<p>Photo: Woman standing in a rice field</p>

Dhal Char, Bangladesh: Swells rise in the Bay of Bengal and splash into Nuzahan Bibi's rice field, which, as global temperatures rise and sea level climbs, becomes an ever more precarious means of support for the widowed Bangladeshi.

Photograph by Peter Essick

By Joel Achenbach, Heidi Schultz

Republished from the pages of National Geographic magazine

Global warming can seem too remote to worry about, or too uncertain—something projected by the same computer techniques that often can't get next week's weather right. On a raw winter day you might think that a few degrees of warming wouldn't be such a bad thing anyway. And no doubt about it: Warnings about climate change can sound like an environmentalist scare tactic, meant to force us out of our cars and cramp our lifestyles.

Comforting thoughts, perhaps. But turn to "Goesigns" the first chapter in our report on the changing planet. The Earth has some unsettling news.

From Alaska to the snowy peaks of the Andes the world is heating up right now, and fast. Globally, the temperature is up one degree Fahrenheit (0.5 degrees Celsius) over the past century, but some of the coldest, most remote spots have warmed much more. The results aren't pretty. Ice is melting, rivers are running dry, and coasts are eroding, threatening communities. Flora and fauna are feeling the heat too, as you'll read in "Ecosigns." These aren't projections; they are facts on the ground.

The changes are happening largely out of sight. But they shouldn't be out of mind, because they are omens of what's in store for the rest of the planet.

Wait a minute, some doubters say. Climate is notoriously fickle. A thousand years ago Europe was balmy and wine grapes grew in England; by 400 years ago the climate had turned chilly and the Thames froze repeatedly. Maybe the current warming is another natural vagary, just a passing thing?

Don't bet on it, say climate experts. Sure, the natural rhythms of climate might explain a few of the warming signs you'll read about in the following pages. But something else is driving the planet-wide fever.

For centuries we've been clearing forests and burning coal, oil, and gas, pouring carbon dioxide and other heat-trapping gases into the atmosphere faster than plants and oceans can soak them up (see "The Case of the Missing Carbon," February 2004). The atmosphere's level of carbon dioxide now is higher than it has been for hundreds of thousands of years. "We're now geological agents, capable of affecting the processes that determine climate," says George Philander, a climate expert at Princeton University. In effect, we're piling extra blankets on our planet.

Human activity almost certainly drove most of the past century's warming, a landmark report from the United Nations Intergovernmental Panel on Climate Change (IPCC) declared in 2001. Global temperatures are shooting up faster than at any other time in the past thousand years. And climate models show that natural forces, such as volcanic eruptions and the slow flickers of the sun, can't explain all that warming.

As carbon dioxide continues to rise, so will the mercury—another three to ten degrees Fahrenheit (1.6 to 5.5 degrees Celsius) by the end of the century, the IPCC projects. But the warming may not be gradual. The records of ancient climate described in "Timesigns" suggest that the planet has a sticky thermostat. Some experts fear today's temperature rise could accelerate into a devastating climate lurch. Continuing to fiddle with the global thermostat, says Philander, "is just not a wise thing to do."

Already we've pumped out enough greenhouse gases to warm the planet for many decades to come. "We have created the environment in which our children and grandchildren are going to live," says Tim Barnett of the Scripps Institution of Oceanography. We owe it to them to prepare for higher temperatures and changed weather—and to avoid compounding the damage.

It won't be easy for a world addicted to fossil fuels to limit emissions. Three years ago the United States spurned the Kyoto Protocol, citing cost. But even Kyoto would barely slow the rise in heat-trapping gases. Controlling the increase "would take 40 successful Kyotos," says Jerry Mahlman of the National Center for Atmospheric Research. "But we've got to do it."

The signs of warming in the following pages are striking enough, but they are just a taste of the havoc the next century could bring. Can we act in time to avert the worst of it? The Earth will tell.