Guest Commentary from Urs Neu
While analyzing tropical cyclone records is difficult enough (see ‘Tropical cylone history – part I’), it is even more challenging to reliably estimate hurricane activity back in time. Recently, Nature published an attempt to reconstruct past major hurricane activity back to 1730 (Nyberg et al. 2007). The authors concluded that the phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity in the record and thus appears to represent a recovery to normal hurricane activity. The paper was advertised in a press release put out by Nature and received broad media attention.
Although the approach outlined by the authors is interesting, the study contains in my view a number of problems, as outlined in a comment published in Nature today (Neu 2008):
The authors use a couple of coral records and a marine sediment core from the Caribbean to reconstruct first wind shear and then major hurricane activity in the tropical Atlantic. First they find a good correlation of their proxy records to wind shear measurements in the mean hurricane development region (MDR). There are two interesting features here: the coral proxies show a negative correlation to wind shear over the MDR, but a positive correlation north of it. Thus, in relation to hurricane activity there is an opposite effect of wind shear in different regions (Fig. 2 of Nyberg et al.). We’ll come back to that later. The sediment proxy shows a positive correlation to wind shear over the MDR and no correlation north of it. Thus, since the two proxy records are correlated to wind shear in the opposite direction, one would expect that the proxies show opposite patterns and trends. This is more or less true for the period 1950-1990 used to calibrate (or ‘train’) the statistical model. However, for the preceding 230 years, the long-term trend is the same for both proxies. This clearly indicates, that at least one of the proxies has a wrong long-term trend, because they show an opposite long-term trend of wind shear. Thus the long-term trend of a reconstruction using both proxies does not seem very reliable. Nyberg et al. did not comment on this basic problem in their reply (Nyberg et al. 2008).
For the reconstruction of past major hurricane activity they use these two proxies again together with a proxy for SSTs. Besides the problem of two proxies with opposite long-term trend, the coral proxy has, as mentioned, an opposite correlation to wind shear within and outside the MDR. Since 1944 about 50% of major Atlantic hurricanes have reached major hurricane strength only outside the area of positive correlation (in the area north of 20ºN between 50 to 75ºW and north of 25ºN outside this section), i.e. in an area with no or an inverse correlation to the proxies. Since wind shear seems to be high in this area at times when it is low in the MDR, the relation of major Atlantic hurricane frequency to the coral proxy does not seem so evident from physical considerations. Moreover, the fraction of major hurricanes observed outside the MDR varies with time (Figure 1) and might have changed over the last decade, which further complicates the relationship.
Figure 1. Major hurricanes outside the MDR. The annual number of major hurricane tracks where major hurricane status is only reached north of 25ºN or north of 20ºN between 50-75ºW (red line) and the total annual number of major hurricanes in the Atlantic (blue line), shown both as 5-year-running mean. Data is from the NOAA National Hurricane Center best track data set (HURDAT).
Although the correlation patterns presented by Nyberg et al. suggest a good correlation of their Caribbean proxies to wind shear over a large part of the Northern tropical Atlantic, this does not mean automatically that the correlation of the proxies to total Atlantic major hurricane activity is representative for the whole Atlantic as well. Firstly, hurricane activity is influenced by other factors than wind shear (which might vary over different areas), and secondly the effect of wind shear evidently is opposite in different regions.
There is a correlation of the proxies to major Atlantic hurricane activity over the period 1946-1995, however, the large activity increase already starts more than 5 years earlier in the proxies than in the hurricane record. The authors explain this time lag by the El Niño in the early 1990ies, however, since the influence of El Niño partly works over wind shear, this influence should be already included in the proxies.
Neither the reconstruction nor the calibration period covers the strong increase in major hurricane activity in the 1990ies, but it is obvious from observed wind shear, and recognized by the authors, that this increase cannot be explained by wind shear and is probably due to the increase in SSTs as several studies have pointed out (e.g. Hoyos et al. 2006). Since the training period of their reconstruction model covers a phase, where the main variation of hurricane activity seems to be mostly through wind-shear (1946-1990), but not the following phase, where SST changes are the most important factor, the model probably does not represent the influence of SSTs very well.
Now we come back to the question of the reliability of the hurricane record. One of the big problems of the Nyberg et al. reconstruction is, that it is far off the hurricane record before 1944 (Figure 3 in Nyberg et al). For the period 1850-1944, the reconstruction shows more than 3 (about 3.3) major hurricanes per year on average, while the hurricane record has less than 1.5 per year. Nyberg et al (2007) explain this discrepancy by the unreliability of the hurricane record. This explanation seems too simple. There are, as discussed above, uncertainties in the hurricane record. These are, however, not infinitely large. I have tried to estimate an upper error bar for major hurricane activity. Since there are no estimations of underreporting biases of major hurricanes, I assumed a constant proportion of major hurricanes to total tropical storm number. As Holland and Webster (2007) have shown, this proportion has some multidecadal variation, but these variations are not very large on the 50 year time scale, and there seems to be no significant long-term trend. I used the highest reporting bias estimations of tropical storms by Landsea (2004, 2007) – which are probably too high, as discussed in ‘Tropical cyclone history – Part I’, i.e. 1851-1885 plus 3 tropical storms per year on average; 1885-1965 plus 2; 2003-2006 minus 1; see Figure 2, black line).
Figure 2. Major hurricanes compared to the total number of tropical storms. The red line shows the 5-year moving average of the ratio between major hurricanes and the total number of tropical storms calculated from the best track data of the NOAA National Hurricane Center (HURDAT). The black line shows the ratio after correction of the observational bias proposed by Landsea et al. The green line shows the ratio after correction for a possible observational bias of major hurricanes before 1910.
Nyberg et al. claim that Landsea suggests 0-6 additional storms per year before 1885 and 0-4 per year before 1900 and thus the ‘upper limit’ correction should be 6 and 4 storms, respectively. However, the range given by Landsea represents the annual variation of the bias, not an uncertainty range of the average (I very much doubt that Landsea thinks a zero correction on average to be possible).
The average ratio from 1910 to 1965, i.e. before the satellite area, is the same as after 1965 (21%). Thus there is no evidence to assume a significant underestimation of major hurricanes. However, for the period before 1910 a correction of plus 1 major hurricane per year is needed to attain a similar mean ratio (21% major hurricanes) as in the satellite period (Fig. 2, green line). The adjusted record has a mean of just 2 major hurricanes per year between 1851 and 1940. Thus even if taking into account a high observation bias, the Nyberg et al (2007) reconstruction overestimates major hurricane frequency before 1940 by at least 50%. Observation uncertainties therefore do not explain the mismatch of reconstruction and observation as supposed by the authors.
Even if using an extreme correction of plus 5 TCs before 1900, the corrected record does not overlap the error bars given by Nyberg et al (2007). Or, looking the other way round, to get major hurricane frequency as shown by the reconstruction, the observational bias prior to 1900 would have to be in the order of 10 tropical storms per year on average, which is not very likely.
In summary, there are serious doubts about the representativeness of the proxies used by Nyberg et al., and there is a clear mismatch of reconstruction and earlier observation. Therefore the conclusions drawn in that paper are on very weak ground.
Well, what can we conclude from the discussion: Paleo-tempestology is a brand new field of study and there is undoubtedly a long way to go before the reconstruction of extreme events (like hurricanes) in the past will be anything more than suggestive. However, there’s a lot more data out there waiting to be collected and analysed.
References:
Holland, G.J., and P.J. Webster (2007): Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Philos. Trans. R. Soc. Ser. A, 365, 2695– 2716, doi:10.1098/rsta.2007.2083.
Hoyos, C.D., P.A. Agudelo, P.J. Webster, & J.A. Curry (2006). Deconvolution of the Factors Contributing to the Increase in Global Hurricane Intensity. Science, 312, 94-97.
Landsea, C. W. (2007), Counting Atlantic Tropical Cyclones Back to 1900. EOS, 18, 197-208.
Landsea, C. W., C. Anderson, N. Charles, G. Clark, J. Dunion, J. Fernandez-Partagas, P. Hungerford, C. Neumann, and M. Zimmer (2004), The Atlantic hurricane database re-analysis project: Documentation for the 1851–1910 alterations and additions to the HURDAT database, in Hurricanes and Typhoons: Past, Present and Future, edited by R. J. Murname and K.-B. Liu, pp. 177–221, Columbia Univ. Press, New York.
Neu, U. (2008): Is recent hurricane activity normal? Nature, 451, E5 (21 February 2008)
Nyberg, J., B.A. Malmgren, A. Winter, M.R. Jury, K.H. Kilbourne & T.M. Quinn (2007): Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature, 447, 698-701.
Nyberg, J., B.A. Malmgren, A. Winter, M.R. Jury, K.H. Kilbourne & T.M. Quinn (2008): Reply to ‘Is recent hurricane activity normal?’ Nature, 451, E6 (21 February 2008)
RE #45, I’m thinking if the tables are turned here and we’re in the future with GW going full force, say 2C increase, and we take “GW increases hurricane intensity” as our null hypothesis (as environmentalists and policy-makers and potential & actual victims of GW should), then we would also have just as a serious problem in proving GW is NOT causing the landfalling hurricanes that hit densely populated, expensive property areas & cause enormous damage, harm, and expense.
And since it is the GHGs we’re emitting today (& yesterday) that will contribute to this scenario in the future, then in addition to building better hurricane-proof buildings and higher levees, we should also be seriously reducing our GHGs right now.
I understand that scientists and lawyers defending clients against being sued for GW need to have high standards in avoiding false positives, but the rest of us don’t need such high standards. Especially when reducing GHGs here in the U.S. by 75% could save us a great deal of money without lowering living standards or productivity (some of that money could then be plowed into building more hurricane-proof buildings). With new compressed air car tech on the horizon to supersede the hydrogen concept, it looks like we may be able to reduce GHGs even further; see:
http://www.youtube.com/watch?v=QmqpGZv0YT4
http://green.yahoo.com/blog/ecogeek/66/air-car-ready-for-mass-production.html
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RE “the return period is more than 200 years” is that 200 days?
Sorry Dhogaza, I was slacking on ornithological erudition. I was actually referring to sunbirds, that are not quite adept at hovering and can not fly backwards. They’re also larger, therefore easier to spot and identify, even without binoculars, which are not very useful in the forest most of the time :)
Chris S (46) — Drought in central Chile, attributed to La Nina. Recent flooding in several countries in southeastern Africa.
That is all I noticed, following the news daily. (Try BBC)
Re: 46. Part of what is happening is that the Great Lakes are bleeding themselves out. Less ice cover in the winter and warmer Lake waters attributed to the warming climate have increased surface evaporation and are producing more lake effect snow. Unfortunately a lot of this snow falls out of the Lake’s watersheds. The upper Great Lakes are at near record low water levels.
Re #47 and the appearance of Reid Bryson on Bob Clipperton’s list.
Something I have wondered about: Back in the 70s, Reid Bryson was perhaps the best-credentialled of those worrying about The Next Ice Age arriving soon. Pat Michaels got his Ph.D. in ecological climatology from the University of Wisconsin. Although I have looked at both bios, I have found no indication of whether Michaels was one of Bryson’s Ph.D. students. Just curious.
Best Regards.
Jim Dukelow
Re #55: Jointly authored papers from the time are usually a good indication.
Re #16: Hank, the fossil fuel industry and similar very much are investing in these denialist information outlets. They don;t get direct dividends, but from their point of view delay = $. Such investments are arguably their most lucrative. What’s odd is that ICECAP would describe the investments in such terms. OTOH maybe their use of the term can be explained by the extreme libertarian POV that virtue tracks self-interest.
RE: The Daily Tech News Cool down article. “Warning: The Content in this Article May be InaccurateReaders have reported that this story contains information that may not be accurate.”
LOL! Yeah.
RE 57: Right, they feel the message warrants private support to save the world economy from CO2 taxes. This is the fear. It’s a State of Fear in those circles.
Anyone care to comment on a new study that says we have to go to zero emissions to stabilize our climate — from NEW SCIENTIST, but I read it on ClimateArk at http://www.climateark.org/shared/reader/welcome.aspx?linkid=93768
Lynn… Hansen has recently presented his expectation that CO2 levels must be brought below 350ppm and maybe as low as 300ppm for the long term health of our planet. This will require that we must go below zero emissions and begin to actively sink CO2 as soon as possible. See his Feb08 pdf or power point on his web site. http://www.columbia.edu/~jeh1/ I understand he has a paper in the works backing up his statements.
Re # 60 Lynn Vincentnathan
The New Scientist article merely summarized a paper by Damon Matthews and Ken Caldeira published in Geophysical Research Letters:
Only zero emissions can prevent a warmer planet
13:49 29 February 2008
NewScientist.com news service
Kate Ravilious
Greenhouse gas emissions will have to be eliminated completely to stabilise the Earth’s climate and prevent temperatures from rising. That’s the conclusion of climatologists in the US who say that our current efforts to merely stabilise emissions will not be enough.
Damon Matthews, from Concordia University in Canada, and Ken Caldeira, from the Carnegie Institution for Science, Stanford, USA, used a global climate model to study how greenhouse emissions would need to change in order to stabilise global temperatures over the next few hundred years. Previous studies have only looked at what happens when emissions are stabilised…So far industrial emissions total around 450 billion tonnes. “Even if we eliminated carbon dioxide today we are still committed to a global temperature rise of around 0.8 ºC lasting at least 500 years,” says Caldeira…Roger Pielke, a climate policy expert at the University of Colorado in Boulder, agrees with the findings. “This research makes the case that simply stabilising concentrations is insufficient to stabilise temperatures. Their argument, if widely accepted, raises the bar on what it means to mitigate climate change,” he says.
There isn’t any lack of good data about hurricanes making landfall in the US. Until someone can demonstrate that hurricanes have for some reason become less likely to hit the US, this argument and discussion is a ruse.
A prominent Australian economist came to that conclusion as well as he handed our current gov his interim paper of the economic effects of climate change and pressed our gov for a 90% reduction in emmissions by 2050 intead of the proposed 60% limit to avert a disater for us economically stating that australia is extremely vunerable to the worsening effects of CC. I was heartened to hear a few weeks back that the head of GM in america said that future cars will have to be significantly cleaner and that the days of the big ‘yank tank’ were numbered and that new fuel sources will have to be discovered or improved upon. Here a leading US corporation is speaking up and saying we will have to have radical change in the products it sells. Anything less than almost total global cessation in the burning of fossil fuels is the only way to eventually mitigate CC and to prevent uncontrollable +ve loops from happening.
The concept of “stabilizing” the climate implies two things.
1. The climate is unstable
2. The climate is normally stable without human interference.
There is little evidence to support either or both of those ideas. For example, essentially all of the long term GISS data shows that Greenland was warmer in the 1930s. Particularly in the areas which are now experiencing the most melt.
http://data.giss.nasa.gov/cgi-bin/gistemp/gistemp_station.py?id=431042500000&data_set=1&num_neighbors=1
I know this sounds stupid but, are there any peer reviewed papers that definitively show that CO2 drives temperature? If so, will they be presented at the skeptics convention in New York?
pbview (66) — You might care to start looking into the literature with the amusing
http://climateprogress.org/2008/02/11/how-do-we-really-know-humans-are-causing-global-warming/
but also the IPCC AR4, linked in the Science Links section of the sidebar.
Patrick Henry (65) — The same advice, but you might also care to start reading with The Discovery of Global Warming, first link in the same section of the sidebar.
RE #65, I think they are looking at it from a “what’s best for human and other life” POV. I don’t think anyone here actually suggests that the climate is static. There have been extreme global warming hysteresis cases in the past and even without human intervention one would expect such sometime in the future. For instance, 95% of life on earth died 251 mya during the end-Permian extinction.
But since we people have been smart enough to develop the technology that may be triggering a new global warming hysteresis wipe out of life on earth, you’d think we might be smart enough to halt or reduce this. Time will tell.
For me these statistics appear not too interesting. Few data points, noisy observation quality. Interesting as discussion topics, but nothing reliable can come out of them as such.
How about the basic physics that generate the storms? So far I have found that there are about 9 preconditions for a hurricane to form. Right season, suitable latitude bands, oceanic environment, adequate surface water temperature, depth of warm water, high humidity profile (no dry layers), low wind shear, presence of an easterly wave, possibly a low CCN (dust) count.
Several of these factors relate to what happens in Africa. Recent changes include drying out of Lake Chad in central Africa, which turned an intensely evaporating surface into a dust bowl. Some 25000 sqkm (in 1960) diminished into 1600 sqkm now …
I saw recently a report saying that the tropical band has expanded by about 4 degrees. In West Africa there is a long linear east-west coastline, roughly matching the northern edge of the tropical band. Band expansion turns some relevant precursor processes from oceanic into coastal/inland types. In general the change in that area appears uncertain as climate models can not decide on a trend on humidity/rain (or can they?).
Some 8000 years ago when the global temperatures were about 2 degrees warmer, Sahara was a grass savanna with lakes and rivers. There even was a huge Lake Super-Chad (one million sqkm), some 60 times larger than today. Recent trend is the direct opposite, a drier climate despite of general warming. A different input, though. Then it was high solar intensity, now it is strong greenhouse effect and major land use change.
Pacific storms should be in some respects different. There is little impact from a large, dry continent up-stream.
Re 69
The basic physic is pretty straightforward to me, atmospheric instability, a parcel of relative cold air situated above a relative warm surface is all it takes to form an hurricane like whirlwind.
Picture of an arctic hurricane:
http://nsidc.org/arcticmet/patterns/polar_low.html
“In satellite imagery polar lows show characteristic spiral or comma shaped patterns of deep clouds, sometimes with an inner “eye” similar to those seen in tropical cyclones. Convective cloud bands occupy the surroundings (see figure below). Analysis of aircraft and radiosonde data collected during field experiments reveals that polar lows may possess warm cores. This finding, coupled with their appearance in satellite imagery, has prompted some investigators to refer to polar lows as “arctic hurricanes,” although they seldom, if ever, possess hurricane strength winds”
Pictures of bone dry whirlwinds possibly sucking up hurricane inhibiting dust:
http://en.wikipedia.org/wiki/Dust_devils
Roger Pielke Jr is at it again, this time via the LA Times-
http://www.latimes.com/news/science/la-sci-adapt26mar26,0,4227673.story?page=1
His “research” (published in Natural Hazards Review) demonstrates that hurricanes are not getting more destructive, and that six of the ten most destructive storms occurred before 1945. The article also refers to “23 inches of sea level rise this century” under business as usual as though that is the current expectation.