2023 summer hottest in non-tropical Northern Hemisphere in two millennia

Researchers said while this warming trend could not be applied globally, the findings demonstrated the 'unparalleled nature of present-day warming', and pressed for urgent action in reducing carbon emissions.

New Delhi: The summer of 2023 was the hottest in the northern hemisphere in the past two thousand years, scientists reported on Tuesday, noting that the global warming limit of 1.5 degrees Celsius temperature set by the Paris climate accord has already been breached.

Although 2023 has already been reported by the World Meteorological Organisation as the hottest year on record, the instrumental evidence dates back only as far as 1850 at best, and most records are limited to narrower geographic regions.

Now using year-by-year temperature records extracted from patterns of tree rings over two millennia, scientists from the University of Cambridge and the Johannes Gutenberg University Mainz have shown how exceptionally hot the summer of 2023 was.

Even allowing for natural climate variations over hundreds of years, 2023 was still the hottest summer since the height of the Roman Empire. For India, it was the hottest since the country was ruled by the Kushans and Satavahanas.

“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” co-author Ulf Büntgen, from Cambridge’s Department of Geography said in a media statement. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”

The results, reported in the Nature, also demonstrate that in the northern hemisphere, the 2015 Paris Agreement to limit warming to 1.5 degrees C above pre-industrial levels has already been breached.

The researchers used instrumental data for the 1850-1900 period even though the information was sparse and inconsistent. They subsequently calibrated it using tree-ring data and also used the same to learn more about past weather trends.

Tree rings contain annually-resolved and absolutely-dated information about past summer temperatures. Using tree-ring chronologies allows researchers to look much further back in time without the uncertainty associated with some early instrumental measurements.

The available tree-ring data carry the details of most of the cooler periods over the past 2000 years, such as the Little Antique Ice Age in the 6th century and the Little Ice Age in the early 19th century.

The coldest summer of the past two thousand years, was recorded in 536 CE and was 3.93C colder than the summer of 2023.

Most of the warmer periods can be attributed to the El Nino climate pattern, or El Nino-Southern Oscillation (ENSO) that impacts weather worldwide due to weakened trade winds in the Pacific Ocean and often results in warmer summers in the northern hemisphere.

While El Nino events were first noted by fishermen in the 17th century, they can be observed in the tree ring data much further back in time.

But over the past 60 years, global warming caused by greenhouse gas emissions are causing El Nino events to become stronger, resulting in hotter summers. The current spell of El Nino is expected to continue into early summer 2024, making it likely that this summer will break temperature records once again.