Human activities contribute to climate change by causing changes in Earth's atmosphere in the amounts of greenhouse gases, aerosols (small particles), and loudiness. The largest known contribution comes from the burning of fossil fuels, which releases carbon dioxide gas to the atmosphere.
How do Human Activities
Contribute to Climate Change and How do They Compare with Natural Influences?
Human activities contribute to climate change by causing
changes in Earth's atmosphere in the amounts of greenhouse gases, aerosols (small
particles), and cloudiness. The largest known contribution comes from the
burning of fossil fuels, which releases carbon dioxide gas to the atmosphere.
Greenhouse gases and aerosols affect climate by altering incoming solar
radiation and outgoing infrared (thermal) radiation that are part of Earth's
energy balance. Changing the atmospheric abundance or properties of these gases
and particles can lead to a warming or cooling of the climate system. Since the
start of the industrial era (about 1750), the overall effect of human
activities on climate has been a warming influence. The human impact on climate
during this era greatly exceeds that due to known changes in natural processes,
such as solar changes and volcanic eruptions.
Greenhouse Gases Human activities result in emissions of four
principal greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O) and the halocarbons (a group of gases containing fluorine, chlorine and
bromine). These gases accumulate in the atmosphere, causing concentrations to
increase with time. Significant increases in all of these gases have occurred
in the industrial era (see Figure 1). All of these increases are attributable
to human activities.
dioxide has increased from fossil fuel use in transportation, building heating
and cooling and the manufacture of cement and other goods. Deforestation
releases CO2 and reduces its uptake by plants. Carbon dioxide is also released
in natural processes such as the decay of plant matter.
has increased as a result of human activities related to agriculture, natural
gas distribution and landfills. Methane is also released from natural processes
that occur, for example, in wetlands. Methane concentrations are not currently
increasing in the atmosphere because growth rates decreased over the last two
oxide is also emitted by human activities such as fertilizer use and fossil
fuel burning. Natural processes in soils and the oceans also release N2O.
gas concentrations have increased primarily due to human activities. Natural
processes are also a small source. Principal halocarbons include the
chlorofluorocarbons (e.g., CFC-11 and CFC-12), which were used extensively as
refrigeration agents and in other industrial processes before their presence in
the atmosphere was found to cause stratospheric ozone depletion. The abundance
of chlorofluorocarbon gases is decreasing as a result of international
regulations designed to protect the ozone layer.
is a greenhouse gas that is continually produced and destroyed in the
atmosphere by chemical reactions. In the troposphere, human activities have
increased ozone through the release of gases such as carbon monoxide,
hydrocarbons and nitrogen oxide, which chemically react to produce ozone. As
mentioned above, halocarbons released by human activities destroy ozone in the
stratosphere and have caused the ozone hole over Antarctica.
vapour is the most abundant and important greenhouse gas in the atmosphere.
However, human activities have only a small direct influence on the amount of
atmospheric water vapour. Indirectly, humans have the potential to affect water
vapour substantially by changing climate. For example, a warmer atmosphere
contains more water vapour. Human activities also influence water vapour
through CH4 emissions, because CH4 undergoes chemical destruction in the
stratosphere, producing a small amount of water vapour.
are small particles present in the atmosphere with widely varying size,
concentration and chemical composition. Some aerosols are emitted directly into
the atmosphere while others are formed from emitted compounds. Aerosols contain
both naturally occurring compounds and those emitted as a result of human
activities. Fossil fuel and biomass burning have increased aerosols containing
sulphur compounds, organic compounds and black carbon (soot). Human activities
such as surface mining and industrial processes have increased dust in the
atmosphere. Natural aerosols include mineral dust released from the surface,
sea salt aerosols, biogenic emissions from the land and oceans and sulphate and
dust aerosols produced by volcanic eruptions.
Radiative Forcing of
Factors Affected by Human Activities
The contributions to radiative forcing from some of the
factors influenced by human activities are shown in Figure 2. The values
reflect the total forcing relative to the start of the industrial era (about
1750). The forcings for all greenhouse gas increases, which are the best
understood of those due to human activities, are positive because each gas
absorbs outgoing infrared radiation in the atmosphere. Among the greenhouse
gases, CO2 increases have caused the largest forcing over this period.
Tropospheric ozone increases have also contributed to warming, while
stratospheric ozone decreases have contributed to cooling.
Aerosol particles influence radiative forcing directly
through reflection and absorption of solar and infrared radiation in the
atmosphere. Some aerosols cause a positive forcing while others cause a
negative forcing. The direct radiative forcing summed over all aerosol types is
negative. Aerosols also cause a negative radiative forcing indirectly through
the changes they cause in cloud properties.
Human activities since the industrial era have altered the
nature of land cover over the globe, principally through changes in croplands,
pastures and forests. They have also modified the reflective properties of ice
and snow. Overall, it is likely that more solar radiation is now being
reflected from Earth's surface as a result of human activities. This change
results in a negative forcing.
Aircraft produce persistent linear trails of condensation
('contrails') in regions that have suitably low temperatures and high humidity.
Contrails are a form of cirrus cloud that reflect solar radiation and absorb
infrared radiation. Linear contrails from global aircraft operations have
increased Earth's cloudiness and are estimated to cause a small positive
Radiative Forcing from
Natural forcings arise
due to solar changes and explosive volcanic eruptions. Solar output has
increased gradually in the industrial era, causing a small positive radiative
forcing (see Figure 2). This is in addition to the cyclic changes in solar
radiation that follow an 11-year cycle. Solar energy directly heats the climate
system and can also affect the atmospheric abundance of some greenhouse gases,
such as stratospheric ozone. Explosive volcanic eruptions can create a
short-lived (2 to 3 years) negative forcing through the temporary increases
that occur in sulphate aerosol in the stratosphere. The stratosphere is
currently free of volcanic aerosol, since the last major eruption was in 1991
The differences in radiative forcing estimates between the
present day and the start of the industrial era for solar irradiance changes
and volcanoes are both very small compared to the differences in radiative
forcing estimated to have resulted from human activities. As a result, in
today's atmosphere, the radiative forcing from human activities is much more
important for current and future climate change than the estimated radiative
forcing from changes in natural processes.
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