New Model Test Experiments, Same old Results
Meehl, G.A., Washington, W.M., Arblaster, J.M., Hu, A., Teng, H. Tebaldi, C., Sanderson, B.N, Lamarque, J.F., Conley, A., Strand, W.G. and White III, J.B. 2012. Climate system response to external forcing and climate change projections in CCSM4. Journal of Climate 25: 3661-3683.
In the latest round of testing the ability of climate models to reproduce the climate of the 20th century and beyond, the same old assumptions produce the same old results. And for the climate scenarios of the future, they produce the same old frightening projections. Meehl et al. (2012) discuss their results of testing the climate system response to external forcing in the Community Climate Model Version 4 (CCSM4) for the Coupled Model Intercomparison Project phase 5 (CMIP5).
The CCSM4 is the latest version in that model series and features fine resolution in the horizontal (about one degree latitude and longitude) and vertical. It also features a fine resolution dynamic ocean model, along with some of the latest improvements in our understanding of atmospheric chemistry, time evolving greenhouse gas (GHG) concentrations, land-use and cover, and even solar cycles. Some new developments, however, are not included such as the indirect effect of aerosols or the solar cycle impact on ozone chemistry.
The modeling strategy in Meehl et al. (2012) is to run an ensemble of six simulations combining natural and anthropogenic effects. Other runs are performed with only anthropogenic forcing, only natural forcing, and combinations of natural and anthropogenic forcing. The future climate scenarios are produced by showing that certain GHG concentrations produce a certain amount of radiative forcing by 2100. For example, the "representative concentration pathway 2.6" (RCP2.6) means that GHG are adding 2.6 Watts per square meter by the year 2100. This corresponds to roughly 350 ppm of carbon dioxide, or a reduction from today's value. They test four different RCPs, including RCP8.5.
The results are not much different from the last round of tests. Only by adding anthropogenic effects do you get the right global temperature trend, and by the end of the 20th century even the right sign! In Fig. 1 the actual global temperatures versus the CCSM4 model results for natural forcing only are shown. The model does a good job until about 1930 (the authors say until about the 1970s), and then natural forcing should show only cooling. Only after anthropogenic forcing is added do we get generally good agreement (Fig. 1). This is the same old result as previous tests with previous model versions.
Figure 1. Adapted from Meehl et al. (2012) their Fig. 2. The time series of annual mean globally averaged surface temperature anomalies from 1850 through 2005, relative to 1900-1919 (solid thick). The natural forcing only is in blue (thinner line) and the natural plus anthropogenic forcing is in red. The shading indicates range of ensemble members. The observations are from the Met Office Hadley Centre-University of East Anglia Climatic Research Unit (HadCRUT3).
Perhaps the most significant result from the article is that the CCSM4 produces a climate that is more sensitive to forcing than CCSM3. "Equilibrium climate sensitivity of CCSM4 is 3.20°C, and the transient climate response is 1.73°C ... The value of the former is higher by about 25% compared to CCSM3." The authors also produce climate scenarios to 2100, then to 2300. These all commit the world to warming, the amount depending on the scenario. After 2100, the RCP2.6 scenario leads to a global cooling, while the RCP 8.5 scenarios add an additional warming equal to the warming to 2100. For some parts of the globe, this is a warming of more than 16° C above today. Finally, they project that polar summer sea ice will be reduced, and nearly disappear by 2100 under the RCP8.5 scenarios.
The article produces interesting results. With no real changes in the basic formulations or assumptions from previous versions of the model and testing, however, the fact that latest version of model produces the same general results should not be surprising. In assuming that anthropogenic GHG are the largest forcing in the model, the outcome will only reinforce the notion of a human control over climate. Perhaps the global cooling produced by the decreased GHG after 2100 makes their point more clear, at least to them. Simpler models (e.g., Loehle and Scafetta, 2011) with more realistic assumptions about the relative sizes of natural and anthropogenic forcing show more modest climate change results.
Additional Reference
Loehle, C. and Scafetta, N. 2011. Climate Change Attribution Using Empirical Decomposition of Climatic Data. The Open Atmospheric Science Journal 5: 74-86.
