GLOBAL WARMING

Global warming is the increase in the average temperature of Earth's near-surface air and
oceans since the mid-20th century and its projected continuation. According to the 2007
Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC),
global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 20th
century. Most of the observed temperature increase since the middle of the 20th century
was caused by increasing concentrations of greenhouse gases, which results from human
activity such as fossil fuel burning and deforestation, also as a result of increasing
concentrations of atmospheric aerosols that block sunlight from reaching the surface, has
partially countered the effects of greenhouse gas induced warming.
An increase in global temperature will cause sea levels to rise and will change the amount
and pattern of precipitation, probably including expansion of subtropical deserts.

The greenhouse effect is a process by which radiative energy leaving a planetary

surface is absorbed by some atmospheric gases, called greenhouse gases. They transfer this
energy to other components of the atmosphere, and it is re-radiated in all directions,
including back down towards the surface. This is absorbed and re-emitted in all directions
by greenhouse gas molecules and clouds. The effect is to warm the earth surface and the
lower atmosphere.
Solar variation refers here to changes in the amount of total solar radiation and its
spectral distribution over multi-annual to multi-millennial time-scales. There are periodic
components to these variations, the principal one being the 11-year solar cycle (or sunspot
cycle), as well as aperiodic fluctuations.

Temperature changes
Most climate change scenarios project that greenhouse gas concentrations will increase
through 2100 with a continued increase in average global temperatures (IPCC 2007).
How much and how quickly the Earth's temperature will increase remains unknown
given the uncertainty of future greenhouse gas, aerosol emissions and the Earth's
response to changing conditions.
 Attributed and expected effects
Environmental

Natural systems

Sparse records indicate that glaciers have been retreating since the early 1800s. In the
1950s measurements began that allow the monitoring of glacial mass balance.

Global warming has been detected in a number of systems. Rising sea levels and observed
decreases in snow and ice extent are consistent with warming. Most of the increase in
global average temperature since the mid-20th century is, with high probability, atttributable
to human-induced changes in greenhouse gas concentrations.

Even with current policies to reduce emissions, global emissions are still expected to
continue to grow over the coming decades. Over the course of the 21 st century, increases in
emissions at or above their current rate would very likely induce changes in the climate
system larger than those observed in the 20th century.

Changes in regional climate are expected to include greater warming over land, with most
warming at high northern latitudes, and least warming over the Southern Ocean and parts of
the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease. The
frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.

Economical
The scientific consensus is that anthropogenic global warming is occurring. Nevertheless,
political and public debate continues. The Kyoto Protocol is aimed at stabilizing
greenhouse gas concentration to prevent a "dangerous anthropogenic interference". As of
November 2009, 187 states have signed and ratified the protocol.
There is some evidence of regional climate change affecting systems related to human
activities, including agricultural and forestry management activities at higher latitudes in
the Northern Hemisphere. Future climate change is expected to particularly affect some
sectors and systems related to human activities. Water resources may be stressed in some
dry regions at mid-latitudes, the dry tropics, and areas that depend on snow and ice melt.
Reduced water availability may affect agriculture in low latitudes. Low-lying coastal
systems are vulnerable to sea level rise and storm surge. Human health in populations with
limited capacity to adapt to climate change. It is expected that some regions will be
particularly affected by climate change, including the Arctic, Africa, small islands, and
Asian and African megadeltas. Some people, such as the poor, young children, and the
elderly, are particularly at risk, even in high-income areas.

Global warming mitigation

Reducing the amount of future climate change is called mitigation of climate change. The
IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or
enhance the capacity of carbon sinks to absorb GHGs from the atmosphere. Many
countries, both developing and developed, are aiming to use cleaner, less polluting,
technologies. Use of these technologies aids mitigation and could result in substantial
reductions in CO2 emissions. Policies include targets for emissions reductions, increased
use of renewable energy, and increased energy efficiency. Studies indicate substantial
potential for future reductions in emissions. Since even in the most optimistic scenario,
fossil fuels are going to be used for years to come, mitigation may also involve carbon
capture and storage, a process that traps CO 2 produced by factories and gas or coal power
stations and then stores it, usually underground.

Global warming adaptation

Other policy responses include adaptation to climate change. Adaptation to climate change
may be planned, e.g., by local or national government, or spontaneous, done privately
without government intervention. The ability to adapt is closely linked to social and
economic development. Even societies with high capacities to adapt are still vulnerable to
climate change. Planned adaptation is already occurring on a limited basis. The barriers,
limits, and costs of future adaptation are not fully understood.

Another policy response is engineering of the climate (geoengineering). This policy

response is sometimes grouped together with mitigation. Geoengineering is largely
unproven, and reliable cost estimates for it have not yet been published.