CARBON DIOXIDE AND THE "GREENHOUSE EFFECT"

CARBON DIOXIDE AND THE "GREENHOUSE EFFECT"

Let's start with carbon dioxide – CO₂. CO₂ is a normal component of the atmosphere, and until recently has not been considered an air pollutant. But average global CO₂ concentrations have been increasing since 1860 with a particularly sharp increase since 1958. The main reason for this continuous increase in CO₂ build-up is the burning of fossil fuels. In the past 100 years, the CO₂ content of the atmosphere has already risen by about 15%.

How does an increased concentration of CO₂ in the atmosphere influence the Earth's temperature? Well, the answer is that although the CO₂ content in the atmosphere is only about 0,032%, it's a major factor in determining average global temperature, in what's known as the "Greenhouse Effect". You see, incoming sunlight consists of many wavelengths, including some very dangerous ones. But ozone in the upper atmosphere, and water vapour and CO₂ in the lower atmosphere filter out or destroy most of the harmful wavelengths. So what reaches the Earth is mostly visible light. It's absorbed by land, sea and cloud, and is reradiated into the atmosphere as longer wavelength infrared (IR) radiation, or heat, as the Earth cools. Now this is where we run across the "Greenhouse Effect". Much of this IR radiation is absorbed by CO₂. The CO₂ then radiates a portion of the absorbed heat energy back to the Earth, to warm the atmosphere. Rather like the glass in a greenhouse or a car window on a sunny day, we see that the CO₂ in the atmosphere acts as a one-way filter that allows visible light to enter the Earth's atmosphere, but prevents longer wave-length heat radiation from leaving. Assuming that energy is arriving from the sun at a constant rate, then as the level of CO₂ increases, the average surface temperature of the Earth should rise.

This possible effect of CO₂ on the Earth's climate was first mooted in 1863, but it was only taken up by scientists as a serious matter in 1956. It's now held that a projected increase in CO₂ concentration in the year 2000 could cause the average air temperature near the Earth to increase by about 0,5°C. A doubling of CO₂ levels - which with increasing fossil fuel consumption might occur by the year 2050 - could raise the average temperature by about 2°C.

A 1 to 2°C change would significantly modify global climate. It could trigger the relatively rapid melting of the floating Arctic ice pack. Eventually, even the land-based Antarctic ice might slowly melt. Once set in motion, these changes would be irreversible, and would probably last for millions of years.

And what would be the effects of all this? Gradual melting of the land-based Antarctic ice could eventually raise world sea levels by 70 to 100 metres. This would flood about 20% of the Earth's present land area, including most of the major cities and the flood plains that produce most of our food. This process, if it occurred, would probably take place very slowly over at least 1000 years.

Melting of the Arctic ice cap and glaciers, however, would be much more rapid. Since the Arctic ice pack is afloat, its melting would not raise the water level in the oceans. But the absence of the Arctic ice pack would change ocean currents, and undoubtedly would trigger major unpredictable changes in climate. Subpolar regions would probably become warmer; and the weather in many other parts of the world would become drier, thus affecting water supplies and food-growing capacity.

This is the hypothesis, then. But what's actually happening? According to this "greenhouse model", the Earth's average temperature should have increased by about 0,2°C between I860 and the present day because of the increase in CO₂. But the facts are puzzling. Between 1880 and 1940, the average global temperature rose about 0,6°C, but it has fallen about 0,3°C since 1945 - the period of greatest expansion in the burning of fossil fuels. This fall since 1945 certainly doesn't tie in with the "greenhouse model". Nonetheless, there has still been a net rise of 0,3°C in the last 100 years. And (as we have seen) even a small rise such as this could eventually have a significant effect on the Earth's climate.

carbon dioxide to trigger

build up ice pack

fossil fuels glacier

wavelength to tie in with

IR net rise

to moot one-way filter

II. What is the role of one-way filter?

III. Speak of the hypothetic dangers of the “Greenhouse effect”.

IV. Make a semantic map of the text.

V. Study the map of Great Britain and America and say which territory is likely to disappear under water and how it’s going to affect the country’s economy.

SPACE-ACG cabs

The Economist, 1999

Although it looks like the latest model of a conventional London cab, Zevco's taxi is actually a hybrid electric vehicle. The cells that power it generate their electricity by reacting hydrogen and oxygen together in the presence of a catalyst. When the cab is moving, this electricity turns the motor. When the cab is stationary, it is used to charge a battery that acts as a supplementary power source. And because the outcome of reacting hydrogen with oxygen is water, the taxi is a "zero-emission" vehicle-hence the company's name.

Zevco is by no means the only firm trying to develop a fuel-cell-powered car. But while outfits such as Daimler-Benz, Toyota and Ford are betting hundreds of millions of dollars on a form of fuel cell known as the proton-exchange membrane (PEM), which uses relatively cheap materials, Zevco is sticking with an adaptation of the original (and expensive) alkaline fuel cell (AFC) technology used in spacecraft.

The virtues of AFC are its superior power-to-weight ratio and its relative simplicity (it needs fewer peripheral pumps and compressors than PEM cells). But for mundane applications, it has always laboured under two expensive disadvantages. The first is that, rather than drawing its oxygen directly from the air, it needs bottled (and therefore expensive) pure oxygen. This is to prevent its electrodes being gummed up with the potassium carbonate that would be formed by the reaction between the potassium-hydroxide electrolyte inside the cell and carbon dioxide from the air. The second disadvantage is that the catalyst, which is used to plate the electrodes, has traditionally been platinum, one of the world's more costly metals.