In a New Climate Model, Short-Term Cooling in a Warmer World
One of the first attempts to look ahead a decade, using computer simulations and measurements of ocean temperatures, predicts a slight cooling of Europe and North America, probably related to shifting currents and patterns in the oceans. The team that generated the forecast, whose members come from two German ocean and climate research centers, acknowledged that it was a preliminary effort. But in a short paper published in the May 1 issue of the journal Nature, they said their modeling method was able to reasonably replicate climate patterns in those regions in recent decades, providing some confidence in their prediction for the next one. The authors stressed that the pause in warming represented only a temporary blunting of the centuries of rising temperatures that scientists have projected if carbon dioxide and other heat-trapping gases continue accumulating in the atmosphere. Were learning that internal climate variability is important and can mask the effects of human-induced global change, said the papers lead author, Noel Keenlyside of the Leibniz Institute of Marine Sciences in Kiel, Germany. In the end this gives more confidence in the long-term projections. The new study focused on relationships between short-term climate trends and a system of currents in the Atlantic Ocean, called the meridional overturning circulation, which undergo periodic changes. The predictions were made by repeatedly running a simulation of the global climate and adjusting conditions in the simulated oceans to match temperature measurements. To get a computer-generated simulation of the climate for the 1990s, for example, the model ran from the 1950s through the 1980s, with sea temperatures adjusted to reflect the real world, then ran without further inputs for 10 more years. The model is a rough replica of conditions, the scientists said. While it reliably reproduced climate patterns in Europe and North America, the model could not replicate patterns over central Africa, for example. In e-mail exchanges, several climate experts not associated with the study expressed a variety of views on the new cooling forecast. But they agreed that the work served the important function of at least trying to chart what will assuredly be a winding climatic journey toward a generally warmer world. Other researchers, including NASA scientists at the Jet Propulsion Laboratory in Pasadena, Calif., reported separately on April 21 that a slowly fluctuating oscillation in Pacific Ocean temperatures had shifted into its cool phase, a condition that is also thought to exert an overall temporary cooling of the climate. These natural variations can also amplify warming, and that is likely to happen in future decades on and off as well, experts say. The global climate will continue to be influenced in any particular decade by a mix of natural variability and the building greenhouse effect, said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. He said efforts to build forecasts by mixing modeling and measurements were vital in a world with rising populations in places where poverty leads to vulnerability from climate-related threats like flooding and famine. It should also help the public and policy makers understand that a cool phase does not mean the overall theory of human-driven warming is flawed, Dr. Trenberth said. Too many think global warming means monotonic relentless warming everywhere year after year, Dr. Trenberth said. It does not happen that way.