New data raises questions about how much the Earth has warmed

Chris Mooney is a Pulitzer Prize-winning journalist and CNN climate contributor. He is currently a professor of practice at the University of Virginia’s Environmental Institute.

After World War II, pollution rates that warmed the planet rose dramatically. James Watt’s steam engine sparked the Industrial Revolution in 1769. Before that, for thousands of years humans had cleared forest 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—what you thought the Earth’s normal temperature was before humans started changing it. But which year should mark this moment?

That’s why the groundbreaking new temperature data set released by a team of British scientists is so remarkable. The data sets used to diagnose the modern history of Earth’s climate and declare that the world is currently warming very close to 1.5 degrees Celsius (2.7 degrees Fahrenheit) typically start in 1850.

The new history dates back to 1781.

This extended time frame is important because greenhouse gases in the atmosphere increased by 2.5% between 1750 and 1850, enough to cause some warming that the data have not yet accounted for.

The new temperature record, known as GloSAT, contributes to scientists’ growing understanding that the Earth is warming beyond calculations based on a year starting in 1850.

“Given the available information, the choice of a start date of 1850 was largely pragmatic,” said Colin Morice, lead author of the new study and a scientist at the Met Office’s Hadley Centre. “Of course, 1850 was not the start of industrialization.”

New data set, published in Earth system science data The study by 16 scientists showed that the Earth was much cooler from the late 1700s to 1849 than from 1850 to 1900 – which scientists define as the “pre-industrial” baseline period used to assess changes in Earth’s temperature.

However, scientists warn that not all the warming between these two earlier periods can be attributed to human activity.

Among other factors, two very powerful volcanic eruptions in the early 1800s had a significant cooling effect on the Earth. Particles from these eruptions scatter around Earth’s stratosphere, blocking some sunlight.

“We know that 1815 was Tambora, and its effects are well documented,” said Ed Hawkins, a researcher at the National Center for Atmospheric Science and one of the study’s authors. “The 1808 eruption was almost as severe, but we don’t know where it happened.”

Some of the warming that occurred in the late 19th century was a natural resumption of the cooling effects of these volcanic eruptions. But maybe it’s more than that.

The United Nations Intergovernmental Panel on Climate Change, a leading authority on climate science, concluded in 2021 that some human-caused warming may have occurred between 1750 and 1850, with temperatures assessed at between 0 and 0.2 degrees.

The scientists behind GloSAT are right in the middle of that spectrum.

Morris and many of the same researchers were involved in a second study, accepted for publication in the journal Environmental Research Letters, that used new data sets and climate models to analyze how much additional warming humans might have caused between 1750 and 1850. The study, led by Andrew Ballinger of the University of Edinburgh, determined that humans were responsible for 0.09 degrees of warming compared with other factors, such as the waning effects of early massive volcanic eruptions. 1800s.

“This was a very interesting time, with a lot of volcanic activity,” said study co-author Andrew Schurer, a researcher at the University of Edinburgh in the United Kingdom.

Piers Forster, a climate scientist at the University of Leeds in the United Kingdom, arrived at similar figures in a study last year using a very different approach. Foster exploited the very powerful and simple relationship between the Earth’s temperature and the amount of carbon dioxide in the atmosphere.

This led him to conclude that an early rise in carbon dioxide levels could have a significant impact.

“If you try to look at the total impact of human activity a little earlier, we think you’d get an additional 0.1 to 0.2 degrees,” Foster said.

Old thermometers and ship records

No one was tracking global temperature changes 300 years ago. But many people have documented local temperature changes. Combining their efforts could help modern researchers understand what’s happening on a global scale.

The study of the oldest temperatures recorded by human observers is intertwined with science, the history of exploration, and the commercial activities that often require companies to collect data.

Early modern records date back to the 17th century. Overall, along the path of the Scientific Revolution, measurement began in Europe and then spread to North America and around the globe. The Central England Temperature Series is the longest of its kind, beginning in 1659 and bringing together the work of many observers. The temperature record in Uppsala, Sweden, was set in 1722 with the help of Anders Celsius himself.

These very old temperature records were included in the new GloSAT analysis – and in some places they show very large warmings.

Consider the records kept at Hohenpeissenberg at the foot of the Bavarian Alps since 1781, when Augustinian priests first began recording temperatures. They contributed to the Societas Meteorologica Palatina, an early coordinated scientific effort that created a network of weather stations centered in today’s Germany but as far away as St. Petersburg, Russia.

Wolfgang Steinbrecht, a scientist at the German Weather Service who works at the observatory, said it was lucky that the record had survived.

“During the Napoleonic Wars, all the monasteries were dissolved,” Steinbrecht said. “All the measurements were taken by people in the monastery, people who could read and write. But Hornpeisenberg survived and now we have more or less gaps in our measurements.”

Compared to temperatures between 1781 and 1849, the Hornpeisenberg temperature record over the past 10 years shows a regional warming of almost 3 degrees.

But how many local measurements (such as this one) need to be made before you can start inferring the temperature of the entire planet? What do you do with the ocean?

After all, measurements at Uppsala, Hohenpesenberg and other similar sites tracked early temperature changes on Earth’s land masses. But the ocean covers about 70% of the earth. Reconstructing temperatures here is also an important addition to the GloSAT record.

Ships began measuring temperatures in the 18th century, but not as systematically as people did on land. For example, in the late 18th century, the British East India Company frequently took barometric pressure, temperature, and other measurements as they sailed and traded silks, spices, and sometimes slaves between Europe and India, China, and other locations.

The ships provided particularly detailed records, although scientists had to carefully adjust the measurements due to biases (such as the ship heating up dramatically during the day) and other factors.

“The East India Company’s surveying was to gain a competitive advantage,” said Elizabeth Kent, a scientist at Britain’s National Oceanographic Center and one of the study’s authors.

“They want to know which way the prevailing winds are so they can trade more quickly, and the temperature of the ocean currents can help them understand if they’re following the currents,” Kent continued. “So, we know they’re seriously trying to measure these things and their [could]”.

Many whalers also took temperature measurements in the Atlantic Ocean off the U.S. coastline, records of which were compiled by Matthew Fountaine Maury, a U.S. naval officer and early oceanographer. Trade routes mean ocean temperatures in the Atlantic and Indian oceans are easier to measure than in the vast Pacific.

The new data set relies on all of these sources. It uses measurements of ocean air temperature rather than the temperature of the water itself like other datasets, since these were more commonly measured on ships in the early days. (The authors note that this approach actually shows slightly less warming in years that overlap with the current temperature data set.)

The scientists fully acknowledge that measurements become sparser over time, and in fact, their map of Earth’s temperature between 1781 and 1800 has many gaps. For this reason, researchers attribute higher uncertainty to estimates of Earth’s true temperature in early years.

“As the authors themselves make clear, there was greater uncertainty before 1850. But it is cooler now, undoubtedly cooler,” said Peter Thorne, a climate scientist at Maynooth University in Ireland and a reviewer of the new study.

what does that mean

However, if additional warming does occur, what does this mean for how we understand the condition of our planet today and how much humans have changed it?

It may seem daunting or depressing to think that climate change is a bigger problem than we think, or that we are doing more to cause climate change than we realize. But scientists say it’s not that simple.

Berkeley Earth researcher Zeke Hausfather, who is familiar with GloSAT’s work, said he thinks this is a major advance. But he warned against jumping to the conclusion that achieving some additional early warming would undermine climate goals, such as those written into the Paris climate agreement, because they are generally thought to be based on a baseline of 1850 to 1900.

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

Thorne, who reviewed the new research, agrees. But he says the discovery of early warming is not inconsequential — you just have to think carefully about what it does and doesn’t mean.

“It does mean that we’re warming the planet more, but it doesn’t mean that the effects are suddenly going to happen faster,” Thorne said. “They’re right where they are. And they’re almost always calculated relative to the most recent reference period.”

Still, additional warming cannot be ignored when assessing humanity’s overall impact on the planet. It may even be part of the forced changes in some Earth systems.

Ultimately, for Thorne, it’s all part of a growing possibility that we should be increasingly concerned about.

“It changes our view of the extent to which important progress has been made in the climate system,” he said.

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