TOPSHOT - This satellite image obtained from NOAA/RAMMB, shows Tropical Storm Dorian as it approaching the Bahamas and Florida at 13:430UTC on August 31, 2019. - Dorian changed course slightly on Saturday, possibly putting it on track to hit the Carolinas rather than Florida as previously forecast, after a dangerous blast through the Bahamas. Meteorologists said Dorian has grown into an extremely dangerous Category 4 storm as it heads toward land. (Photo by HO / NOAA/RAMMB / AFP) / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / NOAA/RAMMB/HANDOUT" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS

Tropical cyclone numbers decreased by about 13 percent in the 20th century compared to the second half of the 19th century. This is the result of a recent study in which researchers combined weather data with computer simulations.

However, the resolution of the simulations is not high enough to make statements about the development of the intensities of the storms, they write in the journal “Nature Climate Change”. The trend can be explained with climate change, which has become particularly apparent since around 1950.

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Because there are large natural fluctuations in the currents in the oceans and in the atmosphere, it is generally difficult to identify trends in tropical cyclones, emphasize the scientists led by Savin Chand from Federation University in Ballarat, Australia. Therefore, previous studies have shown either a tendency towards more or fewer tropical cyclones, depending on the calculation model.

Storm probability should increase as surface seawater warms. However, shear forces from altered air currents in the atmosphere could disrupt the formation of the vortices.

“Before the advent of geostationary weather satellite monitoring in the 1970s, historical global records of tropical cyclones were more prone to disruption and sampling problems,” the researchers write. They are therefore considered problematic for trend analyzes of the influence of climate change.

Chand’s team focused on the air pressure near the sea surface as a unified measure. Using a new hurricane detection and tracking scheme, the researchers developed a unified data set for the period 1850 to 2010 to identify trends spanning more than a century and a half. They discovered that in four out of seven tropical sea regions the number of tropical cyclones increased from 1850 to 1900. During this period, numbers remained about the same in the South Pacific, but decreased in the western North Pacific and North Atlantic.

For most tropical cyclone basins, they found an accelerated decline since the 1950s. The only exception to this trend is the North Atlantic Basin, where tropical cyclone numbers have increased in recent decades. The authors surmise that this may be due to the basin recovering from a decline in tropical cyclone numbers resulting from human-caused aerosol emissions in the late twentieth century. However, the number of annual storms is still lower than in pre-industrial times.

In the period from 1850 to 1900 there were an average of about 15 hurricanes per year, while in the first half of the 20th century the figure was closer to ten to eleven. The upward trend since about 1960 therefore corresponds more to an approximation to the level of 1850 to 1900 than to an increase in times of climate change.

Chand’s team also has an explanation for the declining trend in tropical cyclones: the weakening of large-scale air circulation between the tropical and subtropical climate zones. According to the researchers, higher temperatures at the sea surface cause more evaporation, which leads to less buoyancy of the water-saturated air masses. As a result, more dry air is pulled upwards with vertical air movements, which makes the formation of tropical cyclones less likely. The hurricanes need moist, warm air.

“These results place the observed global decline since 1990, which has been dominated by North Pacific trends, in a longer-term context,” writes Alexander Baker of Britain’s University of Reading in a comment, also in Nature Climate Change. The authors of the study contrasted a period of exclusively natural fluctuations (1850 to 1900) with a period of natural fluctuations and the influence of increased greenhouse gas emissions (1900 to 2010). In doing so, they showed the influence of man-made climate change on the frequency of tropical cyclones.