The summer of 2023 will be the hottest summer in the non-tropical regions of the northern hemisphere in 2000 years, a new study has shown.
Surface air temperatures in June, July and August 2023 across this vast tract spanning Europe, Asia and North America were 2°C higher than the average summer temperature between AD 1 and 1890, as reconstructed from tree-ring records.
Although climate change is a global phenomenon, warming is often stronger on a regional scale. And what people are experiencing is not global average temperature, but regional climate change.
The Paris agreement aims to limit climate change to below 2°C and ideally to 1.5°C of warming, but these figures generally refer to global temperature change with a 20-year average. The authors of the new study suggest that these targets are already violated on a regional scale during the northern hemisphere summer months.
There have been 2,295 deaths in the UK from five heatwaves in the summer of 2023. The authors of the new study wanted to understand how unusually warm the summer of 2023 was in the northern hemisphere compared to the past.
To do this, they resorted to one of the most useful tools for measuring Earth’s temperature over millennia: rings that grow each year on tree trunks anywhere on the planet where the climate is seasonal.
Two thousand year old tree rings
The extreme heat in 2023 was caused by greenhouse gas emissions and was further amplified by El Niño, the warm phase of the natural climate cycle in the Pacific Ocean.
To find out what the climate was like in the recent past, scientists analyze weather station records to see whether a particular year was warmer or colder than previous years. The longest existing instrumental record is the Central England Temperature series, which dates back only to 1659; This isn’t long enough to put the final warm-up in context.
To find out how warm it is now compared to two thousand years ago, scientists use climate proxies. These are natural archives, such as ice cores and sediments, that preserve a record of climate in the layers they accumulate over time.
Trees grow in many different places and accurately record historical climate information for much of the world. Trees form rings annually in seasonal climates, so there is no doubt as to the date when a particular ring was formed.
To study past climate, tree ring scientists analyze how wide or narrow a ring was in a given year, how dense the wood was, or its chemical composition.
Tree ring growth is sensitive to many climate variables, but in conifers growing in northern hemisphere woodlands (the point at which trees can no longer grow due to cold temperatures, high winds, or low humidity) it is summer temperature that most strongly controls trees. ring growth. During hot summer months such trees will tend to form large, dense rings.
A period in which all trees in a given area grow in wide or narrow rings, if correctly cross-referenced with many trees, indicates an unusual phase in the climate affecting the growth of trees.
The authors of the new study were looking for trees that faithfully recorded past summer temperatures. They combined records of thousands of these trees from hundreds of sites in North America and Canada, England and Europe, Scandinavia, Russia, Mongolia and Japan. Tree ring records are produced by scientists who work diligently to sample tree cores, measure the rings, and share the data.
This massive tree-ring archive revealed that the northern hemisphere summer of 2023 will be 2.2°C warmer than the average for every year between AD 1 and 1890. Compared to AD536, the coldest year of the last two millennia when a massive volcano erupted and cooled the planet for several years, the summer of 2023 was found to be almost 4°C warmer than that year.
What will future forests reveal?
As greenhouse gas emissions increase, humans can expect more frequent and severe climate events as Earth’s greenhouse effect increases. In the past, very hot years were experienced globally during El Niño events, such as in 2016.
But greenhouse gas levels are now so high that in 2017 the planet experienced a very hot year for the first time in the La Niña phase, which is the opposite of El Niño and has a cooling effect on global temperatures.
The new study found that 2023 even broke the 2016 record as it was 0.23°C warmer than the last summer when El Niño intensified. Greenhouse gas emissions are now so high that when climate records are broken, they are broken in large incremental changes rather than small increments.
Thousands of trees sampled from mountain forests in the northern hemisphere for this study face increasingly challenging growing conditions. If scientists were to visit them again, they would find trees damaged by heat and drought in many places.
A single drought in California between 2012 and 2016 killed more than 200 million trees. Tree mortality in France has increased by over 80% in the last decade; This is also seen around the world as a result of warmer droughts.
Forests shelter 80% of terrestrial biodiversity and support the livelihoods of 1.6 billion people. Trees also regulate the global climate and store carbon in the air, slowly creating a record in their rings of our failure to stop dangerous climate change.
The extreme heat of 2023 highlights the need for urgent climate action. If the world rapidly decarbonizes, future scientists will hope that climate improvement is quietly recorded in the tree-ring records of Earth’s remaining forests.
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Mary Gagen does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic duties.