STATE OF THE SCIENCE: Greenhouse gas emissions and sea levels continue to rise, outpacing previous predictions in the latest research.
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Climate change is "unequivocal" and it is 90 percent certain that the "net effect of human activities since 1750 has been one of warming," the Intergovernmental Panel on Climate Change (IPCC) —a panel of more than 2,500 scientists and other experts—wrote in its first report on the physical science of global warming earlier this year. In its second assessment, the IPCC stated that human-induced warming is having a discernible influence on the planet, from species migration to thawing permafrost. Despite these findings, emissions of the greenhouse gases driving this process continue to rise thanks to increased burning of fossil fuels while cost-effective options for decreasing them have not been adopted, the panel found in its third report.

The IPCC's fourth and final assessment of the climate change problem—known as the Synthesis Report—combines all of these reports and adds that "warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change." Although countries continue to debate the best way to address this finding, 130 nations, including the U.S., China, Australia, Canada and even Saudi Arabia, have concurred with it.

"The governments now require, in fact, that the authors report on risks that are high and 'key' because of their potentially very high consequence," says economist Gary Yohe, a lead author on the IPCC Synthesis Report. "They have, perhaps, given the planet a chance to save itself."

Among those risks:

Warming Temperatures—Continued global warming is virtually certain (or more than 99 percent likely to occur) at this point, leading to both good and bad impacts. On the positive side, fewer people will die from freezing temperatures and agricultural yield will increase in colder areas. The negatives include reduced crop production in the tropics and subtropics, increased insect outbreaks, diminished water supply caused by dwindling snowpack, and increasingly poor air quality in cities.

Heat Waves—Scientists are more than 90 percent certain that episodes of extreme heat will increase worldwide, leading to increased danger of wildfires, human deaths and water quality issues such as algal blooms.

Heavy Rains—Scientific estimates suggest that extreme precipitation events—from downpours to whiteouts—are more than 90 percent likely to become more common, resulting in diminished water quality and increased flooding, crop damage, soil erosion and disease risk.

Drought—Scientists estimate that there is a more than 66 percent chance that droughts will become more frequent and widespread, making water scarcer, upping the risk of starvation through failed crops and further increasing the risk of wildfires.

Stronger Storms—Warming ocean waters will likely increase the power of tropical cyclones (variously known as hurricanes and typhoons), raising the risk of human death, injury and disease as well as destroying coral reefs and property.

Biodiversity—As many as a third of the species known to science may be at risk of extinction if average temperatures rise by more than 1.5 degrees Celsius.