How Sensitive is Earth's Climate?

This is the third part in a series breaking down the fundamentals of climate science. Here’s Part 1 and Part 2.

To predict how much the world is going to warm in the future, one of the key numbers to understand is Earth’s climate sensitivity. This is the answer to the question: how much will Earth’s temperature rise if we double carbon dioxide levels?

It turns out that this question is as old as the field of climate science. In 1896, the Nobel Prize winning Swedish chemist Svante Arrhenius took a creative approach to solving this problem. By cleverly reinterpreting data on the intensity of moonlight, Arrhenius was able to make the first modern prediction of Earth’s climate sensitivity. (If you’re interested in how he did this, here’s an in-depth video.)

Arrhenius’s answer — 5 to 6 ℃ — is on the high end, compared to our current understanding. Today climate scientists predict this number to be between 1.5 and 4.5 ℃. But the fact that Arrhenius was even in the right ballpark is impressive, given that he was working with indirect data, had to fill in many gaps in the theory, and spent an entire year crunching the numbers by hand!

By the time Arrhenius carried out his calculation, it was well known that carbon dioxide was a greenhouse gas (a fact that we owe to Eunice Newton Foote, although typically credited to John Tyndall). This was the basis for his work, which he combined with the then brand-new theory of heat.

However, Arrhenius wasn’t particularly concerned about global warming. His main motivation was to understand why the ice ages happened. By extrapolating forwards from 1896, Arrhenius worked out that it would take thousands of years for humans to double carbon dioxide levels.

Which was a perfectly reasonable prediction to make in 1896, unless of course humans somehow managed to exponentially increase their carbon emissions.

Well… we all know how that turned out.

Today, we’re in the process of actually conducting Arrhenius’s alarming thought experiment. We’re nearly halfway to a CO₂ doubling compared to pre-industrial levels, and our carbon emissions are accelerating.

So it’s easy to see why Earth’s climate sensitivity is an important number to understand. It helps us understand the future.

We don’t really know where our carbon dioxide levels will end up, that depends on the extent ...