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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.

What the science says...

Global warming is increasing the frequency, duration and intensity of heatwaves.

Global warming is causing more frequent heatwaves. Record-breaking temperatures are already happening five times more often than they would without any 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.

Figure 1: Likelihood of heat records compared to those expected in a world without global warming.

What will heatwaves be like in the future? If we contine to rely heavily on fossil fuels, extreme heatwaves will become the norm across most of the world by the late 21st century. However, if we take major steps to reduce human greenhouse gas emissions, the number of extreme heatwaves will stabilize after 2040. Either way, we will see more heatwaves, but how much more depends on us.

However, the growing risk from heatwaves is ignored by some who argue that heatwaves have happened in the past, hence current heatwaves must be natural. This line of argument is logically flawed, using a logical fallacy called a non sequitor (Latin for 'it does not follow'). This is a fallacy where your starting statement does not lead to your conclusion. For example, this is like arguing that people have died of cancer long before cigarettes were invented, hence smoking can't cause cancer.

The longer we continue to rely on fossil fuels and the higher our greenhouse gas emissions, the more extreme heat we'll lock in. If we manage to take serious action to reduce our greenhouse gas emissions, we can limit global warming to a level where extreme heat events will become more commonplace, but we can manage to adapt to.

Basic Intermediate rebuttal written by John Cook

Update July 2015:

Updated on 2015-07-07 by pattimer.

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