Seeking a link between climate change and tropical cyclones

(Second of two parts)

While meteorology awaits its Einstein, the world must rely on climatologists sifting through data to separate natural periodic signals in tropical cyclone activity from trends that can be identified as anthropogenic in origin. Results of one such effort were published last week in the leading journal Science by a group led by two more big names from the 2001 IPCC report, Peter Webster of Georgia Tech and Greg Holland of the National Center for Atmospheric Research. Strongly supporting Kerry Emanuel’s findings, they found a distinct climb in the number of strong cyclones (though not weak ones) over the past 35 years. Despite their upward-pointing graphs, Webster et al. concludes gingerly that this trend in intense hurricanes is "not inconsistent" with predictions of global warming.

To observers in the Philippines, their restraint is understandable. Counting the annual number of named disturbances that entered the so-called Philippine Area of Responsibility (PAR), we find the same upsurge in incidents up to the early 1990s. However, this was soon followed by a decrease in activity between the mid-1990s and 2002, despite the continuous rise in sea surface temperatures during this period. This even includes an unusually strong warming event in 1998 in the South China Sea just west of the PAR, yet the number of typhoons remained below normal that year. For the past two years, the number has again been rising, just as it has always been in other tropical cyclone corridors.

Webster hints that 35 years are much longer than the period of any known natural phenomenon known to influence tropical cyclones, but the data are simply too short to attribute the increasing trend purely to a human cause. Our capacity to monitor hurricanes and typhoons only reached its present level with the advent of the satellite era in the early 70s. It is for this reason that Emanuel limited his analysis to the last 30 years, and even then he had to resort to some data tweaking in order to allow comparability among the decades. Such shortcomings in the data inevitably foster doubt in any research that relies on them.

There are, however, key points that are not in dispute. The Pielke and Emanuel papers agree that the planet is warming up. Also not in dispute is the net increase in the number of strong cyclones in the recent years. The disagreement is only whether the two phenomena can be linked to human activity. Pielke’s paper also does not discourage taking action against climate change, and warns only that justifying its mitigation based on faulty science will be counterproductive and ineffective. This position is sympathetic to the US administration, which believes that the scientific uncertainty and the massive costs of reducing its CO2 emissions do not warrant the actions chosen by the rest of the world in the Kyoto Protocol. This belief is backed by an ever-narrowing circle of climate scientists with reservations over global warming but who seem to be influential in the White House.

That Emanuel’s critics include the powerful raises eyebrows over the timing of the release of the Pielke paper to come right after his, as if aimed at dampening its impact. Emanuel is quick to dismiss this conspiracy. Here’s a scoop: he was supposed to be among the co-authors of the Pielke article, but withdrew when results of his separate study contradicted the team’s conservative conclusions. The reality is that many scientists who believe in humankind’s role in changing the climate are often skeptical of any specific claims about our ability to influence tropical cyclones.

What seems to be missing from the skeptics is a clear statement on how strong a change or what kind of signal they require before they start believing that fossil fuel burning leads to stronger disturbances. Emanuel’s paper showed a doubling in the strength of Atlantic hurricanes; none of his critics state how much strengthening is needed before they start agreeing that what we are observing goes beyond natural variability. Will a Category 5 hurricane (the strongest class in the US system) in January be sufficient evidence? A hurricane in New York City, or a Category 6?

The science is uncertain, but as activists remind us, the uncertainty should not be an excuse for inaction. In the opinion of many governments who have decided to join the call to curb greenhouse gas emissions, it may be too late to do anything by the time definite signs appear. When they do, there is no doubt that these signs will not be pleasant, as the aftermath of Hurricane "Katrina" reminds the world.

Clarifying the connection between global warming and tropical cyclones is critical to the Philippines not just because the record-setting number of weather disturbances that affect us every year puts to shame the few but well-televised calamities that plague the US. It is also because as ominous as the possibility of suffering too many typhoons is the prospect of droughts caused by too few. Either scenario cannot be ruled out as global warming progresses. If no such link exists, we may be spared from some of its future effects and allow us to focus on our many urgent problems.

Although there is uncertainty in how much the frequency and intensity of tropical cyclones will be altered by climate change, this risk is still only part of the issue. The Pielke review devotes its second half to the problem of vulnerability of US cities, specifically on how humans have placed themselves in harm’s way. Stop worrying about the connection between CO2 emissions and "Katrina," the authors say, because any damage that climate change can inflict via stronger tropical cyclones will be miniscule compared to the increased vulnerability to disasters arising from the building of homes in unstable mountainsides, coastal floodplains and natural drainage channels. Few in the Philippines dispute the wisdom of this warning, but it has long been raised – and ignored. As Hurricane "Katrina" and Supertyphoon "Yoyong" showed us, our countries have something deadly in common.

(The author is an associate professor of the Department of Physics of the Ateneo de Manila University. E-mail him at [email protected])