![]() While most climate model experiments project a likely decline in global TC numbers due to greenhouse warming 12, studies that have attempted to assess these changes empirically are often constrained by the lack of observational data covering the pre-industrial period 2. Given the unprecedented level of warming since the mid-twentieth century in the context of at least the past 2,000 years 1, 20 and its associated impact on the major atmospheric circulations in the tropics 21, 22, 23, 24, one would expect such changes to manifest in the total number of TCs observed globally. But those areas of active convection caused by non-uniform SST increase are also likely to be influenced by other factors such as internal climate variability and localized aerosol effects, making it difficult to detect any long-term changes in regional TC numbers. At a regional scale, non-uniform sea surface temperature (SST) increase (that is, warming relative to the mean tropical SST) can cause shifts in the areas of active convection with associated changes in wind shear and middle-troposphere dryness, leading to shifts in TC formation locations 10, 18, 19. In a warming climate, changes in deep convection, wind shear and middle tropospheric humidity are likely to contribute to an even more hostile TC formation environment globally 8, 9, 10, 11. Even in the current climate, much of the tropics are typically hostile to TC formation with the middle troposphere being too dry and wind shear often too strong 17. To summarize these studies, we note that TCs can form only when an initial circulation is protected from environmental wind shear and dry air intrusions 15, within which prolonged deep convection can moisten the protected region sufficiently to allow a precursor to develop into a TC 16. Several hypotheses tested using climate model experiments point to a plausible link between anthropogenic-induced greenhouse warming and changes in TC numbers at global and regional scales 8, 9, 10, 11, 12, 13, 14. Trends in regional TC numbers based on observations can be obscured by natural climate variability, including at decadal to multi-decadal time scales, leading to conflicting conclusions on detection and attribution of TC frequency trends 2, 3, 4, 5, 6, 7. On the basis of the few decades of reliable historical data, there is no clear evidence of an observed trend in global TC numbers. TC observations have improved substantially since the 1970s, but this relatively short period of high-quality data does not provide consensus on the detection of trends or on the attribution of trends to anthropogenic influences. Before the commencement of geostationary weather satellite monitoring in the 1970s, historical global ‘best track’ records of TCs were more prone to discontinuities and sampling issues and are therefore considered problematic for climate change trend analysis 2, 3. This warming may have already impacted the number of tropical cyclone (TC) occurrences at global and regional scales, but so far changes are unclear-and often controversial-due to several confounding factors, including data quality issues that create major challenges for detection and attribution of TC trends 2. Human activities are estimated to have caused ~1.0 ☌ of global warming above pre-industrial levels, with most of the warming occurring since the mid-twentieth century 1. The declining trends found are consistent with the twentieth century weakening of the Hadley and Walker circulations, which make conditions for TC formation less favourable. ![]() It can also capture TC signatures from the pre-satellite era reasonably well. The Twentieth Century Reanalysis (20CR) dataset is used for reconstruction because, compared with other reanalyses, it assimilates only sea-level pressure fields rather than utilize all available observations in the troposphere, making it less sensitive to temporal inhomogeneities in the observations. ![]() Here, using a reconstructed long-term proxy of annual TC numbers together with high-resolution climate model experiments, we show robust declining trends in the annual number of TCs at global and regional scales during the twentieth century. Assessing the role of anthropogenic warming from temporally inhomogeneous historical data in the presence of large natural variability is difficult and has caused conflicting conclusions on detection and attribution of tropical cyclone (TC) trends.
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