IELTS band 8 plan, week 3: Matching and summary completion

Trees in the city

A For much of the twentieth century, street trees were planted mainly for their appearance. City engineers and planners tended to regard them as decoration, to be removed whenever a road needed widening or a pipe needed replacing. That view has been challenged as researchers have begun to measure what trees actually do.

B The clearest benefit is shade. On a summer afternoon, an unshaded footpath can be many degrees hotter than one beneath a full canopy, and the air under large trees is noticeably cooler. Ecologists point out, however, that the effect depends heavily on the size of the canopy. A young tree may take fifteen years or more to provide meaningful shade, which is why they argue that protecting existing mature trees should come before planting new ones.

C Claims about air quality are less straightforward. Leaves do trap some fine particles, but in narrow streets lined with tall buildings, a dense canopy can slow the movement of air and hold traffic fumes close to the ground. Public health researchers have therefore cautioned against presenting trees as a remedy for pollution, although they note that the link between green streets and lower stress levels is well supported.

D Trees also intercept rain. Water caught on leaves or taken up by roots does not reach the drains, which can ease the load on stormwater systems during heavy storms. Engineers, once among the strongest critics of street trees because roots lift pavements and crack pipes, have in some cities become their advocates, designing underground soil cells that give roots room to grow while also holding water.

E Putting a value on these services has proved difficult. Economists have tried to estimate the worth of a single street tree by adding up energy savings, reduced drainage costs and higher property prices, but they acknowledge that such totals leave out benefits that are hard to price, such as the pleasure of a shaded walk. Some also warn that rising property values can push out the very residents a planting programme was meant to help.

F That concern reflects a wider pattern. In many cities, tree cover is lowest in poorer neighbourhoods, often because they were built with narrow streets and little open space. Programmes to correct this have found that planting alone is not enough: young trees that are not watered through their first summers frequently die, so the most successful schemes involve local residents in their care from the start.

Reading the ice

A The great ice sheets of Greenland and Antarctica are built from snow that has fallen over many thousands of years. As each year's snow is buried by the next, it is compressed into firn, a dense, granular material, and eventually into solid ice. Crucially, as firn turns to ice, small pockets of air are sealed inside, preserving samples of the atmosphere.

B Scientists recover this record by drilling cores: cylinders of ice extracted in short sections, which together may be several kilometres long. In the upper part of a core, individual years can often be counted, because summer and winter snow differ in density and chemistry, rather like the rings of a tree. Deeper down, however, the weight of the ice above thins the layers until they can no longer be told apart, and researchers must instead rely on models of how the ice flows and on markers of known age.

C Among the most useful markers are the traces of volcanic eruptions. A large eruption leaves a thin layer of ash or acid over a wide area, and where the date of the eruption is known from other sources, that layer fixes a point in the core. Identifying the same layer in cores drilled at different sites also allows their records to be aligned with one another.

D One complication is that the air in the bubbles is younger than the ice around it. Because air can still move through firn until its pores close, the gas is sealed decades or even centuries after the snow fell, and the gap is greatest where snowfall is low. Researchers correct for this difference, but it adds uncertainty to any comparison between temperature, which is inferred from the ice itself, and the concentration of gases in the trapped air.

E Despite these difficulties, ice cores have provided one of the most detailed records of past climate. The longest continuous cores reach back hundreds of thousands of years, and they show that carbon dioxide levels and temperature have tended to rise and fall together over that period.