What the New IPCC Global Warming Projections Should Have Looked Like

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The United Nations’ Intergovernmental Panel on Climate Change (IPCC) released its Fifth Assessment Report (AR5) last week to fanfare and stinging criticism.

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.


Figure 1. (Top) Temperature evolution from 1950 to 2100 as portrayed by the IPCC in its Fifth Assessment Reportaccording to its worst case (red) and best case (blue) emissions scenarios (RCP8.5 and RC2.6, respectively). (Bottom) Temperature evolution from 1950 to 2100 from the same emission scenarios adjusted such that the equilibrium climate sensitivity values better matched the latest science.


The result of such adjustments is that the IPCC’s worst-case emissions scenario produces a global average temperature rise (from the 1986-2005 average) by the end of the century that has a centralized value of about 2.6°C, compared with a rise of 4.1°C given in the IPCC AR5. The more moderate emissions scenarios (the more likely course as the natural gas revolution will see lower carbon dioxide-emitting natural gas replace higher-emitting coal in energy production), once properly adjusted, produce a temperature rise by century’s end of only about 1.25°C (of which, about 0.15°C has already happened).

Since virtually all climate impacts are related to the change in the earth’s temperature, the smaller the future temperature increase, the smaller the resulting impacts. Another degree of so of temperature rise during the next 87 years is not going to lead to climate catastrophe.

Had the IPCC been more interested in reflecting the actual science rather than in preserving a quickly crumbling consensus (that human greenhouse gas emissions are leading to dangerous climate change that requires urgent action), its Fifth Assessment Report would have been a much kindler and gentler document—as it well should have been.