Verifying Climate Forecasts

The most perplexing question about climate models is whether they can be trusted as a reliable basis for altering social policies, such as those governing CO2 emissions or the shape and location of wildlife reserves. Even though these models are fraught with uncertainties, several methods are available for verification tests. Although no method is sufficient by itself, several methods together can provide significant, albeit circumstantial, evidence of a forecast's credibility.

The first validation-testing method involves checking the model's ability to simulate the present climate. The seasonal cycle is one good test because temperature changes in a seasonal cycle are larger on a hemispheric average than the change from an ice age to an interglacial period (that is, 15°C seasonal range in the Northern Hemisphere versus 5-7°C glacial/interglacial cycle). General circulation models map the seasonal cycle well. This supports the scientific consensus about the plausibility of global warming of several degrees in the 21st century. The seasonal test, however, does not indicate how well a model simulates slow processes such as changes in deep ocean circulation, ice cover, forests, or soil carbon storage, which may have significant effects on the decade- to century-long time scales over which atmospheric CO2 is expected to double.

A second verification technique involves isolating individual physical components of the model and testing them against actual data. A model should reproduce the flow of thermal energy among the atmosphere, the surface, and space with no more than about a 10 to 20% error. Together, these energy flows make up the well-established natural greenhouse effect on Earth and constitute a formidable and necessary test for all models. A model's performance in simulating these energy flows is an example of physical validation of model components.

A third validation method involves a model's ability to reproduce the diverse climates of the past. This method is aided by recording instrumental observations made during the past few centuries and paleorecords that serve as a proxy for climatic conditions of the ancient Earth. This method may even include testing the model's ability to simulate climates of other planets (Kasting et al. 1988). Paleoclimatic simulations of the Mesozoic (age of the dinosaurs), glacial-interglacial cycles, or other extreme past climates help scientists understand the coevolution of the Earth's climate and living things (Schneider and Londer 1984). As verification tests of climate models, they are also crucial to predicting future climates and changes in biological systems.

Using these techniques, much has been learned from examining the global climatic trends of the past century. The years 1997 and 1998 were the warmest years on record for the Earth's surface in the past century; at the same time the stratosphere was at its coldest (IPCC 1996a and 2001a). These data are consistent with an enhanced greenhouse-effect signal that might be anticipated from the greenhouse-gas injections over the past 150 years, which saw a 30% increase in CO2, a 150% increase in CH4, and the introduction of human-generated heat-trapping chemicals such as chloroflu-orocarbons and halons. Industrial activities since the 1950s have contributed to the increase of sulfur dioxide and other aerosol particles into the atmosphere, the net effect of which is likely to reduce surface temperature by reflecting sunlight back to space. This is complicated by the fact that light hazes like sulfur oxides reflect much more radiation than they absorb but dark particles like soot produced in fires or diesel engines can warm the climate by absorbing more energy than they reflect. The IPCC (2001a) estimates the net effect to be a cooling, with a large range of uncertainty. Although such cooling effects may have counteracted global warming by only several tenths of a degree, the hazes occur regionally and could be producing ecologically significant, unexpected regional changes in climate patterns (Schneider 1994).

Although the 0.6°C ± 0.2°C surface warming in the 20th century is consistent with the human-induced greenhouse gas buildup, some have argued that this warming could possibly be largely nat-ural—either a natural internal fluctuation of the system or driven by natural forcing like a change in the energy output of the sun. However, the IPCC (2001a) assessment states on page 11 of the Summary for Policymakers that there is too much consistent evidence of a human influence to assign all the 20th century warming to natural causes: "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities" (for estimates of the subjective probability of human induced global warming amounts given a doubling of CO2, see Morgan and Keith, 1995). However, if one argues that there could have been a natural warming trend in the 20th century, then by symmetry it is also not possible to rule out the converse that, independent of the enhanced greenhouse effect due to human activity, there was a natural cooling fluctuation taking place during the 20th century. If so, the world would then have warmed up much more than observed had we not had such a fortuitous natural cooling trend. One could even speculate that the dramatic temperature rise since the 1970s with global high temperature records reflects the termination of a natural cooling trend combined with the rapid establishment of the expected enhanced greenhouse effect. We are certainly not suggesting this to be the most probable case, simply showing why a fairly wide range of possibilities is plausible given the remaining uncertainties.

Santer et al. (1996) suggest that when aerosols and greenhouse gas forcings are combined, climate models more closely match 30 years of observations. Nevertheless, wide ranges of climate sensitivities—from as low as a 0.5°C warming to well above a 5.0°C warming (e.g., Wigley and Raper 2001, Andronova and Schlesinger 2000)—are still consistent with current observations. Several reasons exist for such a wide range of uncertainty: difficulty in knowing how to model delays in global warming because of the large heat capacity of the oceans; not knowing what other global-change forcings may have opposed warming (e.g., sulfate aerosols from burning high-sulfur coal and oil or undetectable changes in the sun's light output before 1980); and large, unknown, internal, natural climatic fluctuations. As mentioned previously, though, the ecologically important forecasts of time-evolving regional climatic changes are much less credible than global-average projections and require that ecologists use many alternative scenarios of possible climatic changes. IPCC (2001a) suggests that by the end of the 21st century, there will likely be overall warming between about 1.5 and 6°C; land will warm more than oceans, and higher latitudes more than tropical latitudes; precipitation will increase on average; midlati-tude, midcontinental drying in summer is likely in some areas, and the hydrological cycle will intensify thus raising the possibility of enhanced extremes like droughts and floods, as well as more extreme heat waves and fewer cold snaps. Increased intensity of tropical cyclones is considered likely, but frequency changes remain speculative. In short, the future climate could be very altered from that to which modern ecosystems have become adapted.

In summary, no clear physical objection or direct empirical evidence has contradicted the consensus of scientists (IPCC 1990, 1996a, 2001a; NAS 2001) that the world is warming, nor has credible evidence emerged to contradict the substantial probability that temperatures will rise because of increases in greenhouse gases (Morgan and Keith 1995). Even in the mid-1990s many scientists thought the evidence sufficient to believe that recently observed climatic variations and human activities are probably connected (Karl et al. 1995).The IPCC (1996a, page 5) carefully weighed the uncertainties and concluded that "[n]evertheless, the balance of evidence suggests that there is a discernable human influence on global climate." The IPCC (2001a) reinforces and strengthens that judgment, particularly since recent studies of the past 1000 years (Mann et al. 1999) show that for the Northern Hemisphere the last half of the 20th century is likely to be warmer than at any time during the past 1000 years.

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