Scientists have a concept called climate sensitivity which refers to the amount of global warming in degrees Celsius we might expect from a doubling of CO2 concentrations from a pre-industrial 280ppm to 560ppm (we are currently at about 380ppm today and will reach 560ppm between 2065 and 2100, depending on how aggressive a forecast you want to adopt).
A simple way to estimate sensitivity is from experience over the past century. At the same time CO2 has gone up by 100ppm, global temperatures have gone up by at most 0.6 Celsius (from the 4th IPCC report). I actually believe this number is over-stated due to uncorrected urban effects and other surface temperature measurement issues, but let’s assume 0.6ºC. Only a part of that 0.6ºC is due to man – some is likely do to natural cyclical effects, but again to avoid argument, let’s assume man’s CO2 has heated the earth 0.6 Celsius. From these data points, we can project forward:
As you can see, the projection is actually a diminishing curve. For reasons I will not go into again (you can read much more in this post) this relationship HAS to be a diminishing curve. It’s a fact accepted by everyone. True climate consensus. We can argue about the slope and exact shape, but I have chosen midpoint values from a reasonable range. The answer is not that sensitive to different assumptions anyway. Even a linear extrapolation, which is clearly wrong scientifically, would only yield a sensitivity projection a few tenths of a degree higher.
What we arrive at is a sensitivity of about 1.2 degrees Celsius for a CO2 doubling (where the blue line crosses 560ppm). In other words, we can expect another 0.6ºC increase over the next century, about the same amount we experienced (and most of us failed to notice) over the last century.
But, you are saying, global warming catastrophists get so much higher numbers. Yes they do, with warming as high as 9-10C in the next century. In fact, most global warming catastrophists believe the climate sensitivity is at least 3ºC per doubling, and many use estimates as high as 5ºC or 6ºC. Do these numbers make sense? Well, let’s draw the same curve for a sensitivity of 3ºC, the low end of the catastrophists’ estimates, this time in red:
To get a sensitivity of 3.0ºC, one has to assume that global warming due solely to man’s CO2 (nothing else) would have to be 1.5ºC to date (where the red line intersects the current concentration of 380ppm). But no one, not the IPCC or anyone else, believes measured past warming has been anywhere near this high. So to believe the catastrophic man-made global warming case, you have to accept a sensitivity three or more times higher than historical empirical data would support. Rather than fighting against climate consensus, which is how we are so often portrayed, skeptics in fact have history and empirical data on our side. For me, this second chart is the smoking gun of climate skepticism. We have a lot of other issues — measurement biases, problems with historical reconstructions, role of the sun, etc — but this chart highlights the central problem — that catastrophic warming forecasts make no sense based on the last 100+ years of actual data.
Global warming catastrophists in fact have to argue against historical data, and say it is flawed in two ways: First, they argue there are positive feedbacks in climate that will take hold in the future and accelerate warming; and second, they argue there are other anthropogenic effects, specifically sulphate aerosols, that are masking man-made warming. Rather than just repeat myself (and in the interest in proving I can actually be succinct) I will point you to this post, the second half of which deals in depth with these two issues.
As always, you can find my Layman’s Guide to Skepticism about Man-made Global Warming here. It is available for free in HTML or pdf download, or you can order the printed book that I sell at cost.