Climate changes = more hurricanes? Simple maths or false numbers?

In last week’s article on Hurricane Sandy I stated out and out that the storm’s strength and northerly track were mainly the effects of climate change. Scientifically such a black and white statement should probably have been supported with evidence and as such I have decided to review a recent paper from the journal Nature Geoscience on the links between climate change and tropical cyclones. Hopefully this will serve to support the stance I made in the previous article and inform readers who wish to explore some of the scientific evidence of a link between climate and the intensity of hurricanes. Is it a simple relationship or a more complex one?

The paper, titled ‘Topical Cyclones and Climate Change’ and authored by Knutson et al, analyses both the past correlation between climate change and tropical storm characteristics and uses data from modelling to project the effect of climate change on tropical cyclone intensity in the future. Sea surface temperature (SST) has been tracked and found to have increased by several tenths of a degree over the last few decades. The paper cites the IPCC fourth report and US Climate Change Science program report 3.3 that states that these changes are ‘very likely’ due to anthropogenic global warming.

When considering past data the picture painted by the study is confused. While the rise in SST is easy to follow how this has effected Tropical Cyclone strength is less certain due to both the suggestion of natural climatic variability in the Atlantic (relevant for Atlantic tropical cyclone frequency) and a number of biases in the data especially as data is more readily available and reliable for the Atlantic Ocean compared to the Pacific. The paper states that without adjustment for possible missing data hurricanes have increased in frequency from the late 1800s onwards but shows no significant trend from the 1850s to the present. In terms of intensity (measured via the power dissipation index or PDI) it is also difficult to find an overall trend. The paper compiles information from a number of studies as the graphs below show until very recently there has been no rise in PDI over a prolonged period but rather a number of short term peaks and troughs with no significant long term trend.

However when it comes to the projection of future intensity and frequency of tropical storms (and therefore hurricanes) the paper presents more defined conclusions. It is projected that frequency of tropical cyclones will either remain the same or reduce on a global scale as greenhouse warming continues. The decrease may be as much as -34% although in some individual basins there is an error factor of +/-50% on these projections. In terms of intensity the maximum wind speed is predicted to rise (on average) by +2 to 11% while rainfall is modelled to increase by +20% within a 100km radius of the cyclone centre. There is an error margin of +/-15% associated with some of the methods of modelling (usually for individual basins).

There is still a degree of uncertainty with these figures, especially the figures for cyclone frequency which the authors state they have ‘very low confidence in’. However the models have been calibrated against past observed data so at the very least the overall trends predicted by modelling can be relied upon.

This brief summary of the paper has attempted to highlight the relevant data and trends within the data. The conclusions that can be drawn from it are that it is likely that climate change will cause an increase in the intensity of tropical storms (and therefore hurricanes) but that it is not yet certain whether it will have the same effect on frequency. Despite this these conclusions provide a strong link between climate change and hurricanes.