NASA said the years 2013, 2009, and 2006 were tied for the title of "seventh warmest year since 1880." They also called to attention a "long-term warming trend. "
In 2013 the average global temperature was 58.3 degrees Fahrenheit; this is about one degree higher than the baseline in the 20th century and 1.4 degrees higher than 1880, a NASA news release reported.
"Long-term trends in surface temperatures are unusual and 2013 adds to the evidence for ongoing climate change," GISS climatologist Gavin Schmidt said. "While one year or one season can be affected by random weather events, this analysis shows the necessity for continued, long-term monitoring."
Weather patterns can cause temperatures to vary from year to year, but greenhouse gas emissions are believed to be spurring a long-term warming trend. This year may not be warmer than last, but each decade the average temperature will gradually rise as greenhouse gas emissions increase in the atmosphere.
Carbon dioxide is a commonly-released greenhouse gas, and it traps heat inside Earth's atmosphere. The gas does occur naturally, but massive amounts are released when fossil fuels are burned.
"Driven by increasing man-made emissions, the level of carbon dioxide in Earth's atmosphere presently is higher than at any time in the last 800,000 years," the news release reported.
The researchers looked at "weather data from more than 1,000 meteorological stations" as well as special software that differentiates between surface temperature over the period of a month and average temperature from the same region for other years.
In the year 1880 the atmospheric carbon dioxide level was at only 285 parts per million (this was the first time NASA's Goddard Institute for Space Studies took its first temperature record), by 1960 atmospheric carbon dioxide concentration was at about 315 parts per million. It has since peaked 400 parts per million in 2013.
In 2013 the continental U.S. experienced its "42nd warmest year on record." In Australia, 2013 was the hottest year ever recorded.
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