Tropical cyclones, climate, and consensus

A favorite denialist canard goes something like this: “You alarmists swore that your global warming caused Katrina and would cause more hurricanes every year, but the last few years have been a bust. Yet another algore lie exposed!” If you think I’m unfairly exaggerating, read the comments section of any major newspaper article or online forum discussing climate change.

Lest you believe that this (genuine or otherwise) confusion about the mainstream view of climate change and hurricanes is restricted to random commenters, you see something similar (although less frothy) from even mainstream science writers, like the Houston Chronicle’s science blogger, Eric Berger [emphasis mine]:

When An Inconvenient Truth came out I believed the movie to be scientifically accurate. Carbon dioxide levels were rising and so were temperatures. And hurricane activity, especially after the disastrous 2005 season, was out of control.

But a funny thing happened on the way to the end of the world: hurricane activity on the global scale is near historical lows. And the Earth seems to have, at least temporarily, stopped warming.

This, despite the fact that some of the country’s leading climate scientists say there is unequivocally a link between major hurricanes and climate change. And despite the fact that other leading climate scientists predicted 2009 or 2010 will go down as the warmest year in recorded history. Either prediction, if true, would be alarming.

Yet both of these predictions seem, at the present moment, to be off.

We’re in some familiar denialist territory here- even if there were an alleged consensus about an increase in “hurricane activity”, drawing conclusions based on a period of 2005-2009 is, well, inexcusable. Especially for a science writer. Similarly, he seems to believe that global warming should likewise be monotonic- which is to say that Mr. Berger can’t seem to understand that there can be long term upward trends despite substantial year-to-year variation.

In any event, the idea that there was even a robust, precise consensus about the current and future impacts warming is supposed to have on hurricanes (tropical cyclones, typhoons, etc.) is easily refuted by simply looking at the relevant sections of the IPCC Fourth Assessment Report, which I’ll add to the end of this post.

Whenever someone asks me “What happened to all the hurricanes?” or “What’s the consensus on climate and hurricanes?”, my stock answer is something like this-

First, in terms of a broad, robust consensus- such as those around the issues of whether or not the planet is warming, and the anthropogenic nature of that warming- one doesn’t exist for tropical cyclones. To grossly oversimplify, we could say that in a warming world- all other things being equal- we would expect warmer sea surface temperatures (SSTs) to contribute to an increase in tropical cyclone intensity. However, not all other things will be equal. While SSTs play an enormous role in cyclogenesis, they are not the only factor in play. For example, wind shear is another huge player in tropical cyclone behavior- too much wind shear can both prevent the formation of  and destroy tropical cyclones- and we expect wind shear to increase in a warming world. And of course there are “unknown unknowns”, as well as some known unknowns like the behavior of relevant ocean-atmosphere patterns (like ENSO* and the NAO) in a warmer world.

So roughly speaking, I think it would be fair to say that (setting aside the unknowns) while warmer SSTs imply an increase in intensity, increased wind shear implies a decrease in total storms, so that we’re left with the possibility of a warming world with fewer but more intense hurricanes- something that I’ve categorized in the past as a “proto-consensus”.

There have been a number of recent high profile attempts to model tropical cyclone behavior from some of the big names in the field, like Knutson 2008 and Emanuel 2008. Although Knutson 2008’s and Emanuel 2008’s findings were misrepresented by some in the media and governmental offices as evidence of a shift in opinion that GHGs had little effect on the behavior of tropical cyclones, both papers supported this proto-consensus, albeit with significant caveats and from different angles. One of the biggest areas of concern in these kinds of studies is that the models used often didn’t have the resolution necessary to look at the behavior of the strongest (Cat 3 and higher) storms, which seem to be getting stronger as we warm.

Knutson is part of a new study in Science (or here), lead by Morris Bender, which seeks to remedy this flaw entitled “Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes”. Bender et al. use a downscaling process to examine the effect of anthropogenic warming in GCMs on the behavior of Atlantic storms in an operational hurricane model. They find that while the total number of storms decreases, there is “nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century”.

Tracks for all storms reaching category 4 or 5 intensity, for the control and the warmed 18-model ensemble conditions, as obtained using the GFDL/NWS hurricane model.

This has serious implications for policy-makers, especially in the US, where the strongest storms (Cat 3+) are responsible for the overwhelming majority (80%) of economic losses from Atlantic hurricanes.

So- is this the last word? Have we reached A Consensus? Of course not- although this paper certainly adds weight to the proto-consensus, there is still a tremendous amount of uncertainty on this issue. But the next time someone tries to tell you about how “climate science got it all wrong re: hurricanes”, you’ll know more or less where things stand.

*Intriguingly, while normal El Niños tend to suppress Atlantic hurricanes, so-called Modoki El Niños are actually positively correlated with them, and as the planet warms it looks as though Modoki El Niños are becoming more common.

Relevant sections from the AR4 are below the fold:

From the IPCC AR4 WG1 section on tropical cyclones (“Box 3.5: Tropical Cyclones and Changes in Climate”):

In the summer tropics, outgoing longwave radiative cooling from the surface to space is not effective in the high water vapour, optically thick environment of the tropical oceans. Links to higher latitudes are weakest in the summer tropics, and transports of energy by the atmosphere, such as occur in winter, are also not an effective cooling mechanism, while monsoonal circulations between land and ocean redistribute energy in areas where they are active. However, tropical storms cool the ocean surface through mixing with cooler deeper ocean layers and through evaporation. When the latent heat is realised in precipitation in the storms, the energy is transported high into the troposphere where it can radiate to space, with the system acting somewhat like a Carnot cycle (Emanuel, 2003). Hence, tropical cyclones appear to play a key role in alleviating the heat from the summer Sun over the oceans.

As the climate changes and SSTs continue to increase (see Section 3.2.2.3), the environment in which tropical storms form is changed. Higher SSTs are generally accompanied by increased water vapour in the lower troposphere (see Section 3.4.2.1 and Figure 3.20), thus the moist static energy that fuels convection and thunderstorms is also increased. Hurricanes and typhoons currently form from pre-existing disturbances only where SSTs exceed about 26°C and, as SSTs have increased, it thereby potentially expands the areas over which such storms can form. However, many other environmental factors also influence the generation and tracks of disturbances, and wind shear in the atmosphere greatly influences whether or not these disturbances can develop into tropical storms. The El Niño-Southern Oscillation and variations in monsoons as well as other factors also affect where storms form and track (e.g., Gray, 1984). Whether the large-scale thermodynamic environment and atmospheric static stability (often measured by Convective Available Potential Energy, CAPE) becomes more favourable for tropical storms depends on how changes in atmospheric circulation, especially subsidence, affect the static stability of the atmosphere, and how the wind shear changes. The potential intensity, defined as the maximum wind speed achievable in a given thermodynamic environment (e.g., Emanuel, 2003), similarly depends critically on SSTs and atmospheric structure. The tropospheric lapse rate is maintained mostly by convective transports of heat upwards, in thunderstorms and thunderstorm complexes, including mesoscale disturbances, various waves and tropical storms, while radiative processes serve to cool the troposphere. Increases in greenhouse gases decrease radiative cooling aloft, thus potentially stabilising the atmosphere. In models, the parametrization of sub-grid scale convection plays a critical role in determining whether this stabilisation is realised and whether CAPE is released or not. All of these factors, in addition to SSTs, determine whether convective complexes become organised as rotating storms and form a vortex.

While attention has often been focussed simply on the frequency or number of storms, the intensity, size and duration likely matter more. NOAA’s Accumulated Cyclone Energy (ACE) index (Levinson and Waple, 2004) approximates the collective intensity and duration of tropical storms and hurricanes during a given season and is proportional to maximum surface sustained winds squared. The power dissipation of a storm is proportional to the wind speed cubed (Emanuel, 2005a), as the main dissipation is from surface friction and wind stress eff ects, and is measured by a Power Dissipation Index (PDI). Consequently, the effects of these storms are highly nonlinear and one big storm may have much greater impacts on the environment and climate system than several smaller storms.

From an observational perspective then, key issues are the tropical storm formation regions, the frequency, intensity, duration and tracks of tropical storms, and associated precipitation. For landfalling storms, the damage from winds and flooding, as well as storm surges, are especially of concern, but often depend more on human factors, including whether people place themselves in harm’s way, their vulnerability and their resilience through such things as building codes.

From the AR4 WG2 section on tropical cyclones:

Variations in tropical and extra-tropical cyclones, hurricanes and typhoons in many small-island regions are dominated by ENSO and decadal variability which result in a redistribution of tropical storms and their tracks, so that increases in one basin are often compensated by decreases in other basins. For example, during an El Niño event, the incidence of tropical storms typically decreases in theAtlantic and far-western Pacific and the Australian regions, but increases in the central and eastern Pacific, and vice versa. Clear evidence exists that the number of storms reaching categories 4 and 5 globally have increased since 1970, along with increases in the Power Dissipation Index (Emanuel, 2005) due to increases in their intensity and duration (Trenberth et al., 2007). The total number of cyclones and cyclone days decreased slightly in most basins. The largest increase was in the North Pacific, Indian and South- West Pacific oceans. The global view of tropical storm activity highlights the important role of ENSO in all basins. The most active year was 1997, when a very strong El Niño began, suggesting that the observed record sea surface temperatures (SSTs) played a key role (Trenberth et al., 2007). For extratropical cyclones, positive trends in storm frequency and intensity dominate during recent decades in most regional studies performed. Longer records for the North Atlantic suggest that the recent extreme period may be similar in level to that of the late 19th century (Trenberth et al., 2007).

In the tropical South Pacific, small islands to the east of the dateline are highly likely to receive a higher number of tropical storms during an El Niño event compared with a La Niña event and vice versa (Brazdil et al., 2002). Observed tropical cyclone activity in the South Pacific east of 160°E indicates an increase in level of activity, with the most active years associated with El Niño events, especially during the strong 1982/1983 and 1997/1998 events (Levinson, 2005).Webster et al. (2005) found more than a doubling in the number of category 4 and 5 storms in the South-West Pacific from the period 1975–1989 to the period 1990–2004. In the 2005/2006 season, La Niña influences shifted tropical storm activity away from the South Pacific region to the Australian region and, in March and April 2006, four category 5 typhoons occurred (Trenberth et al., 2007).

In the Caribbean, hurricane activity was greater from the 1930s to the 1960s, in comparison with the 1970s and 1980s and the first half of the 1990s. Beginning with 1995, all but two Atlantic hurricane seasons have been above normal (relative to the 1981-2000 baseline). The exceptions are the two El Niño years of 1997 and 2002. El Niño acts to reduce activity and La Niña acts to increase activity in the North Atlantic. The increase contrasts sharply with the generally below-normal seasons observed during the previous 25-year period, 1975 to 1994. These multi-decadal fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms first named in the tropical Atlantic and Caribbean Sea.

In the Indian Ocean, tropical storm activity (May to December) in the northern Indian Ocean has been near normal in recent years. For the southern Indian Ocean, the tropical cyclone season is normally active from December to April. A lack of historical record-keeping severely hinders trend analysis (Trenberth et al., 2007).

From the AR4 Summary for Policy Makers:

There is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

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9 responses to “Tropical cyclones, climate, and consensus

  1. I like your message here. It’s a nuanced and complicated topic, and not one given to simple statements. I’ve felt that laypeople and activists were out in front of the overall state of the literature on this one. I never saw the Al Gore movie, but I wonder if it’s related.

    • For all of the crap people have thrown at An Inconvenient Truth, surprisingly little of it actually sticks. I haven’t watched it recently, but IIRC Gore makes the point about SSTs and intensity and says something to the effect of all other things being equal. He got attacked mercilessly for allegedly saying that anthropogenic warming caused Katrina, which he absolutely did not. People like Pielke Jr. half-smeared him as suggesting that anthropogenic warming caused Cyclone Nargis (which was on par with the Haiti disaster, for some perspective) when Gore did no such thing there, either.

      Should anyone interested in learning rigorously about climate science use An Inconvenient Truth as gospel? Of course not. For a lay audience, given the constraints of the medium, I think that it does a commendable job of communicating an incredibly complex issue.

      • The book, which is virtually a script for the movie, says (p. 81):

        “A growing number of new scientific studies are confirming that warmer water in the top layer of the ocean can drive more convection energy to fuel more powerful hurricanes.
        There is less agreement among scientists about the relationship between the total number of hurricanes each year and global warming–because a multi-decade natural pattern has a powerful influence on hurricane frequency. But there is now a strong, new emerging consensus that global warming is indeed linked to a significant increase in both the duration and intensity of hurricanes.”

  2. Well, that statement is appropriately ambiguous, though the last sentence is maybe a bit stronger than it ought to be.

    It appears to me that people just make up statements and then attribute them to Gore. They do the same thing with climate model results.

    Still, some of the confusion is perhaps understandable. People don’t go and read the transcript of the movie, nor follow up by reading the cited literature. They just come away with vague impressions and recollections, and if hurricanes were mentioned in the movie, they likely won’t remember the nuance. However, I don’t excuse people in the media who easily could double-check, but don’t.

  3. ThingsBreak, thanks for this. Another resource to refer the denialists to. In a sound bit, b/c alas that is only what many people are capable of taking in nowadays, fewer but more frequent intense tropical storms expected down the road.

    I still get the impression that many ‘skeptics’ expect that we should be experiencing mass armageddon right now, well, yesterday even. They fail to recognise that most of the scenarios were developed for a world in which CO2 has doubled, and that the IPCC was not predicting armageddon to start of with. Well, we still have quite a ways to go yet until we exceed 560 ppmv– yes, we are easily going to exceed doubling CO2 b/c of procrastination. Talking of which, I should get back to work.

  4. I’m told this issue receives reasonably good coverage in Chris Mooney’s book _Storm World_, though I haven’t had time to check that out. He covers the earlier ruckus over Christopher Landsea resigning from the IPCC over his view that the lead authors were putting too much weight on Emmanuel et al.’s “alarmist” views about SST rise -> more intense storms, and conversely too little weigh on his writings about wind shear.
    (I’d welcome anyone who’s read Mooney or is more up on exactly what Landsea’s problem was to set me straight.)

  5. Pingback: Hurricane Sandy and the Climate Hens | The Way Things Break

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