Global warming causes

The climate system varies through natural,CO2 values this high were last attained 20 million
internal processes and in response to variations inyears ago. "About three-quarters of the
external forcing factors including solar activity,anthropogenic [man-made] emissions of CO2 to
volcanic emissions, variations in the earth's orbitthe atmosphere during the past 20 years are due
(orbital forcing) and greenhouse gases. Theto fossil fuel burning. The rest of the
detailed causes of the recent warming remain ananthropogenic emissions are predominantly due to
active field of research, but the scientificland-use change, especially deforestation."
consensus[6] identifies increased levels ofThe present atmospheric concentration of CO2 is
greenhouse gases due to human activity as theabout 383 parts per million (ppm) by volume.
main influence. This attribution is clearest for theFuture CO2 levels are expected to rise due to
most recent 50 years, for which the mostongoing burning of fossil fuels and land-use change.
detailed data are available. Contrasting with theThe rate of rise will depend on uncertain
scientific consensus, other hypotheses have beeneconomic, sociological, technological, natural
proposed to explain some of the observeddevelopments, but may be ultimately limited by
increase in global temperatures, including: thethe availability of fossil fuels. The IPCC Special
warming is within the range of natural variation;Report on Emissions Scenarios gives a wide range
the warming is a consequence of coming out of aof future CO2 scenarios, ranging from 541 to 970
prior cool period, namely the Little Ice Age; or theppm by the year 2100. Fossil fuel reserves are
warming is primarily a result of variances in solarsufficient to reach this level and continue
radiation.emissions past 2100, if coal, tar sands or methane
None of the effects of forcing are instantaneous.clathrates are extensively used.
Due to the thermal inertia of the Earth's oceansPositive feedback effects such as the expected
and slow responses of other indirect effects, therelease of CH4 from the melting of permafrost
Earth's current climate is not in equilibrium with thepeat bogs in Siberia (possibly up to 70,000 million
forcing imposed. Climate commitment studiestonnes) may lead to significant additional sources
indicate that even if greenhouse gases wereof greenhouse gas emissions not included in
stabilized at present day levels, a further warmingclimate models cited by the IPCC.
of about 0.5 °C (0.9 °F) would still occurFeedbacks
Greenhouse gases in the atmosphereThe effects of forcing agents on the climate are
Recent increases in atmospheric carbon dioxidecomplicated by various feedback processes.
(CO2). The monthly CO2 measurements displayOne of the most pronounced feedback effects
small seasonal oscillations in an overall yearlyrelates to the evaporation of water. CO2 injected
uptrend; each year's maximum is reached duringinto the atmosphere causes a warming of the
the northern hemisphere's late spring, and declinesatmosphere and the earth's surface. The warming
during the northern hemisphere growing season ascauses more water to be evaporated into the
plants remove some CO2 from the atmosphere.atmosphere. Since water vapor itself acts as a
Recent increases in atmospheric carbon dioxidegreenhouse gas, this causes still more warming;
(CO2). The monthly CO2 measurements displaythe warming causes more water vapor to be
small seasonal oscillations in an overall yearlyevaporated, and so forth until a new dynamic
uptrend; each year's maximum is reached duringequilibrium concentration of water vapor is
the northern hemisphere's late spring, and declinesreached at a slight increase in humidity and with a
during the northern hemisphere growing season asmuch larger greenhouse effect than that due to
plants remove some CO2 from the atmosphere.CO2 alone. This feedback effect can only be
The greenhouse effect was discovered byreversed slowly as CO2 has a long average
Joseph Fourier in 1824 and was first investigatedatmospheric lifetime.
quantitatively by Svante Arrhenius in 1896. It isFeedback effects due to clouds are an area of
the process by which absorption and emission ofongoing research and debate. Seen from below,
infrared radiation by atmospheric gases warms aclouds absorb infrared radiation and so exert a
planet's atmosphere and surface.warming effect. Seen from above, the same
Greenhouse gases create a natural greenhouseclouds reflect sunlight and so exert a cooling
effect, without which, mean temperatures oneffect. Increased global water vapor concentration
Earth would be an estimated 30 °C (54may or may not cause an increase in global
°F) lower, so that Earth would beaverage cloud cover. The net effect of clouds
uninhabitable. Thus scientists do not "believe in" orthus has not been well modeled, however, cloud
"oppose" the greenhouse effect as such; rather,feedback is second only to water vapor feedback
the debate concerns the net effect of theand is positive in all the models that contributed to
addition of greenhouse gases, while allowing forthe IPCC Fourth Assessment Report.
associated positive and negative feedbackAnother important feedback process is ice-albedo
mechanisms.feedback. The increased CO2 in the atmosphere
On Earth, the major natural greenhouse gases arewarms the Earth's surface and leads to melting of
water vapor, which causes aboutice near the poles. As the ice melts, land or open
36–70% of the greenhouse effectwater takes its place. Both land and open water
(not including clouds); carbon dioxide (CO2), whichare on average less reflective than ice, and thus
causes 9–26%; methane (CH4), whichabsorb more solar radiation. This causes more
causes 4–9%; and ozone, whichwarming, which in turn causes more melting, and
causes 3–7%. The atmosphericthis cycle continues.
concentrations of CO2 and CH4 have increasedPositive feedback due to release of CO2 and CH4
by 31% and 149% respectively abovefrom thawing permafrost is an additional
pre-industrial levels since 1750. These levels aremechanism contributing to warming. Possible
considerably higher than at any time during thepositive feedback due to CH4 release from
last 650,000 years, the period for which reliablemelting seabed ices is a further mechanism to be
data has been extracted from ice cores. Fromconsidered.
less direct geological evidence it is believed that