CAUSES OF GLOBAL WARMING

CAUSES OF GLOBAL WARMING

The greenhouse effect was discovered by Joseph Fourier in 1824. It is the process by

which absorption and emission of infrared radiation by atmospheric gases warm the

planet's surface.

Existence of the greenhouse effect as such is not disputed. Naturally occurring

greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which

Earth would be uninhabitable. Rather, the issue is how the strength of the

greenhouse effect changes when human activity increases the atmospheric

concentrations of some greenhouse gases.

On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of

the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–

26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%. Molecule for

molecule, methane is a more effective greenhouse gas than carbon dioxide, in

addition to nitrous oxide (N2O), which is increasing in concentration owing to human

activity such as agriculture.

From less direct geological evidence it is believed that CO2 values this high were

last attained 20 million years ago. Fossil fuel burning has produced about three-

quarters of the increase in CO2 from human activity over the past 20 years. Most of

the rest is due to land-use change, in particular deforestation.