Guy Callendar, the man who discovered global warming in 1938

83 years ago a little-known amateur scientist called Guy Stewart Callendar made history by discovering the planet had warmed. For our History of Climate Research series, we catch up with climate scientist Ed Hawkins MBE to learn more about Callendar’s work, and how it inspires his research today.

Photograph: G.S. Callendar Archive, University of East Anglia

In 1938, Guy Callendar decided to a take a break from his day job as a renowned steam engineer and begin painstakingly collecting records from 147 weather stations across the world. Doing all his calculations by hand without the aid of a computer, he discovered that average global temperatures had risen 0.3°C over the previous 50 years.

Callendar argued that carbon dioxide (CO2) emissions from industry were responsible.

“The theory of how greenhouse gases could change the planet’s temperature had actually been understood a long time before that,” explains Ed Hawkins, professor of climate science at the University of Reading.

“It was first suggested in the 1820s that there must be something in the atmosphere that made it behave like a greenhouse. By the time we got to the 1860s it was understood that there were various gases in the atmosphere such as carbon dioxide, water vapour and methane that trap heat.”

Although it was generally understood that changing the amount of greenhouse gases in the atmosphere would impact Earth’s climate, Callendar was the first to actually demonstrate that the planet had warmed.

He also pieced together all the various estimates of carbon dioxide in the atmosphere, showed that CO2 was rising, and linked this with how much coal and fossil fuels had been burnt.

“He was the first to bring all of those estimates and pieces of the jigsaw together,” says Hawkins.

Remarkably, despite his crude methods, Callendar’s estimates of global warming were extremely accurate and in line with modern assessments. However, his work was largely ignored or dismissed by other scientists at the time.

“I think there was a lot of scepticism that humans could influence something as large as the planet,” says Hawkins.

“It was a brand new suggestion in many ways, so it took a long time for the implications to be understood. Also, because he was regarded as an amateur operating outside his field, he was perhaps easier to dismiss.”

Nevertheless some people did take notice. In 1953 Charles David Keeling, a young postgraduate geochemist, was inspired by Callendar’s findings, and decided to measure the amount of carbon dioxide in the air directly.

Nobody had ever really tried to measure the level of CO2 in the atmosphere before, so there was no off-the-shelf equipment he could use.

He designed his own apparatus and set off to a weather observation station on the top of the Mauna Loa volcano in Hawaii. Once there, he took meticulous measurements every day. Within five years he proved definitively that CO2 levels were rising.

Since then, daily readings at Mauna Loa have continued almost uninterrupted for more than 60 years. The “Keeling curve”, which documents changes in CO2 levels over time, is the longest continuous record of carbon dioxide concentrations in the world.

“Callendar’s work ultimately helped inspire the construction of a regular carbon dioxide monitoring service on Hawaii by Charles David Keeling,” says Hawkins.

“It also eventually led to the monitoring of global temperatures as well, but it took time.”

Inspiring a new generation of climate science

Ed Hawkins’ Climate Stripes graphic, showing average temperatures across the globe from 1850–2020. Generate your own at

Although he may not have made much of a splash in 1938, eventually the rest of the world did take notice.

“Callendar was really the forerunner of what we do now to understand how the planet is changing,” says Hawkins.

“For me, he is an inspiring character and scientist who was trying to piece together all the lines of evidence he could find to understand what was going on.”

“That’s what we continue to do today. We bring together our theoretical understanding and our observations. However unlike Callendar, who did all his calculations by hand, we now have supercomputers that are able to do all our complex calculations for us.”

“Also Callendar only looked over land areas, because that’s all the data he had available to him, but we now measure temperatures over land, the oceans and over sea ice. We look deep into the oceans as well as higher into the atmosphere.”

Hawkins’ research continues the legacy of Callendar’s work by trying to improve our understanding of how the climate has changed over the past 200 years.

“One large component of what I do is to try to recover lots of old weather observations which are still stuck in various archives on paper,” says Hawkins.

“It’s quite surprising that there are millions of pages of very detailed and accurate weather observations which have never been digitised from their original paper copies.

“We are only using a fraction of the data that we could be, so what I do is run various projects involving volunteers who help us digitise some of this useful old data to improve our understanding of how things have changed in the past.”

Want to know more?

If you’re a UK taxpayer, your contributions help fund the work of researchers like Professor Hawkins and his colleagues at NCAS, via UK Research and Innovation — the funding body that allocates government funds for research — and the nine research councils, including the Natural Environment Research Council. You can read more about what we do here.

Ed Hawkins is a professor of climate science within NCAS at the University of Reading. He is a Lead Author for the IPCC 6th Assessment Report and leads the Weather Rescue citizen science project.

He created the viral ‘climate spiral’ and ‘warming stripes’ visualisations of climate change, which have been used worldwide, including during the opening ceremony of the 2016 Rio Olympics. Ed was awarded the RMetS Climate Communication Prize in 2017, the Royal Society Kavli Medal in 2019 and was made an MBE for services to climate science and science communication in 2019. Follow him on Twitter at @ed_hawkins.

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We need a green revolution: ways to reduce waste and emissions; new designs for cities and homes; more sustainable food systems; successful circular economies. For more than 50 years the UK has been supporting research on climate change and solutions. Here’s what we know.