US study warns of extreme heat, more severe storms

WASHINGTON: A government report warned Friday that the United States could face more frequent severe weather including heat waves and storms for decades to come as temperatures rise far beyond levels being planned for.

The draft Third National Climate Assessment, a scientific study legally mandated to advise US policymakers, made few bones that carbon emissions have been causing climate change -- a source of controversy among some lawmakers.

"Evidence for climate change abounds, from the top of the atmosphere to the depths of the oceans," the study said. "The sum total of this evidence tells an unambiguous story: The planet is warming."

The study, which was submitted for public and expert review and could be revised, said there was "strong evidence" that human activity had already roughly doubled the probability of extreme heat of the kind seen in Texas and Oklahoma in the summer of 2011.

The assessment expected temperatures to keep rising and offered different scenarios for the future -- including temperatures rising between 2.8 and 5.6 degrees Celsius (5 and 10 degrees Fahrenheit) after 2050 if emissions climb further.

Such a rise would be far beyond the level anticipated by world leaders in UN-backed climate change negotiations, which have committed to holding warming to no more than two degrees Celsius above pre-industrial levels.

The report warned that climate change "threatens human health and well-being in many ways," including through more frequent storms, wildfires, diseases and worse air quality.

Rising sea levels have already damaged infrastructure and climate change could increasingly reduce the reliability of water supplies, particularly in the southern half of the United States and Great Plains, the report said.

While US agriculture will likely remain resilient in the next 25 years, yields of major crops could start declining by mid-century and warming oceans could threaten fish, the study said.

The draft assessment said that some additional climate change was "now unavoidable" but said that "Americans face choices."

"Beyond the next few decades, the amount of climate change will still largely be determined by choices society makes about emissions," it said.

"Lower emissions mean less future warming and less severe impacts; higher emissions would mean more warming and more severe impacts," it said.