August 8, 2006

A Hot Urban Legend

According to news reports, the recent heat wave in California was responsible for about 150 fatalities. Many people including the United Nations’ Intergovernmental Panel on Climate Change believe this can only get worse with global warming, predicting a doubling or a tripling of heat-related deaths in North American cities in the next decade.

Global warming or not, our cities have been warming, and will continue to do so. Sprawling masonry and blacktop retain heat and impede the flow of ventilating winds. (Here in DC, there’s an additional warming: waste heat from all the money changing hands). So, this makes the notion of increasing heat-related deaths, as cities warm, a very testable hypothesis.

The United Nations is dead wrong.

In fact, in several refereed papers published in recent years, my colleague Robert Davis and I demonstrated that heat-related deaths have, in aggregate, declined significantly as our cities have warmed. In fact, in a statistical sense, we have completely engineered heat-related mortality out of several of our urban cores, particularly in the eastern U.S.

The UN’s notion that people will simply sit around and slowly fry to death as cities warm is known to climate researchers as the “Stupid People Hypothesis”

A plot of daily mortality versus high temperature generally shows a negative relationship, meaning that, the warmer it is, the fewer deaths there are. It’s well-known, for example, that there are more deaths in the winter than in the summer. But, on the few very hottest days of summer, there’s a big jump in fatalities.

So, reasons the United Nations, if the highest summer temperatures in our cities go up a few degrees, that could lead to a massive increase in mortality—assuming, of course, that people are stupid.

Rather than looking all the decades of mortality data at once, perhaps it might be better to examine it decade-by-decade to see if people are indeed this dumb.

This deconstruction of the data reveals a remarkable adaptation. As cities warmed, the “threshold” temperature at which mortality begins to increase has also risen—more than the temperature of the cities!

For example, in the 1960s, mortality began to increase in Philadelphia once the high temperature exceeded 83°. In the 1970’s, the mortality threshold rose to the low 90s. In the last decade there’s little evidence for any threshold at which mortality increases. In other words, people adapted to their changing climate.

Instead of simmering, people buy and sell air conditioning. They vote for leaders who work to improve emergency medical care. All levels of government warn of the danger of excessive exposure to heat.

Social adaptation can take place very quickly. In mid-July, 1995, over 500 people died from an intense weekend heat wave in Chicago. Research by climatologist Dr. Mike Palecki and colleagues shows that a 1999 Chicago heat wave, which was virtually the same in intensity, resulted in only 15% of the number of deaths that occurred in 1995.

Having said that, this summer’s heat is a bit unusual. Commonly, when it’s very hot in the eastern U.S., temperatures are normal or below average in the West, or vice-versa. This year it’s hot everywhere.

Is history merely repeating itself, or is it global warming?

Many summers in the 1930s were known for their nationally widespread and intense heat. 1930 was a scorcher. In rural Virginia, away from the Washington’s sprawl, there were 21 100°-plus days. Even with the excess heat contribution from the growth of the city, Washington only averages one per year.

Or, instead of repeating history, is the widespread distribution of 2006’s heat the kind of thing we would expect from a more global warming?

The fact is that we cannot completely discriminate between repetitive history and prospective warming when it comes to a single summer. The better place to look for warming is in the winter. Greenhouse-effect theory predicts that the coldest temperatures of winter should warm much more sharply than the hottest ones of the summer. Indeed, there has been a greatly disproportionate warming of winter’s coldest days for the last several decades.

All of which leads to the obvious complexity of global warming. Would people accept (or welcome) climate change that greatly alleviated winter discomfort if the cost were slightly higher summer temperatures? They clearly have adapted to the heat.

The evidence shows that, the warmer the city is to begin with, the more quickly people adapt. There’s only one major U.S. city where heat-related deaths are increasing: chilly Seattle. San Francisco and Los Angeles, two other cities that are relatively cool in the summer compared to every other one to their east, show no change in mortality.

This forces the conclusion that heat waves are dangerous when they are rare and unexpected because people are unfamiliar with them and don’t know the appropriate actions to take to avoid the dangerous impacts. As they become more common, we will simply be better prepared for them and incorporate them into our daily lives and routines–just as the people of Phoenix and Dallas and Houston and New Orleans do, every summer day. People aren’t stupid.


Davis, R.E., et al., 2003. Changing heat-related mortality in the United States. Environmental Health Perspectives, 111, 1712-1718.

Palecki, M.A., S.A. Changnon, and K.E. Kunkel, 2001. The nature and impacts of the July 1999 heat wave in the midwestern United States: Learning from the lessons of 1995, Bulletin of the American Meteorological Society, 82, 1353–1367.

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