“It’s very clear global warming is intensifying cyclones in the Pacific.” – Greens Leader Chistine Milne, comments to reporters, March 17, 2015.
Christine Milne’s statements on global warming linked the devastation wreaked by Cyclone Pam in Vanuatu last weekend with climate change.
But while there is very real possibility that there is a causal relationship between cyclone behaviour and anthropogenic global warming, most climate scientists and meteorologists are hesitant to attribute any single event such as an extreme intensity tropical cyclone to global warming. This is because we know that the globe has experienced such events for many thousands of years.
The key to understanding whether global warming is causing a change in the behaviour of tropical cyclones is to look at the trends, usually over as many years as possible.
The scientific evidence
When asked for evidence to support her statement, a spokesperson for Senator Milne referred to a 2013 Intergovernmental Panel on Climate Change (IPCC) report that said that “precipitation will likely be more extreme near the centres of tropical cyclones making landfall in North and Central America, East Africa, West, East, South and Southeast Asia as well as in Australia and many Pacific islands… and the frequency of the most intense storms will more likely than not increase substantially in some basins.”
The spokesperson referred to a 2007 IPCC report that said “earlier studies… showed that future tropical cyclones would likely become more severe with greater wind speeds and more intense precipitation.” He also pointed to the US Union of Concerned Scientists statement on hurricanes and cyclones, and the US Geophysical Fluid Dynamics Laboratory’s website.
Recent studies suggest that there has been an increase in the occurrence of high intensity tropical cyclones globally.
But this has been offset by a decrease in the number of weaker tropical cyclones. The most intense tropical cyclones have increased their wind speeds in all ocean basins including the Pacific. The proportion of intense tropical cyclones versus weaker ones has increased both regionally and globally by 25% to 30% per degree Celsius of global warming over the past four to five decades.
As the seas warm, the ocean has more energy available to be converted to tropical cyclone wind. So with increasing sea surface temperatures we can expect to see higher tropical cyclone wind speeds. This has been the case as the wind speeds of the most intense tropical cyclones have been increasing in all ocean basins.
Not all ocean basins, however, are responding at the same rates. The North Atlantic and North and South Indian Ocean basins appear to be increasing at the fastest rates. The South Pacific does not show an increase except for the very strongest cyclones.
At the moment, though, it is difficult to definitively determine whether this increase in tropical cyclone intensities is greater than that which has occurred in the past or, in other words, has exceeded the natural variability.
There are a number of modelling studies that suggest the frequency or total number of cyclones in some ocean basins, including the South Indian and South Pacific, will decrease as a function of global warming.
One recent study examining the frequency of tropical cyclone activity in the Australian region showed that total seasonal cyclone activity was at its lowest level in 1500 years in Western Australia and in 500 years in north Queensland.
These results accord well with the modelled forecasts and especially because the most dramatic decline in seasonal cyclone activity has been occurring since approximately 1970. But, again, it is difficult to state whether this is due to anthropogenic warming or natural variability.
Studies examining the long-term history of tropical cyclones (centuries to several millennia) show that cyclone activity can be cyclic. There are periods of substantial activity and relative quiescence that can last for decades to centuries. Eastern Queensland, for example, has seen a 62% fall in the number of severe landfalling cyclones since the late 19th century.
Fact: the strongest cyclones are getting stronger
The recognition of long-term cycles in cyclone activity, however, does not negate the very real possibility that there is a causal relationship between cyclone behaviour and anthropogenic global warming either now or in the future. The notion that cyclones should increase in intensity as sea surface temperatures rise is theoretically sound and the facts are that the strongest cyclones are getting stronger.
Whether we can directly attribute recent very intense cyclones such as Pam and its impact on Vanuatu to global warming remains to be determined.
Either way, the future doesn’t look good for all locations that are prone to these events. The message is clear that we need to understand that the physical risk from tropical cyclones is increasing.
Senator Milne’s statement that global warming is intensifying cyclones in the Pacific cannot be verified at present but cyclones are intensifying in concert with rising sea surface temperatures.
This summary is consistent with our current understanding of this topic. An additional factor relevant to the damage caused by tropical cyclones is sea level rise. We are confident that sea level rise is caused by man-made global warming. Since the most damaging impact of a tropical cyclone along the coast is sea flooding, sea level rise has already increased the amount of this flooding, and will continue to do when the sea level becomes even higher in the future.– Kevin Walsh