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Assignment on

Effects of Global warming

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This assignment is about the current change in Earth's climate. For general discussion of how the

climate can change, see Climate change . For other uses, see Global warming (disambiguation) .

Global mean land-ocean temperature change from 1880–2012, relative to the 1951–1980 mean.

The black line is the annual mean and the red line is the 5-year running mean . The green bars

show uncertainty estimates. Source: NASA GISS .

The map shows the 10-year average (2000–2009) global mean temperature anomaly relative to

the 1951–1980 mean. The largest temperature increases are in the Arctic and the Antarctic

Peninsula. Source: NASA Earth Observatory

Fossil fuel related CO 2 emissions compared to five of the IPCC's " SRES " emissions scenarios.

The dips are related to global recessions. Image source: Skeptical Science .

Global warming is the rise in the average temperature of Earth's atmosphere and oceans since

the late 19th century and its projected continuation. Since the early 20th century, Earth's mean

surface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increase

occurring since 1980. [2] Warming of the climate system is unequivocal, and scientists are more

than 90% certain that it is primarily caused by increasing concentrations of greenhouse gases

produced by human activities such as the burning of fossil fuels and deforestation . [3][4][5][6] These

findings are recognized by the national science academies of all major industrialized nations. [7][A]

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 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. [8] The ranges

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of these estimates arise from the use of models with differing sensitivity to greenhouse gas

concentrations . [9][10]

Future warming and related changes will vary from region to region around the globe. 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 a probable expansion of subtropical deserts .

Warming is expected to be strongest in the Arctic and would be associated with the continuing

retreat of glaciers , permafrost and sea ice . Other likely effects of the warming include a more

frequent occurrence of extreme-weather events 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 the loss

of habitat from inundation.

Proposed policy responses to global warming include mitigation by emissions reduction,

adaptation to its effects, and possible future geoengineering . Most countries are parties to the

United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate

objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change . Parties to

the UNFCCC have adopted a range of policies designed to reduce greenhouse gas

emissions [17] :10 [18][19][20] :9 and to assist in adaptation to global warming . Parties to the UNFCCC have

agreed that deep cuts in emissions are required, and that future global warming should be limited

to below 2.0 °C (3.6 °F) relative to the pre-industrial level. Reports published in 2011 by the

United Nations Environment Programme and the International Energy Agency suggest that

efforts as of the early 21st century to reduce emissions may be inadequate to meet the

UNFCCC's 2 °C target.

Observed temperature changes

Main article: Instrumental temperature record

The increase in ocean heat content is much larger than any other store of energy in the Earth’s

heat balance over the two periods 1961 to 2003 and 1993 to 2003, and accounts for more than

90% of the possible increase in heat content of the Earth system during these periods.

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Two millennia of mean surface temperatures according to different reconstructions from climate

proxies , each smoothed on a decadal scale, with the instrumental temperature record overlaid in

black.

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. [27]

Temperatures in the lower troposphere 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 . [28]

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. [29] Examples

include sea level rise ( water expands as it warms), [30] widespread melting of snow and ice , [31]

increased heat content of the oceans , [29] increased humidity , [29] and the earlier timing of spring

events, [32] e.g., the flowering of plants . [33] The probability that these changes could have occurred

by chance is virtually zero. [29]

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. [34][35][36] 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." [37] The World Meteorological Organization (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..." [38]

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NOAA graph of Global Annual Temperature Anomalies 1950–2012, showing the El Niño-

Southern Oscillation

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. [39] Global temperature is subject to short-term fluctuations that overlay long term trends

and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is

consistent with such an episode. [40][41] 2010 was also an El Niño year. On the low swing of the

oscillation, 2011 as an 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. [42]

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). [43] 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. [44] The northern

hemisphere warms faster than the southern hemisphere because it has more land and because it

has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback . [ citation needed ]

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. [45] 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. [46]

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 commitment studies indicate

that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C

(0.9 °F) would still occur. [47]

Initial causes of temperature changes (external forcings)

Main article: Attribution of recent climate change

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Greenhouse effect schematic showing energy flows 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 of atmospheric carbon dioxide

(CO 2 ) concentrations from 1958–2008. Monthly CO 2 measurements display seasonal oscillations

in an upward trend; each year's maximum occurs during the Northern Hemisphere 's late spring,

and declines during its growing season as plants remove some atmospheric CO 2 .

The climate system can respond to changes in external forcings. [48][49] External forcings can

"push" the climate in the direction of warming or cooling. [50] 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. [51]

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 an ice age , but the 20th century instrumental

temperature record shows a sudden rise in global temperatures. [52]