The summer of 2023 in large parts of the Northern Hemisphere was the hottest for more than 2,000 years

The summer of 2023 was hot – in fact, it was very hot. Images of the devastating forest fires in various regions of our planet, such as those in France, Greece, and Canada, dominated our screens during television newscasts. Geographers at Johannes Gutenberg University Mainz (JGU) in Germany and the University of Cambridge recently looked far back into the past and have determined that the summer of last year was the hottest over wide areas of the Northern Hemisphere since year 1 of the Common Era. They report their findings in an article published yesterday in the online edition of the journal Nature. The team led by Professor Jan Esper of the Institute of Geography of Mainz University describes how they compared temperatures of the landmasses of the Northern Hemisphere located between the 30th and 90th degrees of latitude. Here in an area that includes the whole of Europe as well as large parts of North America and Asia are and were based the largest number of meteorological stations worldwide. The researchers initially correlated measured temperatures for the months of June, July, and August of 2023 with those recorded in the period 1850 to 1900. They discovered that the average temperature in summer 2023 was 2.07°C warmer than that of the summers of the phase from 1850 to 1900, which the Intergovernmental Panel on Climate Change (IPCC) uses as a reference period for pre-industrial temperatures. To generate a more extensive comparison, the team then made use of an existing international archive of meteorological data that had been reconstructed with the help of tree rings and reaches back as far as year 1 of the Common Era. "What we found as a result was that summer 2023 was the hottest even over this very long period of time and was 2.20°C warmer than the mean summer temperature since year 1 CE," noted Esper. "This clearly demonstrates the unparalleled nature of present-day warming of the Earth and how important it is that we take immediate action to reduce levels of emission of greenhouse gases."

Pre-industrial levels of temperature in the 1850 to 1900 period were cooler than has been assumed

Their analysis of tree ring data led the researchers to unearth another unsettling fact. "Our calculations show that the mean temperature in the period 1850 to 1900 was actually 0.24°C lower than had been presumed on the basis of the data collected at the time by meteorological stations," said Esper. "This would mean that global warming is more advanced than had been thought and that our defined climate targets need to be reformulated." For instance, the current goal of the 2015 Paris Agreement is to ideally limit warming globally to 1.5°C in comparison with pre-industrial temperature levels. At the same time, Esper's team is convinced that their historical temperature record produced with the help of tree rings is more accurate than the data collected by the meteorological stations in the second half of the 19th century. "In the region we have been considering, there were at the time only 58 continuously operating meteorological stations, of which 45 were in Europe. This means that there are insufficient weather records for extensive zones of the Northern Hemisphere and for the whole of the Southern Hemisphere," Esper explained. It has also been acknowledged that much of the observational data collected back then is inaccurate because at least some of it was measured using thermometers that were inadequately sheltered against direct solar radiation.

The research outlined in the article published in Nature was financed in part by an ERC Advanced Grant awarded to Jan Esper by the European Research Council.