Science In Action: Storm Warning -- Hurricane Katrina and Global Warming

Is Hurricane Katrina an example of "Global Warming" affecting the local weather? Recent evidence suggests that the warming of the oceans caused by our generation of greenhouse gases over the past century or two may be causing more intense hurricanes.

Climate scientists have known for some time that

The Earth is heating up, and
As the Earth's surface and atmospheric temperatures increase, this will affect the distribution and intensity of weather events.

Global Warming Is Real

Among scientists, any remaining arguments about global warming are questions of degree. There are a few lonely holdouts, as there are with any paradigm shift in science (Leading 19th century American scientist Louis Agassiz went to his grave opposing Darwin's theory of evolution by natural selection). But most of the denial is outside the scientific community and is purely political. (That is to be expected -- see my earlier post on science and politics.)

Graph of Global Temperature Over Past 2000 Years

Source: This Wikipedia page

Sea Temperatures Affect Storms

One predicted effect of global warming is an increase in the intensity of hurricanes and tropical storms. As sea surface temperatures increase, these storms can draw more energy and moisture from those warmer waters.

What Drives a Hurricane?
The intense tropical cyclonic storms we call hurricanes (or typhoons, baguio, or cyclones) are driven by transfer of heat from the ocean to the atmosphere. They can form only over warm water (at least 26°C). The air over the sea is warmed, decreases in density, and therefore rises. The coriolis effect is strong enough to cause these rising currents to form spiraling winds. As the warm air expands and rises it cools, and eventually the water vapor in it condenses, releasing heat.

Heat of Condensation
As everyone knows, to boil water into steam (convert it into water vapor) you have to apply heat. When that water vapor recondenses that heat is released. Since water is a polar molecule, it takes a lot of heat to vaporize (evaporate) it, and a correspondingly large amount of heat is released by condensation.
This heat warms the air, causes further expansion, and reduces the atmospheric pressure below even further. As long as the system remains over warm water this cycle can build and the storm grows in size and windspeed.

Recent Research Has Shown:

1. Sea surface temperatures are clearly rising. This is one of the clear signs of global warming.

An influential study demonstrating that human activities have caused these sea surface temperature increases was reported in this press release from the Scripps Institution of Oceanography. Here is a pdf file of the article "Penetration of Human-Induced Warming into the World's Oceans" in Science. (If that link to the pdf doesn't work, get it through Dr. Pierce's publications site.)

Abstract
A warming signal has penetrated into the world's oceans over the past 40 years. The signal is complex, with a vertical structure that varies widely by ocean; it cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences. Changes in advection combine with surface forcing to give the overall warming pattern. The implications of this study suggest that society needs to seriously consider model predictions of future climate change.

2. Increasing sea surface temperatures will cause stronger hurricanes.

A recent paper, "Increasing destructiveness of tropical cyclones over the past 30 years", published in Nature, confirms what theory predicts: hurricanes have increased in intensity as seas have warmed, and the two factors are highly correlated. Here is a pdf file of the article.

Abstract
Theory and modelling predict that hurricane intensity should increase with increasing global mean temperatures, but work on the detection of trends in hurricane activity has focused mostly on their frequency and shows no trend. Here I define an index of the potential destructiveness of hurricanes based on the total dissipation of power, integrated over the lifetime of the cyclone, and show that this index has increased markedly since the mid-1970s. This trend is due to both longer storm lifetimes and greater storm intensities. I find that the record of net hurricane power dissipation is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multi-decadal oscillations in the North Atlantic and North Pacific, and global warming. My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential, and--taking into account an increasing coastal population--a substantial increase in hurricane-related losses in the twenty-first century.

Whether we are seeing an increase in the number of hurricanes and typhoons is less clear. The theoretical basis for such an increase based on higher sea surface temperatures is also not established. [Update: Evidence inSee this more recent post.]

Breaking News

Graph from Science editorial showing upward trend in category 4 and 5 storms

Another study has recently been published: "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment". (Editorial about the findings, which is the source of this graph; link to full text in Science)

Abstract and concluding paragraph
We examined the number of tropical cyclones and cyclone days as well as tropical cyclone intensity over the past 35 years, in an environment of increasing sea surface temperature. A large increase was seen in the number and proportion of hurricanes reaching categories 4 and 5. The largest increase occurred in the North Pacific, Indian, and Southwest Pacific Oceans, and the smallest percentage increase occurred in the North Atlantic Ocean. These increases have taken place while the number of cyclones and cyclone days has decreased in all basins except the North Atlantic during the past decade.

We conclude that global data indicate a 30-year trend toward more frequent and intense hurricanes, corroborated by the results of the recent regional assessment. This trend is not inconsistent with recent climate model simulations that a doubling of CO2 may increase the frequency of the most intense cyclones, although attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state.

The research reported in these articles is not the last word, and some other scientists don't completely agree with these findings. But it sure is beginning to look like the evidence supports the existence of these trends and implications.

Hurricane Warning

These changes have been caused by an increase in the temperature of the top 300 meters of the world's oceans of about one-half degree Celsius over the past fifty years. Global temperatures are expected to rise between 2°C and 4°C over the coming century. Think how this will affect hurricane strength and other weather! And consider the enormous amount of heat that had to be added to the oceans to raise their surface temperatures even 0.5°C. It would take centuries for them to cool down (or even to stop warming up!) even if we immediately stopped the human activities that contribute to global warming. Maybe we and our descendants had better get used to a stormier future.

Update

[2013-03-19 1530GMT: Recent research found a "twofold to sevenfold increase in the frequency of Katrina magnitude events for a 1 °C rise in global temperature". That is pretty scary. Reuters item here. Abstract of PNAS article here.]

Additional Resources

The world scientific consensus on global warming is summarized by the Intergovernmental Panel on Climate Change of the UNEP and WMO. Here is its summary of evidence for sea surface warming trends. Here are graphs of sea surface temperatures from that report.

Wikipedia article on global warming.

The Tropical Storm Risk consortium predicts a record-severe hurricane season this year. Here is a press release regarding their updated forecast of 5 August (here is the pdf file with details).