We really have not addressed Global Warming yet, but let us suppose for now that Global Warming is real. What will it mean for hurricanes? We know that sea surface temperatures (SST) are an important ingredient in the formation and maintenance of hurricanes. Nominally SST must be at least 79 F degrees to form a hurricane. We know that El Nino increases the wind speed as you go up in the atmosphere, and that is called vertical wind shear.
And that vertical wind shear is not conductive to hurricane formation, particularly weak hurricanes – the tops keep getting blown off. Of course all the effects are interrelated with each other and many others. We have already noticed that in El Nino years there is a proclivity for fewer, but stronger storms. This is a clue.
Hurricane frequency and intensity vary a lot from one year to the next. The very first article I wrote this year (which can be found at www.hurricanehunt.com) decried the inaccuracy of annual hurricane forecast as a prelude to this years forecast, one of the worst seasonal forecast. (Although this years over-forecast averages out the under-forecast of 2005 in some perverse sense of the average accuracy). Thus, how can anyone accurately forecast hurricane activity 100, 200, even 300 years hence when each year’s prognostication is of little value? It is difficult.
This highlights the tremendous year-to-year variability and even decadal highs and lows, even if there is a trend. It means the climatological trend can be determined only over many decades. Remember how active the season was from 1995 through the record breaking 2005 season. Many “professionals” claimed this was proof of Global Warming and a harbinger of imminent and even worse years. Then came 2006 though this year so far. Happily for many, very quiet Atlantic seasons, punctuated with exceptions such as Sandy.
But what does the data and theory suggest? We can look at the seminal and landmark paper published on hurricanes by Kerry Emmanuel of MIT who looked at the thermodynamics and suggested an upper limit to hurricane strength. Others have suggested a 3 % - 5% increase in hurricane strength for every 2 F degrees of increase in global temperature. All the observational inferences are based on incomplete data records, and are speculative (although frequently not presented that way).
What the data does suggest that does make some thermodynamic and dynamic sense is that the Hadley circulation will increase, that the vertical wind shear will increase, and that the jet stream will increase and this will produce on the average 1) about the same number of global hurricanes and typhoons, with a tilt toward 2) more stronger hurricanes and fewer weaker ones, and 3) an infrequently met upper limit of hurricane wind speeds of about 200 mph. That’s in several hundred years -- although there some weak evidence in this direction in the last few decades. There will always be much larger annual variations than any trend increase.