Ronald Bailey| Aug. 30, 2013 12:32 pm Reason.com
That's what an interesting new article in Nature Climate Change points out. The article, "Overestimated warming over the past 20 years," by members in good standing of the "climate community" compares model simulations from 37 of the climate models being used by the Intergovernmental Panel on Climate Change to project future temperatures with the actual global temperature increase over the past two decades. The study (since it's behind a paywall I am linking to the version published online at the AGW skeptical website the Hockeyschtick) reports:
Global mean surface temperature over the past 20 years (1993–2012) rose at a rate of 0.14 ± 0.06 °C per decade (95% confidence interval)1. This rate of warming is significantly slower than that simulated by the climate models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). To illustrate this, we considered trends in global mean surface temperature computed from 117 simulations of the climate by 37 CMIP5 models. These models generally simulate natural variability — including that associated with the El Niño–Southern Oscillation and explosive volcanic eruptions — as well as estimate the combined response of climate to changes in greenhouse gas concentrations, aerosol abundance (of sulphate, black carbon and organic carbon, for example), ozone concentrations (tropospheric and stratospheric), land use (for example, deforestation) and solar variability. By averaging simulated temperatures only at locations where corresponding observations exist, we find an average simulated rise in global mean surface temperature of 0.30 ± 0.02 °C per decade (using 95% confidence intervals on the model average). The observed rate of warming given above is less than half of this simulated rate, and only a few simulations provide warming trends within the range of observational uncertainty... (emphasis added). ...
The inconsistency between observed and simulated global warming is even more striking for temperature trends computed over the past fifteen years (1998–2012). For this period, the observed trend of 0.05 ± 0.08 °C per decade is more than four times smaller than the average simulated trend of 0.21 ± 0.03 °C per decade. It is worth noting that the observed trend over this period — not significantly different from zero — suggests a temporary 'hiatus' in global warming. (emphasis added).
The article concludes:
Ultimately the causes of this inconsistency will only be understood after careful comparison of simulated internal climate variability and climate model forcings with observations from the past two decades, and by waiting to see how global temperature responds over the coming decades.
It seems to me what the researchers are saying in so many words is that the current batch of climate models have not been validated using actual temperature trends. One possibility for the mismatch between actual temperature trends and the model projections is that the modelers have set climate sensitivity (response of the climate to a doubling of atmospheric carbon dioxide) too high. As I have reported before, more recent research has significantly lowered estimates for climate sensitivity which suggests that future warming will also be lower.
In June, I reported data from University of Alabama in Birmingham climatologist John Christy in which he compared the 73 CMIP5 climate models with actual temperature trends from 1978 to 2025. See graphic below:
The graphic above depicts the global lower troposphere temperature projections from 73 CMIP5 models from 1979 to 2025 compared to an average of the satellite data from UAH and RSS (blue boxes) and weather balloons (green circles) for the global lower troposphere temperatures since 1979 until now. Note nearly all the model runs project much warmer temperatures than the globe has recently experienced. The thick black line is the average projection of the 73 models.
Next month the Intergovernmental Panel on Climate Change is set to release its update on the physical science of climate change. It will be interesting to see how (or if) its authors try to explain the growing gap between the model projections and the actual climate.
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Ronald Bailey is a science correspondent at Reason magazine and author of Liberation Biology (Prometheus).
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