Just a few weeks ago, a paper in Environmental Research Letters by Rahmstorf, Foster, and Cazenave (2012) confirmed the accuracy of the global surface warming projections made by climate models used in the 2001 and 2007 Intergovernmental Panel on Climate Change (IPCC) reports (the Third and Fourth Assessment Reports). Now a new paper published in Nature Climate Change, Frame and Stone (2012) has confirmed the accuracy of the temperature projections made by the climate models in the 1990 IPCC First Assessment Report through 2011 (Figure 1).
Figure 1: Observed changes in global mean surface temperature over the 1990–2010 period from HadCRUT3 and GISTEMP (red) vs. 1990 IPCC business-as-usual best estimate (dark blue), vs. projections using a one-dimensional energy balance model (EBM) with the measured greenhouse gas (GHG) radiative forcing since 1990 (light blue) and with the overall radiative forcing since 1990 (green). Natural variability from the ensemble of 587 21-year-long segments of control simulations (with constant external forcings) from 24 Coupled Model Intercomparison Project phase 3 (CMIP3) climate models is shown in black and gray. From Frame and Stone (2012).
The paper tests the IPCC warming projections using a simple one-dimensional energy balance model (EBM) comparable to the main model used to make the 1990 prediction, using similar input parameters. Frame and Stone then ran the model using just the radiative forcing (heat imbalance) caused by greenhouse gas (GHG) changes from 1990 through 2011, represented by the light blue line in Figure 1. Because the IPCC model projection is based on GHGs-only, this is the most applicable comparison. They also simulated other radiative forcings like changes in solar activity and particulates in the atmosphere, represented by the green line in Figure 1.
We at Skeptical Science previously conducted a similar analysis to that in Frame and Stone (2012). The 1990 IPCC report ran simulations using models with climate sensitivities (the total amount of global surface warming in response to a doubling of atmospheric CO2 concentrations, including all feedbacks) of 1.5°C (low), 2.5°C (best), and 4.5°C (high) for doubled CO2 (Figure 2).
Figure 2: IPCC FAR projected global warming in the BAU emissions scenario using climate models with equilibrium climate sensitivities of 1.5°C (low), 2.5°C (best), and 4.5°C (high) for double atmospheric CO2
In reality, GHGs have increased about 20% slower than the IPCC's "business-as-usual" scenario, in part because of the success of the Montreal Protocol in reducing chlorofluorocarbon (CFC) emissions (CFCs are GHGs), and in part because of the collapse of the Soviet Union, among other reasons.
As noted above, the light blue line in Figure 1 is the most appropriate for comparison, and is very similar to our own previous analysis at Skeptical Science (Figure 3).
Figure 3: 1990 IPCC business-as-usual "best" global warming projection reflecting the observed GHG changes (blue) vs. observed average global surface temperature change from GISTEMP (red) since 1990.
Frame and Stone (2012) also simulated the possible range of natural temperature variability since 1990 by using the ensemble of 587 21-year-long segments of control simulations with constant external forcings from 24 Coupled Model Intercomparison Project phase 3 (CMIP3) climate models. These give a 90% range of about ±0.19°C, and are shown in black and gray in Figure 1. The observed warming from 1990 through 2011 was approximately 0.39±0.20°C (95% confidence range); thus there is only a very small chance that the observed global surface warming over the past 21 years could be explained purely by natural variability.
Ultimately, Frame and Stone note that while there is a fairly large range in the envelope of all climate model projections, and while to some degree they may have gotten the right answer 'for the wrong reasons', the IPCC has thus far done quite well in projecting future temperature changes.
"...it seems highly likely that even in 1990 we understood the climate system well enough to make credible statements about how its aggregate properties would change on timescales out to a couple of decades, even in the presence of considerable uncertainty surrounding the exact forcing trajectory."
Rahmstorf, Foster, and Cazenave (2012) arrived at a similar conclusion by taking a very different approach, first using the statistical technique of multiple regression to filter out much of the short-term variability, then showing that the IPCC temperature projections have been very accurate (Figure 4).
Figure 4: Observed annual global temperature, unadjusted (pink) and adjusted for short-term variations due to solar variability, volcanoes and ENSO (red) as in Foster and Rahmstorf (2011). 12-month running averages are shown as well as linear trend lines, and compared to the scenarios of the IPCC (blue range and lines from the 2001 report, green from the 2007 report). Projections are aligned in the graph so that they start (in 1990 and 2000, respectively) on the linear trend line of the (adjusted) observational data. From Rahmstorf, Foster, and Cazenave (2012).
Frame and Stone have also shown it is very unlikely that natural variability alone can account for the observed global surface warming over the past two decades.
While it has underestimated many climate impacts, thus far the IPCC has done very well at projecting average global surface temperature changes.
Posted by dana1981 on Tuesday, 18 December, 2012
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