IELTS Academic Reading Practice Test 28

Long before anyone understood the physics of heat, people found ways to keep food and drink cool. Wealthy households in many cultures stored ice cut from frozen lakes and rivers in winter, packing it with straw or sawdust in underground pits or insulated buildings so that it would last into the summer. In the deserts of Persia, builders constructed tall domed structures known as yakhchal, in which ice made on cold winter nights could be preserved through the hot months. By the nineteenth century, harvesting natural ice had become a substantial industry, and ships carried blocks of it from New England as far as the Caribbean and India. Yet natural ice was bulky and expensive to transport, and a mild winter could ruin an entire season's supply.

The key to producing cold artificially lies in a simple observation: when a liquid evaporates, it absorbs heat from its surroundings. This is why wet skin feels cold in a breeze. In the mid-eighteenth century the Scottish physician William Cullen demonstrated the effect at the University of Glasgow, using a pump to lower the pressure above a container of ether. The ether boiled rapidly and drew so much heat from its surroundings that a small amount of ice formed. Cullen, however, saw no practical use for his experiment, and for decades it remained little more than a scientific curiosity.

The first machine designed to apply the principle continuously was patented in London in 1834 by Jacob Perkins, an American inventor living in England. His design contained the essential elements of almost every refrigerator built since. Rather than allowing the evaporated gas to escape, it captured the vapour, compressed it and turned it back into a liquid so that it could be used again in a closed loop. Perkins's apparatus was never manufactured commercially, but the principle it embodied, now known as vapour compression, would eventually dominate the industry.

In a modern domestic refrigerator, the working fluid, known as the refrigerant, circulates continuously through four main components. Inside the cabinet, liquid refrigerant at low pressure passes through a coil called the evaporator, where it boils and absorbs heat from the food compartment. The resulting vapour then travels to the compressor, usually located near the bottom of the appliance, which squeezes it to a high pressure and, in doing so, raises its temperature.

The hot vapour next flows through the condenser, the grid of thin tubes often visible on the back of the refrigerator, where it releases its heat into the kitchen and condenses back into a liquid. Finally, it passes through a narrow expansion valve, which reduces its pressure so sharply that it cools, ready to begin the cycle again. A refrigerator, in other words, does not create cold; it moves heat from inside the box to the room outside.

During the 1850s several inventors working independently turned these ideas into practical ice-making machines. In Florida, the physician John Gorrie, who believed that cooler air would help patients suffering from fevers, patented an ice machine in 1851, although he failed to attract investors and the venture collapsed. In Australia, the Scottish-born printer and journalist James Harrison built vapour-compression machines that used ether, and supplied them to breweries and meat-processing works. In France, Ferdinand Carré developed a different system based on absorption, in which ammonia dissolved in water took the place of a mechanical compressor. In the 1870s the German engineer Carl von Linde designed more efficient and reliable ammonia machines, and the brewing industry, which needed low temperatures to produce lager all year round, became one of his first major customers.

Refrigeration soon transformed international trade. In February 1882 the sailing ship Dunedin left Port Chalmers in New Zealand carrying around 5,000 carcasses, mostly of sheep, which were kept frozen by a machine on board. When the ship reached London about three months later, the meat was found to be in good condition and sold well. The voyage proved that perishable food could be shipped across the world, and within a few decades meat from New Zealand, Australia and Argentina had become a familiar sight in British shops.

Refrigerators for the home appeared in the United States during the 1910s and 1920s, and one of the most successful early models, General Electric's Monitor Top, went on sale in 1927. These machines were expensive, and many relied on gases such as ammonia, sulphur dioxide or methyl chloride, which were toxic and in some cases flammable, so that a leak could injure or even kill. In search of a safer alternative, a team led by the engineer Thomas Midgley developed a family of chlorofluorocarbons, or CFCs, around 1930. At a meeting of the American Chemical Society, Midgley famously inhaled the gas and then used it to blow out a candle, a demonstration intended to show that it was neither poisonous nor flammable.

For four decades CFCs seemed close to ideal. In 1974, however, the chemists Mario Molina and Sherwood Rowland showed that these stable compounds could drift into the upper atmosphere, where ultraviolet light broke them apart and released chlorine that destroyed ozone. Under the Montreal Protocol of 1987, CFCs were phased out worldwide. Their main replacements, hydrofluorocarbons, do not harm the ozone layer but are powerful greenhouse gases, and a 2016 amendment agreed in Kigali, Rwanda, commits countries to reducing their use. The search for a refrigerant that is safe, efficient and environmentally harmless therefore continues, nearly two centuries after Perkins's patent.

Glaciers are among the most sensitive indicators of a changing climate. Outside the great ice sheets of Greenland and Antarctica, there are more than 200,000 glaciers on Earth, ranging from small patches of ice in mountain hollows to vast rivers of ice many kilometres long in Alaska, the Himalayas and Patagonia. Almost all of them are shrinking. A study published in Nature in 2021, led by the glaciologist Romain Hugonnet, used archives of satellite imagery to calculate that between 2000 and 2019 the world's glaciers lost an average of 267 billion tonnes of ice a year, and that the rate of loss was increasing. This meltwater accounted for about a fifth of the rise in global sea level over the same period.

Whether a glacier grows or shrinks depends on what glaciologists call its mass balance: the difference between the snow it gains and the ice it loses. In its upper reaches, the accumulation zone, more snow falls each winter than melts in summer, and over many years the snow is compressed into dense glacial ice. Lower down, in the ablation zone, losses exceed gains. The boundary between the two, known as the equilibrium line, moves up the mountain in a warm year and down in a cold one. Meanwhile, the ice is never static: it flows slowly downhill under its own weight, carrying material from the accumulation zone towards the ablation zone. When losses consistently outweigh gains, the glacier thins, and its front, or terminus, retreats uphill, even though the ice itself continues to move forward.

Mountain glaciers have not always been in retreat. During the cool centuries often called the Little Ice Age, many Alpine glaciers advanced, at times overrunning farmland and threatening villages, and most reached their greatest modern extent around the middle of the nineteenth century. Systematic international monitoring began in 1894, and today the World Glacier Monitoring Service in Zurich gathers measurements from around the world. Some of the most detailed records come from a small number of reference glaciers, where researchers drill stakes into the ice and measure how much of each stake has been exposed by melting. These traditional observations are now combined with satellite measurements of surface height and even of tiny changes in the Earth's gravity field.

Recent years have broken records. In Switzerland, scientists reported that the country's glaciers lost around 6 per cent of their remaining volume in 2022 and a further 4 per cent in 2023, the two worst years since measurements began. The causes were a combination of low winter snowfall, which left the ice poorly protected, and long, hot summers. In 2019 Icelanders held a ceremony to mark the loss of Okjökull, a glacier that had shrunk so much that it no longer moved under its own weight and could therefore no longer be classified as a glacier. A plaque placed at the site carries a message addressed to future generations.

Several processes can accelerate the decline once it has begun. Fresh snow reflects most of the sunlight that falls on it, but as a glacier loses its snow cover, darker ice is exposed, which absorbs more energy and melts faster. Dust, soot from fires and industrial pollution, and even microscopic algae that grow on the ice surface can darken it further. As a glacier thins, its surface also drops to lower and warmer altitudes. For these reasons, many glaciers are already out of balance with today's temperatures and would continue to shrink for decades even if warming halted immediately.

The consequences extend far beyond the mountains. In many regions, glaciers act as natural reservoirs, storing water as ice and releasing it during the hot, dry months when rivers would otherwise run low. A 2020 study led by Walter Immerzeel ranked the world's mountain water towers and estimated that 1.9 billion people live in or directly downstream of them. Paradoxically, a shrinking glacier at first produces more meltwater, not less. Eventually, however, the remaining ice becomes too small to sustain this flow, and runoff passes a maximum, known as peak water, before declining. In parts of the Andes and Central Asia, some rivers are thought to have already passed this point.

Retreating ice also creates hazards. As glaciers withdraw, meltwater often collects behind unstable ridges of rock and debris left by the ice, forming lakes that can grow rapidly. If such a natural dam fails, perhaps after a landslide or avalanche plunges into the lake, the result is a sudden torrent known as a glacial lake outburst flood, which can travel many kilometres downstream and destroy roads, bridges and villages. In the Himalayas and the Andes, authorities monitor the most dangerous lakes and in some cases have lowered water levels artificially by digging channels or installing pipes.

The future of the world's glaciers now depends largely on the course of global temperatures. A 2023 study in the journal Science estimated that even if warming were limited to 1.5 degrees Celsius above pre-industrial levels, roughly half of the world's glaciers would disappear by 2100, most of them small, while greater warming would remove a much larger share. Local measures can only slow the loss in a few places. At the Rhône Glacier in Switzerland, for example, ice near a popular tourist attraction is covered each summer with white fabric that reflects sunlight, but such efforts are far too costly to apply widely. The authors of the 2023 study emphasised that every additional fraction of a degree of warming would mean more glaciers lost, so that the fate of mountain ice is still partly within human control.

Few complaints about education are as persistent as the claim that grades are not what they used to be. Each summer, when examination results are published, commentators greet rising numbers of top grades with suspicion, and some university teachers privately admit to awarding marks that their predecessors would have considered generous. Defenders of the system respond that students are better prepared and work harder than earlier generations did. Both sides have a point, but I believe the debate is often conducted at cross purposes, and that the real damage done by grade inflation is not the one most critics emphasise.

The evidence that grades have risen is strong in several countries. In the United States, the researchers Stuart Rojstaczer and Christopher Healy compiled grading data from more than a hundred colleges and universities and found that, by the late 2000s, about 43 per cent of all letter grades awarded were As, an increase of 28 percentage points since 1960. In 1960 the most common grade had been a C; half a century later it was an A. At a number of highly selective institutions, A-range grades now make up the majority of those awarded.

In the United Kingdom, the most visible measure is the proportion of students awarded first-class degrees. An analysis by the Office for Students, the regulator for higher education in England, found that the share of firsts rose from 16 per cent in 2010–11 to 29 per cent in 2017–18. Crucially, the regulator tried to take account of changes in the student population, such as higher entry qualifications, and concluded that a large part of the increase could not be explained by those factors. Across the UK, official statistics show that the proportion of firsts peaked at 36 per cent in 2020–21, when assessment was disrupted by the pandemic, before falling back to 30 per cent two years later. Schools showed a similar pattern during the pandemic: in England, when examinations were cancelled and teachers assessed their own students, the proportion of A-level entries graded A or above rose from 25.2 per cent in 2019 to 44.3 per cent in 2021.

It would nevertheless be wrong to assume that every increase represents a decline in standards. Average grades can legitimately rise if teaching improves, if students become more motivated, or if assessment shifts from comparing students with one another to measuring whether they have mastered a defined body of knowledge. Under such criterion-referenced systems, there is no reason in principle why most students should not reach a high standard. Critics who treat any rise in top grades as proof of declining rigour are therefore making an assumption that the figures alone cannot support.

Even so, there are good reasons to suspect that pressures unrelated to learning have played a part. Universities compete for students in league tables that reward high proportions of good degrees. In many institutions, students' anonymous evaluations of their teachers influence decisions about hiring and promotion, and several studies have found that students tend to rate courses more favourably when they are graded leniently. Where students pay substantial tuition fees, some academics also report feeling pressure to treat them as customers whose satisfaction must be protected. None of these influences requires anyone to act dishonestly; each simply tilts countless small judgements in the same direction.

The most serious consequence, in my view, is not that students are flattered but that grades lose their ability to distinguish between them. When most students in a course receive an A, the grade conveys little information to employers or graduate schools, which then rely more heavily on other signals, such as the reputation of the institution attended, internships or personal connections. These alternatives tend to favour students from wealthier backgrounds. Paradoxically, therefore, a practice that appears kind to students may disadvantage precisely those who most need a fair and transparent measure of their abilities. Inflation also deprives the strongest students of useful feedback, since there is little incentive to work beyond a mark that can rise no further.

Attempts to reverse the trend unilaterally have proved difficult. In 2004 Princeton University adopted a policy under which departments were expected to award A-range grades to no more than 35 per cent of students in their courses. The proportion of As fell, but the policy was unpopular with students, who feared that their transcripts would compare unfavourably with those of applicants from other universities, and there were concerns that it was discouraging talented applicants. A faculty committee also found that it had not produced consistent grading across departments, and in 2014 the university abandoned it. The lesson is that no single institution can easily act alone while others continue to inflate.

A more promising approach is to add information rather than to ration high grades. Some universities now report on transcripts the distribution of grades in each course, or a student's position within the class, so that an A in a course where almost everyone receives one can be read differently from an A awarded to a small minority. Such measures do not require anyone to be marked down, and they restore at least part of the signal that inflation has eroded. Grade inflation, in short, is less a moral failing than a collective problem, and it will be solved by better information, not by nostalgia for a stricter past.