Cooling or heating, a balancing act

Every year the sun delivers an average of 340 watts of energy to every square metre of the Earth. To produce this amount of energy we would need 440 million large electric power plants, each generating 100 million watts of power (NASA).

It would get uncomfortably hot on Earth with all this energy, but fortunately for us, the amount of heat we receive from the sun is balanced by heat radiated back into space by the atmosphere. Radiative forcing is the change in the balance between radiation coming into the atmosphere and radiation going out. A positive radiative forcing tends on average to warm the surface of the Earth, and negative forcing tends on average to cool the surface. Greenhouse gases, for example, produce positive radiative forcing – they trap outgoing terrestrial (infrared) radiation, which causes a temperature rise at the Earth’s surface – the “greenhouse effect”. In contrast, negative radiative forcing from clouds and aerosols, which can refl ect back into space, acts as a cooling mechanism.

The enhanced greenhouse effect
Greenhouse gases are a natural part of the atmosphere. Without these gases the global average temperature would be around -20ºC. The problem we now face is that human actions – particularly burning fossil fuels (coal, oil and natural gas) and land clearing – are increasing their concentrations. The more of these gases there are, the more heat is trapped. This is known as the enhanced greenhouse effect. Naturally occurring greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases that are not naturally occurring include hydro-fl uorocarbons (HFCs), perfl uorocarbons (PFCs), and sulphur hexafl uoride (SF6), which are generated in a variety of industrial processes.

On average, about one-third of the solar radiation that hits the Earth is reflected back into space. The land and the oceans mostly absorb the rest, with the remainder trapped in the atmosphere. The solar radiation that strikes the Earth’s surface heats it up, and as a result infrared radiation is emitted.

Radiative forcing
The radiative forcing from the increase in anthropogenic greenhouse gases since the pre-industrial era is positive (warming) with a small uncertainty range; that from the direct effects of aerosols is negative (cooling) and smaller; whereas the negative forcing from the indirect effects of aerosols (on clouds and the hydrologic cycle) might be large but is not well quantified. Key anthropogenic and natural factors causing a change in radiative forcing from year 1750 to year 2000 are shown in this figure, where wide, colored bars mark the factors whose radiative forcing can be quantified. Only some of the aerosol effects are estimated here and denoted as ranges. Other factors besides atmospheric constituents -- solar irradiance and land-use change -- are also shown. Stratospheric aerosols from large volcanic eruptions have led to important, but short-lived, negative forcings (particularly during the periods 1880-1920 and 1960-1994), which are not important over the time scale since the pre-industrial era and not shown. The sum of quantified factors in the figure is positive, but this does not include the potentially large, negative forcing from aerosol indirect effects.

Water vapour is the most abundant greenhouse gas.
However, human activities have little direct impact on its concentration in the atmosphere. In contrast, we have a large impact on the concentrations of carbon dioxide, methane and nitrous oxide. In order to be able to compare how different gases contribute to the greenhouse effect, a method has been developed to estimate their global warming potentials (GWP). GWPs depend on the capacity of greenhouse gas molecules to absorb or trap heat and the time the molecules remain in the atmosphere before being removed or broken down. GWPs can be used to define the impact greenhouse gases will have on global warming over different time periods – usually 20 years, 100 years and 500 years. The GWP of carbon dioxide is 1 (constant for all time periods) and the GWPs of other greenhouse gases are measured relative to it. Even though methane and nitrous oxide have much higher GWPs than carbon dioxide, because their concentration in the atmosphere is much lower, carbon dioxide remains the most important greenhouse gas, contributing about 60% to the enhancement of the greenhouse effect (Houghton et al 2001).

Cloud makers
Clouds can either heat or cool the Earth, depending on their altitude and size. An experiment carried out in the 1980s found that in general clouds tend to cool the planet. If we remove all the clouds from the atmosphere the average temperature is estimated to increase by approximately 11°C (NASA). However, one particular “man made” cloud type is implicated in global warming. Water vapour emitted by aircraft, referred to as condensation trails, produces high altitude ice clouds. Like cirrus clouds, these cold wispy trails trap heat, effectively warming the atmosphere. Increasing air travel means that more of these warming clouds will be produced.

The cooling effect
Increasing greenhouse gases in the atmosphere can warm the planet, but other factors can cool it. These include aerosols in the atmosphere, such as volcanic ash, soot, dust and sulphates. Small aerosol particles are very effective at refl ecting incoming solar radiation back into space and consequently cooling the Earth. Increasing the area of refl ective surfaces can also lead to cooling (referred to as increasing albedo). Deforestation is an example, as the exposed ground is more reflective than the forest canopy. Increasing snow cover acts in the same way, as snow and ice are more reflective than the land or the ocean.

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