Human-caused pollution and the burning of fossil fuels such as oil and coal have driven up greenhouse gas levels, leading to this long-term rise in temperatures, said the US space agency NASA. Carbon dioxide is at its highest level in the atmosphere in 800,000 years, having risen from 285 parts per million in 1880 to 400 parts per million last year, NASA said. Unless current trends change, scientists said the world should expect each of the coming decades to be warmer than the last, said NASA climatologist Gavin Schmidt.
A key difference between last year and other top years of the past decade is that 2013 had no El Nino effect to warm the equatorial region, a weather phenomenon that would have been expected to cause an uptick in global temperatures.
Forecasters say El Nino could return in 2014, with the potential to make this coming year even hotter than last. Another concerning effect of global warming is the melting of sea ice in the Arctic, which is expected to cause sea level rises over time that will endanger coastal communities around the world. "Arctic sea ice is down considerably, especially over the past 10 to 11 years," said Tom Karl, director of NOAA's Climatic Data Center. Last year marked the sixth smallest sea ice extent in the Arctic on record, while the Antarctic saw the opposite trend, and sea ice was above average.
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