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Katharine Hayhoe: Here's How Long We've Known About Climate Change

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One of the biggest myths about climate science—a myth that has been deliberately fostered, for decades—is that we just don't know that much, yet.

The field is still in its infancy, people argue and a lot more is needed before coming to consensus. After all, aren't scientists always changing their minds? Just a few decades ago, they were predicting an ice age, not global warming!

Even for those of us on board with the scientific consensus that climate is changing and humans are responsible, might be hard pressed to pick a year when climate science really began. Surely before 1990, when the first Intergovernmental Panel on Climate Change assessment was published? Maybe in 1988, when Jim Hansen testified to Congress? Or in 1981, when he published his first paper on the greenhouse effect of trace gases?

Joseph Fourier (1768- 1830).

Good guesses—but all wrong. The field of climate science stretches back almost 200 years. That's right: Scientists have been studying our planet for that long.

For more than 150 years, we've known that mining coal and burning fossil fuels produces heat-trapping gases. For more than 120 years, we've been able to put numbers on exactly how much the Earth would warm if we artificially increased carbon dioxide levels in the atmosphere. And it's been more than 50 years since the President's Council of Advisors on Science and Technology formally warned a U.S. president—Lyndon B. Johnson—that building up carbon dioxide in the atmosphere would "almost certainly cause significant changes" and "could be deleterious from the point of view of human beings."

It all started in the 1820s, when a French mathematician named Joseph Fourier realized that, for the Earth to be in equilibrium with the energy it was receiving from the sun every day, it should be a lot cooler than it actually is: around 33 degrees Celsius or nearly 60 degrees Fahrenheit cooler. In fact, it should be a ball of frozen ice. But it isn't.

Eunice Foote was an amateur scientist with a lively interest in many topics, from campaigning for women's rights to filing patents for boot soles. In 1856, she wrote a paper for the annual meeting of the American Association for the Advancement of Science, reporting on her measurements of the heat-trapping properties of carbon dioxide. She even speculated that if, "at one period of [Earth's] history the air had mixed with it a larger proportion [of CO2] than at present, an increased temperature from its own action must necessarily have resulted"—in other words, if there were more carbon dioxide in the atmosphere, then it would trap more heat, and the Earth would be warmer.

All this has to do with the planet's natural atmosphere, though. How long have we known that humans can impact climate? Over in England, a scientist and professor at the Royal Institute, John Tyndall, was asking similar questions, at around the same time.

John Tyndall (1820 – 1893).

With his rigorous scientific training and access to a state-of-the-art laboratory, John laid the foundation for our modern understanding of how molecules absorb and emit radiation. He also connected the dots between human activities and heat-trapping gases.

Svante Arrhenius (1859 – 1927).

By extracting and burning coal, oil and natural gas, we're putting extra carbon into the atmosphere. And this thicker blanket traps more heat, making the planet warmer. How much warmer? In the 1890s, a Swedish chemist named Svante Arrhenius decided to calculate, by hand, the very first climate model. It took him two years to figure out how much the world would warm if humans doubled or tripled the amount of carbon in the atmosphere: and his numbers were amazingly close to what the most recent global climate models, run on powerful supercomputers, still find today.

But wait a minute. We know the climate has changed in the past, when there weren't any humans around. How do we know the planet's not just still warming after the last ice age?

During WWI, a Serbian concrete expert named Milutin Milankovic was told he could continue his studies—as long as he focused on something that had nothing at all to do with the war effort. So he thought, why don't I figure out why we had ice ages in the past?

Milutin Milankovic (1879 – 1958).

So he did. He discovered that ice ages, and the warm interglacial periods like we're in right now, are initiated by changes in the shape of the Earth's orbit around the sun and the tilt of its axis of rotation. Over time, these cycles cause the great continental ice sheets to expand and retreat.

Variations in the tilt of Earth's axis and the shape of the orbit around the sun that occur over millennia act as triggers for glacial maxima, or ice ages, and the warm periods in between.

So, does that explain what's happening right now? No, because the warming after the last ice age peaked between four to eight thousand years ago. Today, according to natural cycles, we should be gradually and slowly cooling, in preparation for the next ice age. But, thanks to all the coal, oil and gas we've burned since the Industrial Revolution, that's no longer the next event on our geological calendar. Instead, we're heading into unknown territory—unknown, that is, since the time of the dinosaurs, when there weren't any ice sheets, when the sea level was more than 300 feet higher than today and when the land where a third of the people on this planet currently live would've been under water.

Historical departure from annual global mean surface temperature average (1961-1990), showing that warming after the last glacial maximum peaked between 8,000 and 4,000 years ago.

Yes, it's been warmer before and it's been colder. But human civilization is not built to deal with the changes we are making to this planet, the only one we have. That's why we care about a changing climate.

This essay originally appeared at The Equation, a blog of the Union of Concerned Scientists.


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