Lower Atmosphere

Basics

Greenhouse gases and their effect

The greenhouse effect is very important for life on Earth.  The average temperature of the Earth is 15 ^oC, if there were no greenhouse gases in the air, the average temperature of the Earth would be about 30 ^oC lower. 

We need a natural greenhouse effect.  This acts like a pullover in winter which traps a warm layer of air around our body. However, if the pullover is too thick, we begin to sweat. By putting more and more greenhouse gases into the air, humans have enhanced the natural greenhouse effect and are making the Earth warmer.   It's not the natural greenhouse effect which is causing global warming, it's the additional greenhouse effect caused by humans which is causing the trouble.

So greenhouse gases do the same with the heat radiation from the Earth as a pullover does with our body in winter. They hold back the warmth and cause a warm layer to form around the Earth's surface.

2. Light coming from the Sun is mostly visible light, the dangerous ultra-violet part is absorbed by the ozone layer. This sunlight is either reflected back into space by the light coloured parts of the Earth's surface (ice, snow and clouds) or reaches the Earth's surface and heats it up (symbolised by the red colour).  Author: Elmar Uherek.

3. Warm infrared heat radiation (invisible to our eyes) is emitted by the Earth.  Greenhouse gases in the atmosphere (symbolised by blue ellipses) absorb the infrared radiation and send part of the heat back to the Earth and part of it back into space.  Author: Elmar Uherek.

Which gases contribute to the greenhouse effect?

The most important greenhouse gas is water vapour (which accounts for about 60% of the greenhouse effect) but we don't think that concentrations of water vapour in the atmosphere have changed much over the past few centuries.  So its unlikely that water vapour is responsible for the observed warming of our planet.  However, human activity has dramatically increased the concentration of carbon dioxide in the atmosphere, from 280 ppm in preindustrial times to 385 ppm* in 2008.  Carbon dioxide is the second most important greenhouse gas in the atmosphere, contributing about 20% of the greenhouse effect.  Concentrations of methane and ozone, which are also strong greenhouse gases, have also increased dramatically since the industrial revolution.  Greenhouse gases are trace gases, and beside from CO₂, they account for less than one millionth of the total air mass.

In some scientific publications the contribution of the greenhouse gases to the warming of the Earth is called 'radiative forcing'.  It is measured in watts per square meter (W m^-2).  Between 1750 (when the industrial revolution started) and today, the concentrations of greenhouse gases have increased dramatically as a result of human activity.  The numbers on the right show the increase in radiative forcing during this time. 

* 1ppm = 1 molecule of a gas in 1 million molecules of air

You can learn more about radiation,  greenhouse gases and the energy balance of the Earth in:
Lower Atmosphere - More - Unit 2 - Radiation

About this page:

author: Elmar Uherek - MPI Mainz
scientific reviewer: Dr. Pascal Guyon - MPI for Chemistry, Mainz - 2004-05-12
educational reviewing: Roland Bergmann + students - Velbert Comprehensive School
revised and last published: 2004-05-13