Global warming hiatus
A global warming hiatus, also sometimes referred to as a global warming pause or a global warming slowdown, is a period of a slower rate of increase of the global mean surface temperature (GMST), the globally average land and sea temperature at the bottom of the troposphere. This can occur during continued global warming of the Earth's climate system when overall energy uptake is balanced by increased subsurface–ocean heat uptake.
Compared to the long term warming trend, hiatus periods of fifteen years are common in the surface temperature record. The term is currently used to refer to the period since the exceptionally warm year of 1998. While evidence of continued multi-decadal warming is robust, there is considerable variability on annual to decadal scales, so shorter periods of ten or fifteen years can show weaker or stronger trends. Scientific research has continued into the extent to which this current hiatus is due to natural variability, and mechanisms which might have led to it.
Pauses are part of natural climate variability, and their existence does not refute long-term climate change trends. Also, other means of measuring climate change exist besides global mean surface temperatures, such as sea level rise, which has not stopped in recent years, as well as continuing record high temperatures and Arctic sea ice decline. Short term hiatus periods of global warming are compatible with long-term climate change patterns.
Some evidence suggests that the hiatus over the past 15 years is largely an artifact of insufficient coverage of certain parts of the globe in calculating global temperatures.
Research into mechanisms
There has been research proposing various mechanisms to explain the hiatus from 2000 to 2013. Several studies have proposed a role for the change in temperature of the Pacific or Atlantic Oceans in contributing to the hiatus.
Effects of oceans
One proposal is that the hiatus was a part of natural climate variability, specifically related to decadal cooling in the eastern equatorial Pacific in the La Niña phase of the El Niño–Southern Oscillation. This has been explained as due to unprecedented strengthening of Pacific trade winds in the last 20 years, so that surface warming has been substantially slowed by increased subsurface ocean heat uptake caused by increased subduction in the Pacific shallow overturning cells, and increased equatorial upwelling in the central and eastern Pacific.
A study published on August 3, 2014 reported that the rapid warming of the Atlantic Ocean has increased trade winds, thereby cooling temperatures in the Pacific Ocean. This, the study concluded, contributed to the pause because such winds trap heat in the deep ocean. Another study published later that month found evidence that a cycle of ocean currents in the Atlantic influences global temperatures by sinking large amounts of heat beneath the oceans, and suggested the hiatus might continue for ten more years because each phase of this cycle lasts for thirty years.
Two papers were published by scientists of NASA's Jet Propulsion Laboratory in October 2014 in the same issue of Nature Climate Change. "Global warming is still happening; there is still sea-level rise," said Josh Willis, co-author of one of the studies, and in the study's press release he wrote, "these findings do not throw suspicion on climate change itself." This study was based on the fact that water expands as it gets warmer, and a straightforward subtraction calculation: From the total amount of sea level rise, they subtracted that due to the expected expansion of the upper ocean, and that due to added meltwater worldwide. The remainder, representing the amount of sea level rise caused by warming in the deep ocean, was "essentially zero." According to the other study, the upper layers of the Southern Ocean warmed at a much greater rate between 1970 and 2005 than previously thought, because before the deployment of Argo floats, temperature measurements in the Southern Ocean were "spotty, at best." That the oceans warmed in the past significantly faster than we thought would imply that the effects of climate change could be worse than currently expected, placing the planet's sensitivity to CO
2 toward the higher end of its possible range.
Additional proposed causes of the decreased rate of warming over the past 15 years include increased sulfur emissions from volcanic activity, the emission of pine-smelling vapors from pine forests, which have been shown to turn into aerosols, and the ban on chlorofluorocarbons as a result of the Montreal Protocol, since they were potent greenhouse gases in addition to their ozone-depleting properties.
A joint report from the UK Royal Society and the US National Academy of Sciences in February 2014 said that there is no "pause" in climate change and that the temporary and short-term slowdown in the rate of increase in average global surface temperatures in the non-polar regions is likely to start accelerating again in the near future. "Globally averaged surface temperature has slowed down. I wouldn’t say it's paused. It depends on the datasets you look at. If you look at datasets that include the Arctic, it is clear that global temperatures are still increasing," said Tim Palmer, a co-author of the report and a professor at University of Oxford.
In a presentation to the American Physical Society, William (Bill) Collins of Lawrence Berkeley National Laboratory and lead author of the modeling Chapter 9 of the IPCC AR5 said "Now, I am hedging a bet because, to be honest with you, if the hiatus is still going on as of the sixth IPCC report, that report is going to have a large burden on its shoulders walking in the door, because recent literature has shown that the chances of having a hiatus of 20 years are vanishingly small."
For many years those opposed to action on global warming have been arguing that global warming has stopped since the record-breaking warm El Niño year of 1998: an example appeared as an opinion piece in the Daily Telegraph in 2006, and was rebutted at the time. These arguments were given new prominence in media reporting in March 2013. Research explaining the issue was after the deadline for inclusion in the IPCC Fifth Assessment Report, and a leaked draft of the report was publicised by the press in August with assertions that scientists were struggling to explain the hiatus. When the full report was published that November the wording was clarified, but this minor issue was given prominent attention in media coverage of the publication.
Two independent studies published in August 2014 concluded that, once surface temperatures start rising again, it is most likely that "they will keep going up without a break for the rest of the century, unless we cut greenhouse gas emissions." Watanabe et al said, "this warming hiatus originated from eastern equatorial Pacific cooling associated with strengthening of trade winds," and that while decadal climate variability has a considerable effect on global mean surface temperatures, its influence is gradually decreasing compared to the on-going man-made global warming. Maher et al found that under the existing and projected high rates of greenhouse gas emissions there is little chance of another hiatus decade occurring after 2030, even if there were a large volcanic eruption after that time. They went on to say that most non-volcanic warming hiatuses are associated with enhanced cooling at the surface in the equatorial Pacific, which is linked to the Interdecadal Pacific Oscillation.
World Meteorological Organisation climate report
When announcing the annual World Meteorological Organisation climate report in March 2014, the WMO secretary-general Michel Jarraud said that there had been no pause, with 2013 continuing a long-term warming trend showing "no standstill in global warming". 2013 had been the sixth warmest year on record, and 13 of the 14 warmest years on record had occurred since the start of 2000. He said that "The warming of our oceans has accelerated, and at lower depths. More than 90 percent of the excess energy trapped by greenhouse gases is stored in the oceans."
The report itself stated that "While the rate at which surface air temperatures are rising has slowed in recent years, heat continues to be trapped in the Earth system, mostly as increased ocean heat content. About 93 per cent of the excess heat trapped in the Earth system between 1971 and 2010 was taken up by the ocean." From 2000 to 2013 the oceans had gained around three times as much heat as in the preceding 20 years, and while before 2000 most of the heat had been trapped between the sea surface and 700 metres (2,300 ft) depth, from 2000 to 2013 most heat had been stored between 700 and 2,000 metres (6,600 ft) depth. It proposed this could be due to changes in atmospheric and ocean circulation around the tropical Pacific Ocean, interacting with the El Niño–Southern Oscillation and the Pacific Decadal Oscillation.
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