Hurricanes & climate change: recent U.S. landfalling hurricanes

Here is a link to Judith Curry's blog article "Hurricanes & climate change: recent U.S. landfalling hurricanes".

As Curry points out, the basis for attribution to anthropological warming is includes models and theory that are sufficiently "incomplete" such that statistical conclusions are a joint test of the models, theory, and data.  Since we know that the models and theory are incomplete, there is little basis for attribution to anthropological warming with high confidence.

The message is that the Alarmists are Alarmists - having little reason for their claims.

Some excerpts:
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6.1 Detection and attribution of extreme weather events

Given the challenges to actually detecting a change in extreme weather events owing to the large impact of natural variability, the detection step is often skipped and attribution arguments are made, independent of detection. There are two general types of extreme event attribution methods that do not rely on detection: physical reasoning and fraction of attributable risk (NCA4, 2017),

The fraction of attributable risk approach examines whether the odds of occurrence of a type of extreme event have changed. A conditional approach employs a climate model to estimate the probability of occurrence of a weather or climate event within two climate states: one state with anthropogenic influence and the other state without anthropogenic influence (pre-industrial conditions). The “Fraction of Attributable Risk” framework examines whether the odds of some threshold event occurring have been increased due to manmade climate change.

Participants at the 2012 Workshop on Attribution of Climate-related Events at Oxford University questioned whether extreme event attribution was possible at all, given the inadequacies of the current generation of climate models (Nature, 2012):

“One critic argued that, given the insufficient observational data and the coarse and mathematically far-from-perfect climate models used to generate attribution claims, they are unjustifiably speculative, basically unverifiable and better not made at all.”

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6.2 Hurricane Sandy

Hurricane Sandy made landfall on 10/22/12 near Atlantic City, NJ. Hurricane Sandy’s most substantial impact was a storm surge. The highest measured storm surge from Sandy was 9.4 feet (at The Battery)[2]. The argument is that human-caused global warming worsened the storm surge because of sea level rise.

Curry (2018a) summarized sea level rise at The Battery. Sea level has risen 11 inches over the past century (Figure 6.1), with almost half of this sea level rise caused by subsidence (sinking of the land). Kemp et al. (2017) found that relative sea level in New York City rose by ~1.70 meters [5.5 feet] since ~575 A.D. A recent acceleration in sea level rise between 2000 and 2014 has been attributed to an increase in the Atlantic Multidecadal Oscillation and southward migration of the Gulf Stream North Wall Index. The extent to which manmade warming is accelerating sea level rise remains disputed (as summarized by Curry, 2018a).

The 2017 U.S. Climate Change Special Report (NCA4, 2017) evaluated published analyses seeking to attribute aspects related to Hurricane Sandy to human-caused global warming: e.g. sea surface temperatures, atmospheric temperatures, atmospheric moisture, and hurricane size. The analysis concluded:

“In summary, while there is agreement that sea level rise alone has caused greater storm surge risk in the New York City area, there is low confidence on whether a number of other important determinants of storm surge climate risk, such as the frequency, size, or intensity of Sandy-like storms in the New York region, have increased or decreased due to anthropogenic warming to date.”

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6.3 Hurricane Harvey

Several publications based on model simulations have concluded that as much as 40% of the rainfall from hurricane Harvey was caused by human-caused global warming (Emanuel 2017; Risser and Wehner 2017).

The rationale for these assessments was that prior to the beginning of northern summer of 2017, sea surface temperatures in the western Gulf of Mexico exceeded 30 oC [86 oF] and ocean heat content was the highest on record in the Gulf of Mexico (Trenberth et al. 2017). However, El Niño–Southern Oscillation (ENSO) and Atlantic circulation patterns contributed to this heat content, and hence it is very difficult to separate out any contribution from human-caused global warming.

Landsea (2017) summarizes the arguments for more rainfall from tropical cyclones traveling over a warmer ocean. Intuitively, rainfall from hurricanes might be expected to increase with a warmer ocean, as a warmer atmosphere can hold more moisture. Simple thermodynamic calculations suggest that the amount of rainfall in the tropical latitudes would go up about 4% per oF [7% per oC] sea surface temperature increase. Examining a 300 mile radius circle for nearly all of the rain implies that about 10% more total hurricane rainfall for a warming of 2-2.5 F [1-1.5 C]. The Gulf of Mexico has warmed about 0.7 oF [0.4 oC] in the last few decades. Assuming that all of this warming is due to manmade global warming suggests that roughly 3% of hurricane rainfall today can be reasonably attributed to manmade global warming. Hence, only about 2 inches of Hurricane Harvey’s peak amount of 60 inches can be linked to manmade global warming.Landsea (2017) summarizes the arguments for more rainfall from tropical cyclones traveling over a warmer ocean. Intuitively, rainfall from hurricanes might be expected to increase with a warmer ocean, as a warmer atmosphere can hold more moisture. Simple thermodynamic calculations suggest that the amount of rainfall in the tropical latitudes would go up about 4% per oF [7% per oC] sea surface temperature increase. Examining a 300 mile radius circle for nearly all of the rain implies that about 10% more total hurricane rainfall for a warming of 2-2.5 F [1-1.5 C]. The Gulf of Mexico has warmed about 0.7 oF [0.4 oC] in the last few decades. Assuming that all of this warming is due to manmade global warming suggests that roughly 3% of hurricane rainfall today can be reasonably attributed to manmade global warming. Hence, only about 2 inches of Hurricane Harvey’s peak amount of 60 inches can be linked to manmade global warming.

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6.4 Hurricane Irma

Hurricane Irma made landfall on September 10, 2017 as a Category 4 hurricane. Hurricane Irma set several records. Irma was the 5th strongest Atlantic hurricane on record. Irma was the 2nd strongest Atlantic storm in recorded history in terms of its accumulated cyclone energy – a function both of intensity (wind speed) and duration of the storm. Irma is tied with the 1932 Cuba Hurricane for the longest time spent as a Category 5 hurricane. Hurricane Irma maintained 185-mph winds for 37 hours — longer than any storm on record globally.[3]

Irma formed and rapidly intensified to a major hurricane in the eastern Atlantic, where sea surface temperatures were 26.5 oC (80 oF). The rule of thumb for a major hurricane is 28.5 oC. Clearly, simple thermodynamics associated with SST were not driving this intensification, but rather favorable atmospheric dynamics. In particular, wind shear was very weak. Further, the atmospheric circulation field (e.g. stretching deformation) was very favorable for spinning up this hurricane (Curry, 2017).

While the media made much ado about a global warming link to Irma’s intensity, there have been no published journal articles to date that have examined this issue. This is presumably because the sea surface temperatures during Irma’s development and intensification were relatively cool.

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6.6 Conclusions

Convincing detection and attribution of individual extreme weather events such as hurricanes requires:
a very long time series of high-quality observations of the extreme event
an understanding of the variability of extreme weather events associated with multi-decadal ocean oscillations, which requires at least a century of observations
climate models that accurately simulate both natural internal variability on timescales of years to centuries and the extreme weather events

Of the four hurricanes considered here, only the rainfall in Hurricane Harvey passes the detection test, given that it is an event unprecedented in the historical record for a continental U.S. landfalling hurricane. Arguments attributing the high levels of rainfall to near record ocean heat content in the western Gulf of Mexico are physically plausible. The extent to which the high value of ocean heat content in the western Gulf of Mexico can be attributed to manmade global warming is debated. Owing to the large interannual and decadal variability in the Gulf of Mexico (e.g. ENSO), it is not clear that a dominant contribution from manmade warming can be identified against the background internal climate variability (Chapter 4).