Global Warming

By Jackson Dias

Back in the mid-1980s, Veerabhadran Ramanathan of the Centre for Clouds, Chemistry and Climate in La Jolla, California, began wondering whether such condensation cooling alone was enough to protect the hottest regions of the Earth from overheating. He marvelled at the fact that the upper layers of the tropical oceans never got any warmer than 28 degrees centigrade, though the burning sun beats down on them day after day. Is there a mechanism at work that could even help stop the greenhouse effect?

Water vapour only warms the atmosphere to a point, the point where the shadow-cooling effect of clouds takes over. Like an enormous parasol, the earth’s cloud-cover reflects much of the invisible and ultraviolet radiation from the sun back into space. Because the upper layers of clouds reflect light, they become blindingly white. And the whiter they are on top, the darker the clouds under them, hanging over the earth looking leaden grey, sometimes almost black.

For the most part, cloud-cover is heaviest over those parts of the globe that absorb sunlight and transform it into warmth. These are primarily dark surfaces, including farmland, bare rocky areas, lava flows, but also asphalt and house roofs. Gleaming snow and ice crystals reflect radiation and prevent the sun from warming the earth. Glaciers and polar ice fields work quite like clouds: They send the sun’s radiation bouncing back almost entirely into space.

Without the natural greenhouse effect caused by water vapour, the planet would be cooler by 35 degrees Celsius. On the other hand, without the clouds that always cover at least half the earth, the temperature would be 11 degrees hotter.

The question now arises whether all the man-made greenhouse gases from industry and auto emissions could throw this elegantly balanced natural system out of whack. The picture is a complicated one. In fact, clouds themselves contribute to the greenhouse effect. Even as their upper surfaces reflect the sun’s radiant energy back into space, their lower sides reflect warmth back towards the earth. So, a cloud produces both cold-producing shadows and greenhouse warmth.

It is easy to see how this works in particularly cloudy areas. They remain much warmer than areas under clear skies because the warmth radiated back from the earth cannot escape.

Taken in their entirety, clouds tend to cool the earth - at least at present. But researchers warn that if the warming of the earth leads to more high ice-clouds forming, the greenhouse effect will intensify. Only clouds of very small and dense droplets could slow this, says Erich Roeckner. For, only they can reflect sufficient solar radiation back into space that their dark shadows would stop temperatures from rising.

Meteorologists have classified 10 different types of clouds on our planet, by form, height and stability. There are countless hybrids and subcategories as well. All of them behave differently. Making matters more complicated, there are often layers of different types of clouds between the earth and the sky.

How all these different cloud types would react in a changing climate is something no meteorologist can predict. The climate is influenced not only by the type of clouds but by their number.

For decades, the industrial nations belittled the greenhouse effect eyen while producing clouds of pollution. The aerosol emitted by industry provides fruitful ‘seed’ - the nuclei needed for cloud formation.

For years, some people held that the man-made greenhouse effect would be offset by manmade pollution - and, in fact, the ‘industrial clouds’ of the 1960s and 1970s helped cool the climate. In the long run, however, such calculations did not hold, ironically because of environmental efforts. By insisting that factories use better filters to cleanse their aerial emissions, they were able to achieve a lowering in the amount of dust and aerosol released. Since the 1980s, the earth’s temperature has steadily and inexorably risen.

There has also been a steady rise in exhaust emissions from jet planes. These cloud-seeds produced warmth, not cooling. When high-altitude air is moist enough, the ‘jet trails’ - 10 kilometres and more above the earth - don’t break up, but instead form a feathery cloud layer that gradually covers more and more of the sky, trapping more and more warmth.