By the time they reach retirement age, the changes will be far more dramatic — and perhaps life-threatening on a massive scale, an authoritative U.N. study will say this week.
On Monday, the Intergovernmental Panel on Climate Change, a network of more than 2,000 scientists, opens a five-day meeting in Brussels, Belgium, to finalize a report on how warming will affect the globe and whether humans can do anything about it.
The panel will paint a bleak picture of increasing poverty, paucity of drinking water, melting glaciers and polar ice caps, and a host of vanishing species by mid-century unless action is taken to curb emissions of carbon dioxide and other heat-trapping gases.
Some regions like parts of North America and northern Europe will see some benefits, at least in the short term, from longer growing seasons and milder winters.
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"We are going into a realm the Earth has not seen for a very long time ... over the past 800,000 years," said Camille Parmesan, a University of Texas biologist who has studied the effects of climate change on wildlife and was a reviewer of the upcoming report.
A draft of the IPCC's summary has been obtained by The Associated Press, but policy makers will go over the document line-by-line this week before unveiling the final text Friday. It will then become a guideline for governments to determine policies and draft legislation.
About 285 delegates from 124 countries are attending, along with more than 50 of the scientists who compiled the report and dozens of observers from nongovernment, mostly environmental, organizations.
The closed-door talks are likely to focus on predictions of how many people will be at high risk from changing ecosystems and water cycles, and whether such specific weather events like Hurricane Katrina should be attributed to global warming.
"Do you use examples? And do you use ones that are relatively positive or highly negative?" said Rik Leemans, a co-author from Wageningen University in the Netherlands. "You can tone it down or strengthen it by including examples, and that's always an issue in these discussions."
The summary's final wording must be adopted by consensus among the diplomats, with the approval of the scientists.
While there may be editing for the sake of nuance, the underlying premise of the draft report will not change. "A decade ago, climate impacts were largely hypothetical," said James J. McCarthy, a Harvard University oceanographer who was a main author of the 2001 IPCC report. "That's no longer a question."
It is the second of four reports by the IPCC. The first, issued in February, updated the science of climate change, concluding with near certainty that global warming is caused by human behavior.
That report galvanized the European Union to adopt an ambitious goal of reducing carbon emissions by at least 20 percent from 1990 levels by 2020.
The IPCC's work will be presented at a summit in June of leaders from the world's richest countries, including U.S. President George W. Bush whose administration has declined to take coordinated action with other nations to limit greenhouse gases.
The latest report was six years in the making. Since the IPCC's 2001 assessment, knowledge about climate change has become more precise, and studies have tracked specific shifts on the ground to changing temperatures and weather patterns.
"Many natural systems on all continents and in some oceans are being affected by regional climate changes, particularly temperature increases," reads the final draft.
Parmesan said storms and floods have become more severe in some places, coastlines have eroded and deserts have expanded. Diseases common in the tropics have spread. In the Northern Hemisphere, spring is coming an average two weeks earlier, disrupting bird migrations and causing flowers and trees to bloom too early. At least 70 species have become extinct so far because of global warming, Parmesan said in a telephone conference with reporters.
The report will offer stark warnings for the future.
Within 25 years, hunger and death from diarrhea will threaten poor countries where crops fail and water becomes more scarce. Later in the 21st Century, warmer seas will likely destroy coral reefs and the fish that feed off them, and may lead to the poisoning of shellfish. Tens of millions of people in coastal cities and river basis will likely be affected by flooding, and fresh water supplies will likely be inundated with salt water from sea surges. Small islands will probably be submerged by rising sea levels. Beetles and other pests are expected to infest forests even more, with forest and wild fires more frequent and widespread.
But the scientists say not all these dire consequences have to happen. A third report to be released in May will outline possible ways to slow the affects of global warming.
"These are projections that many of us believe don't have to be the future; many of these can be avoided" by reducing emissions of greenhouse gases from the burning of fossil fuels, said Harvard's McCarthy. Though the scientific projections are solid, he said he is optimistic the worst won't happen — "because we can't be that stupid."