The Climate in Emergency

A weekly blog on science, news, and ideas related to climate change

Hot Enough Yet?


This past May was the warmest May ever since record-keeping began, back in 1880, according to NOAA. That is a global average, so not every place in the world was particularly warm, only that most places were. The figure is derived from both ocean and land-based temperatures, combined. NOAA does not say how May ranked in comparison to other months–whether this was the hottest month ever (the hottest month ever could occur in May, or January, or any other month, since summer in one part of the world is winter in another). Still, this is an impressing figure. Here are some others, again, from NOAA’s website:

  • The previous May record was set recently, in 2010
  • The 1st, 2nd, 3rd, and 5th warmest Mays on record were all within the past five years
  • This May was the 39th consecutive May with global temperatures above average (the average calculated for the 20th century)
  • Every month has been above the 20th century average for the past 351 months–that’s just over 29 years.

Just to put that in context, the last time Planet Earth had a cooler-than-average May, I wasn’t born yet.

The last time the planet had any cooler-than-average month (a February, as it happens), the Space Shuttle Challenger had not yet exploded–Christa McAuliffe was still alive. Mt. St. Helens still had a snow-capped, cone-shaped peak because it had not yet blown up, either. Ronald Reagan had just been sworn in to his second term as US President. Foreigner’s “I Want to Know What Love Is” topped the music charts.

When scientists look for the signs of global warming, they look for statistics. In 2012, James Hansen, then a climate scientist for NASA, and several colleagues, announced that recent heat waves were definitely caused by humans. Their statement was controversial, because climatologists usually say that specific events, like individual heat waves, cannot be traced to global warming. And, indeed, Dr. Hansen et al. did not actually study the cause of any heat waves directly. They looked at statistics.

The issue was that when people ask “is this heat wave” (or hurricane or drought) global warming?” They don’t really care about the details of meteorological cause and effect. They mean “is this what global warming feels like? Is it happening yet?” When scientists assert (correctly) that it is impossible to trace individual weather events to global warming, that sounds as though they’re saying the climate hasn’t really changed yet, or that they just aren’t sure. It’s a classic case of people talking past each other, with experts answering a question nobody really asked. Dr. Hansen answered the question people are actually asking.

What he and his colleagues did was to look at how much of the Earth’s surface has heat wave conditions at any given time and how the frequency of extreme heat has changed over the years. They found that between 1951 and 1980, 0.2% of the planet’s land surface had a heat wave between June and August. In contrast,between 2006 and 2011, 4-13% of the land was under heat waves during those months.

Just to be clear, 4 is twenty times larger than 0.2.

It’s like flipping a coin; everybody knows that a coin comes up heads half the time, and that while you can get a couple of heads in a row by random chance, those heads are balanced by tails later. If a coin came up heads twenty five consecutive flips in a row, everybody would know the coin had been fixed somehow. The statistics used in climatology are a lot more complex, but what Dr. Hansen did is essentially the same; the increase in heat waves is too much to be a random variation of the old climate patterns. The climate has changed.

But how does global warming cause heat waves? After all, the global average temperature rise is still less than a degree. How does that seemingly small change translate into the extreme temperatures we’ve seen this May in Africa, Europe, and Australia?

A detailed answer is hard to pin down. For example, some heat waves are caused by El Niño events and El Niños might or might not be intensified by global warming. In general, we know that climate change involves more extreme weather, including more heat waves.

However, even if the number of heat waves as compared to the current average were not increasing, we would still be getting dramatically more heat waves as compared to the historical average. That is, if the global climate had just gotten warmer by a degree or so, without otherwise changing, the increase in extreme heat would still be dramatic.

Picture a bell curve. The shape is famous enough that you have probably seen it. The bell curve is a graph of how most variable things vary, from test scores to adult height to temperatures in May. The shape visually expresses the statistical principle that extreme events are rare. For example, if you go to a public place and look around, most people will probably be somewhere between five and six feet tall. A few people will be a bit over that or a bit under, but if you spot someone bigger than six six, say, or someone under four ten, you’d probably be surprised. There are people over seven feet tall and adults under four, but it is possible you have never personally met one. That is the bell curve. Extremes are rare.

Apply the bell curve to temperature. Say, for convenience, that in your area 110° F days were extreme relative to the 20th century average. They occupy the narrow tail of the curve, so you get days like that maybe once every ten or twenty years. Move the whole bell over slightly, as global warming has done, and a somewhat thicker part of the curve moves into the 110° F spot. That temperature is now slightly less extreme, so it is much less rare. Now you get a 110° F day every year.

Temperature extremes are not just annoying; they kill people, more people, in fact, than more obviously violent weather events, such as hurricanes, do.

In the United States, about 117 people die of hot weather directly every year, a very small number in relation to the total population of the US, but that’s still more than the death toll from hurricanes, tornadoes, and earthquakes combined.

But on top of that, around 1,800 more people die from other illnesses that the heat makes worse, such as heart disease or respiratory problems. And, since heat-related illnesses increase several days into a heat wave (heat injury is cumulative), more and longer heat waves are going to have a greater impact on human health than the temperature rise alone suggests. A three-day-long heat wave is more than three times worse than a one day heat wave.

The dangers of heat are born disproportionately by children, the elderly, the already ill, and those too poor to afford air conditioning or escape to cooler places (athletes are also at high risk, but they can choose to do something about that).

It is important to remember that not all places are hot at once. For example, this June has been lovely where I am–a few really warm days, but nothing too sticky, and long periods of crisp, cool summer weather, hot enough for shorts but not uncomfortable. India? Not so much. Earlier this month, temperatures in New Delhi stayed above 110° F. for seven days in a row. Parts of India, China, and Tibet rose 22°F above even the 2001-2010 average (a decade, you remember, in which every single month was above the previous century’s average).

Please think about this the next time you consider driving somewhere or buying non-local or factory-farmed food or, better yet, the next time you vote.

Thank you.




Author: Caroline Ailanthus

I am a creative science writer. That is, most of my writing is creative rather than technical, but my topic is usually science. I enjoy explaining things and exploring ideas. I have one published novel and another on the way. I have a master's degree in Conservation Biology and I work full-time as a writer.

14 thoughts on “Hot Enough Yet?

  1. This is a very informative post with some surprising data. Your explanations are very clear.

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