have three primary responses to your comments that I want to cover separately: 1. Projections of changes in the CO2 concentration: The present atmospheric CO2 concentration is about 387 ppm, up from about 280 ppm during preindustrial times, so up about 38%. The rate of increase per year has been accelerating—mostly because the rate of emissions has been increasing—in the 1960s, it was about 1.3 ppm/yr; recently it is about 2.2 ppm/yr. While we do not know exactly what future emissions will be, global emissions have been increasing more rapidly than the highest projected emissions scenario prepared just a decade ago. Your note suggests a linear extrapolation, which is simple, but we really know more than that, given the international energy infrastructure, growing demands for energy, and available energy resources and technologies. Even assuming global emissions remain constant (which is very likely an underestimate given the need for more energy in China, India, and elsewhere) and that ocean and biospheric uptake rates stay the same (and as you note, these rates of uptake might well drop), then the projected increase in CO2 concentration from 2010 to 2060 would be from 390 ppm up to about 500 ppm (not 450 ppm as you suggest). Realistically, if nations cannot reduce emissions, then it is likely that the concentration could well be 550 ppm or higher. And then there are the increasing concentrations of the many other greenhouse gases. The world is in quite a predicament. You are right that cutting down a forest can also lead to an increase in the global CO2 concentration, but the fossil fuel emissions really dominate. There are only about 600 billion tonnes of carbon in above ground vegetation for the world as a whole—at the present rate of emissions, we will burn up that much carbon, we’ll burn that much in about 70 years, and with growing emissions, in even less time—and we will not be destroying all the world’s vegetation. 2. CO2’s warming influence and past changes in climate: First, we are not anywhere near saturating the radiative influence of CO2—we will simply not reach a concentration where adding more CO2 will not exert a further warming influence (recall, that during the Cretaceous, so more than 65,000,000 years ago, the CO2 concentration was likely between 1500-2000 ppm and their were palm trees at Arctic latitudes). So, the world has in the past been a lot warmer, and a higher CO2 concentration had a lot to do with it. It is true that the climate sensitivity is such that the relationship is logarithmic, so the equilibrium warming associated with each CO2 doubling is about the same, but clearly, the world can get a lot warmer. Second, there are several reasons that you cannot simply extrapolate past changes in climate to estimate future change. Even assuming that human factors are the only cause of past changes in climate, there are multiple factors that are affecting the climate—several different greenhouse gases and absorbing aerosols (like soot) are exerting a warming influence, and reflective aerosols (like sulfate) are exerting a cooling influence—and the absolute and relative influences of these factors have been changing in time. For the future, the cooling influence of sulfate aerosols is likely to decrease (as air quality is improved), thus uncovering the warming influence of greenhouse gases that have been offset by sulfates. In addition, the concentrations of greenhouse gases are building up because their atmospheric lifetimes are very long (for CO2, some of the perturbation will last for millennia). So, a simple linear extrapolation is just not going to be a reliable projection of the warming influence of human activities. Third, the response of the climate system to the increasing concentrations of greenhouse gases is slowed by the time it takes for the oceans to warm (and for some other adjustments to occur). So, you cannot just suggest that the warming to date and the rise in the CO2 concentration are linearly coupled, and then extrapolate into the future. 3. Projection of changes in climate: As just noted, while linear extrapolation might be an elementary way to get a sense of future warming, in the 1970s this led some to think the world would continue cooling rather than change over to a very significant warming since the 1970s. A major contributor to the cooling influence was, it appears (and seems reasonable), the buildup of sulfate aerosols, both due to growing emissions and to going to tall stacks for their emissions, which lengthened their atmospheric lifetime from a day or two to a week or two. At the time there were many (including the President’s Science Advisory Council in their 1965 report to President Lyndon Johnson), however, who realized that over time the buildup of greenhouse gases would come to dominate because of the very long (centennial to millennial) atmospheric lifetimes of the perturbations in concentration that were being caused. And that has been shown to be what has happened—understanding of the physics of an issue are a much more reliable approach to estimating future conditions than linear extrapolation. The climate models that have been developed incorporate all the many things that we have learned—they don’t let us just focus on one process or influence—we have to consider them all quantitatively and how they interact. To build confidence in the models, they are tested in many, many ways—not the exact situation, but as many different past cases and situations that we can. Indeed, there are still uncertainties due to limits to understanding (and some due to limitations in computer resources to allow fine enough spatial resolution in the models), but there is really no basis for selecting projections based on simple extrapolations compared to the much more comprehensive representations included in models. The Hadley Centre model results are among the most respected around the world. I just cannot accept your statement that the “science is highly uncertain”—we may not know exactly what the changes will be, but we have very strong reason to project warming of several degrees, and associated shifts in precipitation zones, etc. The representations of processes, including of water vapor, sea ice, and so on, are largely consistent with all that we have learned about climate system behavior—and that the climate sensitivity is very likely within the range 2-4°C, quite likely near the center of the range. Warming over the next few decades is likely to average a few tenths of a degree per decade. This is behind the equilibrium temperature increase associated with the particular CO2 concentration. Even if the world can stabilize the atmospheric CO2 concentration, this means that warming will continue for several decades more, and sea level rise will go on for centuries as the ocean slowly warms and as ice sheets deteriorate. And if the nations of the world cannot stabilize the atmospheric concentrations, the situation is likely to be much worse—polar warming could release a lot of methane or CO2, amplifying the warming influence; warming could accelerate ice sheet deterioration, increasing the rate of sea level rise to well over a meter per century (which was the average rate from about 20,000 to 8,000 years ago as the glacial ice sheets melted—and the warming during that period was much less rapid than the world is now undergoing), and more. In summary, there is compelling evidence that indicates that the extent and challenge of dealing with climate change is as serious as the IPCC has stated, and more likely is even more severe than less, given the potential tipping points and the cautious nature of the IPCC process that is necessary to generate the unanimous endorsements of the participating nations.