The limitations of climate models
yield slightly different results and thus a certain range of projections.
The release notes that one would assume that the longer scientists concentrate on the climate, the more accurate the results of the model calculations should become and hence the projections of the individual models should converge. According to Knutti, however, this assumption might well be true in the long run, but not in the short term. After all, the more complex a model becomes, the more processes are factored into it and, unfortunately, the greater the uncertainty becomes in the short term. “The models might not have become more accurate in the last five years, but they are more reliable, especially since today’s models consider more physical processes more realistically,” says the climate physicist.
Weather more variable than one might think
As Knutti’s results reveal, climate models might well enable tendencies to be calculated reliably, but they eventually reach their limits. One such limitation is also apparent in the present trend of making increasingly small-scale and short-term projections on the climate, says Knutti and refers to another study conducted by him and other climate researchers that was recently published. “Whether there will be an increase in heat waves or especially cold winters in the United States, in Europe or in Russia in the next twenty years certainly doesn’t depend solely on climate change caused by humans,” Knutti points out. The frequency of locally stable weather situations particularly has an impact on this. And these have greatly been influenced by such phenomena as North Atlantic Oscillation, which (unlike the long-term, manmade trend) cannot be predicted several years in advance.
The problem with the new, short-term projections: the shorter the timescale, the smaller the influence of the manmade trend and the greater that of variable weather phenomena. Especially in the mid-latitudes we live in, the weather phenomena vary greatly and the climate change caused by humans is obscured by them. Therefore, as the researchers write in their study, it is difficult to make short and medium-term climate predictions, however good the models are.
Robust heat stress projections
The climate events that are difficult to predict also include extreme weather events such as flooding, periods of drought, or heat waves. Interestingly, however, the combined measures of temperature and atmospheric humidity can be predicted fairly well. All the climate models yield similar results for these measures, as Knutti and Erich Fischer, a senior researcher in his team, were able to demonstrate recently in a third study. “This is significant as the risk of heat stroke is greatest when it’s hot and humid at the same time, for instance,” says Fischer. The fact that the combined measures of temperature and atmospheric humidity can be predicted so well is linked to the fact that temperature and humidity also depend on each other through physical processes. One factor why temperatures were so high during the so-called heat wave of 2003, for example, is that it was so dry and hardly any soil moisture could evaporate anymore.
Even if climate projections sometimes reach their limits because of divergent predictions and the influence of unpredictable weather phenomena, accurate projections are thus perfectly feasible in certain areas, too — projections that will also influence the next IPCC report, which will be published in September 2013.
— Read more in Reto Knutti et al., “Robustness and uncertainties in the new CMIP5 climate model projections,” Nature Climate Change (28 October 2012) (doi:10.1038/nclimate1716); Clara Deser et al., “Communication of the role of natural variability in future North American climate,” Nature Climate Change (26 October 2012): 775–79 (doi:10.1038/nclimate1562); and E. M. Fischer and R. Knutti, “Robust projections of combined humidity and temperature extremes,” Nature Climate Change (2 September 2012) (doi:10.1038/nclimate1682)
