The flooding in Louisiana. The fires burning parched land in California. Our own lack of rain this summer. It all begs the question: Is our weather becoming more extreme?

And what is extreme weather anyway? Superlative descriptions are nothing new, but they often don’t have any scientific basis. If I say there is extreme heat coming, what does that really mean? To some, extreme heat might be a day over 95 degrees, but is that really extreme?

To understand extreme weather, let’s look at a graph from the EPA. The graph shows a typical curve of weather variables, and the peak in the middle is where most weather occurs. These are typical days of warm and cold, average snow, average rain showers, etc.  The ends of the curve are the extreme events—the record cold and record heat, for example. I’ll revisit this chart later.

A typical curve has most weather occuring in the middle, with the extremes at either end.A typical curve has most weather occurring in the middle, with the extremes at either end. —EPA

As the climate has changed over the decades, the curve has shifted to the right into a warmer world.

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(It’s possible the planet would/could trend colder. During colder times, there would still be extreme heat, but not as much of it. However, during the past hundred plus years we’ve been trending warmer.)  As the climate warms, the frequency of extreme cold dissipates, but it isn’t zero.

We only need to go back to February 14, 2016, when the temperature in Boston started the day at 9 below zero and only reached 12. This was the coldest day since the 1950s and now stands as the coldest Valentine’s Day in Boston in the record books.  This period of cold after the mild start to the winter was likely responsible for wiping out much of the peach crop. That’s extreme stuff.

About five years ago, on July 22, 2011, the temperature in Boston reached 103. This was only a degree shy of the all-time record high of 104, set on July 4 100 years earlier in 1911.  Each of these events was certainly extreme.

Of course, there’s also this summer’s lack of rain. This is the driest period ever recorded in the four-month period from mid-April to mid-August.  This, less than two years after the most amount of snow ever recorded in Boston.

Are these types of extremes part of the changing climate? Can we expect them to become more frequent?

The answer to the question of whether or not climate change is bringing us more extreme weather is: It depends. To find the answer, we can look to a relatively new field of climatology and statistics called extreme event attribution.  Scientists perform studies where the results are probabilistic statements. In other words, it’s about odds. We can’t say a particular event or even a  snowy, dry, or cold season was caused by climate change; there are too many other factors at play. It’s not a cause-and-effect problem.

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What we can say, however, is that these events are made more likely by a changing climate, or that a particular event might be more extreme because of climate change. Further, events associated with more warmth, such as heat waves and drought, have a higher likelihood of occurring.

Figure: Global Hurricane Frequency (all & major) -- 12-month running sums. The top time series is the number of global tropical cyclones that reached at least hurricane-force (maximum lifetime wind speed exceeds 64-knots). The bottom time series is the number of global tropical cyclones that reached major hurricane strength (96-knots+). Adapted from Maue (2011) GRL.Figure: Global Hurricane Frequency (all & major) — 12-month running sums. The top time series is the number of global tropical cyclones that reached at least hurricane-force (maximum lifetime wind speed exceeds 64-knots). The bottom time series is the number of global tropical cyclones that reached major hurricane strength (96-knots+). Adapted from Maue (2011) GRL. —WeatherBell-Ryan Maue

Remember that chart from earlier? Predictions of a warmer world will shift the curve to the right. This makes colder records less likely while warmer ones become more frequent. Scientists are less able to connect other weather events, such as hurricanes, tornadoes, and big snowstorms, to climate change because those weather phenomena happen less frequently.

In a warmer world the extremes would be shifted in a warmer direction. Those associated with heat would become more frequent. Record cold would be seen less.In a warmer world the extremes would be shifted in a warmer direction. Those associated with heat would become more frequent. Record cold would be seen less. —EPA

While it may seem like we’ve seen our share of extremes here in New England over the past couple of years, many of the most extreme records still stand.

The wettest year remains 1878, when 65.53 inches of rain and melted snow fell. The coldest winter is still back in 1917-18, and the hottest summer was more than 30 years ago, in 1983. Although it’s been a dry spring and summer, the driest calendar year is still 1965, when only about 23.71 inches of rain fell. Of course, with a full third of the year left, that record could still be in jeopardy.

Weather is, by nature, extreme, and as such, some years or clusters of years can bring more extremes than others. We can observe periods with many extreme events followed by quieter times.  More research in the coming decades will help us understand what  the new normal is and help shape public policy.