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Frequently Asked Question 2.1
How do Human Activities Contribute to Climate Change
and How do They Compare with Natural Influences?
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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 out-going 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.
- Carbon 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.
- Methane 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 decades.
- Nitrous 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.
- Halocarbon 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.
- Ozone is a greenhouse gas
that is continually produced and destroyed in the atmosphere by chemical reactions.
In the troposphere, human activities have increased ozone
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FAQ 2.1, Figure 1.
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.
(Figure adapted from Figure 2.20 of this report.)
- 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
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- 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.
- Water 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.
- Aerosols 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
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FAQ 2.1, Figure 2.
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.
(Figure adapted from Figure 2.20 of this report.)
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FAQ 2.1, Box 1: What is Radiative Forcing?
What is radiative forcing? The influence of a factor that can cause
climate change, such as a greenhouse gas, is often evaluated in terms of its radiative forcing.
Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system
is influenced when factors that affect climate are altered. The word radiative arises because
these factors change the balance between incoming solar radiation and outgoing infrared
radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface
temperature. The term forcing is used to indicate that Earth’s radiative balance is being
pushed away from its normal state.
Radiative forcing is usually quantified as the ‘rate of energy
change per unit area of the globe as measured at the top of the atmosphere’, and is expressed
in units of ‘Watts per square metre’ (see Figure 2). When radiative forcing from a factor
or group of factors is evaluated as positive, the energy of the Earth-atmosphere system
will ultimately increase, leading to a warming of the system. In contrast, for a negative
radiative forcing, the energy will ultimately decrease, leading to a cooling of the system.
Important challenges for climate scientists are to identify all the factors that affect
climate and the mechanisms by which they exert a forcing, to quantify the radiative forcing
of each factor and to evaluate the total radiative forcing from the group of factors.
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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.
Radiative Forcing from Natural Changes
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
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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 (Mt. Pinatubo).
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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