Authored by Louise O'Regan
Each year, hurricanes in the North Atlantic cause catastrophic losses to large areas, including the Caribbean and east coast of the United States of America. To attempt to reduce the impacts of hurricanes in the future we need to understand the impacts of anthropogenically-driven climate change on hurricanes. Are hurricanes going to become more common, or more intense, due to anthropogenically-driven climate change?
Anthropogenically-driven climate change is one of the pressing issues of our time, causing unprecedented changes to the Earth system, from increasing ice melt in the Arctic to increasing the area of the Sahara Desert. Such large-scale changes are likely to alter the characteristics of hydro-meteorological hazards faced by society, such as hurricanes in the North Atlantic. To limit the havoc hurricanes wreak, such as the 1,833 casualties caused by Hurricane Katrina, future changes to hurricanes need to be understood to enable the development of effective management techniques.
Hurricanes are large rotating weather systems, requiring specific conditions to develop. Sea Surface Temperatures (SSTs) need to exceed 26.5°C to allow hurricane formation, as warm ocean water is the main hurricane energy source. SSTs are also considered the main control on hurricane 'Maximum Potential Intensity.' Through increasing atmospheric temperatures, anthropogenically-driven climate change has increased North Atlantic SSTs, with further SST increases likely in the future. Potential for anthropogenically-driven climate change to affect hurricanes now becomes apparent: under a warmer climate, with higher SSTs, an increase in hurricane intensity is likely, as the 'Maximum Potential Intensity' and main energy source will have increased. Increased SSTs will also expand the spatial area able to support hurricane formation.
A low vertical wind shear is needed to develop the large vertical structure of a hurricane. A high vertical wind shear, a large difference between windspeeds at the Earth’s surface and high in the atmosphere, can effectively tear the hurricane apart before it has developed, preventing the hurricane forming. To complicate the impact of climate change on hurricanes, the increase in SSTs is likely to increase vertical wind shear, creating atmospheric conditions less conducive to hurricane formation (Knutson et al., 2010). A decrease in hurricane frequency, therefore, is likely, as hurricanes can no longer develop with high wind shears. This decrease in hurricane frequency has been supported by numerous computer modelling studies, such as Emanuel (2005) and Hoyos et al. (2006).
While hurricane frequency is an "important scientific issue" (Emanuel, 2005), hurricane intensity is more important in controlling the impacts of hurricanes. One intense hurricane will likely cause greater disruption to communities compared to numerous weak hurricanes: indeed, the most intense hurricanes can cause damage upwards of $180 billion, demonstrated by Hurricane Harvey, which made landfall in the United States in 2018. While climate change is likely to decrease future hurricane frequency, will it have an effect on the intensity of hurricanes?
Figure 2 defines the impact of anthropogenically-driven climate change on hurricane intensity; a clear increase in 'Power Dissipation Index' (PDI) from 1975 to 2005 can be seen. Hurricane intensity is commonly quantified based on maximum measured windspeed, however the PDI measures intensity using windspeed, hurricane duration and hurricane diameter measurements. The technique presents a more holistic approach to quantifying hurricane intensity and provides a more representative view of the potential impacts of hurricanes. The increase in this index, therefore, suggests hurricane intensity is increasing, and so the potential impacts of hurricanes are getting worse.
The increase in PDI is attributed to the increase in Atlantic SSTs. Changes in PDI closely mirror changes seen in Atlantic SSTs, which Emanuel (2005) suggests means that the observed increase in SSTs has caused the observed increase in PDI. As the increase in Atlantic SSTs is likely to have been caused by anthropogenically-driven climate change, the increase in PDI is also likely to have been caused by anthropogenically-driven climate change. Atlantic SSTs are likely to further increase in the future, and so PDI is likely to continue increasing. Future changes to hurricane activity, therefore, are likely to be two-fold: a decrease in the overall frequency of hurricanes, but an increase in the intensity, so destructiveness, of hurricanes. This poses numerous management issues, as the most intense, destructive events become more frequent under future climate change scenarios.
As with the majority of climate science, the impact of anthropogenically-driven climate change on hurricane activity is not without contestation. As a result of natural variation in the climate system, the attribution of the increase in PDI and SST from 1975 to anthropogenically-driven climate change has been questioned. The stark increases in PDI and SST from 1975, however, are uncharacteristic of natural climatic oscillations and variation, and so are likely driven by anthropogenically-driven climate change. In addition, whilst Figure 2 shows a correlation, there is potentially a lack of causation between the 2 variables. While Atlantic SSTs and PDI are related, shown by the similarities in Figure 2, observed changes in SSTs did not necessarily cause the changes in PDI. However, modelling results and subsequent studies appear to compliment Emanuel's conclusions, that whilst the frequency of hurricanes in the future is likely to decrease, the intensity of hurricanes is likely to increase. Modelling outputs support the interpretation of Figure 2, that increases in ocean temperatures and hurricane intensity are driven by anthropogenically-driven climate change. While we cannot attribute a single hurricane to anthropogenically-driven climate change, we can attribute long-term trends in hurricane intensity to anthropogenically-driven climate change with a relatively high degree of certainty. We can say with some certainty, therefore, that future anthropogenically-driven climate change is likely to increase hurricane intensity.
An increase in future hurricane intensity will inevitably cause greater disruption when hurricanes make landfall, calling for more effective management techniques to attempt to mitigate the impacts of these more intense hurricanes. Projected future increases in coastal populations are likely to compound the increased impacts of more intense hurricanes; a hurricane can only cause damage if there is infrastructure to damage. Even without the projected increase in hurricane intensity under anthropogenically-driven climate change, hurricanes are likely to become more disastrous as coastal areas at risk of hurricane landfall continue to grow.
While overall hurricane frequency is likely to decrease in the future, the frequency of the most intense, destructive hurricanes is likely to increase, with potentially devastating consequences.