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2015 declared the hottest year on record

January 20, 2016 This article courtesy of Nature News.

Warming in the Pacific helps shatter past marks, and could bring even faster temperature rises.

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It’s official: 2015 was the hottest year on record. Global data show that a powerful El Niño, marked by warmer waters in the tropical Pacific Ocean, helped to drive atmospheric temperatures well past 2014's record highs. Some researchers suggest that broader Pacific trends could spell even more dramatic temperature increases in years to come.

Released on 20 January, the global temperature data come from three independent records maintained by NASA, the US National Oceanic and Atmospheric Administration (NOAA) and the UK Met Office. All three data sets document unprecedented high temperatures in 2015, pushing the global average to more than 1 ºC above pre-industrial levels. Although El Niño boosted temperatures late in the year, US government scientists say that the steady increase in atmospheric greenhouse gases continues to drive the longer-term trend.

“The reason why this is such a warm record year is because of the long-term trend,” says Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York. “And there is no evidence that this long-term trend has slowed.”

Average global surface temperatures in 2015 were 0.16 °C warmer than in 2014, which was previously the warmest year on record, according to NOAA. Virtually all areas of the globe, including land and oceans, experienced above-normal temperatures. Satellite and balloon records of temperatures in the upper atmosphere showed less warming due to a delayed response to El Niño, but are expected to rise faster in 2016.

Overall, global temperatures have increased by 0.1 to 0.2 ºC per decade since the 1970s, says Thomas Karl, director of NOAA’s National Centers for Environmental Information in Ashville, North Carolina.

“Clearly the 2015 data continues the pattern,” Karl said. “This trend will continue.”

Pacific flip-flop

The currrent El Niño is predicted to continue to boost average global temperature over the next several months. This could translate into another year of record temperatures. But the question facing scientists is whether the near-record El Niño that developed in 2015 has helped flip the Pacific Ocean into a warmer state that favours the development of future El Niños and higher global surface temperatures.

The Pacific Decadal Oscillation (PDO) is a 15- to 30-year cycle that produces warmer sea surface temperatures across the eastern Pacific in its positive phase and cooler temperatures in its negative phase. Since 1998, after the last major El Niño and a subsequent La Niña, the PDO has been mostly negative. Some scientists say that the associated cooling helped suppress the increase in global temperatures in the early part of the millennium. Since early 2014 the PDO has been largely positive.

“It sure looks to me like we’ve changed phases in the PDO,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

Some studies have tied the PDO to long-term temperature trends. The PDO was largely negative through the mid-1970s, when global temperatures rose less quickly. It was largely positive in the 1980s and 1990s, when global temperatures registered faster increases. Scientists debate the climatic linkages between the PDO and both El Niño and global temperature.

“If you try to look at PDO and global temperatures, you can come up with a variety of relationships,” says Karl, who questions whether the PDO is an independent phenomenon or merely an extension of El Nino.

Trenberth notes that the PDO is related to the El Niño/La Niña cycle in the tropical Pacific. That leaves open the possibility that it could fade with El Niño, which models predict will diminish over the next several months. But he says the PDO is also a result of longer-term fluctuations in ocean currents that push warmer water deeper into the ocean or maintain it closer to the surface.

Jerry Meehl, a climate modeller at NCAR, has a study under review that suggests the PDO is likely to remain in a positive state over the coming decade. In that analysis, Meehl and his colleagues plugged actual atmospheric and ocean data from 2013 into a global climate model and then ran the model forward in order to simulate how the climate might evolve. The model evolved into a positive PDO and remained there.

“Over the next ten years, we see higher rates of warming,” Meehl says. Meehl says that this does not change the overall assessment that global warming has proceeded apace over the last century. Rather, he says that global temperatures vary and often increase in stepwise fashion over decades.

The question scientists are trying to understand now is how the ocean circulation works and the extent to which changes in ocean circulation can affect global temperatures.

“We are still trying to figure that out,” he says. “It’s really intriguing. That’s why it’s exciting for climate science.”


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