Douglas Sheil is a professor at the Norwegian University of Life Sciences and senior associate at the Center for International Forestry Research.
Hurricane Dorian was not really a surprise. Hurricanes happen at this time of year in the Northern Hemisphere, but severity and location has a strong element of chance. Generally speaking, hurricanes such as Dorian are formed by moist air converging from across a large area, building an energetic storm system that focuses much of the energy around the eye.
The details of this process remain a matter of research and debate, and thus there are different views on how global warming has fed into Hurricane Dorian’s severity. Temperature is underlined in conventional views, in which the energy produced by higher oceanic temperature spurs storms. Newer, revised views point more at moisture that already existed in the air: water vapor has energy, and when vapor condenses, the energy can spur air movement and cause the winds that make storms so powerful.
Some, such as Kerry Emanuel of the Massachusetts Institute of Technology, another leading water cycle researcher, say both play a major role.
I believe that moisture plays a dominant role in determining the power of cyclonic storms, which include typhoons and hurricanes. Having a lot of warm, humid air accumulate over the ocean combines the energy of both forces (temperature and moisture), providing a good place to incubate a powerful cyclone. When storm power is plotted against rainfall, we see a remarkably clear linear relationship.
Are there linkages between Dorian and the weather conditions that contributed to the widespread forest fires this year? Potentially the same processes disrupted water cycles, contributing to dryness over land and leaving moist air over the ocean – it’s plausible. But we don’t have strong evidence, and it would be hard to persuade a skeptic.
Rather, I would highlight the Amazon region’s declining forest cover as a likely contributor to this, as forests play a significant role in creating and maintaining global rainfall patterns. In my career researching the water cycle, I’ve been struck by various theoretical arguments and indications suggesting that loss of forests can contribute to the severity and frequency of cyclones. There are several mechanisms at work in this.
Forests help draw moisture that might otherwise feed storms off the ocean, reducing the energy available to feed the storm. This time of year – when the Atlantic hurricane season runs from August to September, and the Amazon dry season occurs approximately from August to November – the processes drawing moist air into the Amazon are weak. Moisture accumulates over the ocean, providing plenty of “fuel” for hurricanes to develop. Deforestation further reduces the moisture taken off the ocean, leaving more fuel for storms such as Dorian.
Forests also create drag for large-scale winds, extracting energy from nearby storms, potentially slowing them and sapping their growth. Though forests are themselves a source of atmospheric moisture that might be expected to feed storms, this source is only active in the day and resists the local concentration that cyclones can drive over the open ocean.
In short, when we lose forests, we lose their drain on cyclone fuel and break on cyclone formation and growth.
September is known to be the most intense month of storm season, but it’s hard to say what’s next aside from the fact that risks do remain high. Each storm is, like the toss of a dice, difficult to predict. All we know for sure is that the danger is real now; and in years to come, cyclonic storms such as Hurricane Dorian can be expected to become more frequent and more powerful.