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DatePageclimate sEare nch usÄŻ00 Rave 8eveso const uencesim bas chcnan |
| Answer» <p>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.</p><p>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.</p><p>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.Radiative Forcing of Factors Affected byHuman ActivitiesThe contributions to radiative forcingfrom some of the factors influenced by humanactivities are shown in Figure 2. Thevalues reflect the total forcing relative to thestart of the industrial era (about 1750). Theforcings for all greenhouse gas increases,which are the best understood of those dueto human activities, are positive because eachgas absorbs outgoing infrared radiation in theatmosphere. Among the greenhouse gases,CO2 increases have caused the largest forcingover this period. Tropospheric ozone increaseshave also contributed to warming, whilestratospheric ozone decreases have contributedto cooling.Aerosol particles influence radiative forcingdirectly through reflection and absorptionof solar and infrared radiation in the atmosphere.Some aerosols cause a positive forcingwhile others cause a negative forcing. The directradiative forcing summed over all aerosoltypes is negative. Aerosols also cause a negativeradiative forcing indirectly through thechanges they cause in cloud properties.Human activities since the industrial erahave altered the nature of land cover overthe globe, principally through changes in croplands, pastures and forests. They have also modified the reflectiveproperties of ice and snow. Overall, it is likely that more solarradiation is now being reflected from Earth’s surface as a result ofhuman activities. This change results in a negative forcing.Aircraft produce persistent linear trails of condensation (‘contrails’)in regions that have suitably low temperatures and highhumidity. Contrails are a form of cirrus cloud that reflect solar radiationand absorb infrared radiation. Linear contrails from globalaircraft operations have increased Earth’s cloudiness and are estimatedto cause a small positive radiative forcing.Radiative Forcing from Natural ChangesNatural forcings arise due to solar changes and explosivevolcanic eruptions. Solar output has increased gradually in theindustrial era, causing a small positive radiative forcing (see Figure2). This is in addition to the cyclic changes in solar radiation that follow an 11-year cycle. Solar energy directly heats the climatesystem and can also affect the atmospheric abundance of somegreenhouse gases, such as stratospheric ozone. Explosive volcaniceruptions can create a short-lived (2 to 3 years) negative forcingthrough the temporary increases that occur in sulphate aerosolin the stratosphere. The stratosphere is currently free of volcanicaerosol, since the last major eruption was in 1991 (Mt. Pinatubo).The differences in radiative forcing estimates between thepresent day and the start of the industrial era for solar irradiancechanges and volcanoes are both very small compared to the differencesin radiative forcing estimated to have resulted from humanactivities. As a result, in today’s atmosphere, the radiative forcingfrom human activities is much more important for current andfuture climate change than the estimated radiative forcing fromchanges in natural processes. </p> | |