Hotter extremes belie warming 'pause'
LONDON, 1 March - If global warming has paused, nobody told the thermometer. Although global average temperature rises have not kept pace with greenhouse gas emissions in the last decade, the mercury has been higher than ever for longer than ever over increasingly larger areas of land, according to a new study in Nature Climate Change.
Sonia Seneviratne from the Swiss Federal Institute of Technology in Zurich and colleagues in Australia and Canada chose not to look at averages but at extremes of temperature. The scientists examined daytime extremes from 1979 onwards, and compared the temperatures of any particular day anywhere to an average of daily temperatures between 1979 and 2012, to identify the hottest 10%. Any region might normally expect 36.5 hottest days in a year; that is, hotter than the average.
Then they looked more closely at temperatures from 1997 to 2012. Regions that experienced 10, 30 or 50 extremely hot days above this average saw the greatest upward trends in extreme hot days over time – and over area. That is, not only were people experiencing greater heat extremes, but they were experiencing them over more days and over more extended regions.
And this consistent upward trend persisted right through the so-called “hiatus” of 1998 to 2012. The year 1998, at the time the hottest ever, coincided with a major El NiÃ±o event, the peak of a natural cycle of warmth and cooling in the Pacific.
Extreme extremes change most
Thereafter, although 13 of the 14 warmest-ever years have occurred this century, the rate of increase in warming as a global average has fallen. Climate sceptics used the trend to argue that global warming was an illusion, or part of a natural cycle. Dr Seneviratne and her colleagues do not see it that way.
“It quickly became clear the so-called ‘hiatus’ in global average temperatures did not stop the rise in the number, intensity and area of extremely hot days,” said Lisa Alexander of Australia’s Centre of Excellence for Climate System Science.
“Our research has found a steep upward tendency in the temperatures and number of extremely hot days over land and the area they impact, despite the complete absence of a strong El NiÃ±o.”
And her colleague Markus Donat added: “There has been no pause in the increase of warmest daily extremes over the land and the most extreme of the extreme conditions are showing the largest change.
"Another interesting aspect of our research was that those regions that normally saw 50 or more excessive hot days in a year saw the greatest increases in land area impact and the frequency of hot days. In short, the hottest extremes got hotter and the events happened more often.”
However, perhaps because the world is mostly ocean, and the extremes have been measured over land, the average, year-on-year rises in temperatures have been lower in the last decade than in previous decades. There have been a number of inconclusive explanations for this phenomenon.
Cyclic changes in trade winds are one explanation; another is that the heat is there, but has been stored in the deep ocean, where measurements are not systematically taken. It’s there somewhere, waiting to be found.
And US scientists argue in the latest issue of the journal Science that the oceans may have an even bigger influence on climate than anybody foresaw, and that persistent cool conditions in the tropical Pacific may be behind what they call the “pause in global warming since 2000.”
But the latest Nature Climate Change paper puts the case that this pause or hiatus is illusory with – for a scientific paper – unusual clarity. “Based on existing observational evidence,” the authors say, “we highlight that the term pause, as applied to the recent evolution of global annual mean temperatures, is ill-chosen and even misleading in the context of climate change.
"Indeed, an apparently static global mean temperature can mask large trends in temperatures at both regional and seasonal scales.”