When will global warming actually hit the landmark 1.5 ºC limit?
There’s a 66% chance that the annual global average temperature will hit 1.5 ºC above pre-industrial temperatures at some time in the next five years, according to a World Meteorological Organization report released on 17 May. Reaching 1.5 ºC of warming in a single year will be a landmark moment for the planet, which in 2022 was about 1.15 ºC warmer than in pre-industrial times. But it’s not quite the milestone most people mean when they talk about 1.5 ºC of warming — for that, we probably have about a decade to go.
The famous 1.5 ºC figure, widely quoted as the desired ‘maximum’ for planetary warming, stems from the 2015 United Nations Paris agreement on climate change. This treaty declared the goal of keeping the global average temperature well below 2 ºC above pre-industrial levels, with a preferred limit of 1.5 ºC.
The Paris agreement, however, refers to a sustained planetary average of 1.5 ºC warming — not just the average for a single year, which alone could be anomalously hotter or cooler than the longer-term average. The Paris agreement didn’t specify exactly what was meant by 1.5 ºC of warming, but the most recent report of the first working group of the Intergovernmental Panel on Climate Change (IPCC), published in 2021, clarifies that it means the midpoint of the first 20-year period when the average global surface air temperature is 1.5 ºC warmer than the 1850–1900 average.
In 2018, an IPCC special report on 1.5 ºC of warming estimated that the world would probably hit the 1.5 ºC threshold at some stage between 2030 and 2052. By 2021, using a different methodology, that had been pinned down to the early 2030s. “The time frame is getting closer and closer,” says geographer William Solecki at City University of New York, an author on the IPCC special report.
A massive, two-year ‘global stocktake’ of progress on the Paris agreement’s goals is winding up now, and will be presented at the next UN Framework Convention on Climate Change meeting (COP28), which will start on 30 November. So far, the stocktake has found that things aren’t going well. For a 50% chance of limiting warming to 1.5 ºC, a stocktake meeting report notes, global greenhouse-gas emissions need to peak before 2025; this hasn’t happened yet, and national emissions commitments aren’t sufficient to keep the planet within the target.
The lower the better
The 1.5 ºC number was chosen in an attempt to limit the severity of the impacts of warming, taking into account factors such as food security and extreme weather events. However, IPCC experts stressed that 1.5 ºC shouldn’t be seen as a “guardrail” below which everything would be fine, and noted that whatever temperature the world’s warming peaks at, the lower it is, the better. “Obviously there’s a continuum,” says Solecki. “The higher the temperature, the worse the outcome.”
The 2018 IPCC report on 1.5 ºC of warming notes that effects of reaching this threshold could include: extreme hot days in mid-latitudes that are 3 ºC warmer than in pre-industrial times; sea-level rise of up to three-quarters of a metre by 2100; the loss of more than half of the viable habitat for 8% of plants and 4% of vertebrates; and a decrease in annual global fisheries catches of 1.5 million tonnes.
The report also notes that because global warming is uneven, more than one-fifth of the world’s population currently live in regions that have already exceeded 1.5 ºC of warming in at least one season.
More important than when Earth will hit 1.5 ºC of warming is what amount of warming the planet will peak at, and when that will happen. “With every tenth of a degree above 2 ºC, you’re looking at more-sustained, more-systemic impacts,” says Solecki.
Those numbers won’t be apparent for decades. According to the IPCC’s 2021 projections of global temperature under different emissions scenarios, peak temperature could be anything from 1.6 ºC in around 2050 (if the globe hits net zero emissions by then), dropping to 1.4 ºC by 2100; to, with emissions still climbing, 4.4 ºC at 2100, with the peak still to come.
The next few years could bring an anomalously high blip in annual temperatures compared with the longer-term average thanks to an expected El Niño event — a natural climate pattern that brings warmer temperatures to the eastern Pacific Ocean and that tends to warm the planet as a whole. In April, Carbon Brief, a website that reports on climate matters, estimated that 2023 was shaping up to be one of the six hottest years on record, most likely the fourth hottest. And in April, the global ocean spiked at the hottest temperature since records began.