Summer isn’t even half over, and we’ve seen with temperatures that would make news in Death Valley, across North America, and lethal floods of biblical proportions in and .
Scientists have warned for about increases in extreme events arising from subtle changes in average climate, but many people have been shocked by the ferocity of recent weather disasters.
A couple of things are important to understand about climate change’s role in extreme weather like this.
First, humans have pumped into the atmosphere that what’s “” has shifted.
Second, not every extreme weather event is connected to global warming.
Shifting the Bell Curve
Like so many things, temperature statistics follow a bell curve – mathematicians call these “normal distributions.” The most frequent and likely temperatures are near the average, and values farther from the average quickly become much less likely.
All else being equal, a little bit of warming to the right – toward higher temperatures. Even a shift of just a few degrees in the extreme “tail” of the bell happen dramatically more often.
The stream of broken temperature records in the North American West lately is a great example. – 9 degrees above its record before the heat wave. That would be an extreme at the end of the tail. One study determined the heat wave would have been “” without human-caused climate change.
Extreme heat waves that were once ridiculously improbable are on their way to becoming more commonplace, and unimaginable events are becoming possible.
The width of a bell curve is measured by its standard deviation. About two-thirds of all values fall within one standard deviation of the average. Based on historical temperature records, the heat wave in 2003 that people in Europe was five standard deviations above the mean, so it was a 1 in 1 million event.
Without eliminating emissions from fossil fuels, by the time today’s toddlers are retirees.
So, Is Climate Change to Blame?
There’s a basic hierarchy of the extreme events that scientific research so far has shown are most affected by human-caused climate change.
At the top of the list are extreme events like heat waves that are certain to be influenced by global warming. In these, three lines of evidence converge: observations, physics and computer model simulations that predict and explain the changes.
At the bottom of the list are things that might plausibly be caused by rising levels of greenhouse gases but for which the evidence is not yet convincing. Here’s a partial list.
1) Heat Waves:
Studies show these are certain to , and indeed that’s exactly .
2) Coastal flooding:
Heat is causing ocean waters to expand, , and melting ice sheets around the world. Both and will become much more frequent as those events start from a higher average level because of sea level rise.
Warmer air evaporates more water from reservoirs, crops and forests, so , even though vary and are hard to predict.
As the western US and Canada are seeing, heat dries out the soils and vegetation, . Forests lose more water during hotter summers, and
5) Reduced spring snowpack:
Snow starts accumulating later in the fall as temperatures rise, from the snowpack during winter, and the , reducing the flush of water into reservoirs that supports the economies of semiarid regions.
6) Very heavy rainfall:
. Damaging rainstorms are due to strong updrafts that cool the air and condense the vapor as rainfall. The more water is in the air during a strong updraft, the .
7) Hurricanes and tropical storms:
These derive their energy from evaporation from the warm sea surface. As oceans warm, larger regions can spawn these storms and . But changes in winds aloft are expected to , so it’s not clear that global warming will increase damage from tropical storms.
Global Sea Surface Temperature Has Been Rising
Every year since 1977, the average annual global sea surface temperature has been above the 20th century average. The chart shows the temperature difference compared to the 20th century average each year in degrees Celsius.
8) Extreme cold weather:
Some research has attributed cold weather that dips south with the meandering of the jet stream – sometimes referred to as “polar vortex” outbreaks – . Other studies that Arctic warming is likely to affect winter weather farther south, and this idea .
9) Severe thunderstorms, hail and tornadoes:
These storms are triggered by strong surface heating, so it’s in a warming world. But their development depends on the circumstances of each storm. There is that the frequency of tornadoes is increasing.
A Warning That Can't be Ignored
The catastrophic impacts of extreme weather depend at least as much on people as on climate.
The evidence is clear that the more coal, oil and gas are burned, , and the it will be for any given location to experience heat waves that are far outside anything they’ve experienced.
Disaster preparedness can quickly fail when extreme events blow past all previous experience. Portland’s are a good example. How communities develop infrastructure, social and economic systems, planning and preparedness can make them more resilient – or more vulnerable – to extreme events.
(This article was first published in The Conversation and republished here with permission.)