http://www.realclimate.org/index.php?p=140
I recommend it to anyone interested in the issue. I'll comment on a couple of excerpts, which were issues we covered on "Talkin' Tropics" a couple of months back:
The topic regarding tropical storms have also been a contriversial topic in the scientific community. Has there really been a systematic long-term trend in the storm statistics? Chan and Liu (2004) found no systematic change in typhoons that could be related to changes in the sea surface temperature. Because trends in sea surface temperature (SST) have been relative weak over the past 50 years, Knutson & Tuleya (2004) have argued that CO2-induced tropical cyclone intensity changes are unlikely to be detectable in historical observations and will probably not be detectable for decades to come. Although there is no clear linear trend in the Atlantic hurricane number (see Fig. 1), there may nevertheless be other indications which may suggest that the tropical cyclone statistics are changing.
This makes the important point that despite some excited uterrances to the contrary, there's no real evidence that active seasons like last year are caused by global warming. It may be that we will some day look back and see such evidence, but we don't have it now.
That said, there are some strong theoretical reasons for thinking that climate change will have an appreciable effect some time down the line:
Why do we think that tropical cyclones would change because of a global warming? We have some basic physics-based principles that may possibly provide some insight. The main driving mechanism for all storms is the condensation of water vapour releasing energy/heat. Temperature differences cause instabilities and drive winds, and unstable disturbances grow into powerful storms. Tropical cyclones differ from mid-latitude storms in their primary driving mechanism. So-called 'baroclinic instabilities' are more important for the mid-latitude storms, whereas plain convective instabilities are the primary cause for the tropical cyclones - such as an increase in the CAPE.
Some climate models suggest that a global warming may be favourable for more intense mid-latitude storms. There are also a couple of physics-based considerations that can provide an indication of the main features: (i) As the surface and atmosphere warm up, more energy (heat) becomes available in the form of water vapour (more evaporation and the air manage to hold more moisture), but (ii) one factor which may act as a moderating influence, is that a global warming is expected to warm the polar regions faster than the lower latitudes, hence reducing the meridional (north-south) temperature differences (gradient). Mid-latitude storms tend to form where there are sharp temperature gradients, where conditions for instabilities are favourable. There tend to be a sharp temperature drop poleward of the polar fronts, and it is no coincidence that this is the same regions where the storm tracks lie. The mid-latitude storms play an important role in the climate system, as they facilitate the poleward heat transport. Hence, we would expect to see a relationship between the number of storms and the pole-equator temperature differences. Perhaps part of the reason why the poles warm more strongly than the lower latitudes (in addition to reduced sea-ice) may be an increase in storm activity along the storm tracks?
This illustrates the complexity of the topic pretty well. I brought up the issue of a weaker equator-to-pole temperature gradient possibly resulting in fewer total storms on the show (since the fundamental thermodynamic function of these systems is to transport heat poleward). OTOH, it seems likely that, regardless of how many storms do form, more of them would develop to intense strength due to high SSTs and increased CAPE.
Jan
