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Cold periods had fewer storms, natural ocean cycles the largest cause of hurricane variability.

I've always been skeptical of the view that global warming leads to stronger hurricanes. The argument behind it seems overly simplistic -- hurricanes feed off warm water, so warmer sea surface temperatures will lead to more frequent and powerful storms.

Elsewhere in our solar system, the exact opposite seems true. Blisteringly hot Venus, for instance, seems to have extremely weak storm activity, whereas icy Jupiter has massive hurricanes that last centuries, large enough to swallow the entire earth several times over. This is only suggestive rather than conclusive, but clearly there's more to storm activity than just raw temperature.

Basic thermodynamics helps to explain why. Storms are essentially large heat engines. It's not temperature that drives a heat engine, but differences in temperature. Global warming, which warms the poles more than the equator, would seem to decrease the latitude-based differential that helps drive a hurricane's rotational energy.

So went my pet theory, at least— but no hard data supported it.

However, the other side had no hard data either. While climate modelers claimed global warming might strengthen storms, actual hurricanologists were adamant that no actual evidence existed. Some pointed to research on wind shear, which suggested that a warmer climate would reduce the conditions that allow hurricanes to form, despite warmer surface water.

In 2005, one hurricanologist, Emmanuel Kerry, broke ranks and claimed to have actual proof that global warming increased hurricanes. For this, Time Magazine quickly named him "Man of the Year". However, last year Kerry publicly recanted his view, admitting that his earlier work was flawed.

With Kerry's renunciation, hurricane scientists were unanimous in their view that global warming wouldn't lead to measurably stronger storms.

But could it do the reverse? Could global warming actually reduce hurricane activity? A pair of Chinese researchers now says this very well may be true, at least for some parts of the earth.

The researchers, using a new branch of science they call "paleotempestology", looked backwards through several thousand years of the earth's history. Using sedimentary deposits, core samples from caves, and other geological proxies, along with documented historical records of hurricane landfalls, they built the longest record of hurricane activity ever constructed. They then correlated it to the varying temperature at each period..

On the global level, the researchers found no link between climate and hurricane activity. Surprisingly, though, cold periods such as the Little Ice Age had the most hurricanes, at least in some regions, a result the team said "begs adequate explanation".

However, the study found a strong link between natural patterns such as El Nino and hurricanes, a clear pattern of rising and falling activity on decadal time scales. These oscillations, known as "ENSO", tended to suppress and enhance hurricanes on a regular cycle, with the cooler "la Nina" years having the most activity.

The research was published in the Chinese Science Bulletin, and can be viewed here.

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How Hurricanes Work
By BigT383 on 1/14/2009 7:53:35 PM , Rating: 4
I'm not doubting the work of the Chinese Scientists, but I'd like to take a moment to discuss your theory and how hurricanes work. Yes, hurricanes are heat engines, but it's not the temperature difference between latitudes (North-South) that drives them, it the temperature difference between the sea surface and the upper atmosphere.

Warmer seas mean that water evaporates from them more rapidly, and thus air traveling over warm water gets full of water vapor. When this air rises into the upper atmosphere and cools - the air is thinner and as a gas expands it cools - the water condenses into clouds and releases stored energy as heat (which is why tropical cyclones are "warm core" storms), lifting the air further (through convection) and creating a void down below which must be filled with more and more air, which drags in more moisture from the sea, etc. etc.

Hurricanes do aquire a spin- not from temperature difference but from trade winds and the Coriolis force. The spinning stabilizes the system like a top, allowing the convection to centralize around an "eye" and strengthen. This is why the direction of spin depends on weather it's in the north or south hemisphere, and why these storms have trouble spinning up if they're too close to the equator where the Coriolis force is weak. Were the spin due to a temperature difference between latitudes, you'd expect random spin directions or, really, no spin at all since once the air moved north (or south), there would be no mechanism to get it back and complete the circuit.

Storms on Earth generally die out when either they hit land (and the water source is cut off) or they move over water temperatures too cold to support them. Jupiter has neither of these problems, since it heated primarily from within and not by the sun (It's actually quite warm, depending on your altitude!), and has no land. Though, we're not sure what exactly powers the storms on Jupiter so it's hard to compare them to terrestrial hurricanes.

Venus has another problem - no heat transport medium. The clouds are sulfuric acid, and it's way too hot on the surface for any precipitation to ever reach the ground and create seas, which means no evaporation-convection-condensation, which means no hurricanes.

RE: How Hurricanes Work
By BigT383 on 1/14/2009 8:04:51 PM , Rating: 2
I'd also like to add that a good source of information for Hurricanes and Climate Change issues is Dr. Jeff Master's blog over at the Weather Underground:

Here, specifically are two articles I found in the archive that discuss potential Hurricane Strength/Global Warming connections:

RE: How Hurricanes Work
By masher2 on 1/14/2009 9:11:52 PM , Rating: 3
My apologies; I was a bit too brief in the article. Latitude-based temperature differentials don't cause the vertically rising air which is the initial pump for cyclone formation, but they do power the jet streams and ocean currents which have a huge impact on both hurricane genesis and movement.

The Coriolis force is zero at the equator. Weak circulatory storms thus form close to it then, if conditions are right, are steered by the Tropical Trades -- wind patterns that don't exist without latitudinal temperature differentials. Nor do the strong ocean currents exist, which many also think play some role in cyclone formation.

Rising air provides the initial source of energy -- but that motion is vertical. The exact conditions which convert that into a strongly organized horizontal circulatory pattern are still largely unknown, but it seems a fair bet to believe that horizontal wind and ocean flows are involved. After all, if one models a ocean-atmosphere system with only a vertical 1D temperature differential, cyclones don't form at all.

Furthermore, even on a vertical basis, GW is expected to reduce temperature differentials. GW predicts tropospheric amplification (about 1.3X as much warming as the surface receives), and it is the all-important upper troposphere where condensation and cloud formation occurs.

And yes, comparing the weather of any other planet to Earth is dicey indeed. But the fact remains that the data we have today strongly suggests that global warming will not result in measurably stronger cyclones.

RE: How Hurricanes Work
By BigT383 on 1/15/2009 1:04:05 AM , Rating: 3
I understand that the point of this article is to report the findings of the Chinese scientists, and I think it does a good job. :-) </disclaimer>

IMHO, I'm not convinced either way by these arguments because tropical cyclone forecasts for a given season aren't that accurate yet even when made at the beginning of the season. And though we can predict the path of a given cyclone with fairly good accuracy, strength forecasts days or even a few hours out still leave a lot to be desired.

If global warming does indeed raise temperatures in the upper atmosphere more than at the surface, as you say, will the decrease in temperature difference weaken storms, or will the increase in pressure difference strengthen storms (I would think that higher temperatures above would I believe result in lower pressures at altitude, but I'm not sure)? What effect will it have on wind shear, which can make or break a hurricane? Lower wind shear over hurricane generating regions during a season would significantly increase the number and intensity of storms formed.

As always, more research is needed!

RE: How Hurricanes Work
By ang sang on 1/15/09, Rating: 0
RE: How Hurricanes Work
By FITCamaro on 1/16/2009 9:16:57 AM , Rating: 1
So I guess the dinosaurs were wiped out by global warming from all the methane coming from their massive shits then?

Christ you're stupid.

RE: How Hurricanes Work
By masher2 on 1/15/2009 10:04:42 AM , Rating: 3
> "IMHO, I'm not convinced either way by these arguments."

Oh certainly. I didn't intend to suggest any of this was definitive proof, but rather simply one possible explanation for the results these researchers have obtained.

> "Lower wind shear over hurricane generating regions during a season would significantly increase the number and intensity of storms formed."

The recent research indicates that wind shear is increased by GW, decreasing hurricanes. See a paper like Vecchi 2007 for details.

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