In the next week, with the Paris climate conference underway, millions of Americans will hear and read news stories about global warming. They'll hear that allowing global average temperatures to increase by more than 2°C would be dangerous, that nations of the world have pledged to keep warming below 2°C and that, in the absence of policies, global average temperatures are likely to increase by a shocking 4°C by the end of the century.
There's only one problem: Americans think in degrees Fahrenheit.
In Europe or Asia, 40° is a hot summer day. But no one who grew up in the United States of America thinks that way. For someone who grew up in the U.S., 100° is a hot summer day. A 40° day requires a jacket.
In the past month I've asked members of two large crowds at Columbia University in New York City to raise their hands if they knew how to convert degrees Centigrade to degrees Fahrenheit. In one crowd, less than half the hands went up. In another, less than a third did. This was at one of the United States' elite universities, in crowds self-selected for their interest in global warming.
For those of you wondering, here's the answer: a degree Centigrade is roughly twice a degree Fahrenheit -- 9/5 as much to be exact. So 2° Centigrade is roughly 4° Fahrenheit -- or 3.6° Fahrenheit to be exact.
Why does this matter? Because using degrees Centigrade when talking about global warming makes the problem seem smaller to most Americans than it actually is. When people who grew up in the United States hear that "global average temperatures are likely to increase by more than 2°C," their strong instinct is to interpret that as 2°F -- about half the actual increase. It would be surprising in the extreme if most Americans did a quick mental conversion and said to themselves "oh, 2°C, you mean 3.6° in the units I'm most familiar with and use every day."
Consider one of the findings of the Intergovernmental Panel on Climate Change's Fifth Assessment Report -- that in the absence of policies global average temperatures will likely increase roughly 4°C from current levels by the end of the century. That's scary, but likely to be misinterpreted by many Americans. It would improve understanding to say -- accurately -- that without policies global average temperatures will likely increase by roughly 7°F.
Or consider a recent analysis by Climate Action Tracker, an independent research group, which found that even if all the climate action plans national governments have submitted in connection with the Paris conference are fully implemented, global average temperatures will nevertheless increase this century by roughly 2.7°C. Your American friends are more likely to understand that correctly if you say that, even if all the national climate plans are fully implemented, global average temperatures will likely increase by roughly 5°F. (Warming on land will be even greater. Warming in the Poles will probably be double that amount. But that's another topic.)
I suggest that anyone communicating with Americans about global warming take the Fahrenheit Pledge. The Pledge is simple: "When discussing global warming with an American audience, I will never use degrees Centigrade without also using degrees Fahrenheit."
The Pledge has a corollary: "When discussing the 2°C goal with an American audience, I will always refer to the 2°C/3.6°F goal or the 3.6°F goal."
This simple step is especially important for reporters. In recent months several news outlets have begun to use degrees Fahrenheit in global warming stories with U.S. audiences, but the practice is by no means uniform. Accurate communication requires speaking to any audience in a language and in units they understand.
It's also important when talking about global warming with American friends, neighbors and co-workers. Why confuse them by using units that make the problem seem roughly half as large as it actually is?
Please join me in taking the Fahrenheit Pledge.
David Sandalow, Inaugural Fellow at the Center on Global Energy Policy at Columbia University, has served in senior positions at the White House, State Department and U.S. Department of Energy.