What is the greenhouse effect?

What is meant by the greenhouse effect?

Source: Barb Deluisi, NOAA

What is the greenhouse effect and how does it affect our climate? The sun is the primary source of energy here on the earth.  Heat generated from the earth’s interior (geothermal heat) is estimated to be only about 0.027% of Earth’s total energy budget at the surface so can be ignored for most purposes.  The earth reflects about 30% of the incoming solar radiation.  The sunlight that is not reflected then passes through the atmosphere and heats the earth’s surface. The surface then re-radiates that heat (infrared radiation) back through the atmosphere.  This heat is partially absorbed by greenhouse gases warming the atmosphere more than it would otherwise. Without greenhouse gases the earth’s temperature would be about 33°C (59ºF) colder than it is now, and our world would be a giant snow ball.

Molecular vibrations in the Greenhouse Effect

Source: The UCAR Center for Science Education

Chemistry of greenhouse

The major greenhouse gases are carbon dioxide, water vapor, methane, and nitrous oxide. All of these molecules are made of three or more atoms which can vibrate in complex patterns when they interact with infrared photons (heat).  Eventually they will release that heat, some of which escapes into space while some is radiated back down to the earth’s surface and some may be absorbed by other greenhouse gas molecules. Most of the atmosphere consists of nitrogen and oxygen, both of which are made up of two atoms that are bound together tightly and unable to vibrate so they do not act as greenhouse gases.

Water Vapor
Water vapor is a major natural greenhouse gas that responds to temperature changes.  As the temperature of the atmosphere warms, the water vapor content of the atmosphere increases which creates a positive “feed-back” adding to the heat absorption created by the other greenhouse gases. This process is complicated as increasing water vapor also tends to increase the total amount of clouds. Clouds can provide either a positive or a negative feedback by trapping outgoing thermal radiation or increasing the amount of solar radiation reflected back to space, respectively. Water vapor represents around 80 percent of total greenhouse gas mass in the atmosphere and 90 percent of greenhouse gas volume.  It is estimated that water vapor accounts for about 50% of Earth’s total greenhouse effect, with clouds contributing about 25%.

Global Warming and Greenhouse gases
In recent years there has been growing interest in rising global temperatures since the mid 1800’s.  This recent period of global warming  has been mainly attributed to the steady increase of atmospheric trace gases produced largely by human activities, such as carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFC or “Freon”). These gases have been accumulating over time in the atmosphere and thus are adding to the natural greenhouse effect.

CO2 Contribution to Greenhouse Effect
Over the past approximately 150 years the level of carbon dioxide, for example, has undergone a very significant increase of about 40-45 percent, which cannot be accounted for by natural sources alone.  Despite the large increase in atmospheric CO2, the total amount of  CO2 still comprises only a tiny 0.04% of the total atmosphere.

Estimates of CO2’s contribution to the greenhouse effect vary greatly between 2% (0.7 C) to as much as 26% (8.6C) of the total 33 C greenhouse effect.  This means that there is some uncertainty as to CO2’s actual contribution.  In addition, there is also uncertainty about the sensitivity of the climate to CO2. The best estimates I can find is that doubling CO2 in the atmosphere should result in a warming of 0.4 C to 1.2C before factoring in any feedbacks.

Climate Feedbacks

CO2 trends and green house effect

Atmospheric CO2 trend

As the planet gets warmer, more water vapor evaporates from the Earth’s surface adding to the total amount of water vapor in the atmosphere. As stated earlier, water vapor is the most significant greenhouse gas, so more water vapor in the atmosphere leads to even more warming.  Climate feedbacks are processes that amplify or reduce the effect of a particular climate forcing and play a role in determining total climate sensitivity. Based on past glaciations, climate feedbacks could amplify the warming effect of CO2 by a factor of about 2.

Climate Sensitivity to doubling CO2
Model simulations suggest a wide range of estimates regarding how the doubling the CO2 levels could eventually warm plant.  Estimates range from a low of 0.8 C to as much as 4.5 C with the most likely range being between 1.2 C and 1.6 C. The difficulty in estimating future temperature based on CO2 levels is that there are many other factors that come into play, including atmospheric dust from volcanic eruptions, amount of forests and other vegetation, variations in incoming sunlight, aerosol particle pollution and the relationship of the deep global ocean currents.

In future posts, I will explore in more detail the various opinions on global warming or climate change and how this will effect the rate of sea level rise.

Fred Pickhardt





About Fred Pickhardt

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