Global warming will affect different regions in different ways. In 2000, a national assessment by the U.S. Global Change Research Program (USGCRP) warned generally about potential climatic changes in what it called “mega-regions” of the nation. “What we were able to do at that point was very limited,” recalls Michael MacCracken, an atmospheric physicist, now retired, who coördinated the assessment effort. And the study’s climate scenarios were based only on global models: “We really wanted to have more models, and more regional results, but we had very little resources to get that done.” Similar, very general statements about climate change across large regions appeared in the most recent IPCC assessment, the first time the IPCC has narrowed its focus even that much. The report pointed out, for example, that the southwestern United States will probably get even more parched than it is now. But what we need are projections on a far finer scale. With federal climate-science budgets cut to the bone in recent years, a few state and local governments are funding their own efforts in New York, California, and western states eyeing dwindling water supplies with alarm.
A Wet New York City
Rushing into her office near Columbia University, Cynthia Rosenzweig was chipper despite her evident exhaustion. An agronomist by training, she directs the Climate Impacts Group at NASA’s Goddard Institute for Space Studies (GISS) and advises New York City’s government on how climate change will intensify heat waves, stress upstate watersheds, and increase the risk of a devastating storm surge. She had just returned from Delhi, India, where she cowrote a summary of the 2007 IPCC reports, the first of which was released in February. Brightly painted papier-mâché elephants she’d brought back from her trip were arranged on the coffee table in her sixth-floor office overlooking 112th Street (as it happens, some of the highest ground in Manhattan). She sat down and, on her computer screen, called up images from a GISS global climate model.
Honed by a broad range of climate scientists, the model represents atmospheric and oceanic systems. Like other global models, it simulates interrelated processes: for example, the warming of Earth’s surface by solar radiation; the absorption of heat by the oceans; the reflection of solar energy by land surfaces, ice sheets, and particulates in the atmosphere; and the effects of the accumulation of excess carbon dioxide and other atmospheric gases that trap heat. Researchers test the accuracy of such models by seeding them with, for example, data on actual greenhouse-gas emissions over the past 30 years and then seeing whether they return results consistent with temperature and other measurements recorded over that period. The goal, of course, is a model that can predict how much temperatures will continue to rise given various future greenhouse-gas emission levels, and how other parts of the climate system are likely to respond.
But the limitations of global models quickly become clear when Rosenzweig zooms in on a map of the eastern United States showing climate predictions for the 2050s. On the screen, a line cuts from eastern Pennsylvania to western Massachusetts. The area north of the line is yellow, representing a 2 ºC increase over historical averages; the area south of the line is more orange, indicating a 2.25 ºC increase. The entire New York metropolitan area, Connecticut, and much of Massachusetts and New Jersey are lumped together under a single temperature estimate. The same goes for several other variables, such as precipitation and evaporation rate. The problem is that one “grid box” in the typical global climate model–think of it as a pixel in a photograph–is a square of 150 to 200 kilometers per side.
Weather and climate are, obviously, far more localized than that. Mountain ranges–even individual peaks and valleys–make their own weather. A single glacier might grow or dissolve because of temperature and rainfall changes in a very specific area. Differences in air temperature over water and land cause breezes that can dramatically influence climate and weather in coastal areas. Regional models that take such phenomena into account are familiar to any viewer of TV weather news. But where global models are calibrated against data spanning decades, regional models are used to project only a few days ahead. Thus, one goal of climate scientists is to find a way for the twain to meet, to give local precision to predictions about global warming and climate change.