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...And Then How Cold? by MICHAEL D. LEMONICK It seems obvious that trapping more of the sun's heat will make the planet hotter. But what seems obvious isn't always true. According to some respected scientists, there's a chance that global warming could plunge us into, of all things, an ice age. The argument hinges on the Gulf Stream, the ocean current that brings warm surface water north and east and heats Europe. As it travels, some of the water evaporates; what's left is saltier and thus denser. Eventually the dense surface water sinks to the sea bottom, where it flows back southward. And then, near the equator, warm, fresh water from tropical rivers and rain dilutes the salt once again, allowing the water to rise to the surface, warm up and begin flowing north again. But with global warming, melting ice from Greenland and the Arctic Ocean could pump fresh water into the North Atlantic; so could the increased rainfall predicted for northern latitudes in a warmer world. Result: the Gulf Stream's water wouldn't get saltier after all and wouldn't sink so easily. Without adequate resupply, the southerly underwater current would stop, and the Gulf Stream would in turn be shut off. If that happens, Europe will get very cold. Rome is, after all, at the same latitude as Chicago, and Paris is about as far north as North Dakota. More snow will fall, and the bright snow cover will reflect more of the sun's energy back into space, making life even chillier. Beyond that, the Gulf Stream is tied into other ocean currents, and shutting it down could rearrange things in a way that would cause less overall evaporation. Because atmospheric H20 is an important greenhouse gas, its loss would mean even more dramatic cooling--a total of perhaps as much as 8°C (17°F). Worst of all, the experts believe, such changes could come on with astonishing speed--perhaps within a decade or less. And while we might have a great deal of trouble adjusting to a climate that gets 2°C (4°F) warmer over the next century, an ice age by midcentury would be unimaginably devastating. The lingering uncertainty about whether our relentless production of greenhouse gases will keep heating our planet or ultimately cool it suggests that we should make a better effort to leave the earth's thermostat alone. |
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