New data raises questions about how much the Earth has warmed

By Chris Mooney

Planet-warming pollution rates exploded after the end of World War II. James Watt’s steam engine launched the Industrial Revolution in 1769. Before that, for thousands of years, humans were clearing forested land for farming, releasing carbon from trees and plants into the atmosphere.

The severity of global warming has long depended on your frame of reference — on what temperature you think was normal for the Earth before humans began changing it. But what year should mark that moment?

That’s what makes a groundbreaking new temperature dataset released by a group of scientists based in the United Kingdom so striking. The datasets used to diagnose the modern history of the planet’s climate — and to proclaim that the world is now very near to 1.5 degrees Celsius (2.7 degrees Fahrenheit) of warming — typically begin with the year 1850.

The new one goes all the way back to 1781.

This extended time frame matters because greenhouse gases in the atmosphere increased 2.5 percent between 1750 and 1850, enough to have caused some warming that the data hasn’t accounted for.

The new temperature record, dubbed GloSAT, helps contribute to the growing sense among scientists that the Earth has warmed more than what calculations based on the 1850 starting year would suggest.

“That 1850 start time is one that’s chosen for essentially practical considerations, given the information that’s available,” said Colin Morice, lead author of the new study and a scientist with the Met Office Hadley Centre in the UK. “For sure, 1850 is not the start of industrialization.”

The new dataset, published in Earth System Science Data by 16 scientists, shows a significantly cooler Earth from the late 1700s through 1849 compared with 1850-1900 — the latter being what scientists have defined as the “preindustrial” baseline period used to assess the planet’s temperature change.

However, not all of the warming between the two early periods can be attributed to human activities, scientists caution.

Among other factors, two very powerful volcanic eruptions in the early 1800s had a marked cooling effect on the Earth. Particles from those eruptions spread around the planet’s stratosphere and blocked some sunlight.

“We know 1815 was Tambora, with well documented impacts,” said Ed Hawkins, a researcher with the National Centre for Atmospheric Science and one of the study’s authors. “The 1808 eruption was nearly as big, but we have no idea where it happened.”

Some of the warming that occurred by the late 19th century is a natural recovery from the cooling effect of these eruptions. But perhaps not only.

The leading climate science authority, the U.N. Intergovermental Panel on Climate Change, concluded in 2021 that there had probably been some human-caused warming between 1750 and 1850, assessing that it was between 0 and 0.2 degrees.

The scientists behind GloSAT come down right in the middle of that range.

Morice and many of the same researchers contributed to a second study, accepted in the journal Environmental Research Letters, which uses the new dataset and climate models to analyze how much additional warming humans may have caused between 1750 and 1850. That study, led by Andrew Ballinger of the University of Edinburgh, determined that 0.09 degrees of warming occurred that was attributable to humans, as opposed to other factors, such as the waning effect of the large volcanic eruptions seen in the early 1800s.

“This period of time is just so interesting, it’s got this large amount of volcanism in it,” said Andrew Schurer, one of that study’s authors and a researcher at the University of Edinburgh in the UK.

Piers Forster, a climate scientist at the University of Leeds in the UK, landed on a similar number using a very different approach in a study last year. Forster used the very strong and simple relationship between the planet’s temperature and how much carbon dioxide is in the atmosphere.

It led him to conclude that the very early rise in carbon dioxide levels would have had a significant effect.

“We think you get between an extra .1 and .2 degrees if you try and look at the total effect of human activity going back a bit further,” Forster said.

Old thermometers and ship records

No one was keeping track of global temperature changes 300 years ago. But there were lots of people documenting local temperature change. Combinging their efforts helps modern researchers understand what was happening at a global scale.

The study of the oldest temperatures recorded by human observers is interwoven with the history of science, exploration and, frequently, the commercial activities that required businesses to collect the data.

Early modern records go back to the 17th century. Following the path of the scientific revolution generally, measurements began in Europe — and then spread to North America and around the globe. The Central England Temperature series, the longest of its kind, begins in 1659, stitching together the work of many observers. A temperature record in Uppsala, Sweden began in 1722, aided by the work of Anders Celsius, himself.

These very old temperature records feed into the new GloSAT analysis — and in some locations, they show a very large amount of warming.

Consider a record that has been kept in the foothills of the Bavarian Alps at Hohenpeissenberg since 1781, when Augustinian clerics first began recording temperatures. They were contributing to the Societas Meteorologica Palatina, an early coordinated scientific effort that created a network of weather stations centered in today’s Germany, but extending as far afield as St. Petersburg in Russia.

This record is lucky to have survived, said Wolfgang Steinbrecht, a scientist with the German Weather Service who works at the observatory.

“During the Napoleonic wars, all the monasteries were disbanded,” Steinbrecht said. “All the measurements were taken by people at monasteries, they were the ones who could read and write. But Hohenpeissenberg survived, now we have measurements more or less without gaps.”

The Hohenpeissenberg temperature record shows nearly 3 degrees of regional warming when the last 10 years are compared with temperatures between 1781 and 1849.

But how many local measurements, like this one, do you need before you can begin to deduce the the temperature of the entire planet? And how do you deal with the oceans?

After all, measurements at Uppsala, Hoehenpeissenberg and other similar locations track early temperature changes on Earth’s land. But oceans cover about 70% of the planet. Reconstructing temperatures here, too, is a key addition of the GloSAT record.

Ships measured temperature in the 18th century, though not as systematically as people did on land. The British East India Company for instance, often took air pressure, temperature and other measurements as they sailed and traded silk, spices and sometimes slaves between Europe and India, China, and other locations in the late 18th century.

These ships provide a particularly detailed record, although scientists have had to carefully adjust the measurements because of biases — ships heat up dramatically during the day, for instance — and other factors.

“The East India Company made their measurements for competitive advantage,” said Elizabeth Kent, a scientist with the National Oceanography Centre in the UK and one of the study’s authors.

“They wanted to know the prevailing winds so they could do their trading faster, and the temperature of the currents helped them understand if they were catching the currents,” Kent continued. “So, we know that they were seriously trying to measure these things as well as they [could].”

Many whaling vessels also took temperature measurements in the Atlantic ocean off the US coastline, and their records were compiled by Matthew Fountaine Maury, a US naval officer and early oceanographer. Trade routes meant ocean air temperatures were far better measured in the Atlantic and Indian oceans than over the vast Pacific.

The new dataset relies on all of these sources. It uses a measurement of marine air temperatures – not the temperature of the water itself, as in other datasets – because these were more frequently taken aboard ships in these early years. (This approach actually shows a slightly lower level of warming during the years of overlap with current temprature datasets, the authors note.)

The scientists fully admit that measurements become much more sparse as one goes back further in time, and indeed, their map of the Earth’s temperatures in the period from 1781-1800 comes with a lot of gaps. For this reason, the researchers ascribe a higher level of uncertainty to their estimates of the Earth’s true temperature during these early years.

“As the authors themselves make clear, there is larger uncertainty prior to 1850. But it is cooler, it’s undoubtedly cooler,” said Peter Thorne, a climate scientist at Maynooth University in Ireland, who served as a reviewer for the new study.

What it means

If that additional bit of warming did occur, though, what might it mean for how we understand the state of the planet today, and how much humans have changed it?

It might seem daunting or disheartening to contemplate that climate change is an even bigger problem than we thought, or that we’ve done more to cause it than we realized. But it’s not so simple, scientists say.

Zeke Hausfather, a researcher with Berkeley Earth who is familiar with the GloSAT work, says he thinks it’s a significant advance. But he cautions about leaping to the conclusion that this realization of some additional early warming undermines climate goals, such as those written into the Paris climate agreement, as those are generally understood to be based on the 1850 to 1900 baseline.

“I think it tells us something about pre-1850 warming,” Hausfather said, “I just don’t think we should read too much into what that implies for our ability to meet climate goals.”

Thorne, who reviewed the new study, agrees to a point. But he says the finding of earlier warming isn’t irrelevant — you just need to think carefully about what it means and doesn’t mean.

“It does mean we warmed the Earth more, but it doesn’t mean the impacts are suddenly going to occur sooner,” Thorne said. “They are where they are. And they’ve almost invariably been calculated relative to a much more recent reference period.”

Still, that additional warming can’t be ignored in assessing the total human impact on the Earth. It could even be part of what forced changes in some Earth systems.

Ultimately, for Thorne, it’s all part of a picture in which we should be increasingly concerned about the potential for mounting impacts.

“It changes our perception of how far we have already pushed the climate system in important ways,” he said.

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