Anthony Watts recently posted another article (archived here) by one of his WUWT readers, Eric Worrall. Eric was writing about a Sydney Morning Herald article about tropical cyclones in Australia, after TC Marcia made landfall in Queensland.
Eric's blog article showed that he doesn't read much if any climate science. His article appeared under the headline:
Unsubstantiated Claim Over One Cyclone: Climate Change is “Expanding the Tropics”
The headline was wrong. First of all, the "claim" wasn't just "over one cyclone". Secondly, the fact that climates are shifting poleward is not unsubstantiated. There were references substantiating this in the WUWT article under the headline, and in the SMH article on which the WUWT article was based, and there is lots about this in the scientific literature.
How the tropics are expanding
For example, in 2007 there was an excellent "progress article" published in Nature. The article was written by Dien Seidel and co. That article describes how different disciplines describe the tropics. In particular, how climate scientists define the tropics.
...the boundaries of the tropics are not uniquely defined and vary among scientific disciplines. In astronomy and cartography, the edges of the tropical belt are the Tropics of Cancer and Capricorn, at latitudes of ~23.5 degrees north and south, where the sun is directly overhead at solstice. They are determined by the tilt of the Earth’s axis of rotation relative to the planet’s orbital plane, and their location varies slowly, predictably and very slightly — by about 2.5 degrees latitude over 40,000 years. In climatology, on the other hand, there are several indicators of the boundaries of the tropics, and they do not all necessarily yield the same location. Moreover, their positions vary by much larger amounts and much more rapidly and unpredictably than the astronomically defined tropics.
Climatologists and geographers have traditionally defined the tropics using classification systems, notably that of Köppen, based on surface temperature and precipitation patterns. ...
The authors describe various features of atmospheric circulation, for example:
...Within the Hadley circulation cell, atmospheric mass in the lower atmosphere moves towards the equator, whereas outside the cell it moves toward the poles. The latitude at which the net north–south flow is zero can be considered the poleward extent of the Hadley cell and therefore can be used to estimate the width of this tropical circulation. Within the tropical belt, surface winds generally blow from east to west, whereas in midlatitudes they blow from west to east and intensify upward from the surface to form the jet streams.
After describing these various features (not just the Hadley cells), they write:
Thus from various perspectives, climate scientists find clear distinguishing features of the tropics that can be used to estimate the width of this climatic zone. Several recent studies suggest that the tropics have been expanding over the past few decades and that this widening may continue into the future in association with anthropogenic climate change. Such an expansion of the tropical belt could have broad scientific implications and societal impacts.
The paper is written very clearly and is not too technical, which makes it suitable for the lay audience. It has a simple diagram illustrating the main atmospheric circulation patterns:
In the second IPCC report (SAR, 1995), there was mention of research suggesting poleward shifts in storm activity, though it was stated at the time to be highly uncertain, particularly in the northern hemisphere. For example, in Section 188.8.131.52 :
Hall et al. (1994) found an intensification and poleward shift in both the Northern Hemisphere storm tracks in the high resolution UKMO slab model, with the most spectacular change occurring in the eastern Atiantic/western Europe region (Figure 6.31c). Camell et al. (1996) found similar results from the UKMO AOGCM. In contrast, results from the CCC slab model (Lambert, 1995) suggest no obvious shift in the Northern Hemisphere storm tracks but a slight shortening of the Atlantic tracks suggesting a reduction in storm activity over Europe. The ECHAM l AOGCM (Konig et al, 1993) shows a northward shift of North Atlantic cyclones and an eastward movement in the North Pacific. The most significant change is a poleward shift of Southern Hemisphere cyclones in autumn and winter. Clearly, there is little agreement between models on the changes in storminess that might occur in a warmer world. Conclusions regarding extreme storm events are obviously even more uncertain.
Is it all a nefarious plot?
In his WUWT article, Eric asked a question:
My question – if the conditions promoting cyclonic activity are intensifying and expanding, why would we expect *fewer* cyclones? Why wouldn’t cyclones become more intense AND more frequent? Could this prediction of fewer cyclones be a desperate attempt to accommodate an inconvenient observation, that cyclones are becoming more infrequent – an attempt to spin a rather feeble cyclone season into a story of impending doom?
You will notice Eric's conspiratorial thinking. He is surmising nefarious intent on the part of scientists with his "desperate attempt to accommodate and inconvenient observation".
Notice also the inherent contradiction in Eric's comment. He is asking why we would expect fewer cyclones, while apparently assuming that there has already been a drop in the frequency of cyclones.
From what I've read, the scientific jury is still out on the question about frequency and intensity of tropical cyclones. There are different perspectives in the literature. Mostly, as reported in the latest IPCC report, there is general agreement that cyclones will become more intense, further away from the poles.
Tropical cyclones moving poleward
There was a paper by James P. Kossin and colleagues in Nature last year, which was summarised in ScienceDaily.com as:
Powerful, destructive tropical cyclones are now reaching their peak intensity farther from the equator and closer to the poles, according to a new study. The results of the study show that over the last 30 years, tropical cyclones -- also known as hurricanes or typhoons -- are moving poleward at a rate of about 33 miles per decade in the Northern Hemisphere and 38 miles per decade in the Southern Hemisphere.
Fewer, more intense cyclones a possibility
And there have been quite a few published papers suggesting that cyclones will become fewer but more intense. For example, as reported by the ABC in 2010, and discussed just the other day on the CSIRO website. The CSIRO article states:
The underlying warming trend of oceans around the world, which is linked to human-induced climate change, will tend to increase the risk of extreme rainfall events in the short to medium term. Studies in the Australian region point to a potential long-term decrease in the number of tropical cyclones each year in future, on average.
On the other hand, there is a projected increase in their intensity. In other words, we may have fewer cyclones but the ones we do have will be stronger. So there would be a likely increase in the proportion of tropical cyclones in the more intense categories (category 4 or 5). However, confidence in tropical cyclone projections is low.
From the WUWT comments
hunter doesn't believe it:
February 21, 2015 at 8:19 am
More climate hype liars. Tropical Cyclones go into temperate regions on occasion. Always have and always will. And the gobbledegook about stronger but less frequent but worse but rarer is simply sciencey sounding bs.
joelobryan thinks it's all a hoax
February 21, 2015 at 8:35 am
Like the GCM CMIP 3/5 ensembles, the CAGW strategy is cover every possible outcome of our ever-changing climate, warmer, cooler, wetter, drought, more hurricaanes, less hurricanes, etc. Then the Climate Change faithful always can post hoc cherry pick the result that agreed with reality. A naive, gullible, indoctrinated public accepts the pseudoscience results.
Peter Miller is another conspiracy theorist, who seems to think that someone said that cyclones have never been in evidence before. Nutty in the extreme:
February 21, 2015 at 9:25 am
Well, your chart of cyclone tracks makes a complete mockery of the uniqueness of these cyclones.
Anyhow, it’s the same old story of “this subject needs a great deal more study, so gimme lots more money now!”
If there had been no Australian cyclones this year, that would have been firm evidence of global warming, likewise so would have no cyclones or the usual number of cyclones.
Every time there is a bit of bad weather somewhere the snouts lift briefly from the climate change trough and a trotter is imperiously waved demanding more money.
Sam Wright is one of several who is only aware of the cartographic definition of the tropics, and doesn't know that climate scientists define tropical zones differently:
February 21, 2015 at 9:26 am
The tropics is an area of latitude between the Tropic of Cancer and the Tropic of Capricorn and is totally dependent on the tilt of the earth’s axis for it’s location. The angle of the tilt of the earth’s axis is slowly decreasing to the angle of about 22.5 degrees as it normally does in its cycle. The Tropics are in fact shrinking at this time and no amount of weather or climate can change that.
Seidel, Dian J., Qiang Fu, William J. Randel, and Thomas J. Reichler. "Widening of the tropical belt in a changing climate." Nature geoscience 1, no. 1 (2008): 21-24. doi:10.1038/ngeo.2007.38 (open access)
Kossin, James P., Kerry A. Emanuel, and Gabriel A. Vecchi. "The poleward migration of the location of tropical cyclone maximum intensity." Nature 509, no. 7500 (2014): 349-352. doi:10.1038/nature13278 (pdf here)