Here's the conventional view.

The atmosphere contains an increasing concentration of greenhouse gases (CO2 and methane) and other emissions that trap heat. Energy from the Sun passes through the atmosphere and warms the Earth's surface, but greenhouse gases do not allow the heat to radiate back into space.

This effect has come to be called the greenhouse effect by analogy with a common greenhouse, whose glass roof lets in sunlight but traps in heat. The burning of fossil fuels, so crucial for modern civilization, enhances the greenhouse effect and warms the climate.

The proponents of alternative theories of global warming do not agree that anthropogenic factors are responsible for increased concentrations of greenhouse gases in the air.

Rather, some, such as Nikolai Yasamanov of Moscow State University, blame volcanic activity. Heavy magma seeps out of the mantle (the layer between the earth's crust and its core) through mid-oceanic rifts. When it mixes with water, it releases methane, which rises to the surface and then into the atmosphere. Upon reacting with oxygen, it creates CO2.

There are other explanations for the increase in the amount of carbon dioxide in the air.

Criticizing the traditional theory of global warming, geographer Andrei Kapitsa, a member of the Russian Academy of Sciences, points out that the processes of heat exchange in the atmosphere are much more complicated than in a common greenhouse. He believes that the growing concentration of carbon dioxide is a consequence rather than a cause of the warmer climate.

Kapitsa proceeds from the premise that the amount of carbon dioxide dissolved in the ocean is many times greater than in the air. Rising global temperatures cause the CO2 in the Ocean to evaporate, increasing its concentration in the atmosphere.

Oleg Sorokhtin from the Shirshov Institute of Oceanography came up with his own original theory, questioning the very existence of the greenhouse effect.

He maintains that a high concentration of carbon dioxide and methane in the atmosphere will accelerate the transfer of heat to the upper layers of the atmosphere, cooling rather than heating the lower layers.

So what really causes global warming?

Khabibullo Abdusamatov from the Pulkovo Astronomical Observatory believes that we should search for the answer in the Sun. The Earth's climate directly depends on the energy it receives - the more energy the higher the temperature.

He thinks that global warming is a result of a long-term increase in the energy emitted by the Sun. This growth is caused by the change in the temperature of the solar core and hence it's the Sun's size and brightness.

That brightness has already reached its peak for the current cycle, and after 2015 we can expect a gradual decrease in the Earth's average temperature. In 2035-45, the Sun's brightness will reach its lowest point in the cycle, causing a cold spell.

There are many more alternative explanations for global warming, including changes in cloud cover, atmospheric aerosols, cosmic rays and the effects of a prehistoric collision with a comet.

I believe that the best approach is the one taken by the Intergovernmental Panel on Climate Change, which brings together scientists from different fields and insists on a comprehensive analysis of global warming. Its fourth and latest report, published earlier this year, deals with the impact of both man-made and natural factors on the warming process.

The same logic stands behind the UN Framework Convention on Climate Change and the Kyoto Protocol, which drew up a plan for reducing greenhouse emissions.

How can we reduce humankind's influence on the atmosphere? By saving energy and increasing production efficiency, using alternative and renewable energy sources, preserving forests and raising our technological and ecological standards.