National Geographic : 1998 May
could provide an exact description of today's climate. Only very recently have models been developed that are capable of realistically depicting the present global climate without a lot of tinkering-adjustments often called "fudge factors." In part this is because only the most power ful computers are fast enough to handle the job, and in part because some aspects of cli mate change are still mysterious. Even avid proponents caution that GCMs are not yet trustworthy for predicting detailed effects in individual regions: Models divide the world's surface into grids that are typically about 200 miles on a side, but ocean eddies, storms, and cloud activity take place on far smaller scales. The modelers therefore have to compensate with approximations. According to Kevin Trenberth, chief climate analyst at the National Center for Atmospheric Research in Boulder, Colorado, all GCMs pro ject global warming, but they can provide only a range of projected temperature change. The warming could be one degree C over the next century, or it could be as much as three times greater than that. "Used properly, models are useful tools," Trenberth says, "but they cannot solve the problem." A Case of Missing Carbon With oceans and clouds, uncertainty is a formidable problem. Oceans serve as a vast "sink" for carbon dioxide, but precisely how they do so is poorly understood. We do know this: Human activity releases approximately 7 billion metric tons of carbon (in the form of CO2) into the atmosphere every year, add ing to the 750 billion tons that is already there. Yet only about half our emissions-3 billion tons-remains in the air. The rest is taken up by terrestrial and marine plants, buried in ocean sediments, absorbed in seawater, or otherwise sequestered. Of that "missing" amount of carbon, the oceans apparently remove at least 2 billion tons from the atmo sphere each year. This raises a number of compelling but so far unanswered questions: How, exactly, does seawater interact with the air above it to remove CO 2? How much more carbon can the seas hold, and what level of global warming would it take to alter their capacity? And how much has the oceans' ability to soak up and store heat delayed climate change? The role of clouds and airborne suspended particles called aerosols is no easier to factor into models. Clouds shade Earth's surface, promoting cooling, but depending on their altitude, density, and other conditions, they can also trap outgoing heat, promoting warm ing. Aerosols are equally tricky. Some encour age water vapor in the air to condense into tiny droplets. The resulting clouds are dense and shiny, shading the surface for weeks. Similarly, smoke particles block sunlight until they pre cipitate out of the air. The combined effect can be sizable: The volcanic eruption of Mount Pinatubo in 1991 in the Philippines blew a colossal amount of sulfates into the strato sphere, causing a worldwide drop in temper ature that lasted two years. Thus, ironically, our own pollution, mainly from combustion of sulfur-bearing coal and oil, may temporarily have spared us some effects of global warming. The National Oceanic and Atmospheric Administration (NOAA) estimates that during the 20th centu ry, aerosols reduced the amount of warming that would otherwise have occurred by per haps 20 percent. In general, temperatures rose until the 1940s but then dropped significantly until 1970, when they began rising again to set records. Aerosol effects may help explain the odd mid-century cooling. If CO 2 emission increases are to blame for global warming, skeptics say, then temper atures should have risen appreciably during the postwar economic boom, when fossil fuels were burned in escalating quantity. Jerry Mahl man, director of NOAA's Geophysical Fluid Dynamics Laboratory at Princeton, however, has calculated that the surge in coal and oil use quickly increased the amount of sulfates aloft, prompting the cooling. After 1970 the longer term effect of CO 2 and methane overwhelmed the short-lived aerosols, accounting for the temperature rise since then. An enhanced greenhouse effect may not necessarily be catastrophic. Indeed, it might be good news for some farmers. High concen trations of CO 2 can have a fertilizing effect on plants, which is why some commercial UNLOCKING THE CLIMATE PUZZLE radical changes in she span of decades or, even years.