July 2023 was the hottest month on record, the European Centre for Medium-Range Weather Forecasts confirmed on 8 August. More broadly, this year’s overall global average temperature is shaping up to be the hottest ever measured.
The main reason for these extremes is the long-term effect of humans releasing ever more greenhouse gases into the atmosphere, plus the shift to a warm El Niño pattern in the Pacific Ocean, says Michael McPhaden at the US National Oceanic and Atmospheric Administration.
He says El Niño’s effect is especially striking after several cooler years in the Pacific “masked” a steady rise in the atmospheric concentration of carbon dioxide – an increase of about 4 per cent since the last big El Niño in 2016.
The heat was also boosted by a wavy jet stream pattern (in yellow, above) that formed simultaneous “heat domes” of trapped hot air in North America, Europe and China. Whether climate change makes such heat domes themselves more likely is an active area of research.
While this explains the lion’s share of the heat, natural variability and other factors may also be behind local extremes. For instance, a natural warm phase in the North Atlantic Ocean has contributed to record sea surface temperatures there. Other factors, from a deficit of reflective sand blown off the Sahara desert to a drop in sulphur emissions from ships following new regulations in 2020, may also be at play. Water vapour injected into the stratosphere by the 2022 eruption of Hunga-Tonga Hunga Ha’apai in Tonga could also have had a marginal global warming effect.
But, generally, climate researchers aren’t surprised. “Nothing in the physical climate side that is happening now is unexpected,” says Andrew Dessler at Texas A&M University. With El Niño likely to strengthen and emissions rising, don’t expect 2023 to hold on to its records for long.
This story is part of a series in which we explore the most pressing questions about climate change. Read the other articles below: