The world last month matched a record for the hottest September, and some scientists point to global warming as a cause.
It was the third time since 2000 that the world set or tied a heat record for September. Previous hot September records were set in 2003 and 2005. The records go back to 1880.
Last month's average temperature was 60.2 degrees (15.6 Celsius) Fahrenheit worldwide, the U.S. National Oceanic and Atmospheric Administration said Monday.
Why September? It might be the lengthening of the Northern Hemisphere summer as a result of man-made global warming and the loss of Arctic sea ice that indirectly helps cool other parts of the world, said University of Victoria climate scientist Andrew Weaver.
This is the 16th time that the world has set or tied a hot temperature record since 2000, according to NOAA. The last time the world set a cold temperature record was in December 1916, nearly 96 years ago.
These trends are man-made global warming at work, Weaver said.
"What's playing out is precisely what climate said we should expect to see 20 to 30 years ago," Weaver said.
This is the 331st consecutive month with global temperatures above the 20th century average.
For most of the year, world temperatures were warmer than normal, but not near record levels. At the same time, the United States kept setting heat records. But while September was a record hot month for the world, the United States ranked as only the 23rd hottest.
NOAA climate monitoring chief Deke Arndt said the heat was most intense in South America, Japan, Russia, Canada and the Atlantic Ocean.
Two weather factors may be in play, Weaver and Arndt said. The La Nina weather oscillation—which is the flip side of El Nino and tends to depress global temperatures slightly—ended. And the Arctic was unusually warm and had a record amount of sea ice melting, factors that alter weather throughout the Northern Hemisphere.
Some skeptics of global warming have been pointing to British data that a London newspaper claims shows no warming since 1997. But the United Kingdom meteorological office and Weaver said the claims are misleading.
"I don't know what data they are looking at," Weaver said. "2010 is the warmest year. 2005 is the second warmest year."
According to NOAA, all of the top 10 warmest years on record have occurred after 1997, when skeptics claimed global warming stopped.
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