Hottest nth hemisphere summer in 2,000 years: study

A re-evaluation of climate records shows that the 2023 northern hemisphere summer was the warmest in 2,000 years and the Paris Climate Agreement’s ambitious goal of limiting global average temperatures to 1.5°C above pre-industrial levels may have already been exceeded.

The authors behind the surprising evaluation say poor quality records from measuring stations around the mid-19th century, have been used to formulate previous climate history and global warming targets today.

Verifying this data has been difficult based on the quality of temperature records, most of which only extend back to the mid-19th century.

But an investigation by a team of geographers from Johannes Gutenberg University (Germany) and Cambridge University in the UK recalibrated the 1850-1900CE average land temperatures using tree ring data.

Comparing meteorological data from the end of the 19th century to tree ring proxy data, the Gutenberg-Cambridge geographers found weather station data was biased. Because of inconsistencies in early temperature data collection, weather station data from before 1900 is likely “several tenths of a degree colder”, according to the study published in the journal Nature.

That, its authors argue, calls into question the data used to arrive at the 2015 Paris Climate Agreement.

“Many of the conversations we have around global warming are tied to a baseline temperature from the mid-19th century, but why is this the baseline? What is normal, in the context of a constantly changing climate, when we’ve only got 150 years of meteorological measurements?” says Ulf Büntgen, a professor in Cambridge’s geography department who co-authored the research.

“Only when we look at climate reconstructions can we better account for natural variability and put recent anthropogenic climate change into context,” Büntgen says.

“When you look at the long sweep of history, you can see just how dramatic recent global warming is. 2023 was an exceptionally hot year, and this trend will continue unless we reduce greenhouse gas emissions dramatically.”

Time and temperature

The recalibration puts the northern hemisphere’s 2023 summer at 2.07°C hotter than the 1850-1900CE average.

Benchmarking with tree ring data shows the fluctuating global climate, with evidence of mini ice ages in the  6th and  19th centuries on the back of large volcanic eruptions pumping large volumes of aerosols into the planet’s atmosphere.

El Nino events also show up in the ring record. Typically, warmer years have wider tree rings while the reverse is true in cooler ones. Dry conditions may also limit growth in hot years.

The data shows an almost 4°C difference between the summer of 2023 and the coldest summer, which occurred in 536CE.

When represented chronologically, the recent increase in average temperatures in tandem with the unprecedented rise in greenhouse gas emissions from human activity is hard to miss.

Time scale
2023 in context of the past 2,000 years. Credit: Esper et al, 2024

“It’s true that the climate is always changing, but the warming in 2023, caused by greenhouse gases, is additionally amplified by El Niño conditions, so we end up with longer and more severe heat waves and extended periods of drought,” says Jan Esper, the study’s lead author and a geography professor from the Johannes Gutenberg University Mainz in Germany.

“When you look at the big picture, it shows just how urgent it is that we reduce greenhouse gas emissions immediately.”

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