Otherwise, Earth this century could cross a climate threshold or "tipping point that could lead to intolerable impacts on human well-being," says the 166-page report prepared for the United Nations. It was written by 18 experts in climate, water, marine science, physics and other disciplines, seven of them Americans.
"It is still possible to avoid an unmanageable degree of climate change, but the time for action is now," says panelist John Holdren, a Harvard University professor of environmental policy.
Without action, the panel says, a litany of harmful consequences awaits: the spread of disease, less fresh water, more and worse droughts, more extreme storms and widespread economic damage to farming, fishing and forests. In the USA, which emits about 25% of the world's carbon dioxide, it could mean more intense hurricanes, heat waves, wildfires and droughts.
The two-year study, issued by the U.N. Foundation, says the risk of tipping over that climate threshold rises sharply if Earth's temperature increases 3.6 to 4.5 degrees above what it was in 1750 (it is 1.2 degrees above that point now). That year marked the start of the Industrial Revolution. It led to the widespread burning of coal, oil and other fossil fuels that generate carbon dioxide, the main heat-trapping "greenhouse gas" blamed for global warming.
The scientists say a climate turnaround will take a huge effort. They call for the world's carbon emissions to level off by 2015-2020, then to be reduced by another one-third by the end of the century. Without action, they say, temperatures could rise 11 degrees by 2100.
The report comes three weeks after the Intergovernmental Panel on Climate Change laid out the latest evidence that human action — burning fossil fuels and deforesting the land — is mostly to blame for global warming.
Tuesday's report recommends a dramatic increase in development and use of "biofuels" such as ethanol to replace oil. It also calls for an immediate shift in power-plant construction away from traditional coal-burners to high-tech designs that capture and hold the carbon dioxide they produce.
The world should spend three to four times more on such advanced energy technology, the scientists add. The USA spends almost $3 billion a year now.
The report also suggests nations define a new category of "environmental refugee" to prepare for people who must flee the extreme effects of climate change, such as rising sea levels.
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