by Colin Tudge
All kinds of evidence show that the fluctuations in temperature, from global tropical to global freezing and back again, are correlated with the concentration of carbon dioxide in the atmosphere. Some of the most convincing evidence comes from ancient ice in Greenland and Antarctica, which traps atmospheric gas from earliest times that can then be directly analysed. When the concentration of carbon dioxide was high, the fossils show the world was warm. When it was low, the world was obviously cool.
The physics is simple. The earth is constantly warmed by sunlight, and constantly radiates the warmth away again. The solar energy coming in contains light of all wavelengths. The radiant heat that the earth gives out is all in the infra-red spectrum. Carbon dioxide absorbs infra-red. So atmospheric carbon dioxide filters out only a fraction of the solar energy coming in – a proportion of the infra-red; but it traps a great deal of the energy going out, since most of what’s going out is infra-red. So more energy is kept in than is kept out, and the net effect is to warm the atmosphere. The glass in a greenhouse operates in the same way, so this is called the ‘greenhouse effect’ and carbon dioxide is called a ‘greenhouse gas’. Some other gases also have a greenhouse effect, including nitrous oxide, methane and water vapour. But carbon dioxide is the main variable.
It may seem odd that carbon dioxide can have such an effect. After all, its concentration in the atmosphere is low. The present concentration of carbon dioxide in the atmosphere is only around 370 parts per million. Yet is has been calculated that if the atmosphere contained no carbon dioxide at all, the average surface temperature would fall to –18° C. At the time of the ice ages, atmospheric carbon dioxide was down to 190 to 200 parts per million; and it was indeed very cold. When plants first colonized land 400 million years ago, atmospheric carbon dioxide stood at around 7,000 parts per million – and the world was extremely warm. At the time when cyanobacteria first evolved photosynthesis, more than 2 billion years ago, the atmosphere contained no oxygen at all. Carbon dioxide was the chief gas of the atmosphere, apart from nitrogen. The world must then have been ridiculously hot. For creatures like us (or modern trees) the heat alone would have been lethal (let alone the noxiousness).
Modern meteorological records, and the notebooks of gardeners and naturalists, show that the world has been getting warmer this past 150 years or so. The Intergovernmental Panel on Climate Change (IPCC) reported in 2001 that the average temperature of the whole world increased by 0.6° C between 1900 and 2000. Such a rise may not seem great, but ostensibly small changes averaged over the whole world can imply huge changes locally and regionally – enough to have profound effects on environments and people. The trend seems inexorably upwards. 1998 was the warmest year since reliable records began; and the 1990s was the warmest decade.
Changes in temperature cause changes in rainfall. Precipitation rises overall (since more water evaporates from the surface of the oceans), but the distribution is uneven; so some places become much wetter, and others much drier. The IPCC noted an overall increase in precipitation (rain and/or snow) of 0.5 to 1 per cent per decade through the twentieth century: up to 10 per cent over the century. The increase was highest in the mid and high latitudes of northern continents. In the tropics (10° N to 10° S) the increase was only 0.2 to 0.3 per cent per decade – and in the northern subtropics (10° N to 30° N) rainfall actually lessened, by around 0.3 per cent per decade. The subtropics are hugely important to humanity for their agricultural crops, their forests, and as places where people live. Drought in that latitudinal band is seriously bad news. In the southern hemisphere, which lacks a vast continental land mass and has vast oceans instead, there were no comparable changes – or at least the changes were not so regular.
But overall averages are not all that matters. Extremes increased too – just as theory predicts must be the case, since the temperature of the world’s surface changes unevenly. More and more places recorded their hottest days in history. The late twentieth century saw some of the worst-ever storms and hurricanes, floods, droughts and forest fires. The southern hemisphere is hugely affected – one might say plagued – by El Niño, the warm current that flows from west to east across the Pacific every few years as a result of the ‘southern oscillation’. The western South Pacific warms up more than the east, and every so often the warm water in the west escapes, and flows to the east. The El Niño current (so called because it tends to flow around Christmas time – Niño means ‘little boy’ and refers particularly to the Christ Child) causes floods in some places, droughts in others, and also causes fish stocks to fail, with the consequent devastation of seabirds, and potentially of local fisheries. El Niños seem to be getting more frequent and more severe.
The changes in atmospheric carbon dioxide that caused huge climatic fluctuations in the deep past were brought about by natural events. In general, carbon in the earth’s surface takes the form of carbon dioxide that floats free in the atmosphere, or is dissolved in the oceans. Or it is present as ‘organic’ carbon, locked up in the bodies (including the dead bodies) of living creatures – flesh, leaf litter, and timber; and carbonates, in rocks. Rainfall, volcanic action and the movements of tectonic plates ensure that carbon flows between the four sources, changing chemically as it does so – from carbonate to carbon dioxide to flesh (plus dead leaves and timber) to carbon dioxide and back to carbonate.
The general cooling over the past 40 million years or so has apparently been caused by the (fairly) steady loss of carbon dioxide from the atmosphere; and this has been ascribed to the rise of the Himalayas and the Tibetan Plateau, caused by the slow tectonic crunch of India into the south of Asia. The Tibetan Plateau and the Himalayas are a huge mass of rock, which interrupts the saturated winds that blow from the Pacific, and thus causes the rains that manifest as the monsoons. This rain is steeped in atmospheric carbon dioxide, which reacts with the rocks of the Himalayas, and then washes into the sea. Thus, carbon dioxide has been steadily leached from the atmosphere, and the world has cooled accordingly.
But carbon dioxide has been rising again this past 200 years or so – not from natural causes but mostly through the restless activity of humanity. Atmospheric carbon dioxide now stands at around 370 parts per million, but in 1750, at the start of the Western world’s Industrial Revolution, it was a mere 280 parts per million. The annual rise averages around 0.4 per cent. Methane in the same period has risen by 151 per cent, and nitrous oxide by 17 per cent, and both continue to rise. By 2080, at the current rate of increase, carbon dioxide levels will be double present levels, around 750 parts per million. This will make the world between 1.4 and 5.8°C warmer than it was in 1990, which will melt the polar ice caps (as is already happening) and increase sea levels by anywhere between 9 and 88 centimetres. The higher figure, the best part of a metre, would be enough to wipe out many a small island (and not a few major cities). The calculations are rough for all kinds of reasons – not least because more warmth means more cloud, and more cloud would be cooling, and so would have a negative feedback effect. But future changes in cloud cover are difficult to calculate.
In recent years, too – in fact since 2001 – American meteorologists in particular have realized that over the past few decades the full effects of global warming have been masked by ‘global dimming’. It transpires that atmospheric pollution by particles – variations on a theme of soot – has been reducing the energy input from the sun by an astonishing 30 per cent. The world, currently, is cleaning up the soot – which, technically, is fairly easy to do. Catalytic converters on cars do much of what is needed, and these are encouraged because they are big business, and their development and sale increases GDP. But the world is not commensurately reducing its output carbon dioxide. The Kyoto Protocol of 1997 was and is intended to reduce carbon dioxide output, or at least to ensure that whatever extra carbon dioxide is produced, is mopped up. The Protocol was not ratified until the beginning of 2005. The United States, by far the world’s biggest producer of industrial carbon dio
xide, has not signed up to it. But the Protocol would not go far enough, even if everyone did sign up, and acted upon it. Meanwhile, most of the rest of the world has yet to industrialize, and evidently feels that industrialization is necessary. China has already become the world’s second-greatest carbon dioxide emitter.
To reduce carbon dioxide on the necessary scale would mean severe curtailment of the use of fossil fuels, and although this can be made profitable (house insulation could and should become a significant industry), on the whole it is cheaper and easier to continue business as usual. The carbon dioxide output of the United States is virtually unabated, and, although we have yet to see which way China will go at present, it seems hell-bent on further industrialization. As the dimming soot is removed and carbon dioxide continues to rise, both the magnitude and the speed of global warming seem liable to exceed even the most extreme predictions of the late twentieth century.
Trees, as ever, are or should be at the heart of all discussion on climate change. The changes in carbon dioxide, in temperature, and in patterns of rainfall will each affect them in many ways – and each paramater interacts with all the others, so between them these three main variables present a bewildering range of possibilities.
Rising carbon dioxide alone – even if temperature and rainfall stayed the same – should accelerate photosynthesis. This in theory must increase the amount of carbon in each tree, and reduce the amount in the atmosphere – and if enough trees photosynthesized more quickly, then they could in theory reduce atmospheric carbon dioxide enough to lower global temperature. That would produce a negative feedback loop: an excellent outcome. We know that rising carbon dioxide can indeed stimulate photosynthesis. Commercial growers sometimes ply their crops with extra carbon dioxide to boost their growth. In large-scale experiments in forests and plantations, groups of trees that have been semi-enclosed and then given extra carbon dioxide also grow faster, as they fix more carbon.
But all is not so simple. Key players in photosynthesis are the stomata, the holes in the leaf surface that allow the carbon dioxide to enter. Rising levels of carbon dioxide, and increasing photosynthesis, stimulates the stomata to close. This, of course, will slow the increase in photosynthesis. So whatever effect the increase in photosynthesis may have in reducing atmospheric carbon dioxide will be limited. At some point, metaphorically speaking, the trees will cry, ‘Enough!’ They could simply be overwhelmed.
And as carbon dioxide increases so too will temperature – and this complicates the picture again, in many ways. Heat in general increases the rate of all chemical reactions, and since metabolism is the sum total of the body’s chemistry, rising temperature means faster metabolism. Rising temperature should stimulate photosynthesis along with everything else, which is fine: the warmer it gets, the faster the trees should absorb CO2, and so prevent further warming.
But – there is always a but! – rising temperature will also stimulate respiration – the burning of sugars; and respiration may speed up more quickly than photosynthesis does, and if it gets too warm there could be net loss of carbon from any one plant. Worse: the many creatures that live in the soil – bacteria, fungi, invertebrates – will also respire faster as temperature rises. Since they feed primarily on leaf litter, they will break it down faster – and so release the carbon it contains more rapidly. Thus rising temperature could easily cause a net loss of carbon from the forest as a whole – partly from the trees, and partly from the ground. Finally, if temperatures rise too high, then essential enzymes within the plant are damaged, and the plant starts to die. As this happens, photosynthesis stops and the living parts decay. Then, the net release of carbon into the atmosphere becomes massive – and the temperature rises even more. Again, there is some direct experimental evidence to support this scenario.
Of course, tropical trees are adapted to heat, and should not be killed by the kind of temperatures that are envisaged; and all trees can adapt to change to some extent, given time. But what’s truly serious, not to say terrifying, is the rate of climate change. We may well see significant temperature rises in the next half-century, or even sooner if the global dimming hypothesis turns out to be correct. Few trees indeed could adapt in such a time, and none could make the necessary, generation-by-generation genetic changes. Only the creatures with short lifetimes – bacteria, flies, perhaps mice – can effect significant change in such a period. So we could see a massive die-off, pushing even more carbon dioxide into the atmosphere, resulting in higher and higher temperatures, leading to more deaths, and so on.
Then there’s the matter of water. If plants have a design fault, it lies with their stomata. The same neatly guarded apertures that allow the essential carbon dioxide to enter also, inevitably, allow evaporating water to leave. This of course is exacerbated by rising temperatures. So plants that are warmed more than they are used to, in regions where water is the limiting factor (which is often the case even in rainforest – for many rainforests have dry seasons) will wilt and eventually die: the precise opposite of what is desired. Again, there is direct evidence from the field. Scientists in Brazil, including Dr Yadvinder Malhi of Oxford University, have deprived whole hectares of trees of water by covering the ground in polythene panels, so that most of the rain runs off into drains around the edges. As might be predicted (but these things always need to be demonstrated), the trees quickly show signs of suffering. Growth stops – meaning that respiration starts to exceed photosynthesis, meaning that carbon dioxide, in net, is being lost.
As a final botanical complication, it seems likely that when conditions favour more photosynthesis – more carbon dioxide, more warmth (within limits), and more water – then the epiphytes, the mass of ferns, bromeliads, orchids and others that grow on trees, will grow quicker than the trees themselves. This may not matter for the wellbeing of the planet as a whole – so long as some plants absorb more carbon, it does not matter which they are. But overgrowth of epiphytes would be bad for trees, and would profoundly affect overall forest ecology (it could in theory increase biodiversity, though all change is more likely to be destructive in the short term). This last complication proves another general point: that prediction is immensely difficult because we just don’t know enough about the physiology of wild creatures in general, let alone the result of all their interactions; and if we did know enough, the computers that help to make sense of masses of data would be very hard pressed indeed to simulate reality reliably, on the local as well as on the grandest scale.
Then there is fire. More and more fires these days are started by human beings, deliberately or inadvertently, although even when deliberate and for legitimate agricultural or conservational purposes, they sometimes get out of hand. But fires, commonly though not exclusively triggered by lightning, are part of nature. In the places where they naturally occur – virtually everywhere that has anything to burn and is not permanently wet – the local plants (and animals, to a greater or lesser extent) tend to be adapted to them. Grasses need to have their tops burned off if grazing animals do not do the job for them, or the tops become senescent and stifle the fresh growth beneath. As we have already seen, many trees are highly fireproof, like redwoods and eucalypts, and the seeds of many pines and other species will not germinate unless first effectively cooked, whereupon they ‘know’ they can sprout in the nutrient-rich ash provided by their immediate predecessors.
But as with every input – including water, general warmth, light, carbon dioxide, and many minerals that are in small doses essential –there can be too much of a good thing. Fire is lethal to the trees that are not adapted to it, of course – and even for those that need it, timing and intensity are all. If, for whatever reason, the fires come too frequently, or too rarely, or burn too intensely, then the best adapted trees are overwhelmed. Human beings are altering the world in ways that are very much against the interests of wild trees, upsetting even those that seem well adapted.
Human beings have been using fire to influence vegetation
probably for about 500,000 years: that at least is the age of the oldest fires that are thought to have been started by humans. Human beings in those earliest days were not modern, even anatomically. They had smaller brains than we have. But they knew about fire. Sometimes our ancestors set fire to the local plants, as Australian Aborigines may still do, to drive wild animals into traps, or simply to freshen the vegetation (grasses) to attract more grazers. Sometimes they set fire to trees just to get rid of them, and then sow crops or plant grass for cattle in the ashes. Sometimes people just seem to want a better view. As the world becomes more and more crowded and complicated, there are many conflicts of interest, each requiring a different attitude to fire. Conservationists in general want to keep trees, but recognize the need for occasional fires, for example to stimulate germination. Tour operators and people who live in the country commonly want to keep trees, but hate the idea of fires, which reduce some of the forest to ashes and threaten their houses. Farmers commonly want to get rid of trees –but prefer it if the ones that do remain do not catch fire and burn their farms. People in general tend to feel that fires are a bad thing, and politicians have an eye to their votes.
Even in the absence of any specific policy, however, human beings have an immense influence on the likelihood and frequency of fires. Thus fires in forests tend to begin with the leaf litter; and in the savannah, they commonly begin with the grasses. Over the past few decades the Brazilians (and North Americans too) have introduced several grasses from Africa, which they feel make better fodder for cattle. Some of these grasses have crept in to the Cerrado, the dry forest, primarily along the disturbed ground along the verges of the roads that now criss-cross the country. From there they spread into the surrounding land. These particular grasses, as it happens, burn more slowly than the native grasses: and this prolonged burning is far more damaging to other vegetation than the quick, albeit hotter flames generated by the native grasses.