by Cate Montana

Hurricane: a violent tropical cyclonic storm of the western north Atlantic, having wind speeds of, or in excess of, 72 miles per hour; derived from Hurakan, a Mayan creator god who blew his breath across the chaotic waters.
Katrina: A Category 4 hurricane which devastated New Orleans and much of the Louisiana Gulf coast in August, 2005.
Rita: A Category 3 hurricane which heavily damaged western Louisiana and eastern Texas coastal areas, as well as Gulf oil platforms and other production facilities in September, 2005.
Global Warming: A potential cause of both?

Hurricanes are caused when warm air off the ocean surface rises. As it rises, the moisture in the air condenses, releasing more heat energy. The rotation of the earth causes the air to rotate (the Coriolis effect). When moving over warm water, the effect becomes self-reinforcing, gathering more and more warm, moisture-laden air and releasing more energy into the system. As anybody in the world with a TV has learned in the past two months, it’s only when a hurricane moves over colder water, or colder land, that it loses energy and drops in force.

Satellite photo of Hurricane Rita. Courtesy National Center for Atmospheric Research

For two years in a row, the Gulf states, notably Florida and Louisiana, have sustained unheard-of damage in the wake of unexpectedly violent, unexpectedly numerous hurricanes making landfall. What is causing this sudden increase in hurricane activity? What is making these storms so powerful? Although the public finger has tended to point towards global warming as the culprit, about the only thing atmospheric scientists know for sure, is that they don’t know for sure.

“It’s very difficult to say this particular storm, even a series of storms, is the result of global warming,” says Cliff Mass Ph.D., an Atmospheric Scientist at the University of Washington. “A number of people have said that, and that’s very irresponsible. You cannot say that one storm or even a few storms are the result of a phenomenal event.”

Climate changes

Planet Earth has always been subject to natural climate changes and fluctuations over the millennia. The most dramatic climate shifts on a global scale have been the result of heat either being let into, or out of, the atmospheric system. For example, a warming trend can be created either by an increase of heat let into the Earth’s system, or a decrease of heat let out of the system.

Trend in Global Temperatures
Click image for larger view. Courtesy The Intergovernmental Panel on Climate Change (IPCC)

Of course the main source of heat that comes into the system is from the sun, which warms up the oceans and the land masses. But the amount of sunlight coming in varies naturally. The Earth’s orbit fluctuates, bringing the Earth closer or father away from the sun. Volcanic activity and dust storms, which release enormous amounts of particulate matter into the atmosphere, can trigger an increase in cloud cover that reflects the sun’s light, and thereby its heat, away from the Earth.

The amount of heat that radiates away from the Earth’s surface fluctuates as well. The greenhouse gases we’re all so familiar with, carbon dioxide, water vapor, methane and nitrous oxide, allow the heat radiated from the Earth to remain trapped. However, Greenhouse gases have increased in the last hundred years, and therefore less heat is being distributed away from the planet. As a result, global temperatures have been steadily on the rise. Increased release of carbon dioxide from the burning of fossil fuels is a significant factor in this trend.

According to the Intergovernmental Panel on Climate Change, the world’s surface air temperature has increased an average of 0.6°C (1.0 °F) during the 20th Century. That may not sound like very much, but even a one degree increase can trigger worldwide weather changes.

Projected Changes in Global Temperature
Click image for larger view. Courtesy The Intergovernmental Panel on Climate Change (IPCC)

If there is one word that can describe climate, it’s complicated.

“There’s a lot of information, but it’s very hard to go anywhere with it,” says Bill Nicholls, a climate panelist and speaker at the recent Coppercon Science Fiction and Science Conference in Phoenix. “The scientists are very cautious about extrapolating beyond what they’re pretty sure about. And … at this point we can’t connect the dots. We can hypothesize that this affects that, which affects that, which affects that etc… and it all makes a certain amount of sense, and you can say that the physical environments would cause that to happen. But until we have measurements, it’s not science. … It’s like diving into a black hole. There’s just no end of things to study.”

On top of its complexity, atmospheric science is still in its infancy. Accurate historic weather data has only been available since the 1970s when geosynchronous satellites could be launched into orbit to gather data. Because weather patterns are patterns over time, it is vital that sufficient information be gathered before any significant trends and changes can be predicted – let alone attributed to any particular cause.

Click image for larger view. Cliff Mass checking satellite photos. Photo Courtesy A&S Perspectives, Mary Levin photographer

With hurricanes there appears to be a roughly 30 year cycle of more activity and less activity. “Up until about 1995 we were in the down side; we had less hurricanes,” says Mass. “Now we’ve switched over to a period - this is in the Atlantic - where there’s more. So there are certain natural cycles that would tend to produce more hurricanes now. So that’s on top of whatever potential global warming signal there is.”

Despite scientist’s reluctance to point in any one particular direction towards a cause for individual weather events, global warming is showing up in a lot of scenarios. That it has, can and will contribute to hurricane generation is not in doubt. But what aspect of global warming and which effect it is having on what part of hurricane generation isn’t clear. It might be convenient to say that a gradual, overall temperature increase in the oceans is the main factor. But it wouldn’t necessarily be the truth.

According to Nicholls, a physicist and computer scientist who got involved in climate research last year, hurricanes reflect an energy imbalance that has occurred in tropical waters. “We think some of that imbalance is due to a slight slowing of the Gulf Stream, but we do not have the kind of numbers that would prove that,” Nicholls says. “It’s a hypothesis that the Gulf Stream is being slightly slowed because of the large amount of ice water flowing in from Greenland. We can see that the ice is melting. We can see there’s more water there. We can’t tell what effect it has.

“[But] if the mechanisms of energy distribution are possibly being disrupted, which is possible, then we’re going to see significant changes in energy distribution. Which means significant increases in storm intensity and quantity.”