1. Global warming is the unequivocal and continuing rise in the average temperature of
Earth's climate system.[2] Since 1971, 90% of the warming has occurred in the oceans.[3] Despite the
oceans' dominant role in energy storage, the term "global warming" is also used to refer to increases
in average temperature of the air and sea at Earth's surface.[4] Since the early 20th century, the
global air and sea surface temperature has increased about 0.8 °C (1.4 °F), with about two-thirds of
the increase occurring since 1980.[5] Each of the last three decades has been successively warmer
at the Earth's surface than any preceding decade since 1850.[6]
Scientific understanding of the cause of global warming has been increasing. In its fourth
assessment (AR4 2007) of the relevant scientific literature, the Intergovernmental Panel on Climate
Change (IPCC) reported that scientists were more than 90% certain that most of global warming was
being caused by increasing concentrations of greenhouse gasesproduced by human
activities.[7][8][9] In 2010 that finding was recognized by the national science academies of all major
industrialized nations.[10][A] Affirming these findings in 2013, the IPCC stated that the largest driver of
global warming iscarbon dioxide (CO2) emissions from fossil fuel combustion, cement production,
and land use changes such asdeforestation.[11] Its 2013 report states
Human influence has been detected in warming of the atmosphere and the ocean, in changes in
the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes
in some climate extremes. This evidence for human influence has grown since AR4. It is extremely
likely (95-100%) that human influence has been the dominant cause of the observed warming since
the mid-20th century. - IPCC AR5 WG1 Summary for Policymakers[12]
Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by
the Intergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st century
the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2.0 to 5.2 °F) for their
lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest.[13] The ranges of these
estimates arise from the use of models with differing sensitivity to greenhouse gas
concentrations.[14][15]
Future climate change and associated impacts will vary from region to region around the
globe.[16][17] The effects of an increase in global temperature include a rise in sea levels and a
change in the amount and pattern of precipitation, as well as a probable expansion
of subtropical deserts.[18] Warming is expected to be strongest in the Arctic, with the
continuingretreat of glaciers, permafrost and sea ice. Other likely effects of the warming include
more frequent extreme weatherevents including heat waves, droughts and heavy rainfall; ocean
acidification; and species extinctions due to shifting temperature regimes. Effects significant to
humans include the threat to food security from decreasing crop yields and theloss of habitat from
inundation.[19][20]

2. Proposed policy responses to global warming include mitigation by emissions
reduction, adaptation to its effects, building systems resilient to its effects, and possible
future climate engineering. Most countries are parties to the United Nations Framework Convention
on Climate Change (UNFCCC),[21] whose ultimate objective is to prevent dangerous anthropogenic
(i.e., human-induced) climate change.[22] Parties to the UNFCCC have adopted a range of policies
designed to reduce greenhouse gas emissions[23][24][25][26] and to assist in adaptation to global
warming.[23][26][27][28] Parties to the UNFCCC have agreed that deep cuts in emissions are
required,[29] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the
pre-industrial level.[29][B] Reports published in 2011 by the United Nations Environment
Programme[30] and the International Energy Agency[31] suggest that efforts as of the early 21st
century to reduce emissions may be inadequate to meet the UNFCCC's 2 °C target.
Emissions of greenhouse gases grew 2.2% per year between 2000 and 2010, compared with 1.3%
per year from 1970 to 2000.[32]
Contents
[hide]
 1 Observed temperature changes
 2 Initial causes of temperature changes (external forcings)
o 2.1 Greenhouse gases
o 2.2 Particulates and soot
o 2.3 Solar activity
 3 Feedback
 4 Climate models
 5 Observed and expected environmental effects
o 5.1 Natural systems
o 5.2 Ecological systems
o 5.3 Long-term effects
o 5.4 Large-scale and abrupt impacts
 6 Observed and expected effects on social systems
o 6.1 Food security
o 6.2 Habitat inundation
 7 Proposed policy responses to global warming
o 7.1 Mitigation
o 7.2 Adaptation
o 7.3 Climate engineering
 8 Discourse about global warming
o 8.1 Political discussion
o 8.2 Scientific discussion
o 8.3 Discussion by the public and in popular media
 8.3.1 Surveys of public opinion
 9 Etymology
 10 See also
 11 Notes
 12 Citations

3.  13 References
 14 Further reading
 15 External links
Observed temperature changes
Main article: Instrumental temperature record
Two millennia ofmean surface temperatures according to differentreconstructions from climate proxies,each
smoothed on a decadal scale, with the instrumental temperature record overlaid in black.
NOAA graph of Global Annual Temperature Anomalies 1950–2012,showing the El Niño-Southern Oscillation
Earth has been in radiative imbalancesince atleastthe 1970s,where less energyleaves the atmosphere than enters
it. Most of this extra energy has been absorbed by the oceans.[33]
It is very likely that human activities substantially
contributed to this increase in ocean heatcontent.[34]

4. The Earth's average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. The rate
of warming over the last half of that period was almost double that for the period as a whole
(0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade). The urban heat island effect is very
small, estimated to account for less than 0.002 °C of warming per decade since
1900.[35] Temperatures in the lowertroposphere have increased between 0.13 and 0.22 °C (0.22
and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Climate
proxies show the temperature to have been relatively stable over the one or two thousand
years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and
the Little Ice Age.[36]
The warming that is evident in the instrumental temperature record is consistent with a wide range of
observations, as documented by many independent scientific groups.[37] Examples include sea level
rise (due to melting of snow and ice and because water above 3.98 °Cexpands as it
warms),[38] widespread melting of snow and ice,[39] increased heat content of the
oceans,[37] increased humidity,[37] and the earlier timing of spring events,[40] e.g., the flowering of
plants.[41] The probability that these changes could have occurred by chance is virtually zero.[37]
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic
Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread
instrumental measurements became available in the late 19th century, exceeding 1998 by a few
hundredths of a degree.[42][43][44] Estimates by the Climatic Research Unit (CRU) show 2005 as the
second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, "the
error estimate for individual years ... is at least ten times larger than the differences between these
three years."[45] The World Meteorological Organization (WMO) WMO statement on the status of the
global climate in 2010 explains that, "The 2010 nominal value of +0.53 °C ranks just ahead of those
of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not
statistically significant..."[46] Every year from 1986 to 2013 has seen annual average global land and
ocean surface temperatures above the 1961–1990 average.[47][48]
Surface temperatures in 1998 were unusually warm because global temperatures are affected by
the El Niño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred
during that year.[49] Global temperature is subject to short-term fluctuations that overlay long term
trends and can temporarily mask them. The relative stability in surface temperature from 2002 to
2009—which has been dubbed the global warming hiatus by the media and some scientists—[50] is
consistent with such an episode.[51][52] 2010 was also an El Niño year. On the low swing of the
oscillation, 2011 as a La Niña year was cooler but it was still the 11th warmest year since records
began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Over the
more recent record, 2011 was the warmest La Niña year in the period from 1950 to 2011, and was
close to 1997 which was not at the lowest point of the cycle.[53]

5. Temperature changes vary over the globe. Since 1979, land temperatures have increased about
twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[54] Ocean
temperatures increase more slowly than land temperatures because of the larger effective heat
capacity of the oceans and because the ocean loses more heat by evaporation.[55] The northern
hemisphere is also naturally warmer than the southern hemisphere mainly because
of meridional heat transport in the oceans which has a differential of about
0.9 petawatts northwards,[56] with an additional contribution from the albedo differences between the
polar regions. Since the beginning of industrialisation the temperature difference between the
hemispheres has increased due to melting of sea ice and snow in the North.[57] Average arctic
temperatures have been increasing at almost twice the rate of the rest of the world in the past 100
years; however arctic temperatures are also highly variable.[58] Although more greenhouse gases are
emitted in the Northern than Southern Hemisphere this does not contribute to the difference in
warming because the major greenhouse gases persist long enough to mix between hemispheres.[59]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can
take centuries or longer to adjust to changes in forcing. Climate commitmentstudies indicate that
even if greenhouse gases were stabilized at year 2000 levels, a further warming of
about 0.5 °C (0.9 °F) would still occur.[60]
Initial causes of temperature changes (external forcings)
Main article: Attribution of recent climate change
Greenhouse effectschematic showing energyflows between space,the atmosphere,and Earth's surface.Energy
exchanges are expressed in watts per square meter (W/m 2
).
This graph,known as the Keeling Curve, shows the increase ofatmospheric carbon dioxide (CO2) concentrations
from 1958–2013.Monthly CO2 measurements displayseasonal oscillations in an upward trend;each year's

6. maximum occurs during theNorthern Hemisphere's late spring,and declines during its growing season as plants
remove some atmospheric CO2.
The climate system can respond to changes in external forcings.[61][62] External forcings can "push"
the climate in the direction of warming or cooling.[63] Examples of external forcings include changes
in atmospheric composition (e.g., increased concentrations of greenhouse gases),solar
luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.[64] Orbital cycles vary
slowly over tens of thousands of years and at present are in an overall cooling trend which would be
expected to lead towards a glacial period within the current ice age, but the 20th
century instrumental temperature record shows a sudden rise in global temperatures.[65]
Greenhousegases
Main articles: Greenhouse gas, Greenhouse effect, Radiative forcing and Carbon dioxide in Earth's
atmosphere
The greenhouse effect is the process by which absorption and emission of infrared radiation by
gases in a planet's atmosphere warm its lower atmosphere and surface. It was proposed by Joseph
Fourier in 1824, discovered in 1860 by John Tyndall,[66] was first investigated quantitatively
by Svante Arrhenius in 1896,[67] and was developed in the 1930s through 1960s by Guy Stewart
Callendar.[68]
Annual world greenhouse gas emissions,in 2005,by sector.
Percentage share ofglobal cumulative energy-related CO
2 emissions between 1751 and 2012 across differentregions.
On earth, naturally occurring amounts of greenhouse gases have a mean warming effect of about
33 °C (59 °F).[69][C] Without the Earth's atmosphere, the temperature across almost the entire surface
of the Earth would be below freezing.[70] The major greenhouse gases are water vapor, which
causes about 36–70% of the greenhouse effect; carbon dioxide (CO2), which causes 9–
26%;methane (CH4), which causes 4–9%; and ozone (O3), which causes 3–7%.[71][72][73] Clouds also
affect the radiation balance through cloud forcings similar to greenhouse gases.

7. Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the
atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric
ozone, CFCsand nitrous oxide. According to work published in 2007, the concentrations of CO2 and
methane have increased by 36% and 148% respectively since 1750.[74] These levels are much
higher than at any time during the last 800,000 years, the period for which reliable data has been
extracted from ice cores.[75][76][77][78] Less direct geological evidence indicates that CO2 values higher
than this were last seen about 20 million years ago.[79] Fossil fuel burning has produced about three-
quarters of the increase in CO2 from human activity over the past 20 years. The rest of this increase
is caused mostly by changes in land-use, particularly deforestation.[80] Estimates of global
CO2 emissions in 2011 from fossil fuel combustion, including cement production and gas flaring, was
34.8 billion tonnes (9.5 ± 0.5 PgC), an increase of 54% above emissions in 1990. Coal burning was
responsible for 43% of the total emissions, oil 34%, gas 18%, cement 4.9% and gas flaring
0.7%[81] In May 2013, it was reported that readings for CO2 taken at the world's primary benchmark
site in Mauna Loa surpassed 400 ppm. According to professorBrian Hoskins, this is likely the first
time CO2 levels have been this high for about 4.5 million years.[82][83]
Over the last three decades of the 20th century, gross domestic product per capita and population
growth were the main drivers of increases in greenhouse gas emissions.[84] CO2 emissions are
continuing to rise due to the burning of fossil fuels and land-use change.[85][86]:71 Emissions can
be attributed to different regions, e.g., see the figure opposite. Attribution of emissions due to land-
use change is a controversial issue.[87][88]:289
Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have
been projected that depend upon uncertain economic, sociological, technological, and natural
developments.[89] In most scenarios, emissions continue to rise over the century, while in a few,
emissions are reduced.[90][91] Fossil fuel reserves are abundant, and will not limit carbon emissions in
the 21st century.[92] Emission scenarios, combined with modelling of the carbon cycle, have been
used to produce estimates of how atmospheric concentrations of greenhouse gases might change in
the future. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the
atmospheric concentration of CO2 could range between 541 and 970 ppm.[93] This is 90–250%
above the concentration in the year 1750.
The popular media and the public often confuse global warming with ozone depletion, i.e., the
destruction of stratospheric ozone by chlorofluorocarbons.[94][95] Although there are a few areas of
linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a
slight cooling influence on surface temperatures, while increasedtropospheric ozone has had a
somewhat larger warming effect.[96]

8. Atmospheric CO2 concentration from 650,000 years ago to near present,using ice core proxy data and direct
measurements
Particulates and soot
Ship tracks can be seen as lines in these clouds over the Atlantic Ocean on the eastcoastof the United States. The
climatic impacts from particulate forcing could have a large effect on climate through the indirecteffect.
Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface,
was observed from 1961 until at least 1990.[97] The main cause of this dimming is particulates
produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the
reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO2 and
aerosols – have partially offset one another in recent decades, so that net warming has been due to
the increase in non-CO2 greenhouse gases such as methane.[98]Radiative forcing due to particulates
is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one
week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate
concentrations will only delay climate changes due to carbon dioxide.[99] Black carbon is second only
to carbon dioxide for its contribution to global warming.[100] In addition to their direct effect by
scattering and absorbing solar radiation, particulates have indirect effects on the Earth's radiation
budget. Sulfates act as cloud condensation nuclei and thus lead to clouds that have more and
smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer
and larger droplets, known as the Twomey effect.[101] This effect also causes droplets to be of more
uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming
sunlight, known as the Albrecht effect.[102] Indirect effects are most noticeable in marine stratiform

9. clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates
represent the largest uncertainty in radiative forcing.[103]
Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric
soot directly absorbs solar radiation, which heats the atmosphere and cools the surface. In isolated
areas with high soot production, such as rural India, as much as 50% of surface warming due to
greenhouse gases may be masked by atmospheric brown clouds.[104] When deposited, especially on
glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the
surface.[105] The influences of particulates, including black carbon, are most pronounced in the
tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in
the extratropics and southern hemisphere.[106]
Satellite observations of Total Solar Irradiance from 1979–2006.
Contribution ofnatural factors and human activities to radiative forcing of climate change.[107] Radiative forcing values
are for the year 2005,relative to the pre-industrial era (1750).[107]
The contribution of solar irradiance to radiative
forcing is 5% the value of the combined radiative forcing due to increases in the atmospheric concentrations of
carbon dioxide, methane and nitrous oxide.[108]
Solar activity
Main articles: Solar variation and Solar wind

10. Since 1978, output from the Sun has been precisely measured by satellites.[109] These
measurements indicate that the Sun's output has not increased since 1978, so the warming during
the past 30 years cannot be attributed to an increase in solar energy reaching the Earth.
Climate models have been used to examine the role of the sun in recent climate change.[110] Models
are unable to reproduce the rapid warming observed in recent decades when they only take into
account variations in solar output and volcanic activity. Models are, however, able to simulate the
observed 20th century changes in temperature when they include all of the most important external
forcings, including human influences and natural forcings.
Another line of evidence against the sun having caused recent climate change comes from looking
at how temperatures at different levels in the Earth's atmosphere have changed.[111] Models and
observations show that greenhouse warming results in warming of the lower atmosphere (called
the troposphere) but cooling of the upper atmosphere (called the stratosphere).[112][113] Depletion of
the ozone layerby chemical refrigerants has also resulted in a strong cooling effect in the
stratosphere. If the sun were responsible for observed warming, warming of both the troposphere
and stratosphere would be expected.[114]
Feedback
Main articles: Climate change feedback and Climate sensitivity
Sea ice, shown here in Nunavut, in northern Canada,reflects more sunshine,while open ocean absorbs more,
accelerating melting.
The climate system includes a range of feedbacks, which alter the response of the system to
changes in external forcings. Positive feedbacks increase the response of the climate system to an
initial forcing, while negative feedbacks reduce the response of the climate system to an initial
forcing.[115]
There are a range of feedbacks in the climate system, including water vapor, changes in ice-
albedo (snow and ice cover affect how much the Earth's surface absorbs or reflects incoming
sunlight), clouds, and changes in the Earth's carbon cycle (e.g., the release of carbon
fromsoil).[116] The main negative feedback is the energy which the Earth's surface radiates into space

11. as infrared radiation.[117] According to the Stefan-Boltzmann law, if the absolute temperature (as
measured in kelvin) doubles[D], radiated energy increases by a factor of 16 (2 to the 4th power).[118]
Feedbacks are an important factor in determining the sensitivity of the climate system to increased
atmospheric greenhouse gas concentrations. Other factors being equal, a higher climate
sensitivity means that more warming will occur for a given increase in greenhouse gas
forcing.[119] Uncertainty over the effect of feedbacks is a major reason why different climate models
project different magnitudes of warming for a given forcing scenario. More research is needed to
understand the role of clouds[115] and carbon cycle feedbacks in climate projections.[120]
The IPCC projections given in the lede span the "likely" range (greater than 66% probability, based
on expert judgement)[7] for the selected emissions scenarios. However, the IPCC's projections do not
reflect the full range of uncertainty.[121] The lower end of the "likely" range appears to be better
constrained than the upper end of the "likely" range.[121]
Climate models
Main article: Global climate model
Calculations ofglobal warming prepared in or before 2001 from a range of climate models under the SRES A2
emissions scenario,which assumes no action is taken to reduce emissions and regionallydivided economic
development.
Projected change in annual mean surface air temperature from the late 20th century to the middle 21stcentury,
based on a medium emissions scenario (SRES A1B).[122]
This scenario assumes thatno future policies are adopted
to limitgreenhouse gas emissions.Image credit: NOAA GFDL.[123]
A climate model is a computerized representation of the five components of the climate
system: Atmosphere, hydrosphere, cryosphere, land surface, and biosphere.[124] Such models are
based on scientific disciplines such as fluid dynamics, thermodynamics as well as physical
processes such as radiative transfer. The models take into account various components, such as
local air movement, temperature, clouds, and other atmospheric properties; ocean temperature, salt
content, and circulation; ice cover on land and sea; the transfer of heat and moisture from soil and
vegetation to the atmosphere; chemical and biological processes; solar variability and others.

12. Although researchers attempt to include as many processes as possible, simplifications of the actual
climate system are inevitable because of the constraints of available computer power and limitations
in knowledge of the climate system. Results from models can also vary due to different greenhouse
gas inputs and the model's climate sensitivity. For example, the uncertainty in IPCC's 2007
projections is caused by (1) the use of multiple models[121] with differing sensitivity to greenhouse
gas concentrations,[125] (2) the use of differing estimates of humanities' future greenhouse gas
emissions,[121] (3) any additional emissions from climate feedbacks that were not included in the
models IPCC used to prepare its report, i.e., greenhouse gas releases from permafrost.[126]
The models do not assume the climate will warm due to increasing levels of greenhouse gases.
Instead the models predict how greenhouse gases will interact with radiative transfer and other
physical processes. One of the mathematical results of these complex equations is a prediction
whether warming or cooling will occur.[127]
Recent research has called special attention to the need to refine models with respect to the effect of
clouds[128] and the carbon cycle.[129][130][131]
Models are also used to help investigate the causes of recent climate change by comparing the
observed changes to those that the models project from various natural and human-derived causes.
Although these models do not unambiguously attribute the warming that occurred from
approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the
warming since 1970 is dominated by man-made greenhouse gas emissions.[64]
The physical realism of models is tested by examining their ability to simulate contemporary or past
climates.[132] Climate models produce a good match to observations of global temperature changes
over the last century, but do not simulate all aspects of climate.[133] Not alleffects of global
warming are accurately predicted by the climate models used by the IPCC. Observed Arctic
shrinkage has been faster than that predicted.[134] Precipitation increased proportional to
atmospheric humidity, and hence significantly faster than global climate models predict.[135][136]
Observed and expected environmental effects
Main article: Effects of global warming

13. Projections ofglobal mean sea level rise by Parris and others.[137]
Probabilities have notbeen assigned to these
projections.[138]
Therefore,none of these projections should be interpreted as a "bestestimate"of future sea level
rise.Image credit: NOAA.
"Detection" is the process of demonstrating that climate has changed in some
defined statistical sense, without providing a reason for that change. Detection does not imply
attribution of the detected change to a particular cause. "Attribution" of causes of climate change is
the process of establishing the most likely causes for the detected change with some defined level of
confidence.[139] Detection and attribution may also be applied to observed changes in physical,
ecological and social systems.[140]
Natural systems
Main article: Physical impacts of climate change
Global warming has been detected in a number of natural systems. Some of these changes are
described in the section on observed temperature changes, e.g., sea level rise and widespread
decreases in snow and ice extent.[141]Anthropogenic forcing has likely contributed to some of the
observed changes, including sea level rise, changes in climate extremes (such as the number of
warm and cold days), declines in Arctic sea ice extent, and to glacier retreat.[142]

14. Sparse records indicate thatglaciers have been retreating since the early 1800s.In the 1950s measurements began
that allow the monitoring ofglacial mass balance,reported to the World Glacier Monitoring Service (WGMS) and
the National Snow and Ice Data Center (NSIDC)
Over the 21st century,[143] the IPCC projects that global mean sea level could rise by 0.18–
0.59 m.[144] The IPCC do not provide a best estimate of global mean sea level rise, and their upper
estimate of 59 cm is not an upper-bound, i.e., global mean sea level could rise by more than 59 cm
by 2100.[144] The IPCC's projections are conservative, and may underestimate future sea level
rise.[145] Over the 21st century, Parris and others[137] suggest that global mean sea level could rise by
0.2 to 2.0 m (0.7–6.6 ft), relative to mean sea level in 1992.
Widespread coastal flooding would be expected if several degrees of warming is sustained for
millennia.[146] For example, sustained global warming of more than 2 °C (relative to pre-industrial
levels) could lead to eventual sea level rise of around 1 to 4 m due to thermal expansion of sea
water and the melting of glaciers and small ice caps.[146] Melting of the Greenland ice sheet could
contribute an additional 4 to 7.5 m over many thousands of years.[146]
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.[147] During the 21st century, glaciers[148] and snow cover[149] are projected to continue
their widespread retreat. Projections of declines in Arctic sea ice vary.[150][151] Recent projections
suggest that Arctic summers could be ice-free (defined as ice extent less than 1 million square km)
as early as 2025-2030.[152]
Future changes in precipitation are expected to follow existing trends, with reduced precipitation
over subtropical land areas, and increased precipitation at subpolar latitudes and
some equatorial regions.[153] Projections suggest a probable increase in the frequency and severity
of some extreme weather events, such as heat waves.[154]
Ecologicalsystems
Main article: Climate change and ecosystems
In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant
and animal ranges, have been linked with high confidence to recent warming.[141] Future climate
change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral
reefs.[147] It is expected that most ecosystems will be affected by higher atmospheric CO2 levels,
combined with higher global temperatures.[155] Overall, it is expected that climate change will result in
the extinction of many species and reduced diversity of ecosystems.[156]
Increases in atmospheric CO2 concentrations have led to an increase in ocean acidity.[157] Dissolved
CO2 increases ocean acidity, which is measured by lower pH values.[157]Between 1750 to 2000,
surface-ocean pHhas decreased by ≈0.1, from ≈8.2 to ≈8.1.[158] Surface-ocean pH has probably not

15. been below ≈8.1 during the past 2 million years.[158]Projections suggest that surface-ocean pHcould
decrease by an additional 0.3–0.4 units by 2100.[159] Future ocean acidification could threaten coral
reefs, fisheries, protected species, and other natural resources of value to society.[157][160]
Long-term effects
Main article: Long-term effects of global warming
On the timescale of centuries to millennia, the magnitude of global warming will be determined
primarily by anthropogenic CO2 emissions.[161] This is due to carbon dioxide's very long lifetime in the
atmosphere.[161]
Stabilizing global average temperature would require reductions in anthropogenic
CO2 emissions.[161] Reductions in emissions of non-CO2 anthropogenic GHGs (e.g., methane and
nitrous oxide) would also be necessary.[161][162] For CO2, anthropogenic emissions would need to be
reduced by more than 80% relative to their peak level.[161] Even if this were to be achieved, global
average temperatures would remain close to their highest level for many centuries.[161]
Large-scale and abruptimpacts
Main article: Abrupt climate change
Climate change could result in global, large-scale changes in natural and social systems.[163] Two
examples are ocean acidification caused by increased atmospheric concentrations of carbon
dioxide, and the long-term melting of ice sheets, which contributes to sea level rise.[164]
Some large-scale changes could occur abruptly, i.e., over a short time period, and might also
be irreversible. An example of abrupt climate change is the rapid release of methaneand carbon
dioxide from permafrost, which would lead to amplified global warming.[165][166] Scientific
understanding of abrupt climate change is generally poor.[167] However, the probability of abrupt
changes appears to be very low.[165][168] Factors that may increase the probability of abrupt climate
change include higher magnitudes of global warming, warming that occurs more rapidly, and
warming that is sustained over longer time periods.[168]
Observed and expected effects on social systems
Further information: Effects of global warming § Social systems and Regional effects of global
warming § Regional impacts
Vulnerability of human societies to climate change mainly lies in the effects of extreme weather
events rather than gradual climate change.[169] Impacts of climate change so far include adverse
effects on small islands,[170] adverse effects on indigenous populations in high-latitude
areas,[171] and small but discernable effects on human health.[172] Over the 21st century, climate
change is likely to adversely affect hundreds of millions of people through increased coastal flooding,
reductions in water supplies, increased malnutritionand increased health impacts.[173]

16. The economic impacts of climate change are highly uncertain.[174] Small magnitudes of global
warming (0 to 2 °C, relative to pre-industrial levels) could lead to losses or gains in world gross
domestic product (GDP).[175] Above around 2.5 °C, most studies suggest losses in world GDP, with
greater losses at higher temperatures.[175]
Food security
See also: Climate change and agriculture
Maize field in South Africa.
Under present trends, by 2030, maize production in Southern Africa could decrease by up to 30%,
while rice, millet and maize in South Asia could decrease by up to 10%.[176] By 2080, yields
in developing countries could decrease by 10% to 25% on average while India could see a drop of
30% to 40%.[177] By 2100, while the population of three billion is expected to double, rice and maize
yields in the tropics are expected to decrease by 20–40% because of higher temperatures without
accounting for the decrease in yields as a result of soil moisture and water supplies stressed by
rising temperatures.[19]
Future warming of around 3 °C (by 2100, relative to 1990–2000) could result in increased crop
yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the
risk of malnutrition.[170] A similar regional pattern of net benefits and costs could occur for economic
(market-sector) effects.[172] Warming above 3 °C could result in crop yields falling in temperate
regions, leading to a reduction in global food production.[178]
Habitat inundation
Map showing where natural disasters caused/aggravated by global warming mayoccur.
Further information: Effects of climate change on humans § Displacement/migration

17. See also: Climate refugee
In small islands and megadeltas, inundation as a result of sea level rise is expected to threaten vital
infrastructure and human settlements.[179][180] This could lead to issues of homelessness in countries
with low lying areas such as Bangladesh, as well asstatelessness for populations in countries such
as the Maldives and Tuvalu.[181]
Proposed policy responses to global warming
There are different views over what the appropriate policy response to climate change should
be.[182] These competing views weigh the benefits of limiting emissions of greenhouse gases against
the costs. In general, it seems likely that climate change will impose greater damages and risks in
poorer regions.[183]
Mitigation
Main article: Climate change mitigation
The graph on the rightshows three "pathways"to meetthe UNFCCC's 2 °C target, labelled "global technology",
"decentralised solutions",and "consumption change".Each pathwayshows how various measures (e.g.,improved
energy efficiency, increased use ofrenewable energy) could contribute to emissions reductions.Image credit:
PBLNetherlands Environmental AssessmentAgency.[184]
Reducing the amount of future climate change is called mitigation of climate change.[185] 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.[186] Studies indicate substantial
potential for future reductions in emissions by a combination of emission-reducing activities such
as energy conservation, increased energy efficiency, and satisfying more of society's power
demands with renewable energy and nuclear energy sources.[187] Climate mitigation also includes
acts to enhance natural sinks, such as reforestation.[187]
In order to limit warming to within the lower range described in the IPCC's "Summary Report for
Policymakers"[188] it will be necessary to adopt policies that will limit greenhouse gas emissions to
one of several significantly different scenarios described in the full report.[189] This will become more
and more difficult with each year of increasing volumes of emissions and even more drastic
measures will be required in later years to stabilize a desired atmospheric concentration of
greenhouse gases. Energy-related carbon-dioxide (CO2) emissions in 2010 were the highest in
history, breaking the prior record set in 2008.[190]
Adaptation
Main article: Adaptation to global warming

18. Other policy responses include adaptation to climate change. Adaptation to climate change may be
planned, either in reaction to or anticipation of climate change, or spontaneous, i.e., without
government intervention.[191] Planned adaptation is already occurring on a limited basis.[187] The
barriers, limits, and costs of future adaptation are not fully understood.[187]
A concept related to adaptation is "adaptive capacity", which is the ability of a system (human,
natural or managed) to adjust to climate change (including climate variability and extremes) to
moderate potential damages, to take advantage of opportunities, or to cope with
consequences.[192] Unmitigated climate change (i.e., future climate change without efforts to limit
greenhouse gas emissions) would, in the long term, be likely to exceed the capacity of natural,
managed and human systems to adapt.[193]
Environmental organizations and public figures have emphasized changes in the climate and the
risks they entail, while promoting adaptation to changes in infrastructural needs and emissions
reductions.[194]
Climate engineering
Main article: Climate engineering
Climate engineering (sometimes called by the more expansive term 'geoengineering'),is the
deliberate modification of the climate. It has been investigated as a possible response to global
warming, e.g. by NASA[195] and the Royal Society.[196] Techniques under research fall generally into
the categories solar radiation management and carbon dioxide removal, although various other
schemes have been suggested. A study from 2014 investigated the most common climate
engineering methods and concluded they are either ineffective or have potentially severe side
effects and cannot be stopped without causing rapid climate change.[197]
Discourse about global warming
Political discussion
Main article: Politics of global warming
Further information: 2011 United Nations Climate Change Conference, 2012 United Nations Climate
Change Conference and 2013 United Nations Climate Change Conference
Article 2 of the UN Framework Convention refers explicitly to "stabilization of greenhouse gas concentrations."[198] In
order to stabilize the atmospheric concentration of CO
2, emissions worldwide would need to be dramaticallyreduced from their presentlevel.[199]
Most countries are Parties to the United Nations Framework Convention on Climate
Change (UNFCCC).[200] The ultimate objective of the Convention is to prevent dangerous human
interference of the climate system.[201] As is stated in the Convention, this requires that GHG

19. concentrations are stabilized in the atmosphere at a level where ecosystems can adapt naturally to
climate change, food production is not threatened, and economic development can proceed in a
sustainable fashion.[202] The Framework Convention was agreed in 1992, but since then, global
emissions have risen.[203] During negotiations, the G77 (a lobbying group in the United Nations
representing 133 developing nations)[204]:4 pushed for a mandate requiring developed countries to
"[take] the lead" in reducing their emissions.[205] This was justified on the basis that: the developed
world's emissions had contributed most to the stock of GHGs in the atmosphere; per-capita
emissions (i.e., emissions per head of population) were still relatively low in developing countries;
and the emissions of developing countries would grow to meet their development needs.[88]:290 This
mandate was sustained in the Kyoto Protocol to the Framework Convention,[88]:290 which entered into
legal effect in 2005.[206]
In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to
limit their emissions. These first-round commitments expired in 2012.[206] US President George W.
Bush rejected the treaty on the basis that "it exempts 80% of the world, including major population
centers such as China and India, from compliance, and would cause serious harm to the US
economy."[204]:5
At the 15th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC
Parties produced the Copenhagen Accord.[207] Parties associated with the Accord (140 countries, as
of November 2010)[208]:9 aim to limit the future increase in global mean temperature to
below 2 °C.[209] A preliminary assessment published in November 2010 by the United Nations
Environment Programme(UNEP) suggests a possible "emissions gap" between the voluntary
pledges made in the Accord and the emissions cuts necessary to have a "likely" (greater than 66%
probability) chance of meeting the 2 °C objective.[208]:10–14 The UNEP assessment takes
the 2 °C objective as being measured against the pre-industrial global mean temperature level. To
having a likely chance of meeting the 2 °C objective, assessed studies generally indicated the need
for global emissions to peak before 2020, with substantial declines in emissions thereafter.
The 16th Conference of the Parties (COP16) was held at Cancún in 2010. It produced an
agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas
emissions to meet a goal of limiting global warming to 2 °C above pre-industrial temperatures. It also
recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.[210]
Scientific discussion
See also: Scientific opinion on climate change and Surveys of scientists' views on climate change
Most scientists agree that humans are contributing to observed climate change.[85][211] A meta study
of academic papers concerning global warming, published between 1991 and 2011 and accessible
from Web of Knowledge, found that among those whose abstracts expressed a position on the
cause of global warming, 97.2% supported the consensus view that it is man made.[212] In an

20. October 2011 paper published in the International Journal of Public Opinion Research, researchers
from George Mason University analyzed the results of a survey of 489 American scientists working
in academia, government, and industry. Of those surveyed, 97% agreed that that global
temperatures have risen over the past century and 84% agreed that "human-induced greenhouse
warming" is now occurring, only 5% disagreeing that human activity is a significant cause of global
warming.[213][214] National science academies have called on world leaders for policies to cut global
emissions.[215]
In the scientific literature, there is a strong consensus that global surface temperatures have
increased in recent decades and that the trend is caused mainly by human-induced emissions of
greenhouse gases. No scientific body of national or international standing disagrees with this
view.[216][217]
Discussion by the public and in popular media
Main articles: climate change denial and global warming controversy
The global warming controversy refers to a variety of disputes, substantially more pronounced in
the popular media than in the scientific literature,[218][219] regarding the nature, causes, and
consequences of global warming. The disputed issues include the causes of increased global
average air temperature, especially since the mid-20th century, whether this warming trend is
unprecedented or within normal climatic variations, whether humankind has contributed significantly
to it, and whether the increase is wholly or partially an artifact of poor measurements. Additional
disputes concern estimates of climate sensitivity, predictions of additional warming, and what the
consequences of global warming will be.
From 1990–1997 in the United States, conservative think tanks mobilized to challenge the legitimacy
of global warming as a social problem. They challenged the scientific evidence, argued that global
warming will have benefits, and asserted that proposed solutions would do more harm than good.[220]
Some people dispute aspects of climate change science.[211][221] Organizations such as
the libertarian Competitive Enterprise Institute, conservative commentators, and some companies
such as ExxonMobil have challenged IPCC climate change scenarios, funded scientists who
disagree with the scientific consensus, and provided their own projections of the economic cost of
stricter controls.[222][223][224][225] Some fossil fuel companies have scaled back their efforts in recent
years,[226] or called for policies to reduce global warming.[227]
Surveys of public opinion
Main article: Public opinion on climate change
Researchers at the University of Michigan have found that the public's belief as to the causes of
global warming depends on the wording choice used in the polls.[228]

21. In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was
unaware of global warming, with people in developing countries less aware than those in developed,
and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature
changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few
countries from the Former Soviet Union lead in the opposite belief.[229] There is a significant contrast
of the opinions of the concept and the appropriate response between Europe and the United States.
Nick Pidgeon of Cardiff University said that "results show the different stages of engagement about
global warming on each side of the Atlantic", adding, "The debate in Europe is about what action
needs to be taken, while many in the US still debate whether climate change is happening."[230][231] A
2010 poll by the Office of National Statistics found that 75% of UK respondents were at least "fairly
convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006.[232] A
January 2011 ICM poll in the UK found 83% of respondents viewed climate change as a current or
imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll
asking the same question, though there had been a slight polarisation of opposing views.[233]
By 2010, with 111 countries surveyed, Gallup determined that there was a substantial decrease in
the number of Americans and Europeans who viewed global warming as a serious threat. In the US,
a little over half the population (53%) now viewed it as a serious concern for either themselves or
their families; this was 10% below the 2008 poll (63%). Latin America had the biggest rise in
concern, with 73% saying global warming was a serious threat to their families.[234] That global poll
also found that people are more likely to attribute global warming to human activities than to natural
causes, except in the USA where nearly half (47%) of the population attributed global warming to
natural causes.[235]
A March–May 2013 survey by Pew Research Center for the People & the Press polled 39 countries
about global threats. According to 54% of those questioned, global warming featured top of the
perceived global threats.[236] In a January 2013 survey, Pew found that 69% of Americans say there
is solid evidence that the Earth's average temperature has been getting warmer over the past few
decades, up six points since November 2011 and 12 points since 2009.[237]
Etymology
According to Erik M. Conway, global warming became the dominant popular term after June 1988,
when NASA climate scientist James Hansen used the term in a testimony to Congress[238] when he
said: "global warming has reached a level such that we can ascribe with a high degree of confidence
a cause and effect relationship between the greenhouse effect and the observed
warming."[239] Conway claims that this testimony was widely reported in the media and
subsequently global warming became the commonly used term by both the press and in public
discourse. However, he also points out that "global climate change" is the more scientifically
accurate term, because changes in Earth systems are not limited to surface temperatures.[238]

22. Global warming is the term used to describe a gradual increase in the average temperature of the
Earth’s atmosphere and its oceans, a change that is believed to be permanently changing the
Earth’s climate. Even though it is an ongoing debate, it is proved by the scientists that the planet
is warming.
The 29th century is experiencing a continued increase of Earth’s mean atmospheric temperature
by about 1.4 degrees F and about two thirds of it occurring since 1980. This is global warming is
affecting the nature’s balance and has a huge impact on life like continued heat waves, and
sudden occurrence of storms and floods. Don’t we see time to time the epidemics that are
devastating to human life and the flooding of the farmlands that puts economy in a deep hole?
Scientific evidence indicates that since 1950, the world’s climate has been warming, primarily as
a result of emissions from non -stop burning of fossil fuels and the razing of tropical forests.
Since the industrial revolution till this day, there is a constant emission of the carbon into the
atmosphere, everything we do we leave carbon footprints. It is a man made cause of the global
warming. The global emissions jumped 3 percent in 2011 and are expected to jump another 2.6
percent in 2012, researchers reported.
The greenhouse effect is a process by which the greenhouse gases absorb thermal radiation; these
are then reradiated in all directions. But when some of these radiations come back to the surface
and lower atmosphere, it causes increase in the average surface temperature leading to global
warming.
Global warming Causes
The causes are many of which the main culprit is the increase in the greenhouse gases that is
produced by burning fossil fuel and deforestation, thus intensifying the greenhouse effect leading
to global warming. The four main contributors of the greenhouse effect are, water vapor, carbon
dioxide, methane and ozone.
Mining for coal and oil releases methane in the atmosphere. More ever the leakage from natural
gas fields and landfills are additional source of methane. Excessive cutting down of the trees is
another factor causing global warming. When deforestation happens the efficiency by which
carbon dioxide is stored and oxygen released by the green plants are decreased to a huge rate in
turn causing increased concentration of carbon dioxide that leads to increased greenhouse effect.
The nitrous oxide from fertilizers, gases used for refrigeration and industrial processes are other
factors that cannot be forgotten as the cause of Global Warming.

23. Another source of methane is methane clathrate, a compound containing large amounts of
methane trapped in the crystal structure of ice. As methane escapes from the Arctic seabed, the
rate of global warming will increase significantly.
Ice caps and glaciers reflect sunlight, bouncing high temperature sun -rays back into space away
from the Earth. When these icecaps are removed the earth gets warmer as the dark oceans absorb
much thermal radiation from the sun.
Some regions may be wet with rain and some areas will suffer drought due to global warming.
The climatic changes happen due to global warming. Seasonal changes are unpredictable
unexpected thunderstorms might result as mentioned earlier.
The burning of wood (should be reduced to a greater extent) releases oxidizable carbon to the
atmosphere whose presence in greater amount causes the elevation of temperature.
There is strong evidence that emissions of chlorofluorocarbons (CFCs) were the major cause of
the recent abnormal warming. Like carbon CFC do not trap heat but in the presence of UV rays
the chlorine gets detached from CFC, drifts up into the stratosphere and these unattached
chlorines catalytically convert Ozone molecules into Oxygen molecules depleting the ozone
layer.
Global Warming and its effects are discussed as follows:
The effect of global warming is dangerous to life on earth, both the human and animal existence.
Due to the hear agriculture and farming, forestry and fishery are seriously hurt. Water
temperature being elevated, the fauna and flora are deprived of their natural habitat that is
detrimental to their growth and might lead to distinction of the species there.
Higher level of carbon dioxide causes the acidification of the oceans. According to the report of
WWF on coral reefs says that coral populations will collapse by 2100 due to increased
temperatures and ocean acidification.
The climatic changes are seen worldwide. Exceptional warm summers are another effect of this
global warming and Europe and America have already witnessed these changes considerably.
The summer of 1998 predicted to be the warmest in the century. Winter of 2003-2004 was the
33rd coldest in the North Eastern America since the records began in 1896.
Scientists are struggling to predict how the heat building in the seas and atmosphere will affect
the strength, intensity and occurrence of the cyclones, typhoons and storms. The increase in sea

24. level due to the ice cap melt has caused the receding of the shorelines, which have been recorded
by the researchers. According to the researchers the maximum wind speeds of the strongest
tropical cyclones have increased significantly since 1981.
Global warming might cause increased incidence of Lyme disease, dengue fever and malaria
because the mosquitoes that are the transmitting agents for these diseases requires winter
temperatures of about 18 degrees Celsius for their survival.
The increased floods and damages to sewage and water infrastructures further encourage the
spread of diseases.
The melting glaciers, polar ice caps, and other frozen grounds are causing and will continue to
cause the rise in sea levels. This will result in flooding and displacement of human population
worldwide.
The fresh water polar ice caps melting into the salt- water oceans alter the ocean gulf- stream
patterns causing major changes on the temperature pattern around the Earth. This changing
environment and temperature in polar region will endanger the flora and fauna and imbalance in
the ecosystem.
The ozone layer keeps out the ultra violet radiations from the sun that harms lives on the earth.
Some human made gases rises into the atmosphere and destroys the ozone layer. Therefore
global warming involves the pollution of the atmosphere, which in turn leads to the depletion of
the ozone layer.
The global warming will delay the ozone recovery at Arctic regions making these people to be
exposed to a ultra-violet dose, which is relatively higher. The penetration of the more ultra-violet
radiations from the sun to the earth poses threats to lives like skin cancer.
The affect on the food chain could also be disastrous. Because UV rays kill plankton in the sea,
the fish and whales that live off of plankton would eventually starve and disappear. This would
then affect the next link in the chain – those creatures that live off of fish – and so it would
continue throughout the chain.
The chlorine that gets detached from CFC by UV rays, drifts up into the stratosphere and these
unattached chlorines catalytically convert Ozone molecules into Oxygen molecules depleting the
ozone layer, which in turn results in cascade of events leading to global warming and associated
harms to life.

25. globalwarming – Prevention and human efforts on reducing it.
Global warming has become a major problem as we move on to 21st century and beyond. Like
the old say goes, prevention is better than cure. We have done enough damage, knowingly and
unknowingly we have contributed to global warming. Our efforts and determined contribution to
reduce global warming and spare the ecosystem to recover thereby cautiously working restore
the balance of nature is very urgent.
Pollution of the atmosphere has to be checked and anything related to the release of the
greenhouse gases should be monitored. Deforestation should be stopped as it causes the
concentration of carbon at a greater extent. It has been said that it takes roughly 19 trees to make
one ton of paper and that the usage of one ton of recycled paper will save approximately 17 trees.
The burning of wood should be reduced to a greater extent as it releases oxidizable carbon to the
atmosphere whose presence in greater amount causes the elevation of temperature. LPG stoves
can replace the fire wood burning. Bio gas plants can be encouraged in village units. Composting
will help you to dispose of biodegradable waste.
The use of CFCs is a major reason for depletion of the ozone layer causing global warming. The
discharges and exhaust from automobiles also causes global warming. We could reduce it by
identifying fuel-efficient cars, use bio-fuels, correct the engine design, or using an electric car.
By not wasting energy at home we can avoid to some extent the global warming. This includes
shutting down the systems when not in usage and also buying energy efficient equipments. Solar
energy, wind energy and hydro-electricity can be used that reduces the pollution of the
atmosphere to a greater extent.
Global warming is mainly due to the ignorance of the people and the ‘don’t care attitude’ of few.
Even at a younger stage it is wiser to educate the children to be efficient consumers and be
responsible users of energy and resources remembering that the global warming is slowly
destroying the earth and its natural habitat.
People should be reminded that they share the earth and its resources with the other species that
keep the ecosystem balanced, if the balance is gone then the life if every single species is
threatened and even humans will not be spared from this.

26. Media can play a role in educating the people of the phenomenon of global warming that has
already taking a huge toll. This won’t disappear overnight. As we thoughtfully practice ways of
preventing the causes that causes global warming one day the earth will be restored.
Planting trees is a very good start that can help diluting the carbon dioxide in the atmosphere. A
single tree will absorb approximately one ton of carbon dioxide during its lifetime.