Meet the heat: 21st century to be hot, hotter, and hottest

From a new set of projections in Environmental Research Letters:

"Climate change is set to trigger more frequent and severe heat waves in the next 30 years regardless of the amount of carbon dioxide (CO2) we emit into the atmosphere, a new study has shown.

Extreme heat waves such as those that hit the US in 2012 and Australia in 2009 — dubbed three-sigma events by the researchers — are projected to cover double the amount of global land by 2020 and quadruple by 2040.

Meanwhile, more-severe summer heat waves — classified as five-sigma events — will go from being essentially absent in the present day to covering around three per cent of the global land surface by 2040.

The new study, which has been published today, Thursday 15 August, in IOP Publishing's journal Environmental Research Letters, finds that in the first half of the 21st century, these projections will occur regardless of the amount of CO2 emitted into the atmosphere.

After then, the rise in frequency of extreme heat waves becomes dependent on the emission scenario adopted. Under a low emission scenario, the number of extremes will stabilise by 2040, whereas under a high emission scenario, the land area affected by extremes will increase by one per cent a year after 2040."

The tropics and the Mediterranean are expected to be the hardest hit, write authors Dim Coumou and Alexander Robinson. We are locked in to a major increase in heat events to 2040, regardless of what we do with emissions, but if we fail to restrain emissions, the post-2040 future will be horrific.

This is a graphic showing the projected rise in 3 and 5-sigma [standard deviation] heat events, the box at the left for the last few decades, and the two at the right showing projections (with the black line showing observations to date). Note the difference post 2040 is dependent on emission scenarios [click to enlarge]. The authors warn of the risks of inaction:

Unmitigated climate change causes most (>50%) continental regions to move to a new climatic regime with the coldest summer months by the end of the century substantially hotter than the hottest experienced today.