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Climate change could give San Francisco the climate of San Diego, scientists say

Cars drive along Presidio Parkway in front of a span of the Golden Gate Bridge in San Francisco on July 15. (Jeff Chiu/AP)

As you enter Redwood City, Calif., you may notice a sign with a curious slogan printed on it: “Climate best by government test.”

Having the best climate is a bold boast, and the sign got climate scientist Ken Caldeira of Stanford University’s Carnegie Institute of Science thinking about how he might test the town’s claim to fame. His research, published Tuesday in the journal Scientific Reports, indicates which city may actually have the nicest climate — and which city could take the title in the next 100 years if humans fail to curb carbon emissions and mitigate climate change.

Caldeira and Yana Petri, a recent high school graduate and intern in his lab, decided to take a closer look at how climate change could affect the need for heating and cooling in buildings throughout the U.S. by the end of the century under a “business-as-usual” trajectory — that is, if humans do nothing to mitigate carbon emissions in the coming decades. Looking at the number of heating and cooling days a place requires throughout the year is one way of gauging how comfortable its climate is, as more comfortable temperatures will require less air conditioning or heating.

Petri and Caldeira made their calculations using temperature projections from a climate model, assuming that temperatures warmer or cooler than 65 degrees Fahrenheit would prompt people to use heating or cooling. They expressed their results in “degree days,” with one degree day being one day at a temperature one degree above or below 65 degrees. “Ten heating degree days could be one day when it’s 55 degrees outside,” says Caldeira — that is, one day with temperatures 10 degrees below 65 degrees. “Or it could be 10 days when it’s 64 degrees.”

The combined number of heating and cooling degree days — in other words, the number of days any sort of air conditioning was required in a location — indicates how temperate the climate is. Fewer degree days means less combined heating and cooling, suggesting a more comfortable climate that requires less energy use. Caldeira and Petri looked at both historical temperature data from around the country as well as future temperatures projected using the climate model.

Some results were not surprising. The researchers found that the combined number of days per year that either heating or air conditioning are used will decrease in the North and increase in the South. This is largely because of the significant increase in the amount of cooling the South will require if temperatures continue to rise across the country.

In addition to a broad look at heating and cooling needs around the country, Petri and Caldeira also zeroed in on 25 major U.S. cities to see how some of the country’s most populous places could change in the future. They found that, as of now, San Diego is the most “comfortable” city temperature-wise, meaning it has the lowest number of combined heating and cooling degree days. Residents of Minneapolis are less lucky — their city is the least comfortable.

And, in fact, projections indicate that Minneapolis will stay the city with the greatest number of heating and cooling degree days. But San Diego, on the other hand, will lose its title to another California city if nothing is done about climate change. Under a business-as-usual trajectory, the researchers found that San Francisco will become the city with the least number of heating and cooling degree days in the country. “The fact that future San Francisco will be quite similar to San Diego is really quite interesting,” Petri says, as present-day San Francisco is known for having a much cooler climate overall.

Under the same scenario, Los Angeles would heat up enough to match the number of cooling degree days currently experienced by Jacksonville, Fla. And New York City’s combined number of heating and cooling degree days will match that of present-day Oklahoma City, as it will both lose heating days and gain cooling days.

To be clear, Caldeira says, the study should not be taken as a prediction of events that are definitely going to occur, but rather a projection of events that could happen under a specific scenario. Carbon emissions reduction plans are in place in the U.S. and elsewhere around the world, and while they’re unlikely to halt climate change altogether, they make a business-as-usual trajectory much less likely. That means that the climatic changes that take place in the U.S. will hopefully be less dramatic than those described in the paper. And it’s also worth pointing out that Caldeira and Petri’s study is by no means a perfect indicator of the “best” climate. Temperature is just one aspect of climate, Caldeira says, and future related studies might take other factors into account, such as cloud cover or precipitation.

Still, Caldeira adds, the projections are a “useful, concrete way of understanding the magnitude of climate change.” And they could even help inform the real estate industry  by indicating which locations have the potential for the most change. “Hopefully, [the results] will help people decide where it will be convenient to live in the future based on temperature comfort,” Petri says.

It’s a thought-provoking first publication for Petri, who has been an intern in the lab for a little more than a year and will start school at University of California, Berkeley this fall. While closely supervised, Caldeira says Petri handled much of the data herself, and adds, “I think she was as smart and hard-working and as thoughtful as many of my postdocs.”

As far as the Redwood City claim is concerned, it looks like San Diego is calling its bluff — if we’re going just by temperature, that is. On the other hand, Redwood City is in the Bay Area, not far from San Francisco. So if our climate mitigation efforts were to fail and the study’s projections became reality, Redwood City’s moment could still be coming — nearly a hundred years down the road.

Also in Energy & Environment:

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Meet the “clean cow” technology that could help fight climate change

What you need to know about Obama’s biggest climate move yet — the Clean Power Plan

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