A key feature of the new IPCC report is its look into how climate change impacts are likely to be different at 1.5°C and 2°C warming above pre-industrial levels. A comparative look at heat extremes at these two warming levels is among the topics covered. The implications of these kind of projected changes – from adverse effects on our health and safety, to creating pre-conditions for large wildfires – are not difficult to envision after the devastating heat waves of 2018.
To date, there is strong evidence that extremely hot days are becoming hotter around the world as a result of human-caused global warming emissions. The frequency and length of extreme heat events are also increasing in many locations. In the US, the frequency of heat waves has generally been on the rise since the mid-1960s, although the 1930s “Dust Bowl” period still holds records in many parts of the country.
Moving forward, as extreme heat responds directly to increases in global warming emissions, pronounced changes are projected around the world at 1.5°C that worsen at 2°C. In fact, increases in temperatures – and especially extreme temperatures – are projected to be larger on land than the overall increase in the Earth’s average surface temperature. Certain regions are expected to experience disproportionately large changes in extreme heat as the Earth warms, including mid-latitude regions and the tropics.
These projections mean that an increasingly large number of people will be exposed to extreme heat more often as the Earth warms. According to a study led by authors at the European Commission, at 2°C, 420 million more people are likely to be exposed to extreme heat waves at least once every five years than would be if warming is contained to 1.5°C. In the US, if global carbon emissions can be stabilized in the next few decades such that global temperatures average around 2.4°C warmer than pre-industrial levels at the end of the century (the scenario known as Relative Concentration Pathway 4.5), the frequency of heat waves is projected to be 50 percent lower at mid-century than if carbon emissions continue to grow through 2100 and global temperatures average 4.3°C warmer than pre-industrial levels (Relative Concentration Pathway 8.5).
We know from this year that extreme heat poses serious risks for our health and safety. Unprecedented heat waves flared up around the world, setting temperature records in Oman, causing large numbers of deaths in Japan, and exacerbating wildfires in the US and even in the Arctic Circle. Here in the US, extreme heat continues to be one of the most dangerous and deadliest natural hazards, with populations such as the young and elderly at a disproportionate risk.
Extreme heat is already a problem for many places, and many places are already seeing more of it because of human-caused climate change. Even in a 1.5°C world we are likely to see more intense, frequent, and longer-lasting extreme heat events than we do now. However, the science that is in the new IPCC 1.5 report is likely to underscore that we can avoid some of the most dangerous changes in extreme heat and we can prevent the exposure of significant portions of the world’s population – including here in the US – to more frequent heat waves. The science is clear that this requires taking immediate action to ramp down global warming emissions, especially as the world’s current commitments under the Paris Agreement bring us nowhere near neither 1.5 nor 2°C above pre-industrial levels – but rather around 3°C.