IELTS Academic Reading — Practice Test 13

Forests occupy a peculiar place in the arithmetic of climate change. A growing tree draws carbon dioxide from the air and locks the carbon away in its trunk, branches and roots; a forest cleared and burned releases that store back into the atmosphere. Because roughly a tenth of humanity's annual carbon emissions comes from the destruction of forests, and because trees offer one of the few affordable ways of pulling carbon back out of the air, governments and companies alike have become intensely interested in measuring exactly how much carbon the world's forests hold and how fast that quantity is changing. Whole markets and international agreements now rest on those figures, and billions of dollars change hands on the strength of them. The trouble is that this apparently straightforward accounting turns out to be riddled with difficulty at every stage.

Consider first the business of measurement itself. No one can weigh a forest. Instead, researchers combine satellite images, which reveal how tree cover changes across large regions, with measurements taken on the ground, where the girth and height of sample trees are recorded and fed into equations that estimate their mass. From that mass the carbon content can be inferred, since dry wood is about half carbon by weight. The method is ingenious, but each step introduces uncertainty: a satellite cannot easily distinguish a dense natural forest from a spindly plantation, and the equations linking a tree's dimensions to its weight, derived from a limited number of felled specimens, may not hold across the enormous variety of the world's woodlands.

A subtler problem concerns the difference between gross and net change. Headlines usually report a single figure for forest loss, but forests are simultaneously being cleared in one place and regrowing in another. The gross figure counts only the areas where forest has been lost; the net figure subtracts the gains from the losses. Each tells a different story, and which one is quoted can dramatically alter the picture. A country might boast that its net forest cover is stable while quietly felling ancient, carbon-rich forest and replacing it with young trees that will take decades to store an equivalent amount. Counting hectares, in other words, is not the same as counting carbon, because not all forests are equal in the amount of carbon they hold.

This leads to the vexed question of what a forest actually is. International definitions are often surprisingly loose, setting a minimum area, a minimum tree height and a minimum proportion of ground shaded by canopy. By such criteria a plantation of fast-growing eucalyptus, destined to be cut and pulped within a few years, qualifies as forest, as does land that has been logged but is expected to grow back. A definition broad enough to be applied worldwide thus lumps together ecosystems of wildly different value, both for the carbon they store and for the wildlife they shelter. When a government pledges to halt 'deforestation', the strength of that promise depends entirely on the fine print of how the word is defined.

Even where carbon is genuinely being stored, a further doubt hangs over the whole enterprise: permanence. Carbon locked in a tree is not locked away for ever. A forest protected today may burn in a wildfire tomorrow, or be cleared in twenty years when a government changes or a commodity price rises, at which point its carbon returns to the atmosphere. This matters acutely for the carbon-offset market, in which a company that continues to emit carbon pays to protect or plant a forest elsewhere and claims the resulting storage against its own emissions. If the forest later disappears, the emissions were real but the offset was not, and the atmosphere is left with the carbon of both. Guaranteeing that a forest will still be standing in a century — long after the company, and perhaps the country's current government, has gone — is a promise that no one can honestly make.

For these reasons a number of scientists have grown wary of treating forests as a simple carbon ledger, to be debited and credited like a bank account. Their objection is not that forests are unimportant — quite the reverse. It is that reducing a living, complex ecosystem to a single number invites exactly the kind of accounting tricks the numbers were meant to prevent: planting monocultures that look good on a spreadsheet, or claiming credit for protecting forests that were never under threat in the first place. A forest, on this view, is worth protecting for the water it cleans, the species it houses and the communities it sustains, and its carbon is best seen as one benefit among many rather than as a currency to be traded. Whether the world's climate policies can accommodate so untidy a truth remains to be seen.

For much of the twentieth century, the ability to make and use tools was held up as the trait that set humanity apart from the rest of nature. To fashion an object for a purpose, the reasoning went, required foresight, an understanding of cause and effect, and a mind able to hold a goal in view — capacities assumed to be ours alone. That comfortable assumption has been dismantled, piece by piece, over the past sixty years, as patient observation of animals in the wild has revealed tool use in creatures separated from us by hundreds of millions of years of evolution. What remains fiercely contested is not whether animals use tools, but what their doing so actually tells us about the minds behind the behaviour.

The first cracks appeared in 1960, when a young researcher watching chimpanzees in Tanzania saw one strip the leaves from a twig and poke it into a termite mound to extract the insects clinging to it. Here was an animal not merely using an object but modifying one to suit a task. Since then the list has grown long and surprising. Sea otters balance stones on their chests to crack open shellfish; some Egyptian vultures throw rocks to break ostrich eggs; and, most impressively of all, the New Caledonian crow fashions hooks from twigs and cuts barbed tools from the stiff edges of leaves, working the material into shapes that improve its catch. Tool use, it turns out, is scattered across the animal kingdom, appearing in mammals, birds and even some invertebrates.

The difficulty begins the moment one asks what these behaviours mean. To an observer, a crow bending a wire into a hook looks like a creature reasoning its way to a solution. But appearances can mislead. A long tradition in the study of animal behaviour insists that we should never attribute a feat to complex thought if a simpler explanation will serve — a principle, sometimes called Morgan's canon, intended as a guard against the human habit of reading our own kind of mind into everything we see. By this cautious standard, much apparent cleverness might be the product of instinct honed by evolution, or of ordinary trial-and-error learning, in which an animal stumbles on a successful action and simply repeats what is rewarded, no understanding required.

Those who take this sceptical line can point to striking demonstrations in its favour. Behaviour that looks insightful can often be produced, in the laboratory, by nothing more than the gradual shaping of responses through reward. An animal may appear to grasp a problem in a flash when in fact it is drawing on a long history of reinforcement that the observer never witnessed. On this account, the danger is not that animals are stupid but that we are credulous: eager to see kindred intelligence, we mistake the outcome of blind processes for evidence of thought. The burden of proof, the sceptics argue, lies squarely with those who would credit an animal with genuine reasoning.

Yet the evidence has become harder to explain away. In controlled experiments, New Caledonian crows have solved problems they could not have encountered before, selecting tools of the right length for a task on the first attempt, and even using one tool to obtain a second that was needed to reach food — a sequence that is difficult to reduce to simple reward. Some birds appear to plan for the future, saving a tool they will need later rather than one that pays off at once. Such findings suggest that at least some animals do more than react; they seem to represent a problem, weigh options and anticipate outcomes. To dismiss all of this as mere association, the defenders of animal intelligence reply, begins to strain credulity as badly as the anthropomorphism the sceptics fear.

Part of the disagreement, however, is not about the animals at all but about the words we use. 'Tool use' and 'intelligence' are human categories, and there is no settled definition of either that all researchers accept. Is an otter's stone a tool in the same sense as a crow's crafted hook? Does using an object cleverly require the same kind of mind as understanding why it works? The puzzle deepens when one notices that the most sophisticated non-human tool users — certain crows and certain primates — sit on utterly separate branches of the evolutionary tree, their last common ancestor a small creature that used no tools at all. Their abilities cannot have been inherited from a shared toolmaking forebear; they must have arisen independently, more than once. This convergence suggests that complex cognition is not a single ladder with humans at the top but something that evolution can arrive at by different routes, in response to similar pressures.

Where this leaves the old question of human uniqueness is genuinely unclear, and perhaps that is the most honest conclusion available. The discovery of tool use in other species has not so much answered the question of what makes us special as exposed how crude the question was. Intelligence is not one thing that a creature either has or lacks; it is a loose family of abilities, distributed unevenly across the living world, and shading by degrees from the reflexive to the reflective. The task is no longer to draw a line with humanity on one side and everything else on the other, but to map, patiently and without vanity, the many different ways of being clever that evolution has produced. That is a humbler project than the one we began with, and very likely a more interesting one.