Hottest Years on Record: 2005 and 2010

Yesterday, the NY Times reported (here) that new data shows that 2010 was tied with 2005 as the two hottest years on record. It's useful to compare the new data to the revised forecasts from the IPCC "Climate Change 2007: Synthesis Report" (here) and to my own forecasts presented below.

The IPCC projections (forecasts) are based on emission scenarios. There are four basic scenarios generated from assumptions about globalization and economic growth. The A1 scenario is high globalization and high economic growth. The B1 scenario is high globalization and sustainable development. The A2 scenario is based on low globalization and high economic growth. Finally, the B2 scenario is based on low globalization and sustainable development. The scenarios produce different GHG emissions (left panel above, click to enlarged) and different global warming paths (right panel). The global warming projections range from about 1 degree C to over 3 degrees C with error bars over 6 degrees C (the error bars for the scenarios are on the far right of the graphic).
There are a number of alternative global temperature forecasts on the web (here and here, for example). The projections are typically based on single-equation statistical approaches. The basic equation is T = sF +V where T is global temperature, F is radiative forcing from GHG emissions or natural sources, V is natural variability and s is the climate sensitivity parameter. The increase in temperature is a direct result of forcing and climate sensitivity with some natural or cyclical variability thrown in.

My approach is based on state-space models that treat the environment and the economy as two interacting complex systems. The state of the environment is affected by the state of the economy and vice versa. Since the two systems can and do have negative impacts on each other, growth and global temperature peak over time. Notice that although the confidence intervals look wide, the top 98% confidence interval never hits the 2 degree C warming threshold considered critical by the IPCC.

The difference in projections is basically the result of the assumptions underlying the IPCC emission scenarios (the left graphic above). I'll go into more detail about and criticism of the emission scenarios (which are based on neoclassical economic growth theory) in a future post. My projections should not be taken as support for global warming denial. The models predict warming. What the world will look like after peak global warming, the models cannot predict.