Global Science Report is a feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”
Most of the criticism was aimed at the IPCC’s defense of climate models—models that the latest observations of the earth’s climate evolution show to be inaccurate, or at least are strongly indicative that is the case.
There are two prominent and undeniable examples of the models’ insufficiencies: 1) climate models overwhelmingly expected much more warming to have taken place over the past several decades than actually occurred; and 2) the sensitivity of the earth’s average temperature to increases in atmospheric greenhouse gas concentrations (such as carbon dioxide) averages some 60 percent greater in the IPCC’s climate models than it does in reality (according to a large and growing collection of evidence published in the scientific literature).
Had the IPCC addressed these model shortcomings head on, the flavor of their entire report would have been different. Instead of including projections for extreme climate changes as a result of continued human emissions of greenhouse gases resulting from our production of energy, the high-end projections would have featured relatively modest changes and the low-end projections would have been completely unremarkable.
Since changes in the earth’s temperature scale approximately linearly with a property known as the earth’s equilibrium climate sensitivity (how much the earth’s average surface temperature rises as a result of a doubling of the atmosphere’s carbon dioxide concentration), it is pretty straightforward to adjust the IPCC’s projections of future temperature change to bring them closer to what the latest science says the climate sensitivity is. That science suggests the equilibrium climate sensitivity probably lies between 1.5°C and 2.5°C (with an average value of 2.0°C), while the climate models used by the IPCC have climate sensitivities which range from 2.1°C to 4.7°C with an average value of 3.2°C.
To make the IPCC projections of the evolution of the earth’s average temperature better reflect the latest scientific estimates of the climate sensitivity, it is necessary to adjust them downward by about 30% at the low end, about 50% at the high end, and about 40% in the middle.
The figure below the jump shows what happens when we apply such a correction (note: we maintain some internal weather noise). The top panel shows the projections as portrayed by the IPCC in their just-released Fifth Assessment Report, and the lower panel shows what they pretty much would have looked like had the climate models better reflected the latest science. In other words, the lower panel is what the IPCC temperature projections should have looked like.