WHAT鈥橲 WITH THE WEATHER? An unruly beast at the best of times, it now seems to be lurching out of control. There are wildfires raging across the western US. India is reeling from floods last month that left hundreds dead and millions homeless. Then there鈥檚 the blistering heatwave in Greece, a bizarre contrast with Yorkshire鈥檚 recent spectacular hailstorm, which left the streets of Hull looking for all the world as if it had snowed in August.
Open any newspaper or flick on the television and you鈥檒l find the same questions. What have we done? Is global warming upon us? Are we finally seeing the effects of our rash experiment with the world鈥檚 climate?
They鈥檙e hard questions to answer, not least because extreme weather has always been with us. 鈥淵ou can鈥檛 say we had a flood in Mozambique and another in India and that must be down to global warming,鈥� says David Easterling, principal scientist at the National Climate Data Center (NCDC) in Asheville, North Carolina. 鈥淓ven if CO2 levels hadn鈥檛 changed in the 20th century, we would still see these events happen. You鈥檙e going to have extremes in climate somewhere every year. The real question is: are we going to see a lot more of these in the future?鈥�
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To find out, researchers around the world are scouring records of temperature, wind and rain, trying to spot the patterns that will tell them whether things really are changing. Though they鈥檙e still figuring out the details, one thing鈥檚 for sure: even if the recent weather tantrums turn out to be nothing particularly new, the signs are that there鈥檚 much worse to come.
Of all the changes researchers are watching for, warming is the easiest to spot. Reliable temperature records stretch back to the mid-19th century. Before that, the data are more sparse, but there is evidence from ice cores, corals, old documents and tree rings of temperatures going back a thousand years or more. Thanks to these records, there鈥檚 no longer serious argument about whether the Earth is heating up. The past century has seen average global temperature increase by more than half a degree, with the majority of that warming piled up in the latter half. The seven warmest years on record occurred in the 1990s, and 1998 was the hottest of all. 鈥淚t was the warmest year in the warmest decade in the warmest century in the millennium,鈥� says Phil Jones, joint director of the University of East Anglia鈥檚 Climatic Research Unit in Norwich.
The warming has made its mark. Around the world, there are fewer days when the temperature drops below freezing, and minimum temperatures everywhere are creeping upwards. All this seems very gentle, though, and it might even be nice. 鈥淪ome people might like the effects of global warming,鈥� says Chris Folland of Britain鈥檚 Meteorological Office in Bracknell. 鈥淚t makes your holidays better.鈥� But it won鈥檛 be all balmy summer evenings and mild winter days. There鈥檚 a sinister side to warming as well.
Take the effect it could have on storms, for instance. Higher temperatures mean that more water evaporates from the Earth鈥檚 surface. That could help crank up the heat engine that drives the weather. 鈥淲ater vapour is a great energy source,鈥� says Catherine Senior, also from the Met Office, who has been studying the likely effects of global warming on storms in middle latitudes, away from the tropics and poles. By evaporating more water, she says, you鈥檙e sucking energy out of the oceans and dumping it into the atmosphere where it鈥檚 free to do its worst.
It鈥檚 not quite that clear-cut, though. For middle latitudes at least, there鈥檚 a competing effect stemming from global warming that could tend to quieten storms rather than stir them up. Researchers believe that the poles will warm more than the tropics because when you melt white ice at the poles, the darker land or water beneath soaks up more sunlight. The temperature difference between the tropics and poles is a great driver for creating storms, so reducing the temperature gradient ought to make storms less likely.
According to Senior鈥檚 model, water vapour wins this climatic tug of war. Two years ago she published results of a computer model suggesting that global warming will intensify mid-latitude storms, such as the one that battered southern England in October 1987. Other models come up with different answers, but that鈥檚 because nobody鈥檚 sure how these effects will play out. 鈥淚t鈥檚 not well understood what causes storms to form and deepen, and what path they take in today鈥檚 climate,鈥� she says. 鈥淪o trying to understand what might happen under climate change is even more difficult.鈥�
Although not everyone agrees with the results of Senior鈥檚 model, there are signs that we are already experiencing more formidable storms, in the North Atlantic at least. Ulrich Cubasch from the German Institute for Climate Research in Hamburg has found that in the past 20 years, mid-Atlantic depressions have become deeper-and the storms they cause more intense.
Even that may not be the effect of global warming, though. According to Folland, it could be down to a perfectly natural phenomenon known as the North Atlantic Oscillation (New Scientist, 31 January 1998, p 22). This is in essence a climate cycle driven by the difference in pressure between the low that hovers over Iceland and the high over the Azores. If both are at their extremes, the NAO is said to be in its strong phase, and the gradient sets up fierce storms and westerly winds which drag the storms onto land. If the pressures are more equal, there are fewer storms, and easterly winds blowing off Europe prevent many of them from making landfall. Right now, the NAO is in its strong phase. It was also strong in the early 20th century, and Europe experienced more storms then as well. Folland also points out, though, that global warming could interfere with the NAO and cause more storminess in the next few decades.
When it comes to tropical storms, the picture is less ambiguous. It could be just a matter of time before the vicious hurricanes and cyclones that already hit the headlines start picking up energy from global warming and wreak still more havoc. As yet, global climate models are too coarse to resolve hurricanes, but hurricane researcher Kerry Emanuel from the Massachusetts Institute of Technology has found another way to tell what the future is likely to hold.
Hurricanes exist because of differences in temperature between the tropical ocean and atmosphere. Not all storms reach the maximum intensity that this temperature difference can produce. They may be held in check by factors like how long they persist and how much they stir up the ocean, bringing up cooler water to dampen their fervour. But weather scientists observing a particular storm can easily calculate how close it has come to achieving its full potential.
Equal opportunities
In April this year, Emanuel and his colleagues published a study of hurricane records from around the world going back 50 years. They looked at different parts of the world, different seasons and different time frames. To their astonishment they always saw a uniform pattern: each storm had the same chance of reaching any intensity you care to name, up to the maximum. There鈥檚 no particular speed or proportion of the maximum that is more probable than any other. 鈥淭ake a cyclone out in the Atlantic that鈥檚 just developed, and there鈥檚 an equal probability that it will get to 70 mph, 90 mph, 120 mph. It鈥檚 true wherever you look,鈥� he says.
This gives climate prediction 鈥渁 big leg up鈥�, says Emanuel. All you have to predict is the maximum intensity. In a warmer world, the maximum intensity for hurricanes will be greater everywhere. And if you raise the maximum possible intensity, Emanuel鈥檚 research suggests that tropical storms will become more severe. 鈥淟ifting the tide lifts all the boats,鈥� he says.
Are there signs that this is happening? Well, not yet. Sure, tropical storms have been more catastrophic in recent years-in both human and economic cost. Hurricane Mitch, for instance, tore through Nicaragua and Honduras in 1998, causing more than 9000 deaths. But it wasn鈥檛 a particularly severe storm. It just landed in a very unfortunate place. And the big bucks that insurance companies are shelling out for the eastern seaboard of the US owe more to increased development on the coast than they do to increased storm intensity. 鈥淓very time a hurricane comes in you have even more houses to blow down,鈥� says Easterling. Study the data rather than the hype and there鈥檚 no sign yet that tropical storms are getting more frequent or more intense. Shock headlines notwithstanding, such storms are still rare, making the statistics hard to handle.
Regardless of what global warming does to storms, sea level will definitely rise. It鈥檚 already on the move, and some say that it will increase by as much as half a metre over this century, thanks to expansion of the oceans and melting of land ice. 鈥淪torm surges test sea defences very severely,鈥� says Folland. 鈥淓ven if storm levels stayed the same, increasing sea level will bring a much greater threat to low-lying areas around the world.鈥�
Putting more energy into the climate engine could also mean more rainfall, and hence more flooding. Mozambique was desperately unlucky when two tropical cyclones conspired to dump their water there earlier this year, while India was hit by a freakishly intense monsoon. Though global warming can鈥檛 be fingered for any one disaster, there鈥檚 already evidence that rainfall is becoming more widespread and more intense.
Tom Karl, also at the NCDC in North Carolina, published a paper in 1998 showing a marked increase over the past century in the number of days that saw heavy rainfall in the US. Easterling has seen a similar pattern for longer periods of rain. Overall, the US has seen increases in rain, hail and snow of up to 10 per cent.
This is in line with what the models predict. In 1997, Kevin Hennessy from the CSIRO in Victoria, Australia, working with researchers from the Met Office in Britain, modelled the effect of warming on rainfall. He found that in high latitudes there should be more wet days every year, and that all rainstorms would become heavier. At middle and low latitudes there would be fewer wet days, but the rain that did fall would be more intense.
That could also bring more severe droughts. When you crank up the climate system everything becomes more extreme, and if rainstorms are more intense in one place, that could mean less rain somewhere else. So far, there is no sign in the records that droughts are increasing in frequency or severity. But the models predict that parts of the world-especially at low latitudes-will become steadily drier as the world warms.
Even perfectly natural parts of the weather machine could be getting in on the global warming act. There are signs, for instance, that El Ni帽o events are getting stronger. El Ni帽os occur when a pool of warm water migrates across the Pacific, dumping torrential rain on the west coast of the Americas and leaving drought behind on the Asian side. The really strong El Ni帽os cause havoc around the world. In 1998, for instance, Indonesia and the Amazon were ravaged by forest fires, Ecuador, Chile and Peru experienced intense rainfall, and Venezuela faced mudslides that were disastrous. Though the jury is still out on whether global warming could exacerbate El Ni帽o events, they do seem to have become more frequent and more intense in the past few decades and some researchers believe warming is to blame (New Scientist, 9 October 1999, p 36).
Add all this together and the prospects for the future seem decidedly tempestuous. Should we batten down the hatches? 鈥淧repare for change,鈥� says Folland. 鈥淲e鈥檙e not reducing our greenhouse gas emissions anything like fast enough to stop the effects of climate change this century. It鈥檚 too late. We can slow warming down, but we can鈥檛 stop it.鈥� All the signs are that change will mean more extreme weather. So if you think today鈥檚 storms are wild, wait till global warming really kicks in. You ain鈥檛 seen nothing yet.
