Tropical cyclone numbers declined by about 13 percent in the twentieth century compared to the second half of the nineteenth century. This is the result of a recent study in which researchers combined weather data with computer simulations.
However, the simulation’s accuracy is not high enough to make statements about the evolution of storm intensity, they wrote in the journal Nature Climate Change. This trend can be explained by climate change, which has become particularly evident since around 1950.
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An exception in the Atlantic
Because there are large natural fluctuations in ocean and atmospheric currents, it is generally difficult to determine the directions of tropical cyclones, confirmed scientists led by Savin Chand of Union University in Ballarat, Australia. Therefore, previous studies have either shown a tendency toward more or less tropical cyclones, depending on the computation model.
The probability of storms should increase as the surface sea water temperature increases. However, the shear forces from changing air currents in the atmosphere can disrupt the formation of eddies.
“Before the advent of geostationary weather satellite monitoring in the 1970s, global historical records of tropical cyclones were more vulnerable to disruption and sampling problems,” the researchers wrote. It is therefore problematic for trend analyzes of the impact of climate change.
Chand’s team focused on air pressure near sea surface as a standardized measure. Using a new hurricane detection and tracking scheme, researchers developed a standardized data set for the period 1850 to 2010 to identify trends spanning more than a century and a half.
For most tropical cyclone basins, they have found a rapid decline since the 1950s. The only exception to this trend is the North Atlantic Basin, where numbers of tropical cyclones have increased in recent decades. The authors believe this may be due to the basin recovering from declining numbers of tropical cyclones caused by human-caused aerosol emissions in the late 20th century. However, the number of annual storms is still lower than in pre-industrial times.
From 1850 to 1900 there were an average of about 15 hurricanes per year, while in the first half of the twentieth century the number was closer to ten to eleven. Thus the upward trend since about 1960 corresponds to an approximate level of 1850 to 1900 more than corresponds to an increase in times of climate change.
Storms are tough
Chand’s team also has an explanation for the downward trend in tropical cyclones: widespread poor air circulation between tropical and subtropical climates. According to the researchers, higher sea surface temperatures lead to more evaporation, which leads to a lower buoyancy of saturated air masses. As a result, more dry air is drawn upwards by vertical air movements, reducing the potential for tropical cyclone formation. Hurricanes need moist and warm air.
“These findings put the observed global decline since 1990, dominated by North Pacific trends, into a long-term context,” Alexander Becker of Britain’s University of Reading wrote in a commentary, also in Nature Climate Change. The study authors compared a period of purely natural fluctuations (1850 to 1900) with a period of natural fluctuations and the impact of increased greenhouse gas emissions (1900 to 2010). In doing so, they demonstrated the impact of man-made climate change on the frequency of tropical cyclones.