Study Projects Steep Increase in NYC Heat-Related Deaths

By Andrew Freedman

An increasing number of New Yorkers are likely to die from heat-related causes as global warming causes more extreme heat events, according to a new study released Sunday.

The study found that heat-related mortality may rise 20 percent by the 2020s, and in some worst-case scenarios, it could increase by 90 percent or more by the 2080s, and the net temperature-related mortality, which includes the drop in deaths related to cold weather, could jump by a third compared to current levels.

Leading causes of weather-related fatalities in the U.S.
Credit: NOAA.

The study, published in the journal Nature Climate Change, was produced by Tiantian Li of the Chinese Center for Disease Control and Prevention, and Radley Horton and Patrick Kinney of Columbia University. The study is the most thorough examination to date of how global warming is likely to alter monthly temperature-related mortality in a major metropolitan area, at a time when the world is becoming increasingly urbanized and mayors are pursuing climate adaptation projects in the U.S. and abroad.

With global-average surface temperatures increasing in response to manmade emissions of greenhouse gases, the world has already been seeing an uptick in the frequency and severity of deadly extreme heat events, and a decrease in cold-weather extremes.

Some other studies have claimed that as heat wave-related deaths increase, they will be offset by a reduction in cold weather-related deaths, keeping the net change in mortality low or possibly even resulting in fewer temperature-related deaths per year. This study, however, finds the opposite to be true.

Extreme heat is already the No. 1 weather-related killer in the U.S., killing an average of 117 people per year during the 2003-2012 period. Hot temperatures can contribute to cardiovascular disease, aggravate respiratory illness, and cause heat stroke, among other life-threatening conditions.

Even a small amount of global warming can have a large effect on weather extremes, as recent studies have shown.

In the study, each of the 32 projections from 16 different computer models all yielded an increase in net annual temperature-related deaths, mainly due to the increase in heat-related illnesses. The researchers found a net annual increase in mean temperature-related mortality in New York City of between 5.3 percent and 6.2 percent by the 2020s, depending on the emissions scenario used, and between a 15.5 and 31.0 percent increase by the 2080s.

Cities tend to be warmer than surrounding areas due to the urban heat island effect, since buildings and pavement absorb more incoming solar radiation during the day, and give off heat more slowly at night. Between 1901 and 2000, the average temperature at New York’s Central Park rose by about 3.6°F, which was faster than the national average. Last year was New York’s hottest on record, with summer temperatures that exceeded 100ºF on several occasions.

According to a study published in 2012 in the Proceedings of the National Academy of Sciences, during the period from 1951-1980, extremely hot summers covered just 1 percent of Earth’s land area. That had risen to 10 percent of the Earth’s land area by the period from 1981-2010, and even higher during the 2006-2010 period.

In other words, the study found that the odds of such extreme summers were about 1-in-300 during the 1951-1980 timeframe, but that had increased to nearly 1-in-10 by 1981-2010.

A heat wave in Chicago in 1995 killed hundreds, and the 2003 European heat wave, which studies have shown was made more likely by manmade global warming, killed an estimated 40,000 people. More recently, a 2010 heat wave in western Russia also killed thousands and set all-time record high temperatures for the region.

The larger increase in heat-related deaths compared to the drop in cold-related deaths foreseen by the new study makes sense when one considers that what are typically counted as cold weather-related deaths may not in fact be due to the cold.

According to Patrick Kinney, who directs Columbia’s Climate and Health Program at the Mailman School of Public Health and co-authored the new study, previous research mistakenly attributed spikes in cold season deaths to temperature, rather than illnesses such as influenza, which tend to reach their peak during the winter. These diseases will continue to take their toll during the winter.

“In reality, winter respiratory and related cardiovascular deaths won’t go away at warmer temperatures,” Kinney said in an email. “We see evidence of that when we look at cities that already have warmer temperatures — such as Honolulu — they still have just as big of a winter increase in deaths as much colder places, like Detroit.”

The study found the biggest percentage changes in heat-related mortality would occur during the months of May and September, as those months would begin to see extreme heat events that typically only occur in the June-through-August period.

The study assumes that the increase in temperatures on the hottest and coldest days will shift in lock step with the increase in typical summer or winter temperatures. However, other research has shown that this is not necessarily the case, with larger shifts in the most extreme temperatures taking place in response to a generally warming climate. That may mean that the study actually underestimates future temperature-related mortality.

On the other hand, the study does not take into account the impacts of adaptation measures, such as expanding the use of air conditioning, nor does it address possible changes in humidity and air quality, which would also influence mortality rates.

“One critical question is the extent to which as a society we may be able to better adapt to heat events in the future,” said co-author Radley Horton of Columbia. He cited New York’s efforts to plant trees, make rooftops more reflective to reduce the urban heat island effect, and increase access to air conditioned-cooling centers, as examples of steps that have already been taken to minimize heat-related illness.

“Around the U.S., will such adaptation strategies — which are already yielding benefits — be able to keep pace with rising temperatures?” Horton said in an email, “Or will potential non-linear surprises, such as small increases in summer temperature producing large impacts on public health and supporting infrastructure, predominate? Many Americans, of course, do not currently have easy access to air conditioning.”

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