Crazy hot summers could become the new normal - and a lot more people may die as a result

AP

Hotter summers could be bad news for many of us.

The study found that every 1.8-degree-Fahrenheit rise in average summer temperature was linked with a 1% higher annual death rate.

If that doesn't sound like a lot, think of it this way: In a city of 100,000 people, if 1,000 people die there in a typical year (just slightly higher than the average mortality rate for the US), that means about 10 additional people would die for each 1.8-degree rise in temperature.

As you might expect, the opposite effect was seen for warmer winters: The same increase in the average winter temperature was associated with a 0.6% lower death rate.

What ended up being more deadly overall for people were variable temperatures. For each 1.8 degree shift in the average temperature during summer or winter months, 1.3% more people died.

The bottom line is this, according to study co-author Liuhua Shi, a doctoral student at the Harvard Chan School of Public Health:

"We found that in zip codes with a warmer summer than its average, more people die," Shi told Business Insider in an email. This is because people don't adapt to large shifts in temperature very fast, she added. The findings were reported Monday, July 13, in the journal Nature Climate Change.

That's particularly bad news because climate change will likely make these swings more common.

The Intergovernmental Panel on Climate Change (IPCC) forecasts that the average global temperature will increase anywhere from 0.4 to 9.9 degrees Fahrenheit by 2100.

And this worldwide warmup will likely be bad news for our health. Many studies have found a link between short-term changes in temperature and the risk of dying, but we know less about the effects of long-term seasonal temperature changes.

To find out, Shi and her colleagues compared the death rate of adults age 65 and older, who were enrolled in Medicare, with average summer and winter temperatures in New England from 2000 to 2008.

The effect of unusually hot summers was larger than the effect of warmer winters, the researchers found. The higher death rates in summers was entirely due to anomalies in one year's temperature compared to the 9-year average temperature for a particular zip code. (People who lived in places where warm summers were the norm actually lived longer, suggesting people adapt to the heat, the researchers said.)

By contrast, the lower death rate due to higher winter temperatures was a much more subtle effect, and suggests that people don't get used to the cold.

The scientists can only speculate as to why more people die when summers are warmer or when winters are colder than normal for their hometown.

One possibility the researchers suggested is that short-term changes in temperature cause fluctuations in blood pressure, which, over time, can lead to poor health.

Another possibility is that wildly changing temperatures may make it harder to keep an exercise routine, or engage in other healthy habits.

At any rate, the findings suggest that changes in climate may be affecting our health in ways scientists are just beginning to understand.