The Enhanced Greenhouse Effect

The amount of carbon dioxide in the atmosphere has increased by more than 30% in the past two centuries since the beginning of the industrial revolution, an increase that is known to be in part due to combustion of fossil fuels and the removal of forests. Most of this increase has occurred since World War II. Because carbon dioxide is recycled through the atmosphere many times before it is finally removed, it has a long lifetime exceeding 100 years. Thus, emissions lead to a buildup in concen­trations in the atmosphere. In the absence of controls, it is projected that the rate of increase in carbon dioxide may accelerate and concentrations could double from pre-industrial values within approximately the next 60 years.

If the amount of carbon dioxide in the atmosphere were suddenly doubled, with other things remaining the same, the outgoing long-wave radiation would be reduced by approximately 4 W m~2 and instead trapped in the atmosphere, resulting in an enhanced greenhouse effect. To restore the radiative balance, the atmosphere must warm up and, in the absence of other changes, the warming at the surface and throughout the troposphere would be approximately 1.2°C. In reality, many other factors will change, and various feedbacks come into play, so that the best estimate of the average global warming for doubled carbon dioxide is 2.5°C. In other words, the net effect of the feedbacks is positive and approximately doubles the response otherwise expected.

Increased heating therefore increases global mean temperatures and also enhances evaporation of surface moisture. It follows that naturally occurring droughts are likely to be exacerbated by enhanced drying. After the land is dry, all the solar radiation goes into increasing temperature, causing heat waves. Temperature increases signify that the water-holding capacity of the atmosphere increases and, with enhanced evaporation, the atmospheric moisture increases, as is currently observed in many areas. The presence of increased moisture in the atmosphere implies stronger moisture flow converging into all precipitating weather systems, whether they are thunderstorms, extratropical rain, or snow storms. This leads to the expectation of enhanced rainfall or snowfall events, which are also observed to be happening. Globally, there must be an increase in precipitation to balance the enhanced evaporation and hence there is an enhanced hydro­logical cycle.

The main positive feedback comes from water vapor. As the amount of water vapor in the atmo­sphere increases as the earth warms, and because water vapor is an important greenhouse gas, it amplifies the warming. However, increases in clouds may act either to amplify the warming through the greenhouse effect of clouds or reduce it by the increase in albedo; which effect dominates depends on the height and type of clouds and varies greatly with geographic location and time of year. Decreases in sea ice and snow cover, which have high albedo, decrease the radiation reflected back to space and thus produce warming, which may further decrease the sea ice and snow cover, known as ice albedo feedback. However, increased open water may lead to more evaporation and atmospheric water vapor, thereby increasing fog and low clouds, offsetting the change in surface albedo.

Other, more complicated feedbacks may involve the atmosphere and ocean. For example, cold waters off the western coasts of continents (such as California or Peru) encourage development of extensive low stratocumulus cloud decks that block the sun and this helps keep the ocean cold. A warming of the waters, such as during El Nino, eliminates the cloud deck and leads to further sea surface warming through solar radiation.

Updated: March 13, 2016 — 5:03 pm