Is climate change responsible for meteorological disasters?

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Summer isn’t even half over, and we’ve seen heat waves across the Pacific Northwest and across Canada with temperatures that would make the news in Death Valley, huge fires that sent smoke across North America and deadly floods of biblical proportions in Germany and China. Scientists have been warning for more than 50 years of increasing extreme events resulting from subtle changes in the average climate, but many have been shocked by the ferocity of recent weather disasters.

It is important to understand some things about the role of climate change in extreme weather conditions like this.

First, humans have pumped so much carbon dioxide and other warming greenhouse gases out of the atmosphere that what’s normal has changed.

Second, not all extreme weather events are linked to global warming.

Like so many things, temperature statistics follow a bell curve that mathematicians call these normal distributions. The most frequent and probable temperatures are close to the mean, and values ​​further from the mean quickly become much less likely.

All other things being equal, a little warming moves the bell to the right towards higher temperatures. Even a shift of only a few degrees makes the truly improbable temperatures in the extreme tail of the bell to occur considerably more often.

The recent flood of temperature records across western North America is a prime example. Portland hit 116 degrees 9 degrees above its pre-heat wave record. It would be an extreme at the end of the tail. A study determined that the heat wave would have been virtually impossible without man-made climate change. Extreme heat waves that were once ridiculously improbable are becoming more and more common and unimaginable events are becoming possible.

The width of a bell curve is measured by its standard deviation. About two-thirds of all values ​​are within one standard deviation of the mean. Based on historical temperature records, the 2003 heat wave that killed more than 70,000 people in Europe was five standard deviations above average, so it was one in 1 million events.

Without eliminating fossil fuel emissions, such heat is likely to occur several times a decade by the time today’s toddlers retire.

So, is climate change to blame?

There is a basic hierarchy of extreme events that scientific research has shown so far are most affected by man-made climate change.

Topping the list are extreme events like heat waves which will certainly be influenced by global warming. In these, three sources of data converge: observations, physics and simulations of computer models that predict and explain changes. At the bottom of the list are things that could likely be caused by increasing levels of greenhouse gases, but for which the evidence is not yet convincing. Here is a partial list.

1) Heat waves: Studies show that these are sure to increase dramatically with global warming, and this is indeed exactly what was observed.

2) Coastal flooding: Heat causes ocean waters to expand, raising sea levels and melting ice caps around the world. High tide flooding and catastrophic storm surges will become much more frequent as these events start at a higher average level due to sea level rise.

3) Drought: Warmer air evaporates more water from reservoirs, crops and forests, therefore drought will increase due to increased demand for water, although changes in precipitation vary and are difficult predictable.

4) Forest fires: As the western United States and Canada find, heat dries up soils and vegetation, providing drier fuel ready to burn. Forests lose more water during hotter summers and fire seasons lengthen.

5) Reduction of snowpack in spring: snow begins to accumulate later in fall, more water is lost from the snowpack during winter, and snow melts earlier in spring, which reduces the flow of water in reservoirs that support the economies of semi-arid regions.

6) Very heavy rains: warmer water can carry more water vapor. Damaging torrential rains are caused by strong updrafts that cool the air and condense the vapor in the form of rain. The more water there is in the air during a strong updraft, the more rain can fall.

7) Hurricanes and Tropical Storms: They get their energy from evaporation from the warm surface of the sea. As the oceans warm, larger areas can generate these storms and provide more energy. But changes in winds aloft are expected to reduce the intensification of hurricanes, so it’s not clear that global warming will increase damage from tropical storms.

8) Extremely Cold Weather: Some research has attributed the cold weather that moves south with the meanders of the jet stream, sometimes referred to as polar vortex outbreaks, to the warming of the Arctic. Other studies strongly dispute that a warming Arctic is likely to affect winter conditions further south, and the idea remains controversial.

9) Severe thunderstorms, hail and tornadoes: these thunderstorms are triggered by strong surface warming, so it is plausible that they could increase in a warming world. But their development depends on the circumstances of each storm. There is no evidence yet that the frequency of tornadoes is increasing.

The catastrophic impacts of extreme weather conditions depend at least as much on people as they do on climate.

The evidence is clear that the more coal, oil, and gas are burned, the warmer the world will be and the more likely it is that a given location will experience heat waves that are well outside of anything they have experienced. .

Disaster preparedness can quickly fail when extreme events exceed all previous experiences. The melting of the power cables of the Portlands trams is a good example. The way communities develop infrastructure, social and economic systems, planning and preparedness can make them more resilient or vulnerable to extreme events.

Scott Denning is a climatologist and professor of atmospheric science at Colorado State University.

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