This is the first installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.
Most people these days accept that our planet is warming, but some find it hard to believe that mere humans could cause such a large-scale global change. How do we know that the warming is due to human activity? How can we be so sure?
Would it surprise you to know that the scientific community has been investigating the link between CO2 and global warming for more than 175 years? Scientists are born skeptics, and by no means accepted the theory as fact the first time it was proposed.
The idea that gases in the atmosphere could trap heat and warm the earth was first proposed in 1827, in an essay by French mathematician and physicist Joseph Fourier. He was trying to explain why the earth was warmer than physics would predict based just on distance from the sun.
The topic came up again 30 years later when Irish naturalist John Tyndall found clear evidence that glaciers once covered the Alps. The Earth had experienced ice ages. But how could the climate have changed that much? Tyndall was familiar with Fourier's essay, and in 1859 did some experiments that showed water vapor and carbon dioxide (CO2) could both trap heat.
Nearly 40 more years passed before another scientist fascinated with ice ages did some investigating. In 1896, Swedish Nobel Prize winner Svante Arrhenius computed that halving the amount of CO2 in the atmosphere would lower the temperature by 7°F or more – ice age temperature. But he didn't see how CO2 concentrations could change this much.
It was a colleague of Arrhenius, Arvid Högom, who discovered that burning fossil fuels could add significant amounts of CO2 to the atmosphere – as much as natural geochemical processes. But both he and Arrhenius saw this as something that could only cause warming over thousands of years – at first.
In 1908, when coal-burning was more widespread, Arrhenius published a book where he theorized that global warming might take centuries rather than millennia, but no one paid this much mind. It wasn't the main point of the book, and no one believed it anyway.
For the next three decades, most scientists dismissed these ideas. The theories were often flawed or oversimplified, and in any case they thought that excess CO2 would be harmlessly absorbed by the ocean.
Then in 1938, an English engineer named Guy Stewart Callendar took another look. People had been talking about a warming trend, so he checked the record and found that CO2 concentrations had increased 10 percent over the last hundred years. His observation spurred further research.
Finally, in the 1950s, thanks to increased government funding for research after World War II, experiments confirmed the suspicions. There was an unmistakable connection between CO2 emissions from the burning of fossil fuels and global warming. Check out this short produced in 1958 by Frank Capra:
So you see, the greenhouse effect is not something scientists cooked up a few years ago, or even a few decades ago. We have been studying it for almost two centuries! After painstaking measurements and calculations by generations of scientists, the scientific community has reached a consensus: Global warming is caused by human activities. In the next posts in this series, I'll tell you how the measurements and calculations lead us to that conclusion.