Tropical cyclones and climate change

Based on satellite imagery, the Dvorak technique is the primary technique used to estimate globally the tropical cyclone intensity.[9] Today measurement for a named system is also done with the ACE index (Accumulated Cyclone Energy), a wind energy index – the sum of the squares of the estimated six-hour maximum sustained wind speed in knots, to assess the storm duration and 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.[10] Various tropical cyclone scales exist to classify a system.

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.[10] 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.

El Niño (ENSO) shifts the region (warmer water, up and down welling at different locations, due to winds) in the Pacific and Atlantic where more storms form, resulting in nearly constant ACE values in any one basin. The El Niño event typically decreases hurricane formation in the Atlantic, and far western Pacific and Australian regions, but instead increases the odds in the central North and South Pacific and particular in the western North Pacific typhoon region.[10]

When analyzing the correlation between extremely active tropical cyclones and El Niño events in the Eastern Pacific Ocean, it was concluded by Murakami et al. that strong El Niño events during hurricane seasons are not the only responsible factors and can most likely be attributed to greenhouse gases.[11]

Studies on increased rainfall rates are largely based on the Clausius–Clapeyron relation, which yields ≈7% increase in water vapor in the atmosphere per 1 °C warming, and the duration of storms.[12][13][14] Storm tracks indicate a global poleward expansion of the latitude at which the maximum intensity of tropical cyclones occurs.[15]

The National Climate Assessment from 2014 states:

The recent increases in activity are linked, in part, to higher sea surface temperatures in the region that Atlantic hurricanes form in and move through. Numerous factors have been shown to influence these local sea surface temperatures, including natural variability, human-induced emissions of heat-trapping gases, and particulate pollution. Quantifying the relative contributions of natural and human-caused factors is an active focus of research.[16]

The World Meteorological Organization stated 2017 that the quantity of rainfall from Hurricane Harvey had very likely been increased by climate change. The relationship between climate change and the frequency of hurricanes (or tropical cyclones) is still unclear, and is the subject of continued research.[17][18]

There is currently no consensus on how climate change will affect the overall frequency of tropical cyclones.[19] Observations have shown little change in the overall frequency of tropical cyclones worldwide.[20]

While trends in overall frequency are inconclusive, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength with 2°C warming.[19] This finding was from a study that sought to ascertain generalizations regarding how tropical cyclone activity will change if the average mean temperature of the Earth increases by 2°C.

Research conducted by Murakami et al. following the 2015 hurricane season in the eastern and central Pacific Ocean where a record number of tropical cyclones and three simultaneous category-4 hurricanes occurred, concludes that greenhouse gas forcing enhances subtropical Pacific warming which they project will increase the frequency of extremely active tropical cyclones in this area.[11]

Additional sea level rise will likely increase storm surge threats.[21] A 2017 study looked at compounding effects from floods, storm surge, and terrestrial flooding (rivers), and projects an increase due to global warming.[21][22]

P.J. Webster and others published in 2005 an article in Science examining the "changes in tropical cyclone number, duration, and intensity" over the past 35 years, the period when satellite data has been available. Their main finding was although the number of cyclones decreased throughout the planet excluding the north Atlantic Ocean, there was a great increase in the number and proportion of very strong cyclones.[23]

According to 2006 studies by the National Oceanic and Atmospheric Administration, "the strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the Earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere".[24]

The 2007 IPCC report noted many observed changes in the climate, including atmospheric composition, global average temperatures, ocean conditions, and others. The report concluded the observed increase in tropical cyclone intensity is larger than climate models predict. In addition, the report considered that it is likely that storm intensity will continue to increase through the 21st century, and declared it more likely than not that there has been some human contribution to the increases in tropical cyclone intensity.[25] The report also noted, "While attention has often been focused simply on the frequency or number of storms, the intensity, size and duration likely matter more."[10]

Studies published since 2008, by Kerry Emanuel from MIT, indicate that global warming is likely to increase the intensity but decrease the frequency of hurricane and cyclone activity.[26] In an article in Nature, Kerry Emanuel stated that potential hurricane destructiveness, a measure combining hurricane strength, duration, and frequency, "is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multidecadal oscillations in the North Atlantic and North Pacific, and global warming". Emanuel predicted "a substantial increase in hurricane-related losses in the twenty-first century".[27]

Research reported in the September 3, 2008 issue of Nature found that the strongest tropical cyclones are getting stronger, in particular over the North Atlantic and Indian Ocean. Wind speeds for the strongest tropical storms increased from an average of 225 km/h (140 mph) in 1981 to 251 km/h (156 mph) in 2006, while the ocean temperature, averaged globally over all the regions where tropical cyclones form, increased from 28.2 °C (82.8 °F) to 28.5 °C (83.3 °F) during this period.[28][29] The environment is much different than 50 years ago with the ocean being 1 °F warmer and melting sea ice contributing to a five to ten percentage increase in air water vapor potentially leading to more severe storms.[30]

A 2011 study linked increased activity of intense hurricanes in the North Atlantic with a northward shift and amplification of convective activities from the African easterly waves (AEWs).[31] A 2014 study investigated the response of AEWs to high emissions scenarios, and found increases in regional temperature gradients, convergence and uplift along the Intertropical Front of Africa, resulting in strengthening of the African easterly waves, affecting the climate over West Africa and the larger Atlantic basin.[32]

Studies conducted in 2008 and 2016 looked at the duration of the Atlantic hurricane season, and found it may be getting longer, particular south of 30°N and east of 75°W, or the tendency toward more early- and late-season storms, correlated to warming sea surface temperatures. However, uncertainty is still high, and one study found no trend, another mixed results.[33]

As to 2019, increasing evaporation and the larger capability of the atmosphere to hold water vapor linked to climate change, already increase the amount of rainfall from the hurricanes Katrina, Irma and Maria by 4 to 9 percent and in the future can increase it by up to 30 percent[34]

Research based on records from Japan and Hawaii indicate that typhoons in the north-west Pacific intensified by 12–15% on average since 1977. The observed strongest typhoons doubled, or tripled in some regions, the intensity of particular landfalling systems is most pronounced. This uptick in storm intensity affects coastal populations in China, Japan, Korea and the Philippines, and has been attributed to warming ocean waters. The authors noted that it is not yet clear to what extent global warming caused the increased water temperatures, but observations are consistent with what the IPCC projects for warming of sea surface temperatures.[35]

In July 2018 Japan experienced a natural disaster: heavy floods and landslides, who killed more than 200 people, make 2 million to flee. This was the consequence of unprecedented heavy rain and the typhoon Prapiroon. Scientist suggests, that climate change is probably one of the main cause for the disaster.[36] This is in line with government predictions for the impact of climate change on Japan. A 2012 report found that global warming could increase the risk of flooding and landslide disasters due to heavy rain.[37]

In the United States, there have been several concrete initiatives taken to better prepare for the strengthening of hurricanes in the foreseeable future.

North Carolina has been focusing on environmental issues since the 1970s when rising environmental awareness pushed the state to take advantage of the federal funds made available to states to deal with potential natural disasters in the Coastal Zone Management Act of 1972. In the interim, North Carolina has identified nine coastal areas which are most likely to suffer most in the event of hurricane activity. The state required that all municipalities and counties in these areas to prepare comprehensive action plans to be implemented and carried out in the event of a major natural disaster. The localities are further required to periodically and constantly update these action plans to keep them up to date.[40] The state requires these plans to include both long term goals to be achieved such as reforestation, building of levees, finding new fresh water sources, etc. as well as short term goals such as preparing local emergency shelters, increasing funding towards research in how best to mitigate the harm of hurricanes, stocking of emergency supplies, etc.[40]

In South Carolina, the state has been actively monitoring and taking an interest in hurricane aftermath management since 1977 when it passed its first coastal management act. The act was administered by the Coastal Council in the state's executive branch until 1993 when it was incorporated into the Department of Health and Environmental Control. From its inception, the South Carolina model has been management of local flora and fauna habitats to strengthen the coastal areas' resilience to flooding and storm surge from hurricanes. In 1988, the state significantly strengthened its coastal laws to deal with the increasing coastal erosion and oceanfront development. These actions came one year before the landfall of Hurricane Hugo in 1989, which devastated the state, and since then South Carolina has continued to focus on increased preparedness for ensuing hurricanes.[40] The Coastal Council's authority was amended by the legislature to provide and further enforce additional guidelines to shoreline development, which the act required be done only as a last resort. The state has also mandated planning and zoning provisions for local communities which are vulnerable to hurricane impacts, and it has required relocation of certain residents in the coastal areas to inland part of the states to prevent further loss or damage to human life.[40]

Hurricane Hugo near peak intensity on September 15, 1989

The state of Maine has a long history of coastal management in all forms dating back to the 1960s, which was then eventually codified into written law in 1978 after the "Blizzard of 1978".[40] The state passed various laws to mandate the management and preparation of state authorities to prepare for further natural disasters, including the potential landfalls of future hurricanes. The legislature allocated funds for several state executive agencies to take an active role in the regulation of coastal development and residence.[40] Maine also utilized already allocated federal funds to better manage its coastal areas. These agencies and funds were utilized to take several precautionary measures such as building sand dunes to serve as temporary levees, construction of seawalls to fortify the coast against storm surges and flooding, reforestation of the state to strengthen the soil, and the banning of construction in certain extremely flood-prone areas.[40]

The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, shattering numerous records. In response articles appeared in relation to climate change and hurricanes. Time Magazine, published an article titled, "Is Global Warming Fueling Katrina?".[42] Shortly after the hurricane, former Boston Globe reporter Ross Gelbspan wrote an op-ed piece for the Globe titled, "Katrina's Real Name", declaring that the hurricane's "real name is global warming." Gelbspan went on to assert:

"Although Katrina began as a relatively small hurricane that glanced off south Florida, it was supercharged with extraordinary intensity by the relatively blistering sea surface temperatures in the Gulf of Mexico."

Gelbspan did not single out Katrina from other recent storms in that regard; in the article he went on to attribute other major weather events over the preceding year to global warming, including a blizzard in Los Angeles, high winds in Scandinavia, wildfires in Spain, and a drought centered in Missouri. Britain's then deputy prime minister, John Prescott, has linked Katrina with global warming,[43] and statements made shortly after the hurricane by Germany's environment minister, Jürgen Trittin,[44] indicate he believes that global warming is responsible for an increase in the frequency of destructive natural events.

Scientists pointed out that the increase of CO
2 emissions contributes to warmer ocean waters and more moist air for rain.[45] Because of sea level rise it is assumed that the storm surge of Hurricane Irma and other storms will cause greater flooding to vulnerable areas.[45][46] Data collected by NASA showed that ocean surface temperatures in the path of Irma were above 30 °C (86 °F), capable of sustaining a Category 5 hurricane.[47] Prior to affecting the U.S. mainland, Miami’s mayor Tomás Regalado noted on Hurricane Irma, "This is the time to talk about climate change. This is the time that the president and the EPA and whoever makes decisions needs to talk about climate change."[48] A day later the head of the EPA, Scott Pruitt said, "..to discuss the cause and effect of these storms, there’s the… place (and time) to do that, it’s not now."[49] Following Irma's landfall, Donald Trump was asked about the connection between hurricanes and climate change, and stated that "We’ve had bigger storms than this."[50]

United Nations secretary general António Guterres citing the devastation from hurricanes noted in September, "The catastrophic Atlantic hurricane season has been made worse by climate change. Cutting carbon emissions must clearly be part of our response to the disaster. The rise in the surface temperature of the ocean has had an impact on weather patterns and we must do everything possible to bring it down."[51]