An international report has confirmed that 2015 surpassed 2014 as the warmest year since at least the mid-to-late 19th century: a record heat that resulted from the combined influence of long-term global warming and one of the strongest El Niño events the globe has experienced since at least 1950. Not only that, but atmospheric greenhouse gases, sea surface temperatures and global sea levels all reached record highs, while Arctic sea ice continued to melt and tropical cyclones were well above average.
The report, State of the Climate 2015, led by NOAA National Centers for Environmental Information, was based on contributions from more than 450 scientists from 62 countries around the world. And while much short-term focus has been on the impact of El Niño in driving temperature extremes over the past year to 18 months, the report emphasizes that underlying trends are continuing on their trajectory toward a warming planet.
The "climate was shaped both by long-term change and an El Niño event," said Thomas R. Karl, Director of the NOAA National Centers for Environmental Information, in a press release. "Both of these time scales are important to consider. Last year's El Niño was a clear reminder of how short-term events can amplify the relative influence and impacts stemming from longer-term global warming trends."
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Among the extreme temperature records highlighted by the report:
Global surface temperature highest on record. Aided by the strong El Niño, the 2015 annual global surface temperature hit record warmth for the second consecutive year, easily surpassing the previous record set in 2014 by more than 0.1°C (0.2°F). "This was the highest margin to break a previous record since 1998," said report editor Jessica Blunden of NOAA's National Centers of Environmental Information at a press conference to launch the report. "It is also the first year to be more than 1°C (1.8°F) higher than pre-industrial conditions of the mid-to-late 19th century." Across land surfaces, record- to near-record warmth was reported across every inhabited continent.
Sea surface temperatures highest on record. The globally averaged sea surface temperature was also the highest on record, breaking the previous mark set in 2014. The highest temperature departures from average occurred in part of the northeast Pacific and in part of the eastern equatorial Pacific. The North Atlantic southeast of Greenland remained colder than average and was colder than 2014.
Ocean heat content highest on record. Globally, upper ocean heat content exceeded the record set in 2014, reflecting the continuing accumulation of thermal energy in the upper layer of the oceans. Oceans absorb over 90 percent of Earth's excess heat from global warming. "This is a huge amount of heat," entering the ocean, said Gregory Johnson of NOAA's Pacific Marine Environmental Laboratory.
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Climate change is about more than direct measurements of temperature increases, of course, and the report found continuing evidence of related changes, including the one that is most directly responsible for them all: record levels of greenhouse gas concentrations. Concentrations of carbon dioxide (CO2), methane and nitrous oxide, among others rose to new record high values during 2015, with the annual average atmospheric CO2 concentration at Mauna Loa, Hawaii, the location of the world's longest direct measurements of CO2, hitting 400.8 parts per million (ppm). Not only did this surpass 400 ppm for the first time, but its increase of 3.1 ppm on 2014 was the largest annual increase observed in the 58-year record.
Sea levels are continuing to rise: Global average sea level rose to a new record high in 2015 and was about 70 mm (about 2¾ inches) higher than the 1993 average, the year that marks the beginning of the satellite altimeter record. Over the past two decades, sea level has increased at an average rate of 3.3 mm (about 0.15 inch) per year, with the highest rates of increase in the western Pacific and Indian Oceans.
Additionally, significant regional impacts continued to be felt: In the Arctic, land surface temperature tied with 2007 and 2011 as the highest since records began in the early 20th century; sea surface and minimum sea ice extent in September 2015 was 29 percent less than the 1981–2010 average and the fourth lowest value on record.
"Currently, the Arctic is warming at twice the rate of lower latitudes," pointed out Jackie Richter-Menge of the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory, a trend that is amplified as disappearing snow and ice cover reduces the area's albedo and causes the land and sea surface to absorb more heat from the Sun. "In Greenland, melting occurred over more than 50 percent of the ice sheet," Richter-Menge added. (Conversely, Antarctic temperatures were colder than average, and sea ice extent there remains highly variable.)
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Meanwhile, there were 101 tropical cyclones across all ocean basins in 2015, well above the 1981–2010 average of 82 storms (furthermore, said Blunden, "we not only had more storms, we had more intense storms"); unusually warm surface water in the Pacific likely triggered a widespread harmful algal bloom, which stretched off the west coast of North America from central California to British Columbia, Canada, in late spring and summer; and June saw the second-lowest extent of late-spring Northern Hemisphere snow cover in the 49-year satellite record.
Twelve countries, including Russia and China, reported record-high annual temperatures. In June, one of the most severe heat waves since 1980 affected Karachi, Pakistan, claiming over 1,000 lives. On Oct. 27, Vredendal in South Africa, reached 119 degrees F (48.4°C), a new global high temperature record for this month.
The report's completion has not, of course, brought an end to the shattering of climate records. Just last month, Mitribah, Kuwait, recorded a temperature of 129.2°F (54°C) during the height of a heat wave, likely the second-highest temperature on record in the Eastern Hemisphere. And all the while, atmospheric concentrations of carbon dioxide and other greenhouse gases continue to rise.
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