Global warming hiatus

A global warming hiatus,^[1] also sometimes referred to as a global warming pause^[2] or a global warming slowdown,^[3] is a period of relatively little change in globally averaged surface temperatures.^[4] In the current episode of global warming many such 15-year periods appear in the surface temperature record, along with robust evidence of the long-term warming trend.^[1] Such a "hiatus" is shorter than the 30-year periods that climate is classically averaged over.^[5]

Publicity has surrounded claims of a global warming hiatus during the period 1998–2013. The exceptionally warm El Niño year of 1998 was an outlier from the continuing temperature trend, and so subsequent annual temperatures gave the appearance of a hiatus: by January 2006, it appeared to some that global warming had stopped or paused.^[2] A 2009 study showed that decades without warming were not exceptional,^[6] and in 2011 a study showed that if allowances were made for known variability, the rising temperature trend continued unabated.^[6] There was increased public interest in 2013 in the run-up to publication of the IPCC Fifth Assessment Report, and despite concerns that a 15-year period was too short to determine a meaningful trend, the IPCC included a section on a hiatus,^[7] which it defined as a much smaller increasing linear trend over the 15 years from 1998 to 2012, than over the 60 years from 1951 to 2012.^[8] Various studies examined possible causes of the short-term slowdown. Even though the overall climate system has continued to accumulate energy due to Earth's positive energy budget,^[4][9] the available temperature readings at the Earth's surface indicate slower rates of increase in surface warming than in the prior decade. Since measurements at the top of the atmosphere show that Earth is receiving more energy than it is radiating back into space, the retained energy should be producing warming in the Earth's climate system.^[4]

Research reported in July 2015 on an updated NOAA dataset^[8][10] casts doubt on the existence of a hiatus, and it finds no indication of a slowdown even in earlier years.^{[11][12][13][14]} Scientists working on other datasets welcomed this study, though they have expressed the view that the recent warming trend was less than in previous periods of the same length.^[15][16] Subsequently, a detailed study supports the conclusion that warming is continuing, but it also find there was less warming between 2001 and 2010 than climate models had predicted, and that this slowdown might be attributed to short-term variations in the Pacific decadal oscillation (PDO), which was negative during that period.^[17][18][19] Another review finds "no substantive evidence" of a pause in global warming.^[20][21] A statistical study of global temperature data since 1970 concludes that the term "hiatus" or "pause" is not justified.^[22] Some climate scientists, however, have questioned the claim that the hiatus is not supported by evidence, arguing that the recent corrections in data do not negate the existence of a hiatus.^[23]

Independent of these discussions about data and measurements for earlier years, 2015 turned out to be much warmer than any of the earlier years, already before El Niño conditions started. The warmth of 2015 largely ended any remaining scientific credibility of claims that the supposed "hiatus" since 1998 had any significance for the long-term warming trend,^[24] and 2016 was even slightly warmer. In January 2017, a study published in the journal Science Advances cast further doubt on the existence of a recent pause, with more evidence that ocean temperatures have been underestimated.^[25][26] An April 2017 study found the data consistent with a steady warming trend globally since the 1970s, with fluctuations within the expected range of short term variability.^[27] A November 2017 joint study by scientists at the University of Fairbanks and Beijing University found that when missing data from the rapidly warming Arctic were interpolated and included in global temperature averages, the so-called hiatus disappeared entirely.^[28]