The number and intensity of extremely hot days has been increasing steadily despite a "pause" in the rise of average surface temperatures over the past 15 years, a new study has found.
"This analysis shows that not only is there no pause in the evolution of the warmest daily extremes over land but that they have continued unabated over the observational record," said the paper published Wednesday in Nature Climate Change.
"Furthermore, the available evidence suggests that the most 'extreme' extremes show the greatest change."
The average global temperature is a common measure of climate change used by scientists and policymakers, including the Intergovernmental Panel on Climate Change.
'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.' - Sonia Seneviratne and her research team
The panel's most recent report, released in September, noted that between 1998 and 2012, the average global temperature over land changed very little, despite an increase in the greenhouse gas concentrations that are expected to drive global temperature increases. The panel called this a "temperature hiatus" and said the temporary pause in global surface warming may have been caused by natural variability or by oceans absorbing extra heat.
The "hiatus" was used by some lobbyists to argue that climate change is not an urgent problem.
Constant increase in some measures
However, based on their results, the Swiss, Australian and Canadian authors of the new paper argue that in fact, during the so-called hiatus or pause in average global surface warming, there has been a constant increase in other climate change measures, including melting Arctic ice and snow and extreme heat days.
"We highlight that the term pause, as applied to the recent evolution of global annual mean temperatures," the researchers wrote, "is ill-chosen and even misleading in the context of climate change."
The new study, led by Sonia Seneviratne of the Institute of Atmospheric and Climate Science in Zurich, looked at existing temperature data and tracked the number of extremely hot days (days in the hottest 10 per cent for that day of the year) each year from 1997 and 2010 compared to the average for 1979 to 2012. The study, whose co-authors included Environment Canada scientist Brigitte Mueller, then mapped the amount of land area where the number of hot days exceeded a certain cutoff e.g. there were 10 or 30 or 50 more hot days than normal.
What they found was that the amount of land area affected by each threshold level of extreme heat increased steadily over time, and extremely hot events now affect on average more than twice the land area than similar events 30 years ago.
Seneviratne wrote in an email to CBC News that extreme temperatures have increased more than average temperatures even over land.
"We have not definitely identified the reasons for this, but it is likely related in part to more drying over land leading to extra heating of air there."
Ocean temperatures less relevant
The researchers suggest that the average temperatures have likely held steady despite an increase in extreme summer heat due to a cooling of winter temperatures in boreal regions and a cooling of ocean surface temperatures — "because heating has taken place at deeper depths in the oceans during the so-called 'hiatus period,'" Seneviratne wrote in an email to CBC News.
She added: "Areas with high population density, agricultural production, forest and grassland ecosystems, are found on continents not over the oceans."
In their commentary, the researchers argued that because of this, extreme heat events over land have a greater impact on human health, agriculture, ecosystems and infrastructure than average global temperatures.
Canadian climate scientist Gordon McBean said he agrees.
"Things that matter to Canadians are heavy precipitation events, hot days or cold days, and extremes. If the [average] temperature is slightly different now than before, it's not a big deal," he said in a phone interview with CBCNews.ca.
McBean, a professor in the department of geography at Western University, noted that a major heat wave in Western Europe in 2003 caused an estimated 70,000 premature deaths. He added that smog can trigger serious health issues for people with asthma and cardiovascular diseases, and that chemical reactions that produce smog are also sped up by extreme heat.
'Way that makes sense'
While climate scientists use a variety of climate change indicators, McBean acknowledged that the average global temperature has become prominent because "we scientists have tended to make it that way."
He said in part that is because scientists are not always good at communicating.
Other measures like the frequency of hot days can be defined in a variety of ways and involve statistics that confuse the average person.
However, he said that Seneviratne and her colleagues "found a way that seems to make sense."
He added that all the authors of the paper are "very top scientists."
CBC News contacted Mueller and Environment Canada Tuesday to ask about Mueller's contribution to the study. Late Wednesday afternoon, Danny Kingsberry of Environment Canada media relations responded that Mueller's work on the paper was completed during her PhD studies in Zurich.
"As a visiting fellow at Environment Canada, Dr. Mueller is not in the best position to speak to the general findings on the warming hiatus," Kingsberry wrote in an email to CBC News.
CBC News also had trouble reaching University of New South Wales researcher Markus Donat, another co-author of the paper.