Most Americans hear temperature degrees and automatically think Fahrenheit, while almost everyone else in the world thinks in terms of degrees Celsius. And so our minds may conjure up something different when we ponder the 2 degrees that the Paris agreement named as an upper limit for global warming, or the 1.5 degrees now deemed dangerous.

Weather stories in the United States use Fahrenheit, but climate stories switch back and forth between Celsius and Fahrenheit — so no wonder if Americans are confused. The smaller degrees of Fahrenheit may cause Americans to underestimate the magnitude of global warming that’s already happened, and what’s being projected for the near future.

In this global era, why would any nation continue to use a quaint temperature scale that’s out of step with everyone else, and, worse still, out of step with the scientific community? Surely there must be some upside to it.

I asked Gino Segre, a physicist at the University of Pennsylvania and author of “A Matter of Degrees: What Temperature Reveals About the Past and Future of Our Species, Planet and Universe.” Mostly, he said, the advantage is that we’re used to it. But that advantage disappears if you get used to Celsius.

Until relatively recently, there was no quantitative temperature scale. When Shakespeare asked, “Shall I compare thee to a summer’s day?” he would have had no sense that the numbers 75 F or 24 C might characterize his balmy image. In the 17th century, said Segre, several people, including Isaac Newton, created early versions of the thermometer based on the expansion and contraction of liquids. But someone had to decide how to impose a scale — where to put the zero and how big to make the degrees.

During the early 18th century, several scales competed. Dutch-German-Polish inventor Daniel Gabriel Fahrenheit invented a scale that put zero at the freezing point of the salt solution he was using to measure expansion, and 100 at his best guess for body temperature. Around the same time, Swedish physicist Anders Celsius advocated for a scale in which the 0 and 100 matched the freezing and boiling points of water. That scale, better rooted in nature, was originally called centigrade but came to be named for its advocate.

Later in the 18th century, scientists realized that temperature was related to pressure and volume through Boyle’s law, and that when they plotted the relationship, temperature hit a lowest point, absolute zero, close to minus 273 C. In the 19th century, the British Lord Kelvin, William Thomson, developed a new scale, the Kelvin scale, which starts at absolute zero but uses degrees the same size as Celsius degrees.

At that point, the bigger C degrees became the standard for science. Scientists use them — and the Kelvin scale — for noting the temperature of deep space, of the interiors of stars, inside black holes, and in laboratory conditions that get within a millionth of a degree of absolute zero.

For Americans growing up with Fahrenheit’s degrees, it feels right to associate 90 with a hot day, 25 with a cold one, and 100 with the start of a serious fever. But those growing up with Celsius temperatures have the same intuition for their own system.

Fahrenheit’s little degrees might work fine if everyone used them, and in fact they convey finer meaning and are well suited to discussing the temperatures common in the most inhabited places on Earth — ranging roughly between 0 and 100. But the world isn’t going to follow American leadership in this. The global scientific community has decided to go with the bigger C degrees. Our reluctance to follow is probably related to the American failure to adopt the metric system back in the 1970s when it was catching on elsewhere.

And now temperature degrees matter more than ever. If Americans shrug over the idea of 2 degrees of global warming, that’s no small matter: The difference between 2 degrees F and 2 degrees C of global warming is the life or death of thousands of people from heat waves and extreme weather, the extinction of thousands of species, and the flooding of Miami, New York City and other low-lying places.

Even if we are using an impractical system to measure temperature, Americans can take the lead in minimizing the emissions that are at the root of the problem. We can also become more conversant in Celsius degrees, and the rest of the world can be mindful to express global warming in degrees F. It may be impractical to switch, but we and the world can meet half way.

Faye Flam is a Bloomberg Opinion columnist. She has written for The Economist, The New York Times, The Washington Post, Psychology Today, Science and other publications. She has a degree in geophysics from the California Institute of Technology.

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