Cool look behind the hot air

By SIMON COLLINS

Are we heading for "large-scale and possibly irreversible changes in earth systems"? Or is the whole idea of global warming a "delusion"?

When scientists disagree so fundamentally, it is tempting for the rest of us to stay out of the argument and wait.

Yet in the case of global warming, the believers warn that the effects of doing nothing could be catastrophic.

In any case, the Government is forcing the issue. It has already passed legislation to ratify the Kyoto Protocol - which creates international rules to reduce greenhouse gases - and plans to start charging emitters of carbon dioxide from 2007.

Cars emit carbon dioxide, so all car drivers are affected. Perhaps it's time to take a deep breath - away from the car exhaust - and try to make sense of the familiar but often confusing terminology.

All sides of the debate agree there is a natural "greenhouse effect" in which gases such as carbon dioxide absorb heat from the sun and keep the earth roughly 32C warmer than it would be otherwise.

This happens because the sun's rays that get through the atmosphere as visible light get absorbed by the earth. Much of the energy is then radiated back from the earth in longer waves known as infra-red energy.

All this energy gets through in both directions without interacting with nitrogen and oxygen, which between them make up 99 per cent of the atmosphere.

However, some of the infra-red energy does get absorbed by water vapour and some of the "trace gases" which make up the remaining 1 per cent of the atmosphere. Despite being a tiny fraction of the air we breathe, these greenhouse gases make the earth warm enough for us to live on.

The main greenhouse gases (listed in parts per million parts of air) are:

1. Water vapour (variable, 0-20,000ppm)

Studies cited by the Intergovernmental Panel on Climate Change (IPCC) record increases in water vapour content in the air (humidity) of several per cent a decade over parts of the United States, Europe and Asia since the 1960s. However, there were decreases over eastern Canada and parts of central Asia, and there are no significant data from the Southern Hemisphere.

Even if there has been an increase in water vapour, scientists are unsure about its net effect. It may increase the greenhouse effect. On the other hand, it may increase cloud cover and shut out some of the sun's rays, causing a net cooling.

2. Ozone (variable, 0-1000ppm)

Ozone, a molecule of oxygen with three atoms instead of two, is mainly found high in the stratosphere. It absorbs short-wave (ultraviolet) energy from the sun, and without it scientists believe life would never have emerged on land.

Ozone is most often associated with another environmental problem - the hole in the ozone layer - but the gas is also present, in much smaller volumes, in the air closer to the earth's surface, where it acts as a greenhouse gas. The IPCC reports that average near-ground ozone levels have risen and fallen several times during the past 30 years, with no clear trend.

3. Carbon dioxide (361ppm)

Carbon emissions from burning coal, oil and other fossil fuels, and producing cement, have been traced back from virtually zero at the dawn of the industrial revolution in 1750 to about 2.5 billion tonnes a year in 1960 and more than 6 billion tonnes a year today.

However, the extent to which these emissions are raising the concentration of carbon dioxide (COinf2) in the air is disputed.

Studies quoted in the IPCC's report last year suggest COinf2 levels have fluctuated wildly in the very long term, reaching peaks of more than 6000 parts per million (ppm) in the air around 200 million and 450 million years ago.

In the past 400,000 years, the gas has settled down to a range around 200ppm during ice ages and around 280ppm during warm periods.

But in the past 200 years, evidence from Antarctic ice cores and, since 1957, direct measurements at Hawaii's Mauna Loa observatory have shown a steady increase to 361ppm today.

In a new book, The Greenhouse Delusion, a former chief chemist of the Coal Research Association, Dr Vincent Gray, argues there is no need for alarm in these figures because the proportion of carbon emissions that end up in the atmosphere appears to be falling slightly.

But even Gray accepts that carbon emissions have risen. This makes it likely that the level of COinf2 in the air is probably rising to some degree.

4. Methane (1.7ppm)

Antarctic ice cores show methane levels have fluctuated during the past 400,000 years between 0.4ppm in ice ages and 0.7ppm in warm periods, before suddenly rising in the past 200 years to 1.7ppm.

However, this increase slowed during the 1990s and since 1999 has stopped completely. The US National Oceanic and Atmospheric Administration recorded a slight drop in world methane levels in 2000, although there was a slight increase again last year.

The IPCC says there is no consensus on why methane may have stabilised.

5. Nitrous oxide (0.3ppm)

The IPCC says the level of nitrous oxide was about 0.27ppm from about the year 1000 until 1800, but has increased to 0.3 ppm today. The main sources are natural emissions from the soil and oceans, but nitrogenous fertilisers and other farming practices are believed to have caused the increase in the past 200 years.

6. Chlorofluorocarbons (CFCs) (0.001ppm)

As well as reacting with ozone, CFCs absorb infra-red energy and have contributed to the greenhouse effect. The international moves to save the ozone layer have reduced the concentrations of key CFCs in the air since the mid-1990s.

Global warming: past evidence

Throughout history, remarkably small changes in the average temperature have produced huge changes in the global climate.

Scientists believe that the planet's average temperature has fluctuated in a fairly narrow band between 12C and 22C for most of the past 500 million years.

In the past 400,000 years, Antarctic ice cores record four major ice ages when temperatures were about 8C colder than at present. All four coincided with troughs of COinf2 and methane.

The average temperature in New Zealand in recent years peaked between 8000 and 10,000 years ago, when it was about 2C warmer than at present.

In the past 150 years, IPCC figures record a gradual increase in the average surface world temperature of about 0.4C from 1860 to the mid-1940s. Temperatures then fell back marginally until the late 1970s, and have risen again by a further 0.4 per cent in the past 25 years. The global surface average is now around 14C, still near the cold end of the very long-term band.

Sceptics such as Gray and Auckland University geographer Chris de Freitas believe the apparent warming in the past 150 years is distorted by urbanisation around most of the measuring stations.

Measurements from satellites, which have been available since 1979, do not show any statistically significant global warming since then, apart from a single hot year in 1998 when the El Nino effect over the Pacific was unusually strong.

However Dr Jim Salinger, of the National Institute of Water and Atmospheric Research (Niwa), who helped to draft the IPCC report, says scientists filter out any records that appear to be distorted by local changes.

He says all of New Zealand's 21 weather stations have been in place since early last century, seven of them since the 1860s. "All these show warming," he says. The West Coast glaciers have shrunk by a third. Satellites "don't measure surface temperature".

Although measuring global averages is inherently difficult, there is clearly some evidence of warming. Whether it turns out to be a "delusion" is yet to be proved.

Global warming: prospects

The IPCC says the average global temperature is likely to increase by between 1.4C and 5.8C by the year 2100, depending on whether the world acts to cut greenhouse gas emissions.

Even the lowest of these scenarios would represent a much faster rate of warming than in the past 150 years because of the higher volume of greenhouse gases already in the air.



The projections based on computer models are far less certain even than the uncertain changes in past variables on which they are based. Gray notes that most of the projections made in 1990 for the year 2000 turned out to be wrong, and suggests that the new ones may be no better.

Logic suggests that, if the original spread of plants and plankton helped to lower the global temperature, then releasing their carbon back into the atmosphere may raise the temperature again.

History shows that climatic changes can become self-reinforcing.

In particular, global warming could be much worse than the IPCC projections - possibly some time beyond the limit of the projections in 2100 - if the current warming eventually melts vast methane deposits stored in ice under the sea floor, as in the last big warming 55 million years ago.

History also shows that every time there has been a sharp warming or cooling, "negative feedbacks" have eventually reversed the process.

Increased carbon dioxide fed "blooms" of plants and plankton which extracted the CO2 again, keeping the global temperature within a narrow range of 10C or so.

Life itself has survived on earth through all these changes. But with each big swing in climate, many species have died out.

Human beings may be in the unique position of being able to take global action to increase the chances of survival of their own species.

Further reading
nzherald.co.nz/climate

Climate change links

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