02/26/2014 05:00 EST | Updated 04/27/2014 05:59 EDT

Extreme heat days multiply despite global warming 'hiatus'

The number of extremely hot days has been increasing steadily globally 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 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.

However, based on their results, the Swiss, Australian and Canadian authors of the new paper argue that in fact, global average temperatures can hide trends in extreme temperatures.

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 when the temperature was hotter than it was 90 per cent of the time for that day of the year) each year from 1997 and 2012 compared to the average for 1979 to 2010. 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.

In their commentary, they argued that extreme heat events, rather than average temperatures, have a greater impact on human health, agriculture, ecosystems and infrastructure.

"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 paper noted that extremely warm temperatures can be amplified by phenomena such as melting ice and snow in the Arctic or drying soil in temperate regions – different factors than those that affect average global temperatures.

The researchers suggest that the average temperatures have likely held steady despite an increase in extreme summer heat due to a cooling of ocean surface temperatures and a cooling of winter temperatures in boreal regions.