Climate change affects tropical cyclones in a variety of ways: an intensification of rainfall and wind speed, an increase in the frequency of very intense storms and a poleward extension of where the cyclones reach maximum intensity are among the consequences of human-induced climate change.IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York City, US, pp. 8–9; 15–16, doi:10.1017/9781009157896.001. Tropical cyclones use warm, moist air as their source of energy or fuel. As climate change is warming ocean temperatures, there is potentially more of this fuel available.
Between 1979 and 2017, there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir–Simpson scale. The trend was most clear in the north Indian Ocean, North Atlantic and in the Southern Indian Ocean. In the north Indian Ocean, particularly the Arabian Sea, the frequency, duration, and intensity of cyclones have increased significantly. There has been a 52% increase in the number of cyclones in the Arabian Sea, while the number of very severe cyclones have increased by 150%, during 1982–2019. Meanwhile, the total duration of cyclones in the Arabian Sea has increased by 80% while that of very severe cyclones has increased by 260%. In the North Pacific, tropical cyclones have been moving poleward into colder waters and there was no increase in intensity over this period. With warming, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength. A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin. Rapidly intensifying cyclones are hard to forecast and therefore pose additional risk to coastal communities.
Warmer air can hold more water vapor: the theoretical maximum water vapor content is given by the Clausius–Clapeyron relation, which yields ≈7% increase in water vapor in the atmosphere per warming. All models that were assessed in a 2019 review paper show a future increase of rainfall rates. Additional sea level rise will increase storm surge levels. It is plausible that extreme wind waves see an increase as a consequence of changes in tropical cyclones, further exacerbating storm surge dangers to coastal communities. The compounding effects from floods, storm surge, and terrestrial flooding (rivers) are projected to increase due to global warming.
There is currently no consensus on how climate change will affect the overall frequency of tropical cyclones. A majority of climate models show a decreased frequency in future projections. For instance, a 2020 paper comparing nine high-resolution climate models found robust decreases in frequency in the Southern Indian Ocean and the Southern Hemisphere more generally, while finding mixed signals for Northern Hemisphere tropical cyclones. Observations have shown little change in the overall frequency of tropical cyclones worldwide, with increased frequency in the North Atlantic and central Pacific, and significant decreases in the southern Indian Ocean and western North Pacific. There has been a poleward expansion of the latitude at which the maximum intensity of tropical cyclones occurs, which may be associated with climate change. In the North Pacific, there may also have been an eastward expansion. Between 1949 and 2016, there was a slowdown in tropical cyclone translation speeds. It is unclear still to what extent this can be attributed to climate change: climate models do not all show this feature.
Background
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds and a spiral arrangement of thunderstorms that produce heavy rain or squalls. The majority of these systems form each year in one of seven tropical cyclone basins, which are monitored by a variety of meteorological services and warning centres.
The factors that determine tropical cyclone activity are relatively well understood: warmer sea levels are favourable to tropical cyclones, as well as an unstable and moist mid-troposphere, while vertical wind shear suppresses them. All of these factors will change under climate change, but is not always clear which factor dominates.
Tropical cyclones are known as hurricanes in the Atlantic Ocean and the northeastern Pacific Ocean, typhoons in the northwestern Pacific Ocean, and cyclones in the southern Pacific or the Indian Ocean. Fundamentally, they are all the same type of storm.
Data and models
Measurement
Based on satellite imagery, the Dvorak technique is the primary technique used to estimate globally the tropical cyclone intensity.
The Potential Intensity (PI) of tropical cyclones can be computed from observed data, primarily derived from vertical profiles of temperature, humidity and sea surface temperatures (SSTs). The convective available potential energy (CAPE), was computed from radiosonde stations in parts of the tropics from 1958 to 1997, but is considered to be of poor quality. The Power Dissipation Index (PDI) represents the total power dissipation for the North Atlantic and western North Pacific, and is strongly correlated with tropical SSTs. Various tropical cyclone scales exist to classify a system.
Historical record
Since the satellite era, which began around 1970, trends are considered to be robust enough in regards to the connection of storms and sea surface temperatures. Agreement exists that there were active storm periods in the more distant past, but the sea surface temperature related Power Dissipation Index was not as high. Paleotempestology is the science of past tropical cyclone activity by means of geological proxies (flood sediment), or historical documentary records, such as shipwrecks or tree ring anomalies. , paleoclimate studies are not yet sufficiently consistent to draw conclusions for wider regions, but they do provide some useful information about specific locations.
Modelling tropical cyclones Climate models are used to study expected future changes in cyclonic activity. Lower-resolution climate models cannot represent convection directly, and instead use parametrizations to approximate the smaller scale processes. This poses difficulties for tropical cyclones, as convection is an essential part of tropical cyclone physics.
Higher-resolution global models and regional climate models may be more computer-intensive to run, making it difficult to simulate enough tropical cyclones for robust statistical analysis. However, with growing advancements in technology, climate models have improved simulation abilities for tropical cyclone frequency and intensity.
One challenge that scientists face when modeling is determining whether the recent changes in tropical cyclones are associated with anthropogenic forcing, or if these changes are still within their natural variability. This is most apparent when examining tropical cyclones at longer temporal resolutions. One study found a decreasing trend in tropical storms along the eastern Australian coast over a century-long historical record.
Changes in tropical cyclones
Climate change may affect tropical cyclones in a variety of ways: an intensification of rainfall and wind speed, a decrease in overall frequency, an increase in frequency of very intense storms and a poleward extension of where the cyclones reach maximum intensity are among the possible consequences of human-induced climate change.
Rainfall Warmer air can hold more water vapor: the theoretical maximum water vapor content is given by the Clausius–Clapeyron relation, which yields ≈7% increase in water vapor in the atmosphere per 1 °C warming. All models that were assessed in a 2019 review paper show a future increase of rainfall rates, which is the rain that falls per hour. The World Meteorological Organization stated in 2017 that the quantity of rainfall from Hurricane Harvey had very likely been increased by climate change.
A tropical cyclone's rainfall area (in contrast to rate) is primarily controlled by its environmental sea surface temperature (SST) – relative to the tropical mean SST, called the relative sea surface temperature. Rainfall will expand outwards as the relative SST increases, associated with an expansion of a storm wind field. The largest tropical cyclones are observed in the western North Pacific tropics, where the largest values of relative SST and mid-tropospheric relative humidity are located. Assuming that ocean temperatures rise uniformly, a warming climate is not likely to impact rainfall area.
Intensity
Tropical cyclones use warm, moist air as their source of energy or "fuel". As climate change is warming ocean temperatures, there is potentially more of this fuel available. A study published in 2012 suggests that SSTs may be valuable as a proxy to measure potential intensity (PI) of tropical cyclones, as cyclones are sensitive to ocean basin temperatures. Between 1979 and 2017, there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir–Simpson scale, which are cyclones with wind speeds over per hour. The trend was most clear in the North Atlantic and in the Southern Indian Ocean. In the North Pacific, tropical cyclones have been moving poleward into colder waters and there was no increase in intensity over this period. With 2 °C warming, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength. A study of 2020's storms of at least tropical storm-strength concluded that human-induced climate change increased extreme 3-hourly storm rainfall rates by 10%, and extreme 3-day accumulated rainfall amounts by 5%, and for hurricane-strength storms the figures increased to 11% and 8%.
Climate change has likely been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin, with the proportion of storms undergoing intensification nearly doubling over the years 1982 to 2009. Rapidly intensifying cyclones are hard to forecast and pose additional risk to coastal communities. Storms have also begun to decay more slowly once they make landfall, threatening areas further inland than in the past. The 2020 Atlantic hurricane season was exceptionally active and broke numerous records for frequency and intensity of storms.
Frequency There is no consensus on how climate change will affect the overall frequency of tropical cyclones. A majority of climate models show a decreased frequency in future projections. For instance, a 2020 paper comparing nine high-resolution climate models found robust decreases in frequency in the Southern Indian Ocean and the Southern Hemisphere more generally, while finding mixed signals for Northern Hemisphere tropical cyclones. Observations have shown little change in the overall frequency of tropical cyclones worldwide.
A study published in 2015 concluded that there would be more tropical cyclones in a cooler climate, and that tropical cyclone genesis is possible with sea surface temperatures below . With warmer sea surface temperatures, especially in the Southern Hemisphere, in tandem with increased levels of carbon dioxide, it is likely tropical cyclone frequency will be reduced in the future.