Frequently Asked Questions

Frequently Asked Question 1.3

What is the Greenhouse Effect?

The Sun powers Earth’s climate, radiating energy at very short 

wavelengths, predominately in the visible or near-visible (e.g., ultraviolet) part of the spectrum. Roughly one-third of the solar 

energy that reaches the top of Earth’s atmosphere is reflected directly back to space. The remaining two-thirds is absorbed by the 

surface and, to a lesser extent, by the atmosphere. To balance the 

absorbed incoming energy, the Earth must, on average, radiate the 

same amount of energy back to space. Because the Earth is much 

colder than the Sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see Figure 1). Much 

of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back 

to Earth. This is called the greenhouse effect. The glass walls in 

a greenhouse reduce airflow and increase the temperature of the 

air inside. Analogously, but through a different physical process, 

the Earth’s greenhouse effect warms the surface of the planet. 

Without the natural greenhouse effect, the average temperature at 

Earth’s surface would be below the freezing point of water. Thus, 

Earth’s natural greenhouse effect makes life as we know it possible. However, human activities, primarily the burning of fossil 

fuels and clearing of forests, have greatly intensified the natural 

greenhouse effect, causing global warming. 

The two most abundant gases in the atmosphere, nitrogen 

(comprising 78% of the dry atmosphere) and oxygen (comprising 

21%), exert almost no greenhouse effect. Instead, the greenhouse 

effect comes from molecules that are more complex and much less 

common. Water vapour is the most important greenhouse gas, and 

carbon dioxide (CO2) is the second-most important one. Methane, 

nitrous oxide, ozone and several other gases present in the atmosphere in small amounts also contribute to the greenhouse effect. 

In the humid equatorial regions, where there is so much water 

vapour in the air that the greenhouse effect is very large, adding a small additional amount of CO2 or water vapour has only a 

small direct impact on downward infrared radiation. However, in 

the cold, dry polar regions, the effect of a small increase in CO2 or 

FAQ 1.3, Figure 1. An idealised model of the natural greenhouse effect. See text for explanation.

(continued)
Frequently Asked Questions

water vapour is much greater. The same is true for the cold, dry 

upper atmosphere where a small increase in water vapour has a 

greater influence on the greenhouse effect than the same change 

in water vapour would have near the surface.

Several components of the climate system, notably the oceans 

and living things, affect atmospheric concentrations of greenhouse gases. A prime example of this is plants taking CO2 out of 

the atmosphere and converting it (and water) into carbohydrates 

via photosynthesis. In the industrial era, human activities have 

added greenhouse gases to the atmosphere, primarily through the 

burning of fossil fuels and clearing of forests. 

Adding more of a greenhouse gas, such as CO2, to the atmosphere intensifies the greenhouse effect, thus warming Earth’s 

climate. The amount of warming depends on various feedback 

mechanisms. For example, as the atmosphere warms due to rising 

levels of greenhouse gases, its concentration of water vapour 

increases, further intensifying the greenhouse effect. This in turn 

causes more warming, which causes an additional increase in 

water vapour, in a self-reinforcing cycle. This water vapour feedback may be strong enough to approximately double the increase 

in the greenhouse effect due to the added CO2 alone.

Additional important feedback mechanisms involve clouds. 

Clouds are effective at absorbing infrared radiation and therefore 

exert a large greenhouse effect, thus warming the Earth. Clouds 

are also effective at reflecting away incoming solar radiation, thus 

cooling the Earth. A change in almost any aspect of clouds, such 

as their type, location, water content, cloud altitude, particle size 

and shape, or lifetimes, affects the degree to which clouds warm 

or cool the Earth. Some changes amplify warming while others 

diminish it. Much research is in progress to better understand how 

clouds change in response to climate warming, and how these 

changes affect climate through various feedback mechanisms.
From the report accepted by Working Group I 

of the Intergovernmental Panel on Climate Change 

but not approved in detail

Frequently Asked Questions

FAQ Citation:

These Frequently Asked Questions have been taken directly from the chapters of the underlying report and are collected here. 

When referencing specific FAQs, please reference the corresponding chapter in the report from whence the FAQ originated.

When referencing the group of FAQs, please cite as:

IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment 

Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, 

M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.