The 2023 summer temperature was 1.19°C higher than the warmest summer in 246 AD
A hot summer day in West Bengal last year. Photo: iStock The summer of 2023 was the warmest in the last 2,000 years in the Northern Hemisphere, according to a new study. Rising greenhouse gas emissions and a climate pattern are thought to have contributed to this.
The 2023 summer temperature was 1.19°C higher than the warmest summer in the year 246 (0.88°C), the study published in Nature noted. This was recorded during the Late Roman Warm Period, from AD 1 to AD 500, and is considered the warmest period of the last 2,000 years. It was driven by natural processes.
Further, the 2023 summer was almost 4°C warmer than the coldest summer in the year 536 over the last 2,000 years. This cooling followed a large-scale volcanic eruption, which released sulphur dioxide. The gas forms sulphuric acid aerosols in the stratosphere and reflects incoming solar radiation, cooling the Earth’s surface.
“When you look at the long sweep of history, you can see just how dramatic recent global warming is,” Ulf Büntgen, from the University of Cambridge’s Department of Geography and co-author, said in a statement. “2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically,” the expert added.
The paper highlighted that the 2015 Paris Agreement to limit warming globally to 1.5°C has already been breached, the researchers wrote in their paper.
The 2015 Paris Agreement aims to keep the global average temperature increase to “well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”.
Researchers from the United Kingdom, Germany, and the Czech Republic gleaned past climate information from tree rings over two millennia. Trees are sensitive to local climate conditions as tree rings usually grow wider in warm, wet years and thinner when it is cold and dry.
Read Tree rings can predict climate change
Reconstructing past temperatures, according to the researchers, helps them consider natural variability and put recent anthropogenic climate change into context.
Their analysis showed that the 2023 June, July, and August temperatures over the Northern Hemisphere were 2.07°C warmer than the early instrumental mean between 1850 and 1900.
The study could not reach the same conclusion for the southern hemisphere due to scarce data.
Further, the team attributed warmer periods in the last 2,000 years to El Nino, a warm phase recurring climate pattern across the tropical Pacific—the El Nino-Southern Oscillation, or “ENSO”.
They studied the central Pacific Nino 3.4 sea surface temperatures (SSTs) — the most commonly used indices to define El Nino and La Nina (the cool phase of ENSO) events. El Nino or La Nina events are defined when the Nino 3.4 SSTs exceed above or below 0.4°C for a period of six months or more.
They highlighted that global warming caused by greenhouse gas emissions has caused El Nino events to become stronger over the last six decades, resulting in hotter summers.
“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Nino conditions, so we end up with longer and more severe heat waves and extended periods of drought,” Professor Jan Esper, the lead author of the study from the Johannes Gutenberg University Mainz in Germany, said in a statement.
The 2023 warming was not unexpected as multiple regional heatwaves, exceeding any daily or weekly instrumental measurements, were reported throughout the boreal summer of 2023, the study read.
In May 2024, National Oceanic and Atmospheric Administration predicted that El Nino is likely to transition into ENSO-neutral in June, after which La Nina may develop with a 49 per chance in June-August 2024, or a 69 per cent chance between July-September.
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