Tropical cyclones and public health: how climate change is driving increasingly extreme weather—an essay by Fintan Hughes, Jack Hodkinson, and Hugh MontgomeryBMJ 2017; 359 doi: https://doi.org/10.1136/bmj.j4908 (Published 09 November 2017) Cite this as: BMJ 2017;359:j4908
- Fintan Hughes, human physiology researcher1,
- Jack Hodkinson, PhD candidate2,
- Hugh Montgomery, director1
Throughout the 60 million years of the carboniferous period, which ended 300 million years ago, carbon dioxide was drawn down from the atmosphere and sequestered in fossil fuels. These fuels have only recently been burnt at scale. Cars first outnumbered horses in New York City just over 100 years ago, but there are now 1.2 billion vehicles on the road worldwide, and airlines carry over 400 000 passengers every hour. Burning fuels has increased atmospheric carbon dioxide substantially from 280 to 406 ppm since 1850.
In 1896, Nobel Laureate and physical chemist Svante Arrhenius predicted that such rises could increase global surface temperatures. By 1970 climate scientists widely recognised this causal relation. In 2016, the average land surface temperature was 1.42°C higher than the 20th century average, with an increase in average global temperature of 1.1°C.1
Food shortages and the spread of disease resulting from climate change have been well described.2 However, the oceans have also been affected: water and ice have absorbed 93% of the net global energy gain caused by carbon dioxide and other greenhouse gases, and their surface temperatures have risen by 0.7°C since preindustrial times.3 Polar sea ice is melting—its surface area 2 000 000 km2 smaller in 2017, the minimum Arctic sea ice extent in September being 44% lower than the 1981-2010 average—and sea levels are rising by 3.4 cm a decade.
From heat to cyclones
Heat in our atmosphere and oceans causes air to rise and water to evaporate. This water precipitates as rain, and rising air creates pressure differences that cause wind. The resulting dynamic system is our weather, which intensifies when fuelled by greater heat. Tropical cyclones form as a result of rapid water evaporation from warm oceans. The rising air draws with it further humid air from the ocean surface. The energy (latent heat) is released when the water condenses to form clouds, which are pushed outwards by the rising air at the centre. The Earth’s rotation causes the system to spiral (the Coriolis effect), generating winds that move across the Earth’s surface.
Hurricanes, typhoons and cyclones
The phrase tropical cyclone refers to a system of thunderstorms and clouds organised into a low level rotating structure, which originates over subtropical or tropical waters. When maximum sustained winds of 120 km/hour or more are reached, different names and intensity scales are used depending on where the system is found:
• Hurricanes form over the north Atlantic and northeast Pacific Oceans.
• Cyclones are formed over the south Pacific and Indian Oceans
• Typhoons are formed over the northwest Pacific Ocean
The energy released by a tropical cyclone is truly staggering; in a typical system the latent heat of water condensation is released at a rate of 6×1014 watts—200 times the entire world’s electricity generating power. This destructive power is huge; cyclones convert our planet’s excess heat into wind and rainfall that directly strike exposed populations. Destruction of property and freshwater and sanitation services exposes the surviving population to disease. Hampered transport infrastructure and fuel shortages impede emergency services as hospitals struggle to stay operational.
This year’s north Atlantic hurricane season
On 27 August 2017, the eye of hurricane Harvey, then a category 4 storm, stalled over the Gulf of Mexico, releasing up to 96 mm of rainfall an hour to its north. Total rainfall accumulations of over 500 mm brought unprecedented flooding to the region. Disrupted drinking water supplies and contaminated floodwater prompted the declaration of public health emergencies in Texas and Louisiana.
The US coastguard rescued 16 800 people; traumatic injuries increased pressure on the Houston health system as evacuations began from 12 hospitals. As floodwaters lingered, the US Centers for Disease Control and Prevention warned of potential public health risks from electrocution and exposure to industrial chemicals from surrounding petrochemical plants, along with respiratory infections from mould growth.4 Standing water also increases the risk of proliferation of the endemic Aedes mosquito, with cases of dengue fever, chikungunya, and Zika virus reported in the region.5
Less than two weeks later, the second most powerful recorded Atlantic storm, hurricane Irma, made numerous landfalls throughout the Caribbean, with sustained wind speeds of 295 km/h. Serious damage to healthcare facilities was widely reported, with growing risks from mosquitoes and rodents. Barbuda, in particular, had all healthcare facilities and ambulances destroyed, with much of the island flooded by a 7 m storm surge (a tsunami-like wave generated by atmospheric energy) forcing the evacuation of the entire island as category 4 hurricane Jose approached.6
Medical supplies were severely limited throughout the Caribbean, with countries requesting aid in the form of mosquito repellents, emergency medical kits, and many essential drugs.7 Cuba, in the midst of intense drought and still recovering from 2016’s hurricane Matthew, was exposed to 9 m waves and coastal inundation during Irma’s landfall. Some 220 000 homes were severely damaged, along with 70% of the hospitals in the affected area, which is home to over nine million people.8 Relief efforts were hampered because few airports throughout the Caribbean were operational in the days after the storm. Widespread road damage presented an additional hurdle to humanitarian aid and emergency medical care—for example, 90% of roads on the island of Anguilla were impassable.9
Ten days later, hurricane Maria rapidly intensified to a category 5 cyclone just 30 hours before damaging 90% of buildings in Dominica. All 53 of Dominica’s hospitals were left without electricity or water—and 80% of the population were in urgent need of water and shelter. Similarly, 1.5 million people were without water supply in the Dominican Republic.9 The next landfall cut the power supply for the entire 3.4 million population of Puerto Rico, with catastrophic consequences for patient care; 51 of the island’s 69 hospitals stopped accepting new patients. By the end of the week only a single hospital was fully operational, and 10 days later only 17 hospitals had power from the grid, while access to generator fuel was limited.10
Even more extreme cyclones on the horizon
Increasing ocean temperatures lead to more powerful storms; the maximum wind speed in a tropical cyclone is proportional to the square root of the difference in temperatures between the ocean surface and the stratosphere. Climate change induced by greenhouse gases such as carbon dioxide not only warms the ocean surface but also cools the stratosphere, further amplifying the intensity of tropical cyclones.11 Meanwhile, higher ocean temperatures lead to greater surface water evaporation. The Clausius-Clapeyron relation predicts an increase in atmospheric water content of 6.5% for every 1°C increase in atmospheric temperature, correlating strongly with an increase in extreme rainfall events. Through such effects, climate change drives increasing cyclone intensity. We have seen the number of tropical cyclones reaching categories 4 and 5, such as Harvey, Irma, Jose, and Maria, increase since 1930.
Models predict that global power dissipation (the total energy released by tropical cyclones in a given period) will increase by 10-13% by the end of the century, under conservative estimates of warming.12 However, because of the greater carbon dioxide concentrations and humidity gradients in the troposphere the frequency of tropical storm formation is predicted to reduce by 7-28%.13 This increased power will therefore be dissipated through a smaller number of larger storms.
Models predict an increase in global storm intensity of 4.1%.12 This amplification of mean intensity increases the frequency of the extreme storms at the upper end of the normal distribution. As such the Intergovernmental Panel on Climate Change models predict a 28% increase in frequency of the maximum intensity, category 4 and 5, storms by the end of the century. This is further reflected by a predicted 25% increase in precipitation from tropical cyclones.14
As global temperatures rise, the destructive effects seen from hurricanes Harvey, Irma, and Maria are predicted to become regular occurrences. This hurricane season provides a worrying glimpse of a future Earth in which warming is not curtailed. With an increase in average storm intensity, the time between any two catastrophic storms will decrease.
With less time to rebuild damaged infrastructure before the next storm makes landfall, areas unable to adapt will become uninhabitable. Importantly, a combination of continued sea level rise and the predicted northward shift of the most intense tropical cyclones15 will make the eastern United States increasingly vulnerable to these extreme events, as seen with hurricane Sandy in 2012. Similar effects in the Pacific and Indian Oceans may combine to substantially increase the population exposed to high intensity tropical cyclones over the course of the century.
Key global actions
No viable engineering solutions exist that would protect exposed populations from the impact of these extreme tropical cyclones. Sea walls have proved ineffective against storm surge, affordable buildings are unable to withstand 200 km/h winds, and drainage systems cannot clear the flooding arising from such extreme rainfall.
We do not have the solutions required to adapt to this predicted increase in storm intensity. It is thus essential that greenhouse gas emissions are substantially and rapidly reduced.
As these extreme weather events show, any further warming that can be avoided must be avoided. Governments of the world must go beyond the minimum requirements set out by the 2015 Paris agreement. The United States should rejoin the Paris process. Global fossil fuel subsidies of $5.3tr (£4tr; €4.6tr) annually amount to subsidies on suffering and must stop.16 This money could instead be used to help hasten the transition to renewable energy sources. Solar photovoltaic is now a cheaper energy source than coal.17 Financial institutions should reduce interest rates on renewable energy investments to facilitate their adoption in developing countries.
Academic funders can also play their part, supporting exploration of new renewable energy technologies and helping fund research that clarifies the human cost of climate change. Industries that develop renewable energy technologies, facilitate their widespread adoption, and establish independent renewable energy supplies should be financially rewarded, as they lead by example. Similarly, carbon taxation is an essential tool in reprimanding those industries that knowingly endanger the planet.
Doctors can use their prominent trusted position in the scientific community, and in broader society, to lead the education of politicians, policy makers, and the general public. A medical community well versed in the public health effects of climate change18 and the subtleties of climate science should pressure politicians for immediate and meaningful action on climate change as the major global health opportunity of our time.
The future of human civilisation as we know it is under threat; global temperatures are accelerating towards potentially irreversible tipping points. Now is the time to act—at speed and scale. We must advocate for a concerted global effort; wishful thinking will not calm the storm.
Fintan Hughes works as a human physiology researcher with Hugh Montgomery at UCL. He has a degree in physics and is studying medicine at the National University of Ireland Galway.
Jack Hodkinson is studying for a PhD in physics within the Dutton Research Group in Cambridge.
Hugh Montgomery is a professor of intensive care medicine at University College London. He co-chaired the 2015 Lancet Commission on climate change and health, is a cofounding member of the UK Climate and Health Council, and is co-chair of the Lancet Countdown, a multidisciplinary collaboration tracking the progress of climate change and health.
Competing interests: We have read and understood BMJ policy on declaration of interests and have no relevant interests to declare.
Provenance and peer review: Commissioned; not externally peer reviewed.