The trouble with climate forecasts

Hollywood is jumping on the global-warming bandwagon but in the world of environmental and weather studies things are not so simple, as SIMON COLLINS reports.


Between 1861 and 2000, New Zealand's average temperatures rose from around 12C to 13C. Scientists calculated at that rate we should be seeing significant changes in the landscape. Based on international evidence, for example, trees should now be growing 180 metres higher up the slopes of mountains than in the days of our great-great-great-grandparents.

But they are not. A careful study by Landcare Research at six South Island alpine sites has found the treeline has climbed only 5 to 6 metres in the past 150 years.

The finding is a surprise, because local measurements of carbon dioxide, methane and nitrous oxide all show these "greenhouse gases" increasing in our air, trapping much of the sun's heat close to the Earth's surface.

The warming has shrunk the amount of ice in the Southern Alps from 85 cubic kilometres in 1900 to 59 cubic kilometres today. Big warm blobs are appearing in the vast, remote Pacific Ocean. And the westerly winds that encircle Antarctica have sped up by about 10 per cent in the past 25 years as the "ozone hole" cooled the stratosphere high above the Southern Ocean, sucking warm air more strongly out of the tropics.

Yet the timberline study shows how little we still understand about what is going on. "It's all a bit bizarre, really," says Matt McGlone, the Landcare ecologist who led the study.

He told a climate change symposium organised by the Foundation for Research, Science and Technology this week that some of the more extreme claims - such as one published in January that global warming could wipe out a quarter of the world's species by 2050 - were simply "atrocious. We are falsifying the coin that we deal with in the public if we keep on with alarmist projections," he said.

There is so much natural variability in the climate that it is hard for the experts to make confident predictions even for the next few hours, let alone centuries ahead.

The very existence of global warming is disputed by satellite data, which shows no clear trend since satellites were launched in the late 1970s.

Ground-level temperatures, collected since 1861 in New Zealand, do show a long-term upwards trend, but the trend can be swamped by short-term variations. The average temperature was 13C in 1990, dropped to 11.8C in 1992, peaked at 13.4C in 1998 and was just 12.6C last year.

Sceptics such as Auckland University's Chris de Freitas say car exhaust fumes, industrial pollution and all other human emissions account for a grand total of just 3 per cent of carbon dioxide emissions into the air. The other 97 per cent come from natural sources, such as animals' breathing and the exchange of carbon between the air and the oceans. (Other scientists dispute these proportions.)

On the other hand, there are also facts that alarm people such as James Lovelock, the British scientist who came up with the "Gaia" theory that the Earth sustains life by the actions of living things, and who suggested this week that it would be safer to use nuclear energy.

He is right that living things have transformed our atmosphere. The growth ofmicroscopic algae and later other plants reduced carbon dioxide from a lethal 8 per cent (80,000 parts per million) about two billion years ago to a range between 190 and 280ppm in the past 400,000 years.

Over the same period, plants raised the proportion of oxygen from virtually zero to 21 per cent, making the air fit for mammals such as us to breathe.

The first humans evolved a mere 200,000 years ago, and our population has exploded from five million to six billion and spread around the world only in the past 15,000 years. We have covered the Earth in cities, roads and intensive agriculture in just the past 300 years.

As atmospheric scientist Greg Bodeker puts it: "Quasi-exponential growth in human population and industry over the past 250 years have resulted in significant changes in the chemistry of the atmosphere, including the global spread of air pollution, increases in the concentrations of tropospheric oxidants [including ozone], stratospheric ozone depletion and accumulation of greenhouse gases."

Among the greenhouse gases alone, carbon dioxide has risen to 370ppm, and methane has almost trebled from 0.7 to 1.8ppm, probably at least in part because of emissions from growing numbers of rice paddies and from belching domestic animals.

David Lowe of the National Institute of Water and Atmospheric Research says another factor in the rise of methane is the southern ozone hole, which has reduced the rate at which sunlight interacts with ozone to produce a very rare molecule called hydroxyl, combining one atom each of oxygen and hydrogen (OH).

Hydroxyl is the main compound that reacts with methane and effectively "cleanses" methane out of the air.

Less ozone means less hydroxyl and more methane. There are just 100 tonnes of hydroxyl in the entire global atmosphere.*

Niwa scientists are the first group in the world to successfully measure changes in these minuscule levels of hydroxyl in the air. They have found that the hydroxyl level is the same at Baring Head near Wellington and Scott Base in Antarctica, and therefore probably at any point on Earth.

And that level has been pretty steady throughout the past 15 years, except for two brief dips below average: after the eruption of Mt Pinatubo in the Philippines in 1991, and after forest fires in Indonesia in 1998-99 which were the biggest burning of biomass ever recorded.

The two events poured so much soot into the stratosphere that they reduced the amount of sunlight getting through to react with ozone, hence reduced the amount of hydroxyl - and increased methane levels by as much as 2 per cent in a year. "It shows that major events can affect the ability of the atmosphere to cleanse itself," Lowe says.

His measurements of hydroxyl, and McGlone's observations of the South Island alpine treeline, are just two more bits of data about a global climatic system that, as McGlone says, is still far too complex for scientists to claim that they can understand it yet.

But they suggest that human beings already hold the power to change the atmosphere, and hence the climate. It is now up to us to decide how that power should be used.

* CORRECTION: In the original version of this report, we stated incorrectly that there were only 4kg of hydroxyl in the Earth's atmosphere.


Foundation for Research Science and Technology


Herald Feature: Climate change

Related information and links

*