The summer of 2023 was unusually hot. Scientists have already proven that it was the warmest summer in the Northern Hemisphere since people began systematically measuring and recording temperatures around 1850.
In a new study published in the journal Nature, researchers compared 2023 with the longer temperature record in most parts of the Northern Hemisphere and said it was the hottest year in 2000. The study uses evidence from tree rings to go back to 1 AD, before the advent of thermometers and weather stations.
“This gives us a complete picture of natural climate change,” says Jan Esper, a climatologist at Johannes Gutenberg University in Mainz, Germany and lead author of the paper.
Most of the recent rise in global temperatures is due to extra greenhouse gases in the atmosphere from burning fossil fuels, but other factors such as El Niño, undersea volcanic eruptions, and reduced sulfur dioxide aerosol pollution from container ships may also be affected. This contributed to last year's heat wave.
According to the researchers' tree-ring data, the average temperature from June to August 2023 was 2.20 degrees Celsius higher than the average summer temperature from 1 to 1890.
And last summer was 2.07 degrees warmer than the average summer temperature from 1850 to 1900, generally considered the benchmark for the period before anthropogenic climate change.
New research suggests that Earth's natural temperature has been lower than this baseline. This baseline is frequently used by scientists and policymakers when discussing climate goals, such as limiting global warming to 1.5 degrees Celsius above pre-industrial levels.
“Instruments aren't covered very well this time of year,” Dr. Esper said, adding, “Tree rings can work very well.” So you can use this as a replacement or even a fix. ”
Trees grow wider each year in a distinct pattern, with brightly colored rings appearing in spring and early summer, and darker rings appearing in late summer and fall. Each pair of growth rings represents one year of his life, and differences in the growth rings provide scientists with clues about changing environmental conditions. For example, in warm, wet years, trees tend to grow more and form wider rings.
The study compared temperatures in 2023 to previously published reconstructions of temperatures over the past 2,000 years. More than a dozen research groups collaborated to create this reconstruction using data from about 10,000 trees from nine regions in the Northern Hemisphere from 30 degrees to 90 degrees latitude, or anywhere above the tropics. . Some of the data was obtained by drilling very thin cores from living trees, but most was obtained from dead trees and historic wood samples.
Covering a longer time period means that more volcanic eruptions will be included in the data. Large eruptions, at least on land, can spray sulfur dioxide aerosols into the atmosphere and cool the Earth. Esper said about 20 to 30 such eruptions have occurred over the past 2,000 years, causing average temperatures to drop.
(In contrast, the recent Hunga Tonga eruption occurred underwater and sprayed large amounts of water vapor into the atmosphere. Water vapor is a powerful greenhouse gas.)
Not everyone agrees that tree rings provide a more accurate picture of past temperatures than historical records.
“It's still an active area of research,” says Robert Rohde, principal scientist at Berkeley Earth. Although Dr. Rohde was not directly involved in the new study, data from his organization was used. “This is not, in any sense, the first paper to suggest that there is a warm bias in the early instrumental period. But I don't think it's really settled.”
Zeke Hausfather, another geoscientist at Berkeley, says the small differences in the stories thermometers and tree rings tell us about Earth's past are somewhat irrelevant today.
“This is more of an academic problem than a practical problem,” he says. “Reassessing temperatures in the distant past doesn't tell us much about the effects of climate change today.”
Last year, these impacts included a heat dome that took hold over much of Mexico and the southern United States for several weeks. Japan has experienced its hottest summer on record. Canada is experiencing its worst wildfire season on record, and parts of Europe are also experiencing a spate of destructive wildfires. 2024 is expected to be another hot year.
