Tropical cyclones and climate change
Tropical cyclones and climate change concerns how tropical cyclones have changed (in number, intensity, track or otherwise), and are expected to further change, under global warming. The topic receives considerable attention from climate scientists who study the connections between storms and climate, and notably since 2005 makes news during active storm seasons.
A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a cyclone occurs in the south Pacific or Indian Ocean.
A 2010 review of related sciences found substantial uptake in the frequency of the most intense tropical cyclones, and increases in rainfall rates within 100 km of the storm centre of up to 20%.
A tropical cyclone's rainfall is primarily controlled by its actual environmental sea surface temperature (SST) - relative to the tropical mean SST, called the relative sea surface temperature. The relative SST has also been identified to more precise explain variations in Atlantic hurricane frequency, than absolute SST and tropical convection. The relative SST is defined as the SST in the TC environment minus the tropical (30°S-30°N) mean SST. 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. Flooding by TCs with global warming is projected primarily by increases in rainfall rate, rather than area extent.
Based on satellite imagery, the Dvorak technique is the primary technique used to estimate globally the tropical cyclone intensity. 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. 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. 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.
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.
The tremendous destruction caused by recent Atlantic Ocean tropical cyclones, such as Hurricanes Katrina, Wilma, and Sandy caused a substantial upsurge in interest in the subject of climate change and hurricanes by news media and the wider public, and concerns that global climatic change may have played a significant role in those events. In 2005 and 2017, related polling of populations affected by hurricanes concluded in 2005 that 39 percent of Americans believed climate change helped to fuel the intensity of hurricanes, with 55 percent in September 2017.
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?". 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, and statements made shortly after the hurricane by Germany's environment minister, Jürgen Trittin, 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 CO2 emissions contributes to warmer ocean waters and more moist air for rain. 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. 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. 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." 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." 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." Richard Branson who was directly impacted by hurricane Irma noted, "..hurricanes are the start of things to come. Look, climate change is real. Ninety-nine per cent of scientists know it's real. The whole world knows it's real except for maybe one person in the White House."
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."
The Associated Press looked at the yearly average Accumulated Cyclone Energy (ACE), which accounts for wind speed and storm duration to assess hurricane power of the past 30 years and found it to be 41 percent higher than the previous 30 years. They asked several experts about their opinion, James Kossin from NOAA “There’s no question that the storms are stronger than they were 30 years ago.” Climate scientist Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, “The only caveat being that the increase might be exaggerated somewhat because of undercounting early storms.” Meteorologist Philip Klotzbach noted, “What’s happening with hurricanes — the frequency, the duration, and the energy — is probably a combination of factors caused by both nature and man, a mish-mosh of everything.” Kerry Emanuel who studies hurricanes, told the BBC, "The warming of the climate has increased the underlying probabilities of very heavy rain events like happened in Harvey and very high category hurricanes like Irma. It is just not sensible to say either storm was caused by climate change, but the underlying probabilities are going up."
A review by Walsh et al. 2015 found good understanding of the links between climate and tropical cyclones on various timescales, with significant observed trends in the Atlantic for frequencies and intensities in the past few decades. However, these trends are still heavily studied. The authors note that the theory for maximum tropical cyclone intensity is considered well established, but with cyclone genesis still elusive. Most climate models predict future decreases in global tropical cyclones, project more intense of the strongest storms, with increased rainfall rates. Additional sea level rise will likely increase storm surge threats. A 2017 study looked at compounding effects from floods, storm surge, and terrestrial flooding (rivers), and projects an increase due to global warming.
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. Storm tracks indicate a global poleward expansion of the latitude at which the maximum intensity of tropical cyclones occur. Some model based research predicts a significant increase (+90%) in the frequency of very intense (category 4 and 5) hurricanes, with average climate change projections.
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.
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.
Greenhouse gases are affecting hurricane frequency and severity hugely. 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 3 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.
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.
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".
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. The report also noted, "While attention has often been focussed simply on the frequency or number of storms, the intensity, size and duration likely matter more."
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. 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".
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.
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). 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.
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.
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.
|1||"Labor Day"||1935||892 mbar (hPa)|
|2||Gilbert||1988||900 mbar (hPa)|
|4||Dean||2007||905 mbar (hPa)|
|5||"Cuba"||1924||910 mbar (hPa)|
|6||Janet||1955||914 mbar (hPa)|
|8||Maria||2017||917 mbar (hPa)|
|9||"Cuba"||1932||918 mbar (hPa)|
|10||Katrina||2005||920 mbar (hPa)|
|Sources: Atlantic Hurricane Best Track Data
Documentation of Atlantic Tropical Cyclones
National Hurricane Center
- Atlantic hurricane reanalysis project
- Climate change in the United States
- Effects of global warming on oceans
- Effects of tropical cyclones
- List of Category 5 Atlantic hurricanes
- List of the most intense tropical cyclones
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