I have long accepted the overwhelming evidence our mean global warming is tied to human activity. Despite that, I’ve adhered to the assumption that it is very difficult to attribute the role of climate change in an individual extreme weather event such as a hurricane, or a heat wave, or a drought, or a heavy precipitation event, or even a bitter blast over eastern North America in the winter. How would you quantify such a relationship to a singular event?
It turns out, I wasn’t giving this question enough thought.
Last week, as Florence began to wreak its tragic havoc on North Carolina, I saw a tweet from Dr. Michael Mann, a well-known climate scientist who is director of the Earth Science Center at Penn State. (If I list his credentials, that’s half the article.) Mann was making the warming climate tie-in for Florence even as the hurricane was slowing to a crawl near landfall. I decided to put my two cents in and challenged his tie-in for Florence as premature.
Since then, I’ve taken some time to familiarize myself more with some of his research, and that of other climate scientists such as Dr. Jennifer Francis of the Rutgers Institute of Marine and Coastal Sciences. My conclusion: I was probably guilty of literal thoughtlessness. I simply hadn’t taken the time to connect many of the individual theories and principles I’ve known into a working set of probabilities.
Did I know that sea levels have been rising faster as warming has accelerated in the last four-plus decades? Check.
Did I know that a warming atmosphere and warming oceans (always in the mean; the warming is not uniform around the globe) increased evaporation, adding more water vapor to the atmosphere (up 7 percent since the 1950s)? Check.
Did I remember that water vapor is also a greenhouse gas and that it was increasing as a byproduct of our burning of fossil fuels and the warming that human activity was creating? Check.
These were some of the sure things out there in the scientific literature.
But was I also aware of a growing body of evidence that a weaker jet stream has been developing, often in the summer months, due to accelerating of the well-predicted Arctic warming? Was I paying attention to the concept that this weakening of the jet was leading to it buckling more often into high-amplitude ridges and troughs, creating more blocking patterns more often? And that these blocking patterns allowed both low-pressure storm systems and dry high-pressure ridges to take up near stationary positions for lengthier periods of time? For that, I can honestly answer with “sort of.” I just wasn’t making the time or effort to tie things together.
The more frequent episodes of blocking patterns are a reality in the data. Whether they are directly tied to Arctic warming has become less controversial, but this issue is not quite settled science. The alternative hypotheses I’ve seen are fuzzy and sparse by comparison.
As Florence made landfall, we had a classic, very well-predicted blocking pattern setting up. For days, models and the National Hurricane Center had predicted Florence would slow to a crawl as a huge high pressure ridge blocked its forward progress to the north and west, as shown in the graphic at right.
We don’t know the final death toll from Florence. We do know last year’s Hurricane Harvey killed 106 people in the United States and produced $125 billion in damages. It, too, did its worst after landfall, again hemmed in by a very well-predicted blocking pattern and lack of upper-level steering winds to move it along. Its flooding was epic — it was the wettest cyclone in U.S. history.
I believe my tweet calling Mann’s judgment premature was, itself, premature. Here is some of what one can see when you begin putting things together.
The storm surge was definitely worse than it would have been without warming. Sea levels are rising because of freshwater ice melt from Greenland, Antarctica and other glaciers, along with the expansion of sea water volume as the waters warm. This is absolutely not in dispute. Sea levels are rising faster in some coastal zones than others. In places such as North Carolina, Florida and Louisiana, the land mass is also sinking or subsiding at the same time as the sea level is rising. The latter outweighs the subsidence, but combined, the water level has risen about 1 foot compared to less than a century ago.
When a 3- to 6-foot storm surge can produce real destruction and flooding, a 1-foot increase from warming cannot be dismissed as a minor percentage of the surge impact.
When a storm slows to a crawl due to a blocking pattern, that also increases the storm's devastation from flooding tremendously, as has been the case with Florence and Harvey. While the storm's center is in proximity to the ocean, it also increases the storm's supply of additional water vapor to fuel the storm’s structure and rainfall potential.
Since the case for more frequent blocking patterns tied to Arctic warming is growing strong, if not conclusive, the horrific combination of more rainfall, longer duration of rainfall over the same regions, and a more destructive storm surge are sufficient to make the call Mann made last week. We’ll learn more after more post-storm research and data analysis is completed. However, in my converted view, we know enough now to point the finger at warming driven almost entirely by human activity.
This blocking phenomenon didn’t just begin in the last couple of years. The European heat wave of 2003, which killed 70,000 people, was also irrefutably due to a block shaped like the Greek letter omega.
The blocking can also take its toll during winter months. Buffalo’s coldest month on record, February 2015, was tied to a blocking warm western ridge forcing the polar vortex to dump its frigid air to the east.