The president of Vanuatu, Baldwin Lonsdale, has said that climate change is contributing to the extreme weather conditions seen by his country.
Prof. Gabi Hegerl, Professor of Climate System Science at the University of Edinburgh, said:
“Attributing changing intensity in tropical cyclones is really difficult.
“Although there is more energy for strong storms in a warmer atmosphere, other factors such as the stability of the atmosphere or upper level winds can change too, and those changes are more difficult to predict and may counter some effect of warming.
“However, there is a human contribution to a moister atmosphere, so when there is a cyclone there is more water for rain, and to global sea level rise. Both these things can make tropical cyclones more damaging.”
Dr Peter Stott, Head of Climate Monitoring and Attribution at the Met Office Hadley Centre, said:
“Sea level rise is leading to large increases in the expected frequency of extremes of sea level from storm surges. For example a sea level rise of 0.5 m would likely result in the frequency of sea level extremes increasing by an order of magnitude or more in some regions (IPCC AR5 Chapter 13.7).
The impact of anthropogenic climate change on tropical cyclones themselves is more uncertain.
“The developing science of attribution seeks to put recent extreme events such as this in the context of climate variability and change as we have been doing in the annual reports of the Bulletin of the American Meteorological Society.”
Dr Nick Klingaman, climate scientist at the University of Reading, said:
“There is no clear evidence that climate change affected the formation or intensity of Cyclone Pam. The latest projections from climate models suggest that climate change will reduce the total number of tropical cyclones in the South Pacific, although the average intensity of the cyclones that do form may be stronger than at present.
“Coastal and island communities, such as those in Vanuatu, are also affected by sea-level rise, much of which is caused by global warming. Higher sea levels amplify the destructive storm surges that accompany tropical cyclones. In a warmer world, combination of rising sea levels and more-intense tropical cyclones may increase the damage caused by an individual cyclone, even if the overall number of cyclones decreases.”
Dr Chris Holloway, a tropical storm expert at the University of Reading, said:
“Tropical Cyclone Pam had the strongest winds of any South Pacific tropical cyclone on record, and is tied for having the strongest winds of any Southern Hemisphere tropical cyclone on record. It has caused devastation across Vanuatu, largely because many buildings have been destroyed or severely damaged by the wind.
“The storm passed just to the east of the most populated island in Vanuatu, Efate, which contains the capital Port Vila: the winds felt on this western side of the storm were slightly weaker than the maximum storm winds because they were rotating in the opposite direction to the overall storm motion, and similarly storm surge was slightly lower on this side of the storm. Further south, the storm passed just to the west of the islands of Erromango and Tanna, which placed them in the most dangerous part of the storm with its highest winds and largest storm surge.
“It is impossible to say whether a specific extreme event like Pam was caused by climate change, but we can estimate how the risk of some extreme events might have been increased or decreased by past climate change and how these risks might change in the future. In the case of South Pacific tropical cyclones, recent studies suggest that, although there may be slightly higher risks of the most intense cyclones, there are likely to be fewer cyclones overall, so that it is not clear whether the risks of a storm like Pam in this region have been changed or will be changed by climate change. On the other hand, globally it is most likely that the total number of tropical cyclones will decrease with climate change while the number of the most intense storms, like Pam, will increase.
“Climate change studies also predict that, for a given tropical cyclone, there will be more rainfall on average (leading to potentially more freshwater flooding). Furthermore, global sea levels are rising and will continue to rise, meaning that coastal regions facing a given tropical cyclone storm surge will be more vulnerable to coastal flooding.”
Prof. Myles Allen, Professor of Geosystem Science at the University of Oxford, said:
“It is a perfectly reasonable question for the President to be raising. Basic thermodynamics means that a warmer atmosphere, all other things being equal, makes more intense cyclones possible.
“But this does not mean cyclones have necessarily become more likely: indeed, the latest assessment of the IPCC stated explicitly that there is no clear evidence at present for any human-induced increase in tropic-wide cyclone frequency.
“Other factors such as sea-level rise will exacerbate any storm’s impacts. Some of the observed sea level rise in this region, but probably not all, can be attributed to human-induced global warming.
“On a personal level, I sympathise with the President’s evident frustration: he and the people of Vanuatu deserve an authoritative answer to the question of the role of past greenhouse gas emissions in Cyclone Pam. The science isn’t there yet, but we are getting there.”
Prof. Richard Betts, Head of Climate Impacts Research at the Met Office Hadley Centre and Chair in Climate Impacts at the University of Exeter, said:
“As yet we are not sure whether tropical cyclone activity is changing, and if it is, what the cause is.
“However, when cyclones and other storms occur, there is already a greater risk of coastal flooding because the background sea level has risen, largely due to human-induced global warming. How much more flooding has occurred due to human action is unclear, but ongoing sea level rise can be expected to further increase this risk unless coastal protection can be improved.
“Reducing global greenhouse emissions could slow sea level rise in the longer term, but even if this were to happen, some further rise is already in the pipeline and is unavoidable.”
Dr Christopher Brierley, Lecturer in Climate Modelling at University College London, said:
“Storms as strong as Cyclone Pam have become more likely due to climate change, despite it making us get fewer storms in total. The sea level rise we’re causing makes the surge from any storm even more devastating.”
Dr Ilan Kelman, Reader in Risk, Resilience and Global Health at the Institute for Risk & Disaster Reduction and Institute for Global Health, University College London, said:
“Climate change might or might not have affected the cyclone, but we can all take control of our own vulnerability to ensure that a storm does not become a disaster.
“A cyclone itself does not create a disaster. There must be vulnerability also. Irrespective of climate change, we can do much more to reduce vulnerability to storms in order to prevent a disaster.
“We could have done much more to work with Vanuatu long before the cyclone struck in order to reduce the disaster potential. That is the point of this week’s disaster risk reduction conference in Japan. A cyclone alone does not create a disaster unless we are not ready for it.”