A global warming hiatus, also sometimes referred to as a global warming pause or a global warming slowdown, is a period of relatively little change in globally averaged surface temperatures. 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."Despite the robust multi-decadal warming, there exists substantial interannual to decadal variability in the rate of warming, with several periods exhibiting weaker trends (including the warming hiatus since 1998) ... Fifteen-year-long hiatus periods are common in both the observed and CMIP5 historical GMST time series", "Box TS.3: Climate Models and the Hiatus in Global Mean Surface Warming of the Past 15 Years", IPCC, Climate Change 2013: Technical Summary, p. 37 and pp. 61–63. Such a "hiatus" is shorter than the 30-year periods that climate is classically averaged over.
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. A 2009 study showed that decades without warming were not exceptional, and in 2011 a study showed that if allowances were made for known variability, the rising temperature trend continued unabated. 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, 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. 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, 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.
Research reported in July 2015 on an updated NOAA dataset casts doubt on the existence of a hiatus, and it finds no indication of a slowdown even in earlier years. 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.Vaidyanathan, Gayathri. "Did Global Warming Slow Down in the 2000s, or Not?", Scientific American (February 25, 2016): "Climate models, which are virtual representations of our planet, project that temperatures were much higher in the early 2000s than was the case in reality. ... Fyfe and his colleagues think the Pacific Decadal Oscillation (PDO), a natural variance in the climate system that switches between positive, neutral and negative phases, explains the recent slowdown." 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. Another review finds "no substantive evidence" of a pause in global warming. A statistical study of global temperature data since 1970 concludes that the term "hiatus" or "pause" is not justified. 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.
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, 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. 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. 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.
Evidence Surface temperature changes: hiatus periods Climate is the statistics (usually, mean or variability) of weather: the classical period for averaging weather variables is 30 years in accordance with the definition set by the World Meteorological Organization. Instrumental temperature records have shown a robust multi-decadal long-term trend of global warming since the end of the 19th century, reversing longer term cooling in previous centuries as seen in paleoclimate records. There has been considerable variability at shorter interannual to decadal periods, with hiatus periods showing less certain short-term trends. The 1998–2012 hiatus shows a rise of 0.05 [–0.05 to +0.15] °C per decade, compared with a longer term rise of 0.12 [0.08 to 0.14] °C per decade over the period from 1951 to 2012. The appearance of hiatus is sensitive to the start and end years chosen: a 15-year period starting in 1996 shows a rate of increase of 0.14 [0.03 to 0.24] °C per decade, but taking 15 years from 1997 the rate reduces to 0.07 [–0.02 to 0.18] °C per decade.IPCC, Climate Change 2013: Technical Summary, pp. 37–39.
Other aspects of the climate system While hiatus periods have appeared in surface-air temperature records, other components of the climate system associated with warming have continued. Sea level rise has not stopped in recent years, and Arctic sea ice decline has continued. There have been repeated records set for extreme surface temperatures.
Development of perception of post-1998 hiatus
The warm El Niño year of 1998 was exceptional: the IPCC Third Assessment Report of 2001 highlighted that the "high global temperature associated with the 1997 to 1998 El Niño event stands out as an extreme event, even taking into account the recent rate of warming." Opponents of action on global warming used this peak to misleadingly suggest that warming had stopped; an April 2006 opinion piece by Bob Carter in the Daily Telegraph announced an 8-year halt, but was soon rebutted. The IPCC Fourth Assessment Report in 2007 reported that "2005 and 1998 were the warmest two years in the instrumental global surface-air temperature record since 1850. Surface temperatures in 1998 were enhanced by the major 1997–1998 El Niño but no such strong anomaly was present in 2005. Eleven of the last 12 years (1995 to 2006) – the exception being 1996 – rank among the 12 warmest years on record since 1850." The IPCC report was disputed by an open letter in the National Post with 94 signatories, which said "there has been no net global warming since 1998. That the current temperature plateau follows a late 20th-century period of warming is consistent with the continuation today of natural multi-decadal or millennial climate cycling."
There were further claims in blogs and media of lack of warming since 1998, and an Investor's Business Daily article in 2008 even claimed the planet was cooling.Investor's Business Daily, November 04, 2008 4:20 PM, Alarmists Still Heated Even As World Cools In April 2009, a NOAA study showed that similar short- term periods with no trend or even cooling had occurred previously in the years since 1901, and could even be found during the warming trend since 1975: it was easy to "cherry pick" the period 1998–2008 to support one view, but 1999–2008 showed a strong warming trend. They used computer simulations of future climate to show that it was "possible, and indeed likely, to have a period as long as a decade or two of 'cooling' or no warming superimposed on a longer-term warming trend." pdf In July 2009 Jeb Bush said that global warming might not be occurring as mean temperatures had been cooler over six years. The decade to the end of 2010 was again the warmest on record, but David Rose in the Mail on Sunday argued that, excluding the 1998 "blip", global temperatures had been flat for 15 years. A November 2011 study by statistician Grant Foster and Stefan Rahmstorf showed that after allowing for known short-term variability, there had been unabated warming since 1998 with no reduction from the rate over the preceding decade. In January 2012 Rose claimed that the latest global temperatures showed 15 years without warming: the Met Office described this as "entirely misleading".
In January 2013 James Hansen and colleagues published their updated analysis that temperatures had continued at a high level despite strong La Niña conditions, and said the "5-year mean global temperature has been flat for a decade, which we interpret as a combination of natural variability and a slowdown in the growth rate of the net climate forcing", noting "that the 10 warmest years in the record all occurred since 1998." Under the heading "Global Warming Standstill" they "noted that the 'standstill' temperature is at a much higher level than existed at any year in the prior decade except for the single year 1998, which had the strongest El Nino of the century. However, the standstill has led to a widespread assertion that 'global warming has stopped'."Global Temperature Update Through 2012 by J. Hansen, M. Sato, R. Ruedy (January 15, 2013) The Economist led an article 30 March 2013 with the sentence "Over the past 15 years air temperatures at the Earth's surface have been flat while greenhouse-gas emissions have continued to soar, quoting Hansen as saying that ""the five-year mean global temperature has been flat for a decade." It discussed possible explanations of "the recent hiatus in rising temperatures", and suggested that it implied lower climate sensitivity. There was a surge in media interest setting a misleading narrative, as in the Reuters headline "Climate scientists struggle to explain warming slowdown". At the Science Media Centre in London in July 2013, journalists met Met Office scientists and were given a briefing document with three papers on "the recent pause in global warming" in surface temperatures. These said other indicators continued to show warming, at least part of the pause related to heat being exchanged into deep oceans, and it did not alter the risks of future warming or invalidate the physics behind the models: it meant only a 10% reduction in the most probable projection, so "the warming that we might have expected by 2050 would be delayed by only a few years".
In preparing the IPCC Fifth Assessment Report (AR5), representatives of the U.S. government and the European Union wanted details of the slowdown or "hiatus", Germany and Hungary were concerned that a 15-year period was too short to determine a meaningful trend, but the IPCC included discussion of the topic.WGI AR5 Final Draft SPM Comments