Surabi Menon, a climate modeler and an affiliate scientist at the Lawrence Berkeley National Laboratory in California, cautions that the simulations rely on relatively incomplete estimates of emissions. Menon has been exploring aerosols and the monsoon with the latest model from the NASA Goddard Institute for Space Studies in New York, and says that modelers can at least check their data against measurements of pollution, which were not available even a few years ago. “We are getting there,” she says. “Slowly.”
The global puzzle
As climate researchers test drive the new generation of models, they are particularly keen to measure the models' overall sensitivity: how strongly they warm up in response to increasing concentrations of greenhouse gases. The addition of indirect aerosol effects makes the new model at NCAR more sensitive to greenhouse gases, says NCAR researcher Andrew Gettelman. Simulations show that the additional cooling from aerosol pollution, as well as the direct effect of haze, masked some of the warming from greenhouse gases during the twentieth century; but the model shows enhanced warming in the twenty-first century as curbs on pollution expose the full power of greenhouse gases. In simplified runs that double greenhouse-gas concentrations — which could happen by the end of this century — the new atmospheric model projects a 4 °C rise in global temperatures, whereas the previous model showed a 3.1 °C increase.
The Hadley Centre model is moving in the same direction, but this is not the rule. A model at the Pierre Simon Laplace Institute near Paris produces less warming in response to greenhouse gases than did the previous generation, says Sandrine Bony, a climate modeler there. The improved treatment of clouds may help explain that change, but the researchers have yet to fully analyze the new results.
These are just the first wave of a deluge in modeling data. Scientists in the IPCC's physical science working group have until 31 July 2012 to submit papers for the IPCC process, so the literature will explode with results from climate simulations over the coming year.
Then the real hard work will begin — working out what to believe. Scientists must tease apart the subtle causes and effects in their models and, where possible, test their results against other models and observations. “What we need now is to really understand what the models are doing, and why they differ,” Bony says. “It's really by comparing the results from a spectrum of models that we can assess which results are robust.”