Is it true that hurricanes appear to be slightly less frequent but have been getting bigger over recent years? (I know this season is an exception, with more than usual.)
If so, has this been predicted to be a result of global warming? I read something along these lines in the media recently but wondered how reliable that is.
First off, over the last two years hurricanes have been getting more frequent. It is also true that they have been getting stronger. This could be a result of global warming, but could also be due to other factors. Many people believe that this is just an artifact of natural oscillations in the ocean's thermohaline circulation. We do know that tropical cyclone activity was much more active in the early 20th century than it was in the late 20th century. Still others think that this could be a return to normal caused by less polluting. I know this may see strange but allow me to explain.
Most of our tropical cyclone records extend in detail only since the deployment of weather satellites. During the early part of this time the world was heavy polluting particulate matter (mostly coal ash). This ash had some very visible side effects including increasingly acidic rain, obscured visibility, soiling and erosion of marble and limestone buildings, and respiratory distress. In reality, coal power also releases more radiation per unit of power generated than nuclear power. All of these side effects gathered increasing attention during the 70s and 80s until finally industrialized countries cleaned their act up. Removing particulate matter from coal exhaust was very easy to do using electrostatic precipitators (think giant Ionic Breeze) and by spraying the exhaust plume with water. After these controls were put in place, we realized another interesting side effect. That coal ash actually reflected incoming solar radiation and dimmed the globe by around 10% of 1950's levels. In some areas, that decrease was as much as 22% in Israel and 30% in parts of Russia. Yes, that’s a lot. In reality, the average global dimming may be closer to 5%. Only recently has the issue been well understood, however, it is now being partially ignored. As it turns out, we corrected the 'problem' before we even knew that it existed because we knew that coal ash had other nasty effects. Since the 1990s the globe has actually been brightening again.
However, this ash had other side effects in the climate. There is evidence that ash blowing downwind of industrial centers could have had other unintended consequences. It is well knows that ash from Europe merely caused acid rain, erosion, and illness across the Soviet block as it blew eastward. However, particulates from the industrial area of the USA blew out over the North Atlantic. Here the ash reflected sunlight and may have helped keep the North Atlantic relatively cooler than it otherwise would have been, while the South Atlantic didn't feel as strong of an effect. We know that in years that the North Atlantic doesn't warm as much, the Intertropical Convergence Zone (ITCZ) doesn't move as far north. If this shift fails to occur over Africa, monsoonal rains fail. We think this pollution could have seriously contributed to the desertification of the Sahel region of Africa during the 70s and 80s (and the corresponding famines across the region). Interestingly, since particulates have been cleaned up in the USA the globe has again brightened, heavier rains have returned to the Sahel and the desertification has reversed since the late 90s. However, nobody knows if there is a true connection here.
The same ocean 'cooling' that could cause the failure of the monsoonal rains in Africa could also deprive tropical cyclones of energy that they need to develop. A 10% drop in available energy would seriously limit tropical cyclone development. Plus, if the ITCZ stays further south, areas more favorable to tropical cyclone development would also stay closer to the equator where the rotational effects needed for cyclone development are weaker.
As you can see, by cleaning up our act we may actually be restoring a natural balance. However, that balance may mean stronger and more frequent hurricanes. By reflecting sunlight, the particulate matter also may have been working to counter-balance global warming. By removing the negative feedback, global warming could be even stronger than if we polluted more heavily. Ironic, I realize.