Is climate change to blame for this extreme hurricane season?

With the recent Harvey, Irma and Maria hurricanes, Thomas Hollands investigates whether climate change is responsible for this barrage of deadly natural disasters.

Written by: Thomas Hollands

Art by: Xin Yang

At the moment, one could argue that extreme weather events do not seem so extreme, due to the sheer number we have seen recently: Enter Harvey, Irma and Maria. The trio of names sound like extras from “Gone with the Wind”, but these are some of the most devastating hurricanes North America has seen in generations; Irma and Maria in particular both made landfall as category five hurricanes – only the second time on record that this has happened.

Just a couple of months after President Trump jettisoned the US from the Paris Agreement, the land of the free has been wracked by three major hurricanes. Some on the left argue that the evidence for climate change has literally weathered and pounded America’s shores, whilst many on the right steadfastly deny any link between extreme weather and climate. As is usual with atmospheric science, the reality is more complex than either view.

Expecting the Expected

Forecasting weather events is complicated – even the very best models are only accurate about three days in advance. On any given day, it is extremely difficult to ascertain what level of the weather that we see is a result of anthropogenic climate change, and what is natural and to be expected. However, if we consider changes in climate over a period of many years, scientists can predict the frequency of extreme weather events to a high degree of accuracy. This is not to say that scientists can tell when extreme weather events will occur (crack that problem and you might get a Nobel Prize or a Fields’ Medal), but they can estimate how likely an event is to occur within a certain duration.

According to most atmospheric scientists, this likelihood is set to increase. Storms need only one main ingredient to flourish – the movement of moist air. When air warms up, its saturation point increases, and so it has a higher capacity for moisture. This is one of the reasons why climate scientists expect the frequency of extreme weather events to increase with global warming; warmer air means wetter air, which provides fruitful conditions for hurricanes to develop. There is a solid link between more atmospheric moisture content and higher rates of precipitation, so by the same token, an additional consequence of global warming is that the intensity of extreme weather events will increase.

This Might Not Blow Over

Global warming is much more than the steady rise in the mean global temperatures which are cited as major pieces of evidence. It is also the rapid change of previously dependable weather systems into more erratic patterns. Feedback processes are abundant in climate science, and one such feedback loop, ice-albedo feedback, causes the Arctic to warm at a faster rate than the neighbouring mid-latitudes. The Arctic is predominantly clad in ice, which reflects sunlight particularly well. If the ice melts, it unearths rock or seawater below, which are both much poorer reflectors of sunlight, and so will absorb more, causing Arctic heating to increase. This process will then itself accelerate in a positive feedback loop. One result of this heating asymmetry is slower upper-troposphere winds at mid-latitudes. Wind patterns are primarily driven by a temperature gradient – if the Arctic air is warmer then this gradient will be less.

The end result of this climatological process is “blocking” of large scale weather systems. This phenomenon accounts for Hurricane Harvey’s long stay in Houston: due to slowing of the jetstream, Harvey spent days over a densely populated area that it otherwise might have only spent hours over. With climate change now in full swing, “blocking” of extreme weather systems like hurricanes and floods may soon become the rule rather than the exception.

Since climate change is a long-term average of short-term weather processes, it is meaningless to try to attribute any one extreme weather event to human-driven global warming. However, climate science can reliably predict increases in both the intensity and frequency of extreme weather events, which are only expected to continue as global warming heaves along.

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