Science of Climate Change

Greenhouse Gases and Radiative Forcing

The Greenhouse Effect

The greenhouse effect is the term used to describe the role of the atmosphere in insulating the planet from heat loss. Greenhouse gases (GHG) are gases in the atmosphere that give rise to this greenhouse effect. GHGs allow shortwave or solar radiation to penetrate down to the atmosphere and the Earth's surface. This radiation is absorbed and the Earth's surface warms (Figure 1). GHGs function as a blanket that traps heat in the lower atmosphere - termed the "greenhouse effect."

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This "natural greenhouse effect" is an important phenomenon to biological life on Earth. The average surface temperature is maintained at approximately 15°C because of this effect. If all the radiant energy were to escape out of the atmosphere, the global temperature would be -18°C, a difference of 33°C.

The Earth's temperature and climate system can be thought of as a heat engine driven by energy from the sun. There is an energy balance between incoming and outgoing radiative energy that is partially regulated by the concentrations of GHG gases in the atmosphere. Climate change occurs when the total amount of the sun's energy absorbed, does not equal the amount of energy released, causing an imbalance in the radiative exchange.

Consequently, humans can also cause temperatures and the climate system to change. Human activities such as the burning of fossil fuels, deforestation or land surface change, industrial processes, etc., are increasing the concentration of GHGs in the atmosphere at an alarming rate. This additional increase of GHG is known as the "enhanced greenhouse effect", where more incoming energy is trapped within the atmosphere. This can have serious impacts on the physical and chemical processes, and biological life on Earth.

The proportion of GHGs amounts to less than 1% of the total gases in the atmosphere. However, these gases collectively increase the Earth's surface temperature by 33°C. Because the concentrations of these gases in the atmosphere are so low, it is possible for human emissions to have a drastic effect on the radiative budget and resulting surface temperature.

Greenhouse Gases

There are some GHGs that are present in the atmosphere due to both natural processes and human activities. These include:

• Water vapour (H₂O)
• Carbon dioxide (CO₂)
• Methane (CH₄)
• Nitrous oxide (N₂O)
• Tropospheric ozone (O₃)

There are also some GHGs that are almost entirely due to anthropogenic sources. These include:

• Chlorofluorocarbons (CFC)
• Hydrofluorocarbons (HFC)
• Perfluorocarbons (PFC)
• Sulfur hexafluoride (SF₆)

Greenhouse Gas Concentration Trends in the 20th Century

The atmospheric concentration of CO₂ has increased by 31 % since 1750 and continues to increase, on average, by 1.5 ppm or 0.4 % per year. About 80% of the anthropogenic emissions of CO₂ during the past 20 years is due to fossil fuel burning and cement production. The rest due to deforestation. The atmospheric concentrations of CH₄ and N₂O have increased by 151 % and 17 %, respectively, since 1750. Refer to table 1 for the 20^th century changes in the Earth's atmosphere system for selected GHGs (IPCC Synthesis Report; 2001).

Greenhouse Gas Emission Trends

The Greenhouse Gas Division of Environment Canada (EC) regularly publishes a report on emissions and removals of GHGs with recent trends for Canada, "Canada's Greenhouse Gas Inventory - 2000 Emissions and Removals with Trends". This report provides a summary of the sources and sinks of greenhouse gases in Canada and the current methods used in calculating GHG emissions and removals. Also available is the Greenhouse Gas Emissions Inventory provided by the United Nations Framework on Climate Change (UNFCCC).

Future GHG Concentrations Using Emission Scenarios

In 2000, IPCC published a Special Report on Emissions Scenarios (SRES), which is a new set of emissions scenarios replacing the well-known IS92 scenarios published in 1992. The SRES uses four different narrative storylines that describe the relationship between the forces driving emissions coupled with the wide range of demographic, economic and technological driving forces to determine future GHG and sulphur emissions. The Kaya equation is used to calculate the CO₂ emissions:

Formula - CO₂ Emissions = Population X GDP per Capita (wealth) X energy/GDP (energy efficiency) X CO₂/Energy (energy type)

Figure 2 shows the projected anthropogenic emissions of the three main GHGs together with anthropogenic sulphur dioxide emissions. The projections encompass a wide range of GHG concentrations. Variations of 490 to 1260 ppm (75 to 350 % above the 1750 concentrations) and -10 to + 30 % decrease/increase are projected by 2100.

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Radiative Forcing

Radiative forcing is the term used to describe a change in the balance between net radiation coming into the atmosphere and net radiation going out, thereby 'forcing' the climate to shift from equilibrium. Such a forcing essentially alters the Earth's radiation budget and hence the climate. A positive radiative forcing (such as that due to increasing GHG concentrations) warms the atmosphere, whereas a negative forcing (for example, an increase in concentrations of sulphate aerosols that reflect incoming solar radiation) tends to cool the atmosphere. Understanding climate forcing agents and their influence on the balance of radiation is important in understanding past changes in climate, and to project possible future climate changes. Figures 3a shows the changes in atmospheric concentrations of the three most prominent GHGs over the past 1000 years, as well as their increasing radiative forcings. Figure 3b illustrates the trends in concentration of atmospheric sulphate deposited on the Greenland ice sheet, which resulted in a negative radiative forcing. Note the increase of the concentrations of GHG and sulphate aerosols and resulting radiative forcing since the onset of the industrial revolution.

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Greenhouse Gases are Changing the Climate

Figure 4 shows the observed (gray line) and model- simulated annual global mean surface temperature anomalies (red line) since about 1860. Simulations of the response to natural forcings alone (changes in solar radiance and volcanic eruptions) do not adequately explain the warming of the 20th century, particularly for the past 50 years. Simulations using human forcings alone provide a much better correlation with observed changes over the past 50 years than to natural forcing. As expected, the best agreement between observed and model simulations from 1850 to present is found when all forcings are combined, as shown in Figure 4c. Experts conclude that most of the observed warming over the last 50 years is likely to have been due to human activities.

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Links / For More Information

The Green Lane^TM,
Environment Canada's World Wide Web Site.