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
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
FAQ 2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. (Data combined and simplified from Chapters 6 and 2 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 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.