By Paul Homewood
I mentioned this piece of research a week or so ago. Although it is from 2013, it is still highly relevant.
New research from the Met Office has raised the possibility that man-made aerosols, industrial pollution, may have had an impact on the number of Atlantic hurricanes.
The paper, published in Nature Geoscience, suggests aerosols may have suppressed the number of Atlantic hurricanes over the 20th Century and even controlled the decade-to-decade changes in the number of hurricanes.
Researchers found that aerosols make clouds brighter, causing them to reflect more energy from the sun back into space. This has an impact on ocean temperatures and tropical circulation patterns, effectively making conditions less favourable for hurricanes.
This interaction between aerosols and clouds is a process that is now being included in some of the latest generation climate models.
Dr Nick Dunstone, a Met Office climate prediction scientist and lead author of the research, said: "Industrial emissions from America and Europe over the 20th Century have cooled the North Atlantic relative to other regions of the ocean. Our research suggests that this alters tropical atmosphere circulation – making it less likely that hurricanes will form.
"Since the introduction of the clean air-acts in the 1980s, concentrations of aerosols over the North Atlantic have reduced and model results suggest that this will have contributed to recent increases in hurricane numbers. On the other hand, the reduction in aerosols has been beneficial for human health and has been linked to the recovery of Sahel rains since the devastating drought in the 1980s."
It has long been known that North Atlantic hurricane activity has distinct long-timescale variability. Dr Doug Smith, a Met Office research fellow and co-author of the study, said: "We saw relatively quiet periods between 1900-20 and then again from 1970-80, and active periods between 1930-60 and since 1995. On average, active periods have 40% more hurricanes."
When the authors include changes in man-made aerosol emissions in the latest Met Office Hadley Centre model, which includes a comprehensive treatment of aerosol-cloud interactions, they can reproduce much of the decade-to-decade variability in Atlantic hurricane activity. This supports evidence of a link between the two.
Dr Ben Booth, a Met Office climate processes scientist and another co-author of the study says: "This study, together with work we published last year, suggests that there may be a greater role than previously thought for man-made influence on regional climate changes that have profound impacts on society."
This study motivates future international collaborative research because modelling the impact of aerosols is one of the largest uncertainties in climate science – particularly true for aerosol-cloud interactions now being incorporated in the latest generation of climate models.
Taken at face value, this study suggests the number of Atlantic hurricanes over the next couple of decades will depend on future aerosol emissions and how they interact with natural cycles in the North Atlantic.
This is the actual paper:
ABSTRACT
The frequency of tropical storms in the North Atlantic region varies markedly on decadal timescales1, 2, 3, 4, with profound socio-economic impacts5, 6. Climate models largely reproduce the observed variability when forced by observed sea surface temperatures1, 8, 10. However, the relative importance of natural variability and external influences such as greenhouse gases, dust, sulphate and volcanic aerosols on sea surface temperatures, and hence tropical storms, is highly uncertain11, 12, 13, 14, 15, 16. Here, we assess the effect of individual climate drivers on the frequency of North Atlantic tropical storms between 1860 and 2050, using simulations from a collection of climate models17. We show that anthropogenic aerosols lowered the frequency of tropical storms over the twentieth century. However, sharp declines in anthropogenic aerosol levels over the North Atlantic at the end of the twentieth century allowed the frequency of tropical storms to increase. In simulations with a model that comprehensively incorporates aerosol effects (HadGEM2-ES; ref. 18), decadal variability in tropical storm frequency is well reproduced through aerosol-induced north–south shifts in the Hadley circulation. However, this mechanism changes in future projections. Our results raise the possibility that external factors, particularly anthropogenic aerosols, could be the dominant cause of historical tropical storm variability, and highlight the potential importance of future changes in aerosol emissions.
The impact of these findings goes way beyond hurricanes. What it is effectively saying is that sea temperatures in the North Atlantic are higher now than they were during the 20thC because of reduced aerosols.
There actually should be little surprise about this. Scientists have long argued that the reduction in NH temperatures in the post-war years was due to aerosols cancelling out the effect of increased levels of CO2.
Yet this sort of air pollution did not begin in 1945. It has been around since the mass industrialisation of the mid 19thC.
How much higher higher would Atlantic temperatures have been 50 or 100 years ago, if concentrations of aerosols had been as low as they are today?
And how much of the rise in land temperatures we have seen in Europe and the US in the last few decades has been due to this aerosol effect?
via NOT A LOT OF PEOPLE KNOW THAT
October 1, 2017 at 09:21AM
Iowa Climate Science Education 