June 21, 2019 by Greg Meckbach
If you are using today’s climate models to predict the frequency and severity of tomorrow’s severe weather, your estimates could be off, suggests a recent convocation speaker at a major Canadian university.
Recent unusual patterns in the jet stream are “tied to some of the most damaging extremes we have seen in recent years,” said Michael Mann, an atmospheric science professor at The Pennsylvania State University, in an interview Friday. He cited Hurricane Harvey as an example.
“We also know the climate models, at least not currently, are not able to capture accurately the underlying atmospheric physics that’s responsible for that behaviour,” added Mann. “Why that’s important [for the Canadian property and casualty insurance industry] is it is leading to what we argue is an underestimate of the impact that climate is already having on these damaging weather extremes.”
Mann was the convocation speaker June 13 for the faculty of science at McMaster University in Hamilton.
There is some uncertainty over how frequent severe rain will be in the future, in Canada, compared to today, Mann told Canadian Underwriter Friday.
“It is sort of a one-side uncertainty, which is to say we are fairly confident that [climate] models must be underestimating the rise that we will see in these persistent weather extremes.”
Global warming means Canada will have more frequent heat waves, droughts and precipitation events, Insurance Bureau of Canada CEO Don Forgeron said Apr. 25 at IBC’s annual general meeting.
Scientists expect certain types of extreme weather events to become more extreme in the future because if the atmosphere gets warmer, it can hold more moisture, said Mann.
To find out how this could affect weather, climate scientists use models and run them both with and without the factor of increasing concentration of greenhouse gases in the atmosphere.
Scientists can see how often an extreme event happens in a “control” model – which does not factor in an increase in greenhouse gas emissions, said Mann. They can then – compare that to how often the same event happens where there is an increase in GHG emissions, added Mann, who is also director of Penn State’s Earth System Science Center (ESSC), where researchers make models of the Earth’s climate system.
“You can say, ‘climate change made this event two times more likely, for example. We would argue that those attribution studies are underestimating the impact that climate change has had – the degree to which the various types of extremes have become more frequent.”
This, Mann suggests, is because many climate models are not fully accounting for recent patterns – known as quasi-resonant amplification – in the jet stream, which NASA describes as a belt of westerly winds in the northern hemisphere.
What is unusual is the amplitude – or the distance from the ridge to the trough – in the jet stream.
If you look at a map depicting the jet stream, the ridge is where the jet stream bends north and curves back around to go south. The trough is where it goes south and bends back north.
An unusually large amplitude was associated with some extreme weather events in both the United States and Canada in the summer of 2018, said Mann. What also happened was the extreme weather stayed in the same area a long time.
The mechanism is fairly complicated, Mann explained.
“Current generation climate models don’t resolve that mechanism very well. The models are not run at a high enough spatial resolution to resolve some of the underlying atmospheric processes that are important to this phenomenon.”