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## Climate Change and Heat Waves

One of the most dangerous effects of climate change is its impact on extreme events. The extra energy that’s present on a warmer world doesn’t distribute itself uniformly – it can come out in large bursts, manifesting itself as heat waves, floods, droughts, hurricanes, and tornadoes, to name a few. Consequently, warming the world by an average of 2 degrees is a lot more complicated than adding 2 to every weather station reading around the world.

Scientists have a difficult time studying the impacts of climate change on extreme events, because all these events could happen anyway – how can you tell if Hurricane Something is a direct result of warming, or just a fluke? Indeed, for events involving precipitation, like hurricanes or droughts, it’s not possible to answer this question. However, research is advancing to the point where we can begin to attribute individual heat waves to climate change with fairly high levels of confidence. For example, the recent extended heat wave in Texas, which was particularly devastating for farmers, probably wouldn’t have happened if it weren’t for global warming.

Extreme heat is arguably the easiest event for scientists to model. Temperature is one-dimensional and more or less follows a normal distribution for a given region. As climate change continues, temperatures increase (shifting the bell curve to the right) and become more variable (flattening the bell curve). The end result, as shown in part (c) of the figure below, is a significant increase in extremely hot weather:

Now, imagine that you get a bunch of weather station data from all across the world in 1951-1980, back before the climate had really started to warm. For every single record, find the temperature anomaly (difference from the average value in that place and on that day of the year). Plot the results, and you will get a normal distribution centred at 0. So values in the middle of the bell curve – i.e., temperatures close to the average – are the most likely, and temperatures on the far tails of the bell curve – i.e. much warmer or much colder than the average – are far less likely.

As any statistics student knows, 99.7% of the Earth’s surface should have temperatures within three standard deviations of the mean (this is just an interval, with length dependent on how flat the bell curve is) at any given time. So if we still had the same climate we did between 1951 and 1980, temperatures more than three standard deviations above the mean would cover 0.15% of the Earth’s surface.

However, in the past few years, temperatures three standard deviations above average have covered more like 10% of the Earth’s surface. Even some individual heat waves – like the ones in Texas and Russia over the past few years – have covered so much of the Earth’s surface on their own that they blow the 0.15% statistic right out of the water. Under the “old” climate, they almost certainly wouldn’t have happened. You can only explain them by shifting the bell curve to the right and flattening it. For this reason, we can say that these heat waves were caused by global warming.

Here’s a graph of the bell curves we’re talking about, in this case for the months of June, July, and August. The red, yellow and green lines are the old climate; the blue and purple lines are the new climate. Look at the area under the curve to the right of x = 3: it’s almost nothing beneath the old climate, but quite significant beneath the new climate.

Using basic statistical methods, it’s very exciting that we can now attribute specific heat waves to climate change. On the other hand, it’s very depressing, because it goes to show that such events will become far more likely as the climate continues to change, and the bell curve shifts inexorably to the right.

References:

The New York Times deserves a clap too. Thanks to toby and Eli for the hat tip.

An article just as good as the Associated Press piece made the front page of the New York Times. Justin Gillis wrote In Weather Chaos, a Case for Global Warming, and, as Eli pointed out, there wasn’t even a question mark in the title.

Gillis does a great job explaining how, for example, there will still be record cold days with climate change – just fewer of them. Here’s one of my favourite passages from the article:

The warming has moved in fits and starts, and the cumulative increase may sound modest. But it is an average over the entire planet, representing an immense amount of added heat, and is only the beginning of a trend that most experts believe will worsen substantially.

If the earth were not warming, random variations in the weather should cause about the same number of record-breaking high temperatures and record-breaking low temperatures over a given period. But climatologists have long theorized that in a warming world, the added heat would cause more record highs and fewer record lows.

The statistics suggest that is exactly what is happening. In the United States these days, about two record highs are being set for every record low, telltale evidence that amid all the random variation of weather, the trend is toward a warmer climate.

## The Associated Press Gets it Right

It’s been quite the summer. Moscow has experienced several months of weather more akin to Texas, and is literally burning up. Floods in China have killed more than a thousand and left countless others displaced. Pakistan has experienced similar floods due to a massive monsoon season, and now they have to deal with cholera, too. The Arctic sea ice extent is not much larger than 2007, and, so far, it’s been the warmest year on record globally.

We can’t tie a single extreme event to climate change. We can tie long-term trends, like 30 years of declining Arctic sea ice, to a warming world, but we don’t yet have the technology to attribute a single anomalous season to a particular cause. In 2007, for example, factors other than high temperatures contributed to the lowest Arctic sea ice extent on record.

However, these events are exactly what we expect from anthropogenic climate change. We shouldn’t look at them as evidence for global warming, but as examples of what is to come. This is an important warning that most newspapers have been shying away from. After nearly a year of terrible climate change journalism across the board, they didn’t even mention the connection between extreme events and climate change, or the fact that this summer is a very real glimpse into our future.

I gave up on my local newspaper months ago, and I don’t regret that decision. On the handful of mornings that I’ve flipped through the paper instead of reading the Globe and Mail on the Internet (journalism of much higher quality, and it saves money and paper), I’ve seen far too many op-eds and letters to the editor saying very strange things about climate science.

However, a headline yesterday caught my eye. A fantastic article by Charles J. Hanley, a Pulitzer Prize winning journalist, was distributed by the Associated Press and, consequently, picked up by dozens of newspapers across the continent – including my local paper.

I became more and more pleasantly surprised as I began to read through the article:

Floods, fires, melting ice and feverish heat: From smoke-choked Moscow to water-soaked Iowa and the High Arctic, the planet seems to be having a midsummer breakdown. It’s not just a portent of things to come, scientists say, but a sign of troubling climate change already under way.
The weather-related cataclysms of July and August fit patterns predicted by climate scientists, the Geneva-based World Meteorological Organization says – although those scientists always shy from tying individual disasters directly to global warming.