An oft-heard statement is that the weather is getting more extreme because of global warming. As opposed to, say, the benign, mild weather conditions associated with the Last Glacial Maximum 20,000 years ago.
Many long-term records of extreme weather events are fraught with problems, usually involving changes in the way that extreme events are detected and measured. If you go beyond the extreme weather itself to consider human disasters caused by extreme weather, additional complications such as changes in population exposure over time arise.
In order to lay the groundwork for future discussions of extreme weather events, I’d like to discuss the basic scientific expectations for changes in extreme events. What is extreme weather? What ought to happen with global warming?
What is extreme weather?
Much confusion arises from the fact that “extreme weather” has two different meanings.
One meaning is that of “statistically extreme weather”, weather that happens extremely rarely. An example would be the record high temperature for a particular day and place. By definition, it has only happened once (except for ties) in the history of weather records, and so is extremely rare. For this definition to make sense, some reference period such as the “period of record” must be chosen.
The other meaning is that of “extremely dangerous weather”. An example would be a tornado. Though there are hundreds of tornadoes each year, each one is an extreme weather event, whether or not it happens to cause damage. Other examples are tropical cyclones, floods, and droughts. Sometimes the term “weather” is defined loosely enough to include wildfires.
There is overlap between these two types of extreme weather. For example, a record hot day in the summer is statistically extreme, and it can also be extremely dangerous. A record hot day in the winter is usually not so dangerous, unless it causes dangerous snow or ice melt.
What ought to happen?
For statistically extreme weather, the answer, at least on the surface, is straightforward. If the average value of some weather variable (temperature, wind speed, etc.) has changed compared to a reference period, then one extreme ought to become more common while the other extreme becomes less common. Thus we expect (and see) more maximum temperature records and fewer minimum temperature records.
The expected change is asymmetrical. In other words, the average number of records set during a given year ought to increase rather than staying the same, no matter the direction of the change. Consider a location with a 100-year record of a stable climate. On average, a new daily high temperature record will be set 3.65 times a year and a new daily low temperature record will be set 3.65 times a year, for an average of 7.3 new records per year. Now suppose the temperature suddenly goes up five degrees. The next year, there might be 40 maximum temperature records, but probably zero minimum temperature records. One kind of record has become more frequent, while the other has become less frequent. But the total number of annual records has increased by 32.7!
It is also conceivable that the variability of weather might change. Maybe temperatures will become more erratic, or maybe less erratic, for example. Such a change would not be an obvious consequence of global warming, while an increase in the frequency of maximum temperature records by itself is an obvious consequence of global warming.
Changes in the frequency of extremely dangerous weather must be considered on a case by case basis. Very few changes are obvious. This is because most dangerous weather arises from various types of localized dynamic instability in the atmosphere, and it’s rarely clear whether the generation of atmospheric instability ought to speed up or slow down in a warming atmosphere, let alone whether other environmental changes will make that instability easier or harder to respond to.
A common oversimplification goes that “a warmer atmosphere has more energy, and therefore more energetic weather.” Scientists sixty years ago showed that this was wrong. Sure, a warmer atmosphere has more energy, but most of that energy can’t do anything but radiate away to space. It is the variation of energy (temperature) from place to place that drives the planetary winds. And in the Northern Hemisphere, as the difference in temperature between the equator and pole decreases, the circulation ought to become less energetic.
Perhaps the most obvious tendency for extremely dangerous weather is that the strength of the heaviest downpours ought to increase. This is because rainfall intensity is a product of the rate of upward motion of the air and the amount of water vapor it contains. Since warmer air can contain more water vapor, about 7% more for each degree Celsius of warming, a given updraft will tend to produce heavier rain. This effect might even be felt at the storm scale, more than making up for the decline of temperature variations.
Note that this is different from saying that there will be more rain on average. The general consensus is that there will indeed be more rain on average, but this conclusion is not obvious from simple principles. Sure there will be more water vapor in the air, but what if the air rises more slowly? Might that not compensate for the increased water vapor?
Also, “more rain on average” doesn’t mean “more rain on average everywhere”, it just means that the global average amount of rainfall (and snowfall) is expected to increase. It doesn’t even necessarily mean that more than 50% of the Earth’s surface will see increased rainfall. Along similar lines, increased drought frequency (another expectation) doesn’t mean that drought frequency will increase everywhere, nor even that drought frequency will increase over a majority of the globe.
As a general rule, most changes related to rainfall will be very place-specific. This is because the production of precipitation requires ascending air, and air ascending in one place requires air to be descending in another place.
Let us suppose, for the sake of example, that 60% of the globe will see more and heavier rain, i.e., more floods, and likewise that 60% of the globe will see more droughts. Presumably those will tend to be different parts of the globe, so these numbers would guarantee that only 20% of the globe would see an increase in both drought and flood.
In summary, some aspects of extreme weather are easy to predict, others less so. We’ll have to consider them each in turn.