IELTS Academic Reading Practice Test 480

Giant sequoias grow in a small number of groves on the western slopes of the Sierra Nevada, a mountain range in California. The groves, perhaps seventy in all, occupy a narrow band at roughly 1,500 to 2,200 metres above sea level, where melting snow provides most of the water the trees need. Outside this limited range the species occurs only where people have planted it. Within its native range, however, the tree is unmistakable: a massive reddish-brown trunk, often free of branches for half its height, supporting a crown of blue-green foliage.

The largest known specimen is the General Sherman Tree, which stands in Sequoia National Park and contains around 1,500 cubic metres of wood in its trunk alone, more than any other living tree. It is about 84 metres tall, and its circumference near the ground exceeds 30 metres. Giant sequoias are also extremely long-lived. The oldest known individuals are more than 3,000 years old, while the General Sherman is estimated to be about 2,200 years of age.

A giant sequoia's defence against fire is its bark. In an old tree this may be 30 centimetres thick or more, and it is soft, fibrous and low in resin. Because it contains little resin and a good deal of tannin, it is also resistant to insects and to the fungi that rot wood. When a low-intensity fire passes through a grove, the outer layers of bark scorch and crumble, but the tissue beneath is insulated from the heat. Scars left by earlier fires are common on old trunks, and new wood gradually grows over them.

The tree does not have a deep taproot. Instead, its roots run close to the surface and spread far beyond the canopy, gathering water from a wide area of shallow soil. The wood is light and brittle, and shatters when a felled trunk hits the ground, which is one reason why the species was never important to the timber industry.

Reproduction begins with cones that are surprisingly small, about five centimetres long, for so large a tree. A mature giant sequoia can carry thousands of them, and each cone holds around 200 seeds. The cones take two seasons to ripen, and many remain closed on the tree for years, opening only when they dry out. Each seed is only a few millimetres long and carries a small wing, so that the wind can carry it some distance from the parent. Even so, only a small proportion of the seeds produced are fertile, and the vast majority never germinate.

Seedlings are fragile, and their requirements are exacting. They need moist, bare mineral soil, plenty of light and freedom from competition with grasses and shrubs. Fire supplies all three. By burning away the litter and killing small trees, a fire exposes mineral soil and opens gaps in the canopy through which sunlight reaches the ground. The heat also dries the cones in the crown above, so that seeds are released onto the freshly cleared surface in the weeks after the fire has passed.

Giant sequoias are not the tallest trees in the world; that title belongs to the coast redwood, a close relative that grows in the fog belt nearer the Pacific coast and can exceed 110 metres. Nor are sequoias the oldest living things, though they are among the oldest. What sets them apart is volume: no other species builds so much living material in a single stem.

The first written accounts of the giant sequoia by settlers date from the early 1850s, when a hunter described trees of extraordinary size in a grove in Calaveras County. Specimens were soon sent east and to Europe, where British botanists named the species Wellingtonia after the Duke of Wellington. That name was later replaced by Sequoiadendron, and the tree was placed in a genus of its own, separate from the coast redwood. The origin of the word sequoia is disputed, but it is often said to honour Sequoyah, the Cherokee scholar who devised a system for writing his language.

In the twentieth century the greatest threat to the groves came from the absence of fire rather than its presence. A century of fire suppression allowed litter and small trees to accumulate, so that when fires did start they burned hotter and reached higher into the crowns. The drought of 2012 to 2015 killed a number of mature trees and left others weakened. Fires in 2020 and 2021 then killed thousands of large sequoias, a far higher proportion than in any previously recorded fire.

Most groves now lie within national parks and monuments, where logging is prohibited, but protection of this kind has proved easier than protection from severe fire. Managers are now thinning small trees and setting controlled fires to reduce the fuel that has built up, a task made harder by the steep terrain and by the risk of an escape. Whether the groves will survive in their present form as the climate warms is uncertain. Computer models suggest that suitable habitat will contract and move upward, and the trees cannot move with it.

'Sea level' sounds like a single number, but the sea surface is never still. Tides raise and lower it twice a day in most places, waves and storms add shorter-term variation, and currents and differences in gravity cause the water to stand higher in some regions than in others. Scientists therefore define mean sea level as an average over time and space, usually over many years and over an area large enough for local irregularities to cancel out. Measuring a change of a few millimetres a year in such a restless system is a considerable technical achievement.

Long before anyone worried about climate, ports needed to know the height of the tide. Simple tide poles, graduated staffs fixed to a harbour wall, were read by hand, and the earliest continuous records come from such instruments. Brest in France has a series of measurements beginning in 1807, and the Baltic ports have records going back even further. From these figures astronomers learned to predict tides, and printed tide tables became essential equipment for every ship.

The first self-recording gauges appeared in the nineteenth century. Their central component was a float in a stilling well: a vertical pipe, open to the sea at the bottom, with a small hole that allows water in and out. Because the hole restricts the flow, waves and swell inside the well are damped, while the slow rise and fall of the tide is passed on almost undiminished. The float rose and fell with the water inside the well and moved a pen across a paper chart wound round a clockwork drum.

Modern gauges replace the float with other sensors. A pressure sensor on the seabed measures the weight of water above it, while radar or acoustic instruments measure the distance down to the water surface. In each case the aim is the same: a stable record of the water level, free of the short-term noise of waves.

There is a complication that no instrument can solve on its own. A tide gauge is fixed to the land, so it measures the height of the sea relative to the shore, not relative to the centre of the Earth. If the land is sinking, the gauge will record a rising sea even if the volume of the ocean is unchanged. This is not a small effect. In some delta cities, groundwater pumped from beneath the streets has caused the ground to subside by many centimetres a decade, while parts of Scandinavia are still rising as the crust recovers from the weight of the last ice age.

To separate the two movements, scientists now install a continuous GPS receiver beside each important gauge, so that the movement of the land can be measured directly and subtracted. The records themselves are gathered by the Permanent Service for Mean Sea Level, based in Liverpool, which assembles monthly and annual averages from hundreds of stations into a single database.

Since 1992, satellites have measured the sea from above. A radar altimeter sends a short pulse downwards and records the time the echo takes to return; because the signal travels at a known speed, that time gives the distance to the water. The first high-precision instrument, carried by the satellite TOPEX/Poseidon, was followed by a series of successors. The record they have built is still short by the standards of a tide gauge, but it covers almost the entire ocean every ten days, and averaged globally it shows a rise of roughly three millimetres a year.

Two things must be known with great precision for these measurements to mean anything. The first is the satellite's own position, which is calculated from radio signals, from the global positioning system and from laser ranging by ground stations. The second is the shape of the reference surface. An altimeter measures height above a mathematical ellipsoid, not above sea level, because the ocean does not follow a smooth curve: it piles up around large masses of rock and dips in depressions, forming a surface called the geoid. Gravity-measuring satellites map this surface, and its shape is subtracted from every measurement.

The two methods are complementary rather than rival. Tide gauges provide records stretching back a century or more, and they show what is happening at a particular coast, where people actually live. Satellites provide the global average, and they reveal that this average conceals large regional differences. In some parts of the western Pacific, for example, the sea is rising several times faster than the global mean, while off parts of the American west coast it has fallen slightly. Neither method alone would show this pattern.

The remaining uncertainties are not about the instruments but about the future. The largest concern the ice sheets of Greenland and Antarctica, whose contribution is difficult to predict and could change quickly. Separating a long-term trend from natural cycles such as El Nino is also harder than it sounds, because a few years of data can easily mislead. For all the precision of modern instruments, the most important number in the science remains a matter of genuine debate.

A job guarantee is a simple idea with radical implications. The state would offer a paid job, at a fixed wage and with reasonable conditions, to anyone who wants one and cannot find work elsewhere. The jobs would be in public services or community work, such as care, maintenance and environmental restoration, and the offer would remain open however long a recession lasted. Article 23 of the Universal Declaration of Human Rights, adopted in 1948, declares that everyone has the right to work, but its wording has never been read as an obligation on governments to supply a job. The question is whether it should be.

The idea has a long pedigree. William Beveridge, whose 1942 report shaped the British welfare state, argued two years later that full employment should be a central aim of government, though he expected it to be achieved mainly through managing demand rather than by hiring people directly. In the 1960s and 1970s the American economist Hyman Minsky developed a more specific proposal: the government should act as the employer of last resort, offering work at a minimum wage to anyone who could not find it elsewhere. Minsky's scheme was designed less to create useful output than to put a floor under wages and prices.

Supporters today make three arguments. The first is economic. A pool of public jobs would expand automatically when private demand collapsed and shrink when it recovered, stabilising spending without the delays of discretionary policy. The second concerns poverty: a guarantee pays a wage for work, rather than requiring people to prove need, and it reaches those who fall outside insurance schemes. The third is about bargaining power. Where a credible offer of a public job exists, employers must improve pay and conditions to attract workers, and the threat of dismissal loses some of its force.

Some of this can be tested against experience. India's Mahatma Gandhi National Rural Employment Guarantee Act, passed in 2005, entitles every rural household to 100 days of paid unskilled manual work a year. Studies have found that it raised rural wages, particularly for women, and that it cushioned communities during droughts. It has also been criticised for delays in payment and for the difficulty of creating useful work in places with weak administration. Argentina introduced a large work programme during its crisis of 2002, and the United States employed millions of people in the 1930s through the Works Progress Administration, which built roads, schools and parks. None of these schemes was a permanent guarantee, and each was shaped by its political moment.

Critics raise objections of principle and of practice. The practical objection is cost: paying a wage to everyone who cannot find work is expensive, and the expense grows precisely when tax revenues are falling. A second is that a guaranteed wage may become a floor that employers undercut, or a ceiling that traps participants in low-quality work. A third concerns displacement: if public jobs do work that private firms or volunteers once did, the net gain in employment is smaller than it appears. And a scheme that must find work for everyone may end up creating tasks with little value, which demoralises the people doing them.

Economists also disagree about inflation. Advocates argue that a fixed wage, set below prevailing rates, anchors pay rather than pushing it up, and that the automatic expansion of the programme makes it a stabiliser rather than a source of pressure. Sceptics reply that no government can credibly promise to withdraw jobs when the labour market tightens; the programme will grow, they say, and the wage floor will drift upwards until it distorts the whole labour market.

Much therefore depends on design, and design is where the debate is most useful. A guarantee at a very low wage may do little to reduce hardship; at a high wage it may compete with private employers for the same workers. The hours, the training offered, the right to move into a permanent job, and the treatment of people who are ill, disabled or caring for others all determine whether the scheme is a genuine opportunity or a residual tier of employment for those who fail to find anything else.

Evidence from other programmes also raises the question of evaluation. Work schemes are rarely assessed by comparing similar people who did and did not take part, and the results are often reported as numbers employed rather than as changes in earnings or well-being afterwards. Without better evidence, both the hopes of advocates and the fears of critics remain largely theoretical.

My own view is that a job guarantee is worth taking seriously, but for narrower reasons than its supporters usually give. It is unlikely to be a general cure for unemployment, and it cannot replace the broader policies, in education, housing and health, that determine whether work is available. Its strongest claim is that it changes the terms on which low-paid work is offered, by giving the weakest workers an alternative they can actually take. That claim justifies a modest, carefully designed scheme, piloted and evaluated, rather than a promise on which governments cannot deliver.