Atlantic hurricane activity and climate change

I think it is about time that I mention hurricane Sandy here on my blog. The “Superstorm” is the largest Atlantic hurricane on record and we’ve all heard about the devastating effects it had on parts of the Caribbean and the US east coast. It is easy to conclude that this unusually intense storm was caused by climate change and global warming, as a relatively newly released article in the Guardian Environment discusses.

I decided to do my own little research in the field and read a paper published in Nature by Knutson et al. (2010) on tropical cyclones and climate change. In this study they try to answer whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. First they state that there is a relationship between tropical Atlantic SSTs and the upward trend in the Atlantic hurricane activity. They then go on by looking at cyclone- (1) frequency, (2) intensity, (3) rainfall, and (4) genesis, tracks, duration and surge flooding separately. It is hard to model tropical cyclone activity because it depends on so many constantly changing factors (such as tropical SSTs), but with the improvements in models and analyzing techniques, the authors could raise their confidence level concerning cyclone-activity projections and conclude the following:

1.       It remains uncertain whether past changes in tropical cyclone frequency have exceeded the variability expected through natural causes. In the future it is likely that global mean tropical-cyclone-frequency will either decrease or remain essentially unchanged owing to greenhouse warming. Among the proposed mechanisms is the weakening of the tropical circulation.

2.       Future surface warming and changes in the mean thermodynamic state of the atmosphere (as projected by climate models) will lead to an increase in tropical cyclone intensity – both in the mean intensities and in the frequency of cyclones at higher intensity levels.

3.       Atmospheric moisture content has increased in recent decades in many regions, and will continue to increase as the atmosphere warms. This should increase rainfall rates in systems such as tropical cyclones, but that has not been established by existing studies. Tropical-cyclone-related rainfall rates are likely to increase with greenhouse warming though.

4.       There is no conclusive evidence that any observed changes in tropical cyclone genesis, tracks, duration and surge flooding exceeded the variability expected from natural causes. However, with highly confident predictions of future sea-level rise, costal environments are more vulnerable to storm-surge flooding.

      Time series of late summer tropical Atlantic sea surface temperature (blue) and the Power Dissipation Index (green), a measure of hurricane activity which depends on the frequency, duration, and intensity of hurricanes over a season. From Emanuel (2007). As you can see there exists a very close relationship.

So it seems like climate change and global warming is not directly affecting hurricane activity in the Atlantic Ocean, but more indirectly through ocean warming, sea-level rise and circulation changes. Concerning hurricane Sandy, anthropogenic activity has definitely played at least a supporting role in its intensity and destructiveness.

It was like Sandy was a sign from above to the American people, right before the election, that they and their political leaders should open their eyes and realize that climate change is happening and needs to be integrated in their politics. If hurricane Sandy had a saying in who won the election, we can literally thank God Obama won. That way there will at least be some effort to slow down the rate of anthropogenic climate change, easing future damage by superstorms like Sandy.