Vad som däremot finns är studier över orkanaktivitet i långa tidsperspektiv, efter de spår de lämnar i sedimenten. Intressant därför det då rör sig om större temperaturvariationer. Samma resultat. Inga dramatiska förändringar. Jag har för mig det var en svensk forskare inblandad, men hittar inte referensen nu…

Den förhatliga RC diskuterar Landseas artikel som Christoper nämner:
http://www.realclimate.org/index.php/archives/2008/02/tropical-cyclone-history-part-i-how-reliable-are-past-hurricane-records/langswitch_lang/sw
RC tar Landseas andra diagram och utsträcker det till perioden före 1900, då andelen landorkaner var lägre än senare.

Det här bör man ha i åtanke när man hör om ”rekordantal” cykloner i modern tid eller läser diverse studier som letar trender.

Ett försök att komma runt detta har Chris Landsea gjort med en uppskattning. Han antog, ganska rimligt, att andelen orkaner som når land är någorlunda konstant. Dessa är ju svåra att missa och statistiken betydligt brtydligt bättre. Han fann att proportionen landorkaner var mycket högre före 1964. Följdakligen saknas många därför i statistiken tidigare. Inte konstigt att många hittar en uppåtgående trend… Se här. Och även denna bild.

”In agreement with previous studies, the frequency and intensity of the strongest hurricanes simulated by the model are increased for both surface wind speed and central pressure. More than twice as many hurricanes occur with wind speeds exceeding 45 m s- 1 in the warm-climate runs (twelve versus five). The normalized frequency distributions show, for the warm climate, an increased relative occurrence rate for all storm intensity categories above 35 m s- 1.”

Färre men värre, typ. Totalt bedömer man dock att potentiell förstörelsekraft (PDI) går ner på grund av totalt sett lägre frekvens med vissa förbehåll:

”However, simulated PDI decreases by about 25%, owing to the decrease in tropical storm and hurricane frequency. As both the frequency of major hurricanes and the maximum intensities of the strongest hurricanes are under-predicted in the control model, results related to the most intense simulated storms should be regarded as only suggestive.”

Knutson et al refererade för övrigt till Holland & Websters artikel. Bra diskussion i frågen mer av detta

OBS: Wikipedia har en jättebra översikt om tropiska cykloner:

http://en.wikipedia.org/wiki/Hurricanes#Global_warming

THOMAS R. KNUTSON, JOSEPH J. SIRUTIS, STEPHEN T. GARNER, GABRIEL A. VECCHI AND ISAAC M. HELD (2008). Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience. 18 May 2008.

(Abstract)

Increasing sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950, raising concerns that future greenhouse-gas-induced warming could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions. Here we assess, in our model system, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.

Ovanstående bekräftas även av William Gray vid Colorado State University och Chris Landsea vid NOAA.

Christopher; lite ”nys” i frågan:

Holland & Webster. Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Philosphical Transactions of The Royal Society November 15 2007

(Abstract)

We find that long-period variations in tropical cyclone and hurricane frequency over the past century in the North Atlantic Ocean have occurred as three relatively stable regimes separated by sharp transitions. Each regime has seen 50% more cyclones and hurricanes than the previous regime and is associated with a distinct range of sea surface temperatures (SSTs) in the eastern Atlantic Ocean. Overall, there appears to have been a substantial 100-year trend leading to related increases of over 0.7°C in SST and over 100% in tropical cyclone and hurricane numbers. It is concluded that the overall trend in SSTs, and tropical cyclone and hurricane numbers is substantially influenced by greenhouse warming. Superimposed on the evolving tropical cyclone and hurricane climatology is a completely independent oscillation manifested in the proportions of tropical cyclones that become major and minor hurricanes. This characteristic has no distinguishable net trend and appears to be associated with concomitant variations in the proportion of equatorial and higher latitude hurricane developments, perhaps arising from internal oscillations of the climate system. The period of enhanced major hurricane activity during 1945–1964 is consistent with a peak period in major hurricane proportions.