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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Increased risk from heatwaves

Observed record ratio (the increase in the number of heat records compared to those expected in a world without global warming) for monthly heat records as it changes over time (thin red line is annual data, thick red line smoothed with half-width 5 years). This is compared with predictions from a simple stochastic model based only on the global mean temperature evolution (blue line with uncertainty band directly comparable to the smoothed red curve).

As global surface temperatures have increased, heatwaves are becoming more frequent. Coumou, Robinson, and Rahmstorf 2013 found that record-breaking monthly temperature records are already occurring five times more often than they would in the absence of human-caused global warming. This means that there is an 80% chance that any monthly heat record today is due to human-caused global warming.

Under a moderate emissions scenario, in which human greenhouse gas emissions peak around the year 2040 (RCP 4.5), monthly heat records in 2040 will have become approximately 12 times more likely to occur than in a non-warming world. As lead author Coumou noted, this is even worse than it sounds, because breaking a heat record in 2040 will require much higher temperatures than breaking a record today.

"Now this doesn’t mean there will be 12 times more hot summers in Europe than today – it actually is worse. To count as new records, they actually have to beat heat records set in the 2020s and 2030s, which will already be hotter than anything we have experienced to date."