Climatologist Judith Curry’s take on link between global warming to hurricanes

Full post at Climate Etc…

Well, the 12-year drought of major hurricane landfalls in the U.S. is ending with a vengeance in 2017.  Following the devastation in Texas by Hurricane Harvey, Hurricane Irma is bearing down on Florida.  Landfall is expected this weekend.  This blog post provides forecast history for Irma, and the latest predictions from my company, Climate Forecast Application Network (CFAN).  And finally, I address the debate on the link between hurricanes and global warming.

…snip…

Global warming

Ever since Hurricane Harvey, the global warming/hurricane hysteria has ratcheted up to levels I haven’t seen since 2006.

NOAA GFDL has written a good article on Global Warming and Hurricanes.  Their main conclusions:

1. It is premature to conclude that human activities–and particularly greenhouse gas emissions that cause global warming–have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations, or are not yet confidently modeled (e.g., aerosol effects on regional climate).

2. Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones globally to be more intense on average (by 2 to 11% according to model projections for an IPCC A1B scenario). This change would imply an even larger percentage increase in the destructive potential per storm, assuming no reduction in storm size.

3. There are better than even odds that anthropogenic warming over the next century will lead to an increase in the occurrence of a very intense tropical cyclone in some basins–an increase that would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity. This increase in intense storm occurrence is projected despite a likely decrease (or little change) in the global numbers of all tropical cyclones.

4. Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones to have substantially higher rainfall rates than present-day ones, with a model-projected increase of about 10-15% for rainfall rates averaged within about 100 km of the storm center.

I agree wholeheartedly with #1.  I agree qualitatively with 2-4, but GFDL has much greater faith in the climate models than I do (and of course the 21st-century projections assume substantial warming, which I don’t necessarily agree with).  However, I much prefer their model-based quantitative estimates (but they need some serious uncertainty estimates, including structural uncertainty), relative to hysterical arm waving by Mann and Trenberth using undergraduate basic thermodynamics reasoning.  There is nothing basic or simple about hurricanes.

I include this text from my 2010 post, Hurricane Katrina – 5 years later, particularly relevant given the cool SST values that Irma formed and intensified.

The relationship between sea surface temperature and hurricane intensity. The causal chain for global warming to increase hurricane intensity has been argued to occur via the increase in sea surface temperature (SST) (e.g. Curry et al. 2006 ).  A nominal SST threshold of 26.5-27^oC is used as a criterion for tropical cyclogenesis, and a nominal threshold of 28.5^oC for intensification to a major hurricane. Some new insights into the relationship between a warming climate and hurricane intensity are suggested by Hoyos and Webster (2010, forthcoming).  During the latter half of the 20^th century, the tropical warm pool defined by the isotherm of 28^oC has expanded in area.  However, the region of tropical cyclogenesis has not expanded, owing to the area of convective activity remaining nearly constant.  Hoyos and Webster argue that the temperature threshold for tropical cyclogenesis increases as the average tropical ocean temperature increases.   It is the increasing intensity of the convection with warmer temperatures that seems to be the link between SST increase and hurricane intensity, rather than the absolute value of the SST itself.  Further, the location of the intense convection is related to the difference between the local SST and global tropical average SST, rather than to the absolute value of the SST itself (Vecchi et al. 2008)

This is particularly relevant for interpreting the formation of extremely strong hurricanes in the early half of the 20th century when SSTs were significantly cooler.

At some point, probably in the 2020s, the Atlantic Multidecadal Oscillation (AMO) will flip to the cool phase (which typically lasts several decades), which is associated with low Atlantic hurricane activity.  At this point, natural variability (interannual, decadal, multidecadal) is swamping any potential signal of global warming on hurricanes.

After I wrote the above article, I spotted this article by Chris Mooney:  The science behind the strange hurricane ‘drought’ — and its sudden end.  This graph illustrates the level of disagreement and faulty reasoning on this whole topic:

“A debate continues about why the 1970s and 80s were relatively quiet in the Atlantic,” said MIT hurricane researcher Kerry Emanuel. “Some believe that it was the consequence of a natural climate oscillation called the Atlantic Multi-decadal Oscillation (AMO), while others, including me, think is was mostly a consequence of sulfate aerosol pollution.”

I guess Kerry Emanuel didn’t read the IPCC AR5 or any of the more recent research, showing that sulfate aerosol forcing of climate (generally, and also mid-20th century) is much smaller than thought circa the AR4.  Not to mention the massive amounts of sulfate being emitted by Asia in recent decades.

p.s.  Cliff Mass has a new blog post: Global Warming and Hurricane Harvey, well worth reading.

Recommend reading the full post at Climate Etc.