Human contribution to climate change

Human contribution to climate change

Climate change has often occurred on earth due to natural causes over timescales that vary from decades to hundreds of thousands of years. However, on the basis of considerable evidence, there is a strong consensus in the climate science research community that the changes that have been observed over the past few decades are mainly caused by human activity. The human influences on climate change arise from combustion of fossil fuels (also known as hydrocarbon fuels) and changes in land use. These activities alter the RADIATIVE BALANCE of the earth either by changing its atmospheric composition so as to enhance the natural greenhouse effect, or by changing the reflectivity of the earth's surface or atmosphere.

The main human activities that contribute to an enhancement of the natural greenhouse effect are:

  • Combustion of fossil fuels, which releases greenhouse gases.
  • Clearing of forests for agriculture, which releases carbon dioxide through increased biomass decay.
  • Deforestation, soil tillage and land degradation, which release carbon from the land system and reduce its capacity to absorb and store carbon.

The main human activities that change the reflectivity of the earth's surface and atmosphere are:

  • Fossil fuel combustion; industrial processes; and biomass burning release aerosols and other pollutants into the atmosphere, changing its capacity to reflect or absorb solar radiation. These aerosols may also be deposited onto the land surface and cause changes to the surface albedo.
  • Deforestation, agricultural practices and urbanisation change the reflectivity of the earth's surface.

Some of these influences can have a cooling effect on the earth's surface and lower atmosphere. For example, most aerosols emitted to the atmosphere reflect solar radiation back out to space. Black carbon particles are an exception, as they strongly absorb heat, and when deposited on snow can act as a significant heat sink on the surface that encourages snow melt. Deforestation tends to increase the albedo of the land surface, since green, hydrated leaves reflect less solar radiation than bare earth. This too contributes a negative or cooling effect to the earth's radiative balance.

There are also complex interactions and feedbacks between many of these processes that are not well understood. For example, while aerosols in the atmosphere contribute a direct cooling effect, they also have an indirect contribution through their effect on the formation and properties of clouds, changing their albedo and lifetime.

However, it is clear that the warming factors are dominant and the net effect of human activities is a warming of the planet. The single most important influence is the increase in greenhouse gas concentrations due to fossil fuel combustion. The figure below illustrates the relative contribution of various processes to radiative forcing of the earth's climate between 1750 and 2005, including the influence of natural changes in solar irradiance.

Summary of the principal components of the radiative forcing of climate change.

Summary of the principal components of the radiative forcing of climate change. All these radiative forcings result from one or more factors that affect climate and are associated with human activities or natural processes as discussed in the text. The values represent the forcings in 2005 relative to the start of the industrial era (about 1750). Human activities cause significant changes in long-lived gases, ozone, water vapour, surface albedo, aerosols and contrails. The only increase in natural forcing of any significance between 1750 and 2005 occurred in solar irradiance. Positive forcings lead to warming of climate and negative forcings lead to a cooling. The thin black line attached to each coloured bar represents the range of uncertainty for the respective value.

Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing, Figure 2.20, p. 203.

Increasing greenhouse gas concentrations

Since the industrial revolution, the atmospheric concentrations of carbon dioxide and some other important greenhouse gases have been increasing as a consequence of the burning of fossil fuels, and the development of various industrial processes and chemicals. The accumulation of these heat-trapping gases and the resultant warming of the planet is known as the enhanced greenhouse effect.

Carbon dioxide (CO2) occurs naturally in our atmosphere at a very low concentration. The concentration of this gas in air has remained fairly static for thousands of years, and natural inputs of CO2 (for example from the respiration of animals, the decomposition of biological material and forest fires) are balanced by natural sinks (the photosynthesis of plants and phytoplankton, and absorption by seawater). As a result of this balance, the atmospheric concentration of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the start of the present interglacial period (the Holocene) and the start of the industrial era two hundred years ago. Since then, of course, it has increased dramatically and is now about 383 parts per million (ppm) and continuing to rise.

The main greenhouse gases emitted or generated by human activities are:

CARBON DIOXIDE (CO2).
METHANE (CH4).
HALOCARBONS (including CFCs, HFCs, HCFCsPERFLUOROCARBONS and HYDROFLUOROETHERS).
NITROUS OXIDE (N2O).
OZONE (O3).

The impact of many of these gases is accentuated owing to their long-lived nature in the atmosphere. Their relative contribution to the enhanced greenhouse effect depends on their lifetime as well as on their concentration in the atmosphere. CO2 has the greatest effect, followed by methane, halocarbons and nitrous oxide. Ozone is also a powerful greenhouse gas (one that occurs naturally within the stratosphere) but is not classified as long-lasting because it is constantly being broken down by ultraviolet radiation and reformed in the upper atmosphere. In the lower atmosphere, ozone is often produced as a result of human activities. It is formed by chemical reactions between atmospheric oxygen and 'precursor' gases such as volatile organic compounds and nitrogen oxides that are emitted from burning of fossil fuels and biomass, and other industrial processes. It is the additional ozone in the lower atmosphere that contributes to the enhanced greenhouse effect.

Carbon dioxide, methane and nitrous oxide concentrations have all grown steeply in the last century relative to earlier levels, as shown in the figure below. Increases in concentration are accompanied by increased radiative forcing. This forcing, measured in watts per square metre, arises from the ability of the gases to absorb long wave radiation, thus reducing the amount of heat energy being lost to space, increasing the warming of the earth's surface.

Trends in the main greenhouse gas concentrations in the atmosphere in the last 1000 years

Trends in the main greenhouse gas concentrations in the atmosphere in the last 1000 years

Source: Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003, http://www.bom.gov.au/info/GreenhouseEffectAndClimateChange.pdf.

The main sources of the anthropogenic greenhouse gases and their relative contribution to the enhanced greenhouse effect are listed in the table below.

Gas Principal anthropogenic sources Lifetime Proportional contribution to the enhanced greenhouse effect
Carbon dioxide (CO2) Fossil fuel burning, biomass burning, gas flaring, cement production, land use and land use change 50–200 years
56 %
Methane (CH4) Disturbance of wetlands, rice paddies, ruminant livestock, venting from natural gas wells, biomass burning and decomposition, coal mining, rubbish tips 12 years
16 %
Nitrous oxide (N2O) Fossil fuel combustion, fertiliser production, biomass burning 114 years
5 %
Halocarbons (combined) Industrial production, consumer goods (aerosol can propellants, refrigerants, foam-blowing agents, solvents, fire retardants) 2 to 50,000 years (e.g. CFC-11 is 45 years, CF4 is 50,000 years)
11 %
Tropospheric ozone (O3) Emissions of precursors (carbon monoxide, nitrogen oxides, volatile organic compounds) from fossil fuel combustion and biomass burning Short-lived
12 %

Sources: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate Change 2007: The Physical Science Basis, Chapter 2, Changes in atmospheric constituents and in radiative forcing, Table 2.12, p. 204; Bureau of Meteorology, The greenhouse effect and climate change, 2003, Table 2, p. 18.

Land use change

The way in which we use the land and change the pattern of vegetation alters the reflectivity of the planet's surface and may reduce or promote the ability of soil and vegetation to absorb, store and release carbon and carbon dioxide.

In agriculture, activities such as land-clearing, burning, deforestation and tillage can change the reflectivity and texture of the land surface, and hence the levels of absorbed radiation, evaporation and evapotranspiration. Changes to soil structure through tillage and removal of vegetation—particularly deforestation—reduce the land's capacity to absorb carbon dioxide. Removal of vegetation also reduces the ability of soils to retain moisture and may make it harder for rainwater to infiltrate, commonly exacerbating erosion. The decay of plant biomass, often associated with processes used in land clearing, contributes to CO2 and methane emissions.

Changes in soil structure, soil moisture loss and over-cultivation can also reduce soil organic content by reducing the density of soil organisms and accelerating oxidation of organic carbon compounds to produce carbon dioxide. Modified farming practices can help retain soil carbon and hence maximise the potential of soil to act as a carbon sink.

The soil is thought to contain about twice the amount of carbon as the atmosphere. Some proponents of carbon sequestration in soil claim that if regenerative agriculture were practised on the planet’s 1.4 billion tillable hectares, it could sequester up to 40 per cent of current CO2 emissions.

The figure below illustrates the extent of conversion of land to crops and pasture between 1750 and 1990.

Anthropogenic changes in land cover from 1750 to 1990

Anthropogenic changes in land cover from 1750 to 1990

Anthropogenic modifications of land cover up between 1750 (top panel) and 1990 (bottom panel)—reconstructions from the History Database of the Environment.

Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing, Figure 2.15, p. 181.

Further reading:

Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing.

 


15 November, 2010

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