WASHINGTON — This winter was the warmest on record worldwide, the U.S. government said Thursday in the latest worrisome report focusing on changing climate.
The report comes just over a month after the Intergovernmental Panel on Climate Change said global warming is very likely caused by human actions and is so severe it will continue for centuries.
And a NASA study reported Thursday in the journal Science found that an important counter-balance to warming — sunlight blocked by volcanic gases, dust, pollution and other aerosol particles — appears to have weakened.
As for this winter, the National Oceanic and Atmospheric Administration said the combined land and ocean temperatures for December through February were 1.3 degrees Fahrenheit above average for the period since record keeping began in 1880.
The report said that during the past century, global temperatures have increased at about 0.11 degrees per decade. But that increase has been three times larger since 1976, NOAA's National Climatic Data Center reported.
Greenhouse gases and El Nino
Most scientists attribute the rising temperatures to so-called greenhouse gases produced by industrial activities, automobiles and other processes. These gases build up in the atmosphere and trap heat from the sun somewhat like a greenhouse.
Also contributing to this winter's record warmth was an El Nino, a periodic warming of the tropical Pacific Ocean. It was particularly strong in January — the warmest January ever — but the ocean surface has since begun to cool.
The report noted that in the Northern Hemisphere the combined land and water temperature was the warmest ever at 1.64 degrees above average. In the Southern Hemisphere, where it was summer, the temperature was 0.88 degree above average and the fourth warmest.
The late March date of the vernal equinox noted on most calendars notwithstanding, for weather and climate purposes northern winter is December, January and February.
For the United States, meanwhile, the winter temperature was near average. The season got off to a late start and spring-like temperatures covered most of the eastern half of the country in January, but cold conditions set in in February, which was the third coldest on record.
Less 'sunscreen' for Earth
In its study, NASA described the aerosol effect as a "sunscreen" for Earth.
"When more sunlight can get through the atmosphere and warm Earth's surface, you're going to have an effect on climate and temperature," lead author Michael Mishchenko of NASA's Goddard Institute for Space Studies said in a statement. "Knowing what aerosols are doing globally gives us an important missing piece of the big picture of the forces at work on climate."
Scientists extracted an aerosol signal from satellite measurements originally designed to observe clouds and weather systems that date back to 1978. "The resulting data show large, short-lived spikes in global aerosols caused by major volcanic eruptions in 1982 and 1991, but a gradual decline since about 1990," NASA said in a statement. "By 2005, global aerosols had dropped as much as 20 percent from the relatively stable level between 1986 and 1991."
The Associated Press contributed to this report