Australasian Science: Australia's authority on science since 1938

No Warming Hiatus Observed

Extremely hot temperatures over land have dramatically and unequivocally increased.

Despite claims that global warming has stalled over the past 10–20 years, an international team of scientists has announced that extremely hot temperatures over land have dramatically and unequivocally increased in number and area.

To get their results, the researchers compared the temperatures of every day of the year with temperatures on that exact same calendar day from 1979–2012. The hottest 10% of all days over that period were classified as hot temperature extremes.

“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 research co-author Dr Lisa Alexander of the ARC Centre of Excellence for Climate System Science (ACECSS). “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 Niño since 1998.”

The observations also showed that extremely hot events are now affecting on average more than twice the area compared with similar events 30 years ago.

Globally, regions normally expect around 36.5 extremely hot days per year. The observations showed that regions that experienced 10, 30 or 50 extremely hot days above this average during 1997–2012 saw the greatest increase in extreme hot days over time and the area they impacted. The consistently upward trend persisted right through the “hiatus” period of 1998–2012.

“Our analysis shows there has been no pause in the increase of warmest daily extremes over land, and the most extreme of the extreme conditions are showing the largest change,” said Dr Markus Donat of ACECSS. “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.”

While global annual average near-surface temperatures are a widely used measure of climate change, this latest research reinforces that they do not account for all aspects of the climate system. A stagnation in the increase of global annual mean temperatures, over a relatively short period, does not imply that global warming has stopped. Other measures, such as extreme temperatures, ocean heat content and the disappearance of land-based ice all show continuous changes that are consistent with a warming world.