Weather and climate

Climate change

Emissions that increase greenhouse gasesEmissions that increase greenhouse gases Climates change over time. In the past, the Earth has been both hotter and cooler than it is now. During the Age of Dinosaurs 100 million years ago, there were no polar ice caps and tropical forests grew in temperate lands. More recently, 20,000 years ago, ice caps extended from the North Pole as far as northern Europe during the Ice Ages. Between about 1550 and 1800, in what were called the Little Ice Ages, winters were significantly colder than they are today. Most scientists are agreed that the global climate is now changing once more, this time due to the rapid growth of industry around the world.

Grinnell Glacier, USA, is retreating.Grinnell Glacier, USA, is retreating.

Global warming

The Earth is becoming warmer. The average temperature has risen by more than 0.5°C in the past century and is predicted to rise by 2°C by 2050—although this figure is far from certain. The effects may be felt by more frequent droughts, violent storms and changing climates. Glaciers are retreating (shrinking). The ice caps at both poles may start to melt, resulting in rising sea levels and the flooding of coastal regions where most cities are located.

Why is this happening? Most scientists are convinced that human activities—industries, power stations, cars and planes—has contributed to a huge increase in greenhouse gases in the atmosphere, causing global warming.

Carbon dioxide

The carbon cycleThe carbon cycleCarbon circulates between the Earth's surface and the atmosphere in what is known as the carbon cycle. Carbon dioxide, its gaseous state, is taken in by plants during photosynthesis, by which it is converted to the sugars that the plants need to survive and grow. The plants are, in turn, eaten by animals, which release carbon dioxide into the atmosphere as they breathe.

When plants and animals die, they decompose and release carbon dioxide into the atmosphere. Carbon dioxide is also given off by marine life, the weathering of limestone (formed from the remains of marine animals and plants that lived millions of years ago), erupting volcanoes—and the burning of fossil fuels by humans.

The greenhouse effectThe greenhouse effect

Greenhouse effect

The Sun’s rays warm the surface of the Earth. The surface then reflects this radiation back into space. Some gases in the atmosphere, such as carbon dioxide and methane, trap part of this outgoing radiation, keeping the surface warm—just like a greenhouse. But an excessive amount of these greenhouse gases warms up the Earth too much. Human activities have resulted in a large increase of greenhouse gases, such as carbon dioxide, in the atmosphere. Vehicles and power stations give off exhaust gases from burning oil or coal (“fossil fuels”). These add billions of tonnes of carbon dioxide to the atmosphere.

Plants on both land and in the sea will usually absorb (take in) carbon dioxide as part of their natural cycle. Carbon dioxide is also dissolved in the ocean waters. But the destruction of forests around the world by people means there are fewer plants to absorb the gas. The global balance of the carbon cycle is being upset, leading to much higher levels of carbon dioxide in the atmosphere.

A doomsday scenario? A coastal city is flooded.A doomsday scenario? A coastal city is flooded.

Rising sea levels

Global warming may have an enormous effect on the oceans in future years. Average temperatures worldwide have risen sharply over the last 100 years. There is now a likelihood that the world's ice caps and glaciers will start to melt. The increase in sea levels by only a few metres may result in many coastal cities—New York, Miami, New Orleans, Guangzhou, Mumbai, Nagoya and Osaka are among those at greatest risk—being flooded. Some low-lying islands, especially coral atolls in the Indian Ocean (the Maldives, for example) and South Pacific (Tuvalu, for example), may completely disappear beneath the waves. Ocean currents may also shift direction and, as a result, world climates may change.

Areas (in red) most vulnerable to rising sea levelsAreas (in red) most vulnerable to rising sea levels

 Ian Fairchild

See also in Earth

See also in Life

See also in Geography

Since the beginning of the Industrial Revolution in the mid-18th century, the burning of fossil fuels has contributed to the increase in carbon dioxide in the atmosphere from 280 ppm (parts per million, by volume of air) to over 400 ppm.

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