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Crossing the threshold

Peter Bunyard

1st February, 2004

It takes no more than a gentle nudge to push a man over the edge of a cliff, but it is almost impossible to haul him back before he hits the ground. Given that we show no sign of putting a stop to global warming, Peter Bunyard takes a look at what the future might hold

Since 1990 we have experienced the warmest 10 years on record. This has left some parts of the world ravaged by drought and famine, and others suffering freak storms such as those that flooded much of lowland Britain in 2000. France, having experienced a devastatingly hot summer in 2003 then found itself enduring torrential winter rains and unprecedented floods. According to Phil Jones, head of the Climatic Research Unit of the University of East Anglia, the three months of June, July and August 2003 were the warmest ever recorded in western and central Europe. The average temperature for those months was nearly 4° centigrade above the long-term norm and breaking records everywhere – including the UK, where temperatures exceeded the 100° Fahrenheit mark for the first time.

Satellite data reveals that the planet has lost about 10 per cent of its snow cover since the 1960s, and that lakes and rivers in the high latitudes of the northern hemisphere remain frozen for two weeks less than they did one century ago. Glaciers in non-polar regions are also retreating, while Arctic sea ice has not only thinned by some 40 per cent since the 1950s, the surface area that it covers during the spring and summer is also down by up to 15 per cent.

The financial cost of natural disasters in 1998 amounted to $65.5 billion, and the World Health Organisation estimates that the spread of diseases induced by global warming may have led to 5 million deaths. Given that all this is down to a mere 0.6° centigrade increase in global temperatures, what will the future hold?
    
The doomsday alternatives

As climatologists are now certain that it is our greenhouse gas emissions that have led to global warming, we urgently need to know what will happen if we fail to curb our emissions or, worse, continue to add to them. The Intergovernmental Panel on Climate Change (IPCC) has come up with a range of predictions for the next 100 years, all contingent on different scenarios of fossil fuel use.

If CO2 emissions remain the same as they are today – 375 parts per million (ppm) of the atmosphere

The truth is that the emissions of yesterday will have their impact tomorrow, and, whether we like it or not, we are committed to further warming – even if we were ‘magically’ to level off our greenhouse gas emissions at the level of today: some 375 parts of carbon dioxide per million parts of the atmosphere (375 ppm). According to such a scenario, global temperatures will rise another 1° centigrade on top of what we have already experienced.

Even that ‘best’ scenario will wreak some havoc. Glaciers and sea ice will in all probability vanish, and the number of extreme climate events, such as floods, landslides, heat waves and violent storms, is bound to increase. Agriculture will be affected, as a lack of rain during the growing season and a spate of heat waves have a catastrophic effect on global food supplies. Worst of it all, as conditions get tougher, we are likely to resort to ever increasing uses of energy, so adding to the potential of global warming in the future.

If we curb emissions so they only rise to 550 ppm

If we could stabilise carbon dioxide concentrations in the atmosphere at about double pre-industrial levels (550 ppm, compared to 280 ppm), global temperatures would rise 2° centigrade over the next 100 years, according to the IPCC. With luck, our current climate system could still cope with such a temperature increase without jumping unexpectedly to a very different and hard-to-predict state.

Nevertheless, we would definitely be committed to substantial sea rises, perhaps a foot or more, as sea water expanded in volume as it got hotter; this would be exacerbated by the further melting of glaciers and polar ice. Increased rainfall, particularly over Siberia, would also lead to a significant increase in the flow of cold fresh water into the Arctic Circle, which would curb the flow of the Gulf Stream and its vital transport of heat from the tropics to the high northern latitudes. We would be subjected to ever stronger climate events, including storms and sea surges, torrential rains and their deadly counterpart – drought.

If energy use continues to grow at the current rate

Our insatiable and growing appetite for fossil fuels means we are heading for a fourfold increase in greenhouse gases compared to pre-industrial times. That being so, the UK Met Office’s Hadley Centre for Climate Prediction and Research envisages a catastrophic 8° centigrade rise on today’s global average. We would be then in a range of global temperatures not seen since 40 million years ago, when the planet had no permanent polar ice sheets and sea levels were 12 metres higher than today.

We would lose our major capital cities and much of our best farmland, and be subjected to violent weather on account of the much greater energy trapped at the earth’s surface. The circulation of air and the movement of oceans would be fundamentally different from what we experience today. Survival under such circumstances would most likely be impossible, especially in those parts of the world where we have already ravaged the environment.

If we take into account neglected variables

In its business-as-usual scenario, in which global emissions of greenhouse gases continue to rise uncurbed, the IPCC anticipates that by 2100 the concentration of carbon dioxide in the atmosphere will rise to 700 ppm – double that of today. However, the IPCC’s predictions neglect the impact of global warming on soils and vegetation.

Until now most climate models, especially those used by the IPCC, have assumed that carbon dioxide will be drawn down out of the atmosphere at a constant rate; this would offset up to half our current emissions. Such models are inherently deficient and far removed from the real world in which the interchange of gases between the earth’s surface and the atmosphere is contingent on living processes such as photosynthesis and respiration.

When there is more photosynthesis than respiration the earth’s plant life and soil organisms become a sink for carbon. Such is the situation today. But if respiration exceeds photosynthesis the situation reverses and that store of carbon begins to be consumed; soils and vegetation emit greenhouse gases, and become a source of carbon.

The fact is that the Hadley Centre’s climatologists are now finding that the IPCC’s climate models (used to inform governments) are far too optimistic in their conclusions. Once different vegetation types (ie, broadleaf trees, tropical forests, savannah and grasslands) are integrated into the dynamic of climate change, there is a very different climate story from that when life is left out of the equation. For instance, those scientists who maintain that increased growth of forests in the northern parts of Siberia and Canada will counteract global warming are found to be fundamentally wrong. Why? Because the boreal forests are quick to shed winter snow on account of their shape, thereby exposing their dark needles to the rays of the spring sun. In contrast to the snow-covered tundra and swamps, boreal forests warm themselves and their surrounding environment when all around is cold. As Hadley Centre climatologist Richard Betts has found, the warming from the dark sun-exposed leaves more than counteracts the cooling that accrues from the growing forest taking up carbon dioxide. Forest growth in the Arctic Circle gives us a warmer, not a cooler, planet.

Currently, one half of all global emissions of greenhouse gases are absorbed into soils and oceans during the course of each year. The growth of forests and storage of carbon compounds in soils therefore play an important role in acting as a ‘sink’ for carbons, thus reducing the overall impact of our emissions. But how permanent is the ‘storage’ of that carbon? Could it suddenly be released back into the atmosphere and become an additional ‘source’ of greenhouse gases, just when the heat is on and we least want it?

Most climatologists base their predictions of future climate change on the grounds that the stores of carbon in soils and vegetation will remain intact as if for ever, and that the sinks for our carbon dioxide emissions will continue to operate come what may. Yet when the Hadley Centre climatologists included carbon cycle feedbacks in their climate models they found that disturbing changes would be likely to occur across the planet.

2080 – the nightmare scenario

By 2080 the pattern of rainfall would be fundamentally different, with somewhat greater precipitation over the high latitudes – including the ocean. But across the tropics (except for a region in the Pacific) rainfall would decline by 50 per cent or more over all continents. With far less broadleaf forests in the tropics as a result of declining rainfall, daytime temperatures would be likely to rise by a substantial 10° centigrade. That, and the lack of rain, would be devastating for agriculture right across the planet. It would also be devastating for settlements, cities and industry. The corn-belt of the US would suffer from a 30 per cent decline in rainfall during its critical growing season, quite aside from an increase in heat waves. And, with more energy retained within the tropics (especially in the oceans), our coastlines would be battered by violent sea storms, including hurricanes and typhoons, as well as sea surges on a scale we have never seen before. With raised sea levels, the damage inflicted by such storms on vulnerable coastlines, such as along the Ganges Delta or in Indonesia and in Europe, would be unimaginable. Such climatic horrors would trigger a flood of refugees that would make today’s numbers appear a trickle.

Switching off the Gulf Stream

One probable consequence of wetter, warmer conditions in the northern hemisphere is that the Gulf Stream would judder to a halt, or at least shift much further south – taking its warmth with it. Just imagine the consequences of Labrador-like winters over northern France, all of the UK and Scandinavia: an ice sheet would develop, spreading over northern Europe and certainly covering much of Britain, as happened in the last Ice Age of 100,000 years’ ago.

The Gulf Stream works the way it does because of the saltiness and low temperature of the surface waters in the higher latitudes. The cold, salty water becomes denser than the waters beneath and sinks to the bottom. From there it flows back to the equator and south towards Antarctica. That conveyor belt circulation picks up nutrients on its long passage at the bottom of the Atlantic, and when those same waters, some thousand years’ hence, rise back to the surface to become the Gulf Stream, they are rich in essential elements for the growth of plankton. That’s why the northern Atlantic provides one of the richest fishing grounds in the world.

But global warming is causing glaciers to melt in Greenland and Canada; it is also causing a substantial increase in rainfall over Siberia. Consequently, the flow of fresh water into the Arctic Circle is diluting the saltiness of the northern waters of the Gulf Stream. At some critical level the surface waters will neither be cool nor salty enough to sink, and a log jam of warm water pushing up from behind will cause the system to stall. Climate records gleaned from ice-core samples and from the ocean bottom show that a similar stalling has occurred in the past.

What has surprised geologists and climatologists is the suddenness with which the flowing Gulf Stream can stall and the temperature can change over northern Europe: it can all happen in a matter of years, not centuries or millennia. Marine scientists from Scotland and the US have found a 20 per cent drop in the temperature of the deep-bottom flow of the ‘overturned’ waters from the Gulf Stream close to the Faroe Islands. Again feedbacks are involved. Less Arctic sea ice means that less light is reflected away during the spring, summer and autumn and more is absorbed into an ice-free sea. That will prevent the Gulf Stream waters cooling sufficiently, let alone retaining sufficient saltiness for sinking to occur.

No one knows precisely the critical turning point at which the system will flip. Could we be on the very edge of it now?

The methane time bomb

Currently, several hundred million tonnes of methane leak into the atmosphere every year; most of which comes from poorly maintained gas pipelines, rice paddies, cattle farming, the draining of wetlands and the destruction of forests. Over the past 250 years, largely because of human activities, methane concentration in the atmosphere has more than doubled to 1.72 ppm. It is now accumulating in the atmosphere at the rate of around an extra one per cent per year. Weight per weight, this potent greenhouse gas is 20 times more powerful over a 100-year time span than carbon dioxide.

Fortunately for our climate, most of the methane produced remains trapped a few hundred metres down in the sea as methane hydrate – an ice-like water-methane compound. Methane’s majority ingredient is carbon, and the total methane store could constitute as much as 10,000 billion tonnes of carbon – more than 10 times the carbon now found in the atmosphere. The release of just one 10th of that methane store would not only double atmospheric carbon; its impact on global warming would be more than 10 times greater than an equivalent quantity of carbon dioxide.

The methane store is a bombshell waiting to go off. Methane levels in the atmosphere have not been so high since 160,000 years ago, when the earth was undergoing rapid global warming. Could global warming, combined with sea-level rise, suddenly trigger the release of enough methane to raise temperatures far higher than those projected by the IPCC? Most disturbingly, once global warming gets underway more and more methane will vent into the atmosphere. Global warming will beget more global warming.

This article first appeared in the Ecologist February 2004

 

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