* Unprecedented 15-month warm spell
* Extreme heat hits U.S. Midwest hardest
* Arctic sea ice at record low size
By Deborah Zabarenko
WASHINGTON, Sept 10 (Reuters) - The first eight months of 2012 have been the warmest of any year on record in the contiguous United States, and this has been the third-hottest summer since record-keeping began in 1895, the U.S. National Climate Data Center said on Monday.
Each of the last 15 months has seen above-average temperatures, something that has never happened before in the 117 years of the U.S. record, said Jake Crouch, a climate scientist at the data center.
Winter, spring and summer 2012 have all been among the top-five hottest for their respective seasons, Crouch said by telephone, and that too is unique in the U.S. record. There has never been a warmer September-through-August period than in 2011-2012, he said.
"We're now, in terms of statistics, in unprecedented territory for how long this warm spell has continued in the contiguous U.S.," Crouch said.
He did not specify that human-spurred climate change was the cause of the record heat. However, this kind of warmth is typical of what other climate scientists, including those at the United Nations' Intergovernmental Panel on Climate Change, have suggested would be more likely in a world that is heating up due in part to human activities.
Alyson Kenward of the non-profit research and journalism organization Climate Central said in a statement, "Extreme heat is closely tied to climate change, and this summer's heat wave left a global warming signature in the data, particularly in the ratio of record high to record low temperatures."
Normally, the number of record highs and record lows would balance out, with an average ratio of one to one. This year, 25 states have had high to low temperature ratios of 10 to one or greater; 14 have had a ratio greater than 20 to one; and three have had greater than 40 to one ratios, Climate Central said.
Ohio topped this list with 49 record high temperatures for every record low.
DROUGHT PERVADES U.S. MIDSECTION
Last month was only the 16th warmest August on record, though still hotter than average, according to the climate data center, which is part of the National Oceanic and Atmospheric Administration.
June was also warmer than average, while July broke the all-time heat record, the data center scientists said in a statement.
It was a dry summer: as of Aug. 28, nearly 63 percent of the Lower 48 U.S. states were experiencing drought. Still, precipitation overall was near the long-term average, with the Southwest and Southeast wetter than average and the Northwest and Northern plains drier.
The Midwest has suffered the most extreme heat, according to Climate Central, which crunches the U.S. government's weather and climate numbers.
Wisconsin, Iowa and Missouri ranked first, second and third, respectively, in terms of extreme heat in 2012, Climate Central said, followed by Indiana, Minnesota, North Dakota, Illinois and Ohio. Two states outside the Midwest - Colorado and Arkansas - rounded out the top 10.
Outside the Lower 48, the U.S. National Snow and Ice Data Center has already reported that Arctic sea ice has shrunk to a record small size, and the melting season is not over yet. The Arctic is sometimes characterized as the world's air conditioner.
As of Sept. 5, the ice on the Arctic Ocean was less than 1.54 million square miles (4 million square km), a 45 percent reduction compared to September conditions in the 1980s and 1990s.