In my article this week about Hurricane Harvey, I noted ongoing warming may not have “caused” Harvey, but could have contributed to the enormity of the unprecedented rainfall by heating waters in the nearby Gulf to above average. That would increase the amount of evaporation into the atmosphere, leading to more “precipitable water” available for rainfall.
In scientific principle, what I wrote is probably correct. Those who are familiar with my writing for The News know I have written extensively of the overwhelming evidence of a warming climate, and of the evidence human activity is responsible for most of that warming.
Yet in the midst of this horrific and continuing disaster, some of us may be overstating the role of a warming climate in the ferocity of Harvey’s rainfall. As I mentioned, we have had many storms of Harvey’s intensity long before warming accelerated in the 1980s, and we also have had enormous flooding events from slow-moving, weakening tropical cyclones after landfall before the warming sped up.
We do know the number of extreme rainfall events have increased in the last couple of decades, as had been predicted in earlier climate models. Again, more warming=more evaporation=more precipitable water=heavier rainfall is a fairly clear-cut relationship. The actual contribution of the excess heat in the Gulf of Mexico, upon closer examination, may not be such a big culprit in this particular horror as many are thinking. Renowned meteorology professor Cliff Mass of the University of Washington wrote in his blog this week: “Global warming is a serious issue and mankind must deal with it, but hype and exaggeration of the current effects is counterproductive in the long term..."
Examination of upper air data and sea surface temperatures reveal the amount of extra water vapor which could have come from the above average warmth in the Gulf does not begin to compare to the contribution made by a blocked-up pattern in the steering winds over south Texas. There is a well-known relationship between how much more water vapor air can hold for each degree increase in temperature (Celsius) … about 7 percent more per degree. The Gulf and air temperatures before landfall do not support that large an increase in, at least, Harvey’s case. It was the predicted and realized blocking which caused Harvey to linger for so long in the same place, delivering the catastrophic rainfall amounts. Those weak currents aloft took a system which would have been just about as incredibly wet prior to global warming and made it a monster through its staying power. In other words, had Harvey not stalled, its rainfall potential would not have been as catastrophic.
It’s important to know there is no suggestive evidence to link those weak steering winds with ongoing warming. As for the Gulf, while it is somewhat warmer than it was in the early 20th century, slow-moving tropical storms have always been enormous rain producers after landfall. This time around, models performed very well in forecasting exceptionally slow motion for Harvey. The huge predicted rainfall totals in those models keyed in on agonizingly slow storm motion as the dominant problem, not global warming. The models do factor in climatology.
None of what I’ve written is meant to suggest the relationship between warming and human activity isn’t such a big deal, or that this warming will not make major contributions to more extreme rainfall events in the future. That’s already been happening. Insurance companies have proof in the damage payouts in the last couple of decades which are putting some of them on the edge of bankruptcy.
But the evidence of warming being a dominant force in THIS storm is scant. It was a contributor, but the data shows it was a minor contributor and that Harvey could have developed and behaved this way prior to anthropogenic – that is human-induced – warming. The paving sprawl and infrastructure in the Houston metro area with the massive disappearance of wetlands was a gigantic contributor by comparison. That took what would be disastrous flooding in any metro area and converted it to the cataclysm which unfolded.
After Superstorm Sandy, New York City Mayor Michael Bloomberg and other urban planners knew New York and New Jersey would need to spend money to begin to construct barriers to mitigate the effects on the coastlines from storm surge. Bloomberg recognized we couldn’t afford to wait for reduction in greenhouse emissions to lessen the threat, and we needed to act now. The costs of not undertaking these expensive projects would far exceed the costs of leaving our largest metro area unprotected from storm surge and the continuing rising sea levels which are most definitely linked to warming.
Something akin to similar realistic planning will need to come into play in other flood-prone metropolitan areas, in terms of urban sprawl and protecting wetlands. Houston is the poster child for the lack of such planning. If we continue to focus most of our energies on hoped-for reductions in greenhouse emissions and make mitigation efforts secondary, we will be spinning our wheels toward more disasters.
The staying power of carbon dioxide in the atmosphere makes quick, significant changes in the greenhouse warming all but impossible for the foreseeable future. It is very expensive but faster and more effective to begin to make more physical changes to mitigate the inevitable effects of rising sea levels and more numerous heavy rainfall events.
In other words, if we deal with global warming with a triage approach for our most vulnerable population centers, we will get more tangible results in the next few decades.