by Andrew Sibley, Daily Sceptic:
The Atlantic hurricane season has come to an end, and the global warming believers have been looking for records to ascribe to global warming. According to a NOAA news report, the end-of-season flourish of cyclone activity was as predicted, and a record for the period. The BBC Weather commented that the activity corresponded with Atlantic sea surface temperatures (SST) in 2024 being 1°C above the 30-year average (1991-2020) – a year which notably began with a warm Pacific El Niño event. Warm sea surface temperatures are of course an important factor in the development of tropical cyclones, but not the only one: for example, weak wind shear in the upper troposphere is also necessary for organised hurricanes to develop.
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The increase in the North Atlantic sea surface temperatures is usually considered a feature of the Atlantic Multidecadal Oscillation (AMO), sometimes referred to as a variability (AMV). However, one notable climate scientist, Michael Mann, has tried to dismiss the AMO by flattening the historic curve so that the warmer ocean and increased hurricane activity becomes attributable to global warming instead.
The impact of the natural AMO on the climate was briefly discussed in a recent article, along with a discussion of other factors such as changes in the El Niño Southern Oscillation (ENSO), volcanic activity and air pollution. The AMO, which is measured between the equator and 70°N, is arguably more influential on the global climate than ENSO, because of its longer duration. It leads to annualised variation in sea surface temperatures of up to 0.8°C over a period of about 60 to 80 years. During the 1970s there were fears of the return of an Ice Age as Atlantic Ocean waters cooled. Then from the mid-1990s onwards a recovery in the Atlantic sea surface temperatures was observed. This correlated with and contributed to the rise of the global warming narrative. The transition from the cold to warm phase of the AMO also occurred at a time when detailed satellite measurements of several climate parameters became available. The AMO index has continued positive since.
Scientific thought suggests that changes in the AMO are associated with variability in the Atlantic Meridional Overturning Circulation (AMOC). This larger scale current, which is part of a global thermohaline circulation (a slow-moving system of ocean currents), initially carries warm surface water northwards from the Caribbean Sea. As the water gathers near Greenland it cools and sinks due to an increase in water density. The very slow current then flows as a deep ocean stream to the South Atlantic, before rising back to the surface and completing the circulation via a northward return flow to the equator. The whole process takes about 1,000 years. (The AMOC has also been in the news recently because of fears it might collapse the Gulf Stream and lead to colder winter weather across the U.K.)
Comparisons of the AMO index against the lower atmospheric temperature (from Dr. Roy Spencer’s UAH V6.0 satellite temperature dataset from 1979) shows a correlation between the two. Given this apparent connection to global temperatures, a number of research papers have tried to put a figure on the actual influence of the AMO upon the climate. A paper by Tung and Zhou in 2013 suggested that its influence on global warming was of the order of 40% over a 50-year period, i.e., it explains nearly half of the warming trend. The anthropogenic (human-caused) temperature increase is then given in the paper as less than 0.5°C over this period. Chylek et al. in 2014 suggested a slightly lower percentage, arguing that about one third of the warming since 1975 was caused by the AMO. The paper commented that: “The anthropogenic effects account for about two thirds of the post-1975 global warming with one third being due to the positive phase of the AMO.” So, from this we have an approximate warming contribution from the AMO of 33% to 40% by 2014.
