Global warming is the observed increase in the
average temperature of the Earth's atmosphere and oceans in recent
decades. The Earth's average near-surface atmospheric temperature
rose 0.6 ± 0.2 °Celsius (1.1 ± 0.4 °Fahrenheit) in the 20th century.
The current scientific consensus is that "most
of the observed warming over the last 50 years is likely to have
been attributable to human activities".The main cause of the
human-induced component of warming is the increase in atmospheric
greenhouse gases (GHGs), especially carbon dioxide (CO2), due to
activities such as burning of fossil fuels, land clearing, and agriculture.
Greenhouse gases are gases that contribute to the greenhouse effect.
This effect was first described by Joseph Fourier in 1824, and was
first investigated quantitatively in 1896 by the Swedish chemist
Climate sensitivity is a measure of the equilibrium
response to increased GHGs, and other anthropogenic and natural
climate forcings. It is found by observational and model studies.
This sensitivity is usually expressed in terms of the temperature
response expected from a doubling of CO2 in the atmosphere, which,
according to the 2001 IPCC report, is estimated to be between 1.5
and 4.5 °C (2.7–8.1 °F) (with a statistical likelihood of 66-90%).
This should not be confused with the expected temperature change
by a given date, which also includes a dependence on the future
GHG emissions and a delayed response due to thermal lag, principally
from the oceans. Models referenced by the Intergovernmental Panel
on Climate Change (IPCC), using a range of SRES scenarios, project
that global temperatures will increase between 1.4 and 5.8 °C (2.5
to 10.5 °F) between 1990 and 2100.
An increase in global temperatures can in turn
cause other changes, including a rising sea level and changes in
the amount and pattern of precipitation. These changes may increase
the frequency and intensity of extreme weather events, such as floods,
droughts, heat waves, hurricanes, and tornados. Other consequences
include higher or lower agricultural yields, glacial retreat, reduced
summer stream flows, species extinctions and increases in the ranges
of disease vectors. Warming is expected to affect the number and
magnitude of these events; however, it is difficult to connect particular
events to global warming. Although most studies focus on the period
up to 2100, warming (and sea level rise due to thermal expansion)
is expected to continue past then, since CO2 has an estimated atmospheric
lifetime of 50 to 200 years. Only a small minority of climate scientists
disagree that humanity's actions have played a major role in recent
warming. However, the uncertainty is more significant regarding
how much climate change should be expected in the future, and there
is a hotly contested political and public debate over implementation
of policies that deal with predicted consequences, what, if anything,
should be done to reduce or reverse future warming, and how to deal
with the predicted consequences.
The term "global warming" is a specific case of the more
general term "climate change" (which can also refer to
"global cooling," such as occurs during ice ages). In
principle, "global warming" is neutral as to the causes,
but in common usage, "global warming" generally implies
a human influence. However, the UNFCCC uses "climate change"
for human-caused change, and "climate variability" for
other changes. Some organizations use the term "anthropogenic
climate change" for human-induced changes. Also, "global
warming" may refer to both the observed and the predicted warming.
Two millennia of mean surface temperatures according to different
reconstructions, each smoothed on a decadal scale. The unsmoothed,
annual value for 2004 is also plotted for reference.Relative to
the period 1860–1900, global temperatures on both land and sea have
increased by 0.75 °C (1.4 °F), according to the instrumental temperature
record. Since 1979, land temperatures have increased about twice
as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade
(Smith, 2005). Temperatures in the lower troposphere have increased
between 0.12 and 0.22 °C per decade since 1979, according to satellite
temperature measurements. Over the one or two thousand years before
1850, world temperature is believed to have been relatively stable,
with possibly regional fluctuations such as the Medieval Warm Period
or the Little Ice Age.
Based on estimates by NASA's Goddard Institute
for Space Studies, 2005 was the warmest year since reliable, widespread
instrumental measurements became available in the late 1800s, exceeding
the previous record set in 1998 by a few hundredths of a degree
Celsius. Similar estimates prepared by the World Meteorological
Organization and the UK Climatic Research Unit concluded that 2005
was still only the second warmest year, behind 1998.
Depending on the time frame, a number of temperature
records are available. These are based on different data sets, with
different degrees of precision and reliability. An approximately
global instrumental temperature record begins in about 1860; contamination
from the urban heat island effect is believed to be small and well
controlled for. A longer-term perspective is available from various
proxy records for recent millennia; see temperature record of the
past 1000 years for a discussion of these records and their differences.
The attribution of recent climate change is clearest for the most
recent period of the last 50 years, for which the most detailed
data are available. Satellite temperature measurements of the tropospheric
temperature date from 1979.
Carbon dioxide during the last 400,000 years and
the rapid rise since the Industrial Revolution; changes in the Earth's
orbit around the Sun, known as Milankovitch cycles, are believed
to be the pacemaker of the 100,000 year ice age cycle.The climate
system varies both through natural, "internal" processes
as well as in response to variations in external "forcing"
from both human and non-human causes, including solar activity,
volcanic emissions, and greenhouse gases. Climatologists agree that
the earth has warmed recently. The detailed causes of this change
remain an active field of research, but the scientific consensus
identifies greenhouse gases as the primary cause of the recent warming.
Outside of the scientific community, however, this conclusion can
Adding carbon dioxide (CO2) or methane (CH4) to
Earth's atmosphere, with no other changes, will make the planet's
surface warmer; greenhouse gases create a natural greenhouse effect
without which temperatures on Earth would be an estimated 30 °C
(54 °F) lower, and the Earth uninhabitable. It is therefore not
correct to say that there is a debate between those who "believe
in" and "oppose" the theory that adding carbon dioxide
or methane to the Earth's atmosphere will, absent any mitigating
actions or effects, result in warmer surface temperatures on Earth.
Rather, the debate is about what the net effect of the addition
of carbon dioxide and methane will be, when allowing for compounding
or mitigating factors.
One example of an important feedback process is
ice-albedo feedback. The increased CO2 in the atmosphere warms the
Earth's surface and leads to melting of ice near the poles. As the
ice melts, land or open water takes its place. Both land and open
water are less reflective than ice, and so absorb more solar radiation.
This causes more warming, which in turn causes more melting, and
the cycle continues.Due to the thermal inertia of the earth's oceans
and slow responses of other indirect effects, the Earth's current
climate is not in equilibrium with the forcing imposed by increased
greenhouse gases. Climate commitment studies indicate that, even
if greenhouse gases were stabilized at present day levels, a further
warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.
Greenhouse gases are transparent to shortwave
radiation from the sun, the main source of heat on the Earth. However,
they absorb some of the longer infrared radiation emitted by the
Earth, thereby reducing radiational cooling and hence raising the
temperature of the Earth. How much they warm the world by is shown
in their global warming potential.
The atmospheric concentrations of carbon dioxide
and methane have increased by 31% and 149% respectively above pre-industrial
levels since 1750. This is considerably higher than at any time
during the last 650,000 years, the period for which reliable data
has been extracted from ice cores. From less direct geological evidence
it is believed that carbon dioxide values this high were last attained
40 million years ago. About three-quarters of the anthropogenic
(man-made) emissions of carbon dioxide to the atmosphere during
the past 20 years are due to fossil fuel burning. The rest of the
anthropogenic emissions are predominantly due to land-use change,
The longest continuous instrumental measurement
of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since
then, the annually averaged value has increased monotonically by
approximately 21% from the initial reading of 315 ppmv, as shown
by the Keeling curve, to over 380 ppmv in 2006. The monthly CO2
measurements display small seasonal oscillations in an overall yearly
uptrend; each year's maximum is reached during the northern hemisphere's
late spring and declines during the northern hemisphere growing
season as plants remove some CO2 from the atmosphere.
Methane, the primary constituent of natural gas,
enters the atmosphere both from biological production and leaks
from natural gas pipelines and other infrastructure. Some biological
sources are natural, such as termites or forests, but others have
been increased or created by agricultural activities such as the
cultivation of rice paddies. Recent evidence indicates that methane
concentrations have begun to stabilize, perhaps due to reductions
in leakage from fuel transmission and storage facilities.
Future carbon dioxide levels are expected to continue
rising due to ongoing fossil fuel usage, though the actual trajectory
will depend on uncertain economic, sociological, technological,
and natural developments. The IPCC Special Report on Emissions Scenarios
gives a wide range of future carbon dioxide scenarios , ranging
from 541 to 970 parts per million by the year 2100. Fossil fuel
reserves are sufficient to reach this level and continue emissions
past 2100, if coal and tar sands are extensively used.
Anthropogenic emission of greenhouse gases broken
down by sector for the year 2000.Globally, the majority of anthropogenic
greenhouse gas emissions arise from fuel combustion. The remainder
is accounted for largely by "fugitive fuel" (fuel consumed
in the production and transport of fuel), emissions from industrial
processes (excluding fuel combustion), and agriculture: these contributed
5.8%, 5.2% and 3.3% respectively in 1990. Current figures are broadly
comparable. Around 17% of emissions are accounted for by the combustion
of fuel for the generation of electricity. A small percentage of
emissions come from natural and anthropogenic biological sources,
with approximately 6.3% derived from agriculturally produced methane
and nitrous oxide.
Positive feedback effects, such as the expected
release of methane from the melting of permafrost peat bogs in Siberia
(possibly up to 70,000 million tonnes), may lead to significant
additional sources of greenhouse gas emissions. Note that the anthropogenic
emissions of other pollutants—notably sulfate aerosols—exert a cooling
effect; this partially accounts for the plateau/cooling seen in
the temperature record in the middle of the twentieth century, though
this may also be due to intervening natural cycles.
The extent of the scientific consensus on global
warming—that "most of the observed warming over the last 50
years is likely to have been attributable to human activities"—has
been investigated: In the journal Science in December 2004, Dr Naomi
Oreskes published a study of the abstracts of 928 refereed scientific
articles in the ISI citation database identified with the keywords
"global climate change". This study concluded that 75%
of the 928 articles either explicitly or implicitly accepted the
consensus view — the remainder of the articles covered methods or
paleoclimate and did not take any stance on recent climate change.
The study did not report how many of the 928 abstracts explicitly
endorsed the hypothesis of human-induced warming, but none of the
928 articles surveyed explicitly endorsed an alternative hypothesis.
Contrasting with the consensus view, alternative
hypotheses have been proposed to explain all or part of the observed
increase in global temperatures. Some of these hypotheses (listed
here without comment on their validity or lack thereof) include:
The warming is within the range of natural variation.
The warming is a consequence of coming out of a prior cool period,
namely the Little Ice Age.
The warming is primarily a result of variances in solar irradiance,
possibly via modulation of cloud cover . It is similar in concept
to the operating principles of the Wilson cloud chamber, but on
a global scale where earth's atmosphere acts as the cloud chamber
and the cosmic rays catalyze the production of cloud condensation
The observed warming actually reflects the Urban Heat Island, as
most readings are done in heavily populated areas which are expanding
with growing population.
The solar variation theory
Modeling studies reported in the IPCC Third Assessment Report (TAR)
did not find that changes in solar forcing were needed in order
to explain the climate record for the last four or five decades].
These studies found that volcanic and solar forcings may account
for half of the temperature variations prior to 1950, but the net
effect of such natural forcings has been roughly neutral since then.
In particular, the change in climate forcing from greenhouse gases
since 1750 was estimated to be eight times larger than the change
in forcing due to increasing solar activity over the same period].
Since the TAR, some studies (Lean et al., 2002,
Wang et al., 2005) have suggested that changes in irradiance since
pre-industrial times are less by a factor of 3 to 4 than in the
reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean,
2000.). Other researchers (e.g. Stott et al. 2003) believe that
the effect of solar forcing is being underestimated and propose
that solar forcing accounts for 16% or 36% of recent greenhouse
warming. Others (e.g. Marsh and Svensmark 2000) have proposed that
feedback from clouds or other processes enhance the direct effect
of solar variation, which if true would also suggest that the effect
of solar variability was being underestimated. In general the level
of scientific understanding of the contribution of variations in
solar irradiance to historical climate changes is "very low".
The present level of solar activity is historically
high. Solanki et al. (2004) suggest that solar activity for the
last 60 to 70 years may be at its highest level in 8,000 years;
Muscheler et al. disagree, suggesting that other comparably high
levels of activity have occurred several times in the last few thousand
years. Solanki concluded based on their analysis that there is a
92% probability that solar activity will decrease over the next
50 years. In addition, researchers at Duke University (2005) have
found that 10–30% of the warming over the last two decades may be
due to increased solar output. In a review of existing literature,
Foukal et al. (2006) determined both that the variations in solar
output were too small to have contributed appreciably to global
warming since the mid-1970s and that there was no evidence of a
net increase in brightness during this period.
The expected effects of global warming are many and various, both
for the environment and for human life. These effects include sea
level rise, repercussions to agriculture, reductions in the ozone
layer, increased intensity and frequency of extreme weather events,
and the spread of disease. In some cases, the effects may already
be manifest, although it is difficult to attribute specific incidents
of natural phenomena to long-term global warming. Since the mid-1970s,
the total annual power of hurricanes has increased markedly because
their average intensity and duration have increased; in addition,
there has been a high correlation of hurricane power with tropical
sea-surface temperature. In spite of such strong evidence, the relationship
between global warming and hurricanes is still being debated.] A
draft statement by the World Meteorological Organization acknowledges
the differing viewpoints on this issue.
The extent and probability of these consequences
is a matter of considerable uncertainty. A summary of probable effects
and recent understanding can be found in the report of the IPCC
Working Group II. Some scientists have concluded global warming
is already causing death and disease across the world through flooding,
environmental destruction, heat waves and other extreme weather
events. (Reuters, February 9, 2006; archived)
Effects on ecosystems
Both primary and secondary effects of global warming — such as higher
temperatures, lessened snow cover, rising sea levels and weather
changes — may influence not only human activities, but also ecosystems.
Some species may be forced out of their habitats (possibly to extinction)
because of changing conditions, while others may flourish. Similarly,
changes in timing of life patterns, such as annual migration dates,
may alter regional predator-prey balance. The effect of advanced
spring arrival dates in Scandinavia on birds that over winter in
sub-Saharan Africa has been ascribed to evolutionary adaptation
of the species to climatic warming.
Ocean pH is lowering as a result of increased
carbon dioxide levels. Lowering of ocean pH, along with changing
water temperature and ocean depth will have a damaging effect on
Another suggested mechanism whereby a warming
trend may be amplified involves the thawing of tundra, which can
release significant amounts of the potent greenhouse gas, methane,
which is trapped in permafrost and ice clathrate compounds.
There are also ecological effects of melting polar
ice: for example, polar bears use sea ice to reach their prey and
they must swim to another ice floe when one breaks up. Ice is now
becoming further separated and dead polar bears have been found
in the water, believed to have drowned. More recently, some scientists
have suggested that the observed cannibalistic behavior in polar
bears may be the result of food shortages brought on by global warming
(Amstrup et al. 2006).
Effect on glaciers
Global glacial mass balance in the last fifty
years, reported to the WGMS and the NSIDC. The increased downward
trend in the late 1980s is symptomatic of the increased rate and
number of retreating glaciers.Global warming has led to negative
glacier mass balance, causing glacier retreat around the world.
Oerlemans (2005) showed a net decline in 142 of the 144 mountain
glaciers with records from 1900 to 1980. Since 1980 global glacier
retreat has increased significantly. Similarly, Dyurgerov and Meier
(2005) averaged glacier data across large-scale regions (e.g. Europe)
and found that every region had a net decline from 1960 to 2002,
though a few local regions (e.g. Scandinavia) have shown increases.
Some glaciers that are in disequilibrium with present climate have
already disappeared and increasing temperatures are expected to
cause continued retreat in the majority of alpine glaciers around
the world. Upwards of 90% of glaciers reported to the World Glacier
Monitoring Service have retreated since 1995.
Of particular concern is the potential for failure
of the Hindu Kush and Himalayan glacial melts. The melt of these
glaciers is a large and reliable source of water for China, India,
and much of Asia, and these waters form a principal dry-season water
source. Increased melting would cause greater flow for several decades,
after which "some areas of the most populated region on Earth
are likely to 'run out of water'" (T. P. Barnett, J. C. Adam
and D. P. Lettenmaier 2005)
Miniature rock glaciers
Rock glaciers — caches of ice under boulders — are among other water
signs such as drying meadows and warming lakes that scientists are
studying in the Sierras in the western United States. Connie Millar
searches for the rock glaciers in the Yosemite area of the Sierra
crest. She hypothesizes that rock glaciers will be predictors of
how ecosystems change with rising temperatures. Millar is leading
an effort (the Consortium for Integrated Climate Research in Western
Mountains) to co-ordinate the work of many scientists to see how
the pieces of the Global Warming puzzle may fit.
Destabilization of ocean currents
There is also some speculation that global warming could, via a
shutdown or slowdown of the thermohaline circulation, trigger localized
cooling in the North Atlantic and lead to cooling, or lesser warming,
in that region. This would affect in particular areas like Scandinavia
and Britain that are warmed by the North Atlantic drift.
Sea level rise and environmental refugees
The termini of the glaciers in the Bhutan-Himalaya. Glacial lakes
have been rapidly forming on the surface of the debris-covered glaciers
in this region during the last few decades. According to USGS researchers,
glaciers in the Himalaya are wasting at alarming and accelerating
rates, as indicated by comparisons of satellite and historic data,
and as shown by the widespread, rapid growth of lakes on the glacier
surfaces. The researchers have found a strong correlation between
increasing temperatures and glacier retreat.Rising global temperatures
will melt glaciers and expand the water of the seas through the
mechanism of thermal expansion, leading to sea level rise. Even
a relatively small rise in sea level would make some densely settled
coastal plains uninhabitable and create a significant refugee problem.
If the sea level were to rise in excess of 4 meters (13 ft) almost
every coastal city in the world would be severely affected, with
the potential for major damage to world-wide trade and economy.
Presently, the IPCC predicts sea level rise is most probable to
be just short of half a metre, and at least between 9 and 88 cm
through 2100 - but they also warn that global warming during that
time may lead to irreversible changes in the Earth's glacial system
and ultimately melt enough ice to raise sea level many meters over
the next millennia. It is estimated that around 200 million people
could be affected by sea level rise, especially in Vietnam, Bangladesh,
China, India, Thailand, Philippines, Indonesia, Nigeria and Egypt.
An example of the ambiguity of the concept of
environmental refugees is the emigration from the island nation
of Tuvalu, which has an average elevation of approximately one meter
above sea level. Tuvalu already has an ad hoc agreement with New
Zealand to allow phased relocation and many residents have been
leaving the islands. However, it is far from clear that rising sea
levels from global warming are a substantial factor - best estimates
are that sea level has been rising there at approximately 1–2 millimeters
per year (~1/16th in/yr), but that shorter timescale factors—ENSO,
or tides—have far larger temporary effects.
Spread of disease
One of the largest known outbreaks of Vibrio parahaemolyticus gastroenteritis
has been attributed to generally rising ocean temperature where
infected oysters were harvested in Prince William Sound, Alaska
in 2005. Before this, the northernmost reported risk of such infection
was in British Columbia, 1000 km to the south (McLaughlin JB, et
Global warming may extend the range of vectors
conveying infectious diseases such as malaria. A warmer environment
boosts the reproduction rate of mosquitoes and the number of blood
meals they take, prolongs their breeding season, and shortens the
maturation period for the microbes they disperse]. Global warming
has been implicated in the recent spread to the north Mediterranean
region of bluetongue disease in domesticated ruminants associated
with mite bites (Purse, 2005). Hantavirus infection, Crimean-Congo
hemorrhagic fever, tularemia and rabies increased in wide areas
of Russia during 2004–2005. This was associated with a population
explosion of rodents and their predators but may be partially blamed
on breakdowns in governmental vaccination and rodent control programs.
Similarly, despite the disappearance of malaria in most temperate
regions, the indigenous mosquitoes that transmitted it were never
eliminated and remain common in some areas. Thus, although temperature
is important in the transmission dynamics of malaria, many other
factors are influential.
Financial institutions, including the world's two largest insurance
companies, Munich Re and Swiss Re, warned in a 2002 study (UNEP
summary) that "the increasing frequency of severe climatic
events, coupled with social trends" could cost almost US$150
billion each year in the next decade. These costs would, through
increased costs related to insurance and disaster relief, burden
customers, taxpayers, and industry alike.
According to the Association of British Insurers,
limiting carbon emissions could avoid 80% of the projected additional
annual cost of tropical cyclones by the 2080s. According to Choi
and Fisher (2003) each 1% increase in annual precipitation could
enlarge catastrophe loss by as much as 2.8%.
The United Nations' Environmental Program recently
announced that severe weather around the world has made 2005 the
most costly year on record. Although there is "no way to prove
that [a given hurricane] either was, or was not, affected by global
warming", global warming is thought to increase the probability
of hurricanes emerging. Preliminary estimates presented by the German
insurance foundation Munich Re put the economic losses at more than
US$200 billion, with insured losses running at more than US$70 billion.
Nicholas Stern in the Stern Review has warned
that one percent of global GDP is required to be invested in order
to mitigate the effects of climate change, and that failure to do
so could risk a recession worth up to twenty percent of global GDP.
Stern’s report suggests that climate change threatens to be the
greatest and widest-ranging market failure ever seen. The report
has had significant political effects: Australia reported two days
after the report was released that they would allott AU$60 million
to projects to help cut greenhouse gas emissions. Tony Blair said
the Stern Review showed that scientific evidence of global warming
was "overwhelming" and its consequences "disastrous".
The creation of biomass by plants is influenced by the availability
of water, nutrients, and carbon dioxide. Part of this biomass is
used (directly or indirectly) as the energy source for nearly all
other life forms, including feed-stock for domestic animals, and
fruits and grains for human consumption. It also includes timber
for construction purposes.
While it's thought that an increase in carbon
dioxide levels should speed up plant growth, which would slow down
the effects of global warming, a new study has found the opposite
to be true. Scientists at Stanford have found that "elevated
atmospheric carbon dioxide actually reduces plant growth when combined
with other likely consequences of climate change -- namely, higher
temperatures, increased precipitation or increased nitrogen deposits
in the soil." A rising temperature can also increase the growing
season in colder regions. It is sometimes argued that these effects
can create a greener, richer planet, with more available biomass.
However, there are many other factors involved, and it is currently
unclear if plants really benefit from global warming. Plant growth
can be limited by a number of factors, including soil fertility,
water, temperature, and carbon dioxide concentration. Ocean plants
(phytoplankton) are actually harmed by global warming, presumably
with negative impact on ocean ecosystems.
IPCC models currently predict a possible modest
increase in plant productivity. However, there are several negative
ramifications: decreases in productivity may occur at above-optimal
temperatures; greater variation in temperature is likely to decrease
wheat yields; in experiments, grain and forage quality declines
if CO2 and temperature are increased; and the reductions in soil
moisture in summer, which are likely to occur, would have a negative
effect on productivity.
Satellite data show that the productivity of the
northern hemisphere did indeed increase from 1982 to 1991. However,
more recent studies found that from 1991 to 2002, widespread droughts
had actually caused a decrease in summer photosynthesis in the mid
and high latitudes of the northern hemisphere.
NOAA projects that by the 2050s, there will only
be 54% of the volume of sea ice there was in the 1950s.
Opening up of the Northwest Passage in summer
Melting Arctic ice may open the Northwest Passage in summer in approximately
ten years, which would cut 5,000 nautical miles (9,300 km) from
shipping routes between Europe and Asia. This would be of particular
relevance for supertankers that are too big to fit through the Suez
Canal and currently have to go around the southern tip of Africa.
According to the Canadian Ice Service, the amount of ice in Canada's
eastern Arctic Archipelago decreased by 15% between 1969 and 2004.
A similar opening is possible in the Arctic north of Siberia, allowing
much faster East Asian to Europe transport.
Further global warming (positive feedback)
Some effects of global warming themselves contribute directly to
further global warming, in a vicious circle, the nature of which
may be difficult to predict in advance. Melting could also lead
to increased heat absorption because ice reflects more solar radiation
(i.e., it has higher albedo) than land or water. Because sea ice
and seasonal snow cover are more reflective than the underlying
sea, any meltback may lead to further warming. Warmer temperatures
in the oceans reduce the productivity (growth) of ocean phytoplankton
(algae). This is expected to reduce the amount of carbon dioxide
taken up by photosynthesis in the ocean, which would again increase
the effects of anthropogenic CO2 releases on the overall amount
of CO2 in the atmosphere, and hence increase the greenhouse effect.
This is a concern because ocean photosynthesis is as large a part
of the planet's overall carbon balance as land photosynthesis.
The Energy Information Administration predicts
world energy and fossil fuel usage will rise in the next decades.The
likelihood that global temperatures will continue to significantly
increase has led to proposals to mitigate global warming. Mitigation
covers all actions aimed at reducing the negative effects or the
likelihood of global warming.There are five categories of actions
that can be taken to mitigate global warming:
Reduction of energy use (conservation)
Shifting from carbon-based fossil fuels to alternative energy sources
Carbon capture and storage
Planetary engineering to cool the earth, including screening out
sunlight or increasing albedo.
Strategies for mitigation of global warming include development
of new technologies; carbon offsets; renewable energy such as biodiesel,
wind power, and solar power; nuclear power; electric or hybrid automobiles;
fuel cells; energy conservation; carbon taxes; enhancing natural
carbon dioxide sinks; population control; and carbon capture and
storage. Many environmental groups encourage individual action against
global warming, often aimed at the consumer, and there has been
business action on climate change.
The world's primary international agreement on
combating climate change is the Kyoto Protocol. The Kyoto Protocol
is an amendment to the United Nations Framework Convention on Climate
Change (UNFCCC). Countries that ratify this protocol commit to reduce
their emissions of carbon dioxide and five other greenhouse gases,
or engage in emissions trading if they maintain or increase emissions
of these gases.
Although the governments of 163 countries ratified
the Kyoto Protocol, (notably excluding the United States and Australia),
there is a growing debate about how effective the Kyoto protocol
has been. Some politicians, including President of the United States
George W. Bush, Prime Minister of Australia John Howard had argued
that the cost of mitigating global warming via the Kyoto protocol
is too large to be practical. This view may be proving correct,
as the signatories of the Kyoto protocol are currently struggling
to meet their targets, including Europe and Japan. After only five
years, Canada has given up entirely. Also, of the 163 countries
that have signed and ratified Kyoto, only 31 are actually required
to lower greenhouse emissions.
Some segments of the business community have accepted
global warming and its attribution to anthropogenic causes as valid,
as well as a need for actions such as carbon emissions trading and
carbon taxes.Adaptation strategies accept some warming as a foregone
conclusion and focus on preventing or reducing undesirable consequences.
Examples of such strategies include defense against rising sea levels
or ensuring food security.
Calculations of global warming from a range of climate models under
the SRES A2 emissions scenario, which assumes no action is taken
to reduce emissions.
The geographic distribution of surface warming during the 21st century
calculated by the HadCM3 climate model if a business as usual scenario
is assumed for economic growth and greenhouse gas emissions. In
this figure, the globally averaged warming corresponds to 3.0 °C
Scientists have studied global warming with computer models of the
climate (see below). Before the scientific community accepts a climate
model, it has to be validated against observed climate variations.
As of 2006, sufficiently high-resolution models successfully simulate
summer/winter differences, the North Atlantic Oscillation[citation
needed], and El Niño. All validated current models predict that
the net effect of adding greenhouse gases will be a warmer climate
in the future. However, the amount of predicted warming varies by
model, and there still remains a considerable range of climate sensitivity
predicted by the models which survive these tests; one of the most
important sources of this uncertainty is believed to be different
ways of handling clouds. Part of the technical summary of the IPCC
TAR includes a recognition of the need to quantify this uncertainty:
"In climate research and modeling, we should recognize that
we are dealing with a coupled non-linear system, and therefore that
the prediction of a specific future climate is not possible. Rather
the focus must be on the probability distribution of the system's
possible future states by the generation of ensembles of model solutions."An
example of a study which aims to do this is the Climateprediction.net
project; their methodology is to investigate the range of climate
sensitivities predicted for the 21st century by those models which
are first shown to give a reasonable simulation of late 20th century
As noted above, climate models have been used
by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C (2.5 °F–10.4
°F) between 1990 and 2100 . They have also been used to help
investigate the causes of recent climate change by comparing the
observed changes to those that the models predict from various natural
and human derived forcing factors. In addition to having their own
characteristic climate sensitivity, models have also been used to
derive independent assessments of climate sensitivity.
Climate models can produce a good match to observations
of global temperature changes over the last century . These
models do not unambiguously attribute the warming that occurred
from approximately 1910 to 1945 to either natural variation or human
effects; however, they suggest that the warming since 1975 is dominated
by man-made greenhouse gas emissions. Adding simulation of the carbon
cycle to the models generally shows a positive feedback, though
this response is uncertain (under the A2 SRES scenario, responses
vary between an extra 20 and 200 ppm of CO2). Some observational
studies also show a positive feedback .
Uncertainties in the representation of clouds
are a dominant source of uncertainty in existing models, despite
clear progress in modeling of clouds . There is also an ongoing
discussion as to whether climate models are neglecting important
indirect and feedback effects of solar variability. Further, all
such models are limited by available computational power, so that
they may overlook changes related to small-scale processes and weather
(e.g. storm systems, hurricanes). However, despite these and other
limitations, the IPCC considered climate models "to be suitable
tools to provide useful projections of future climates" .
In December, 2005 Bellouin et al. suggested in
Nature that the reflectivity effect of airborne pollutants was about
double that previously expected, and that therefore some global
warming was being masked. If supported by further studies, this
would imply that existing models under-predict future global warming.
Defining dangerous global warming
Although global warming has been seen as potentially dangerous for
some time, the first international attempt to define what constitutes
a 'dangerous' level occurred at the Avoiding Dangerous Climate Change
scientific conference in February 2005. This took place in Exeter,
United Kingdom under the UK presidency of the G8 .
At the conference it was said that increasing
damage was forecast if the globe warms to about 1 to 3 °Celsius
(1.8 to 5.4 °Fahrenheit) above pre-industrial levels. It was concluded
that the stabilization of greenhouse gases at the equivalent of
450 ppmv CO2 would provide a 50% likelihood of limiting global warming
to the average figure of 2 °C (3.6 °F). Stabilization below 400
ppm would give a relatively high certainty of not exceeding 2 °C,
while stabilization at 550 ppm would mean it was likely that 2 °C
would be exceeded.
It was stated that unless 'urgent and strenuous
mitigation actions' were taken in the next 20 years, it was almost
certain that by 2050 global temperatures will have risen to between
0.5 and 2 °C (0.9 and 3.6°F) above current levels. With carbon dioxide
levels currently around 381 ppm and rising by 2ppm per year, without
such action greenhouse gasses are likely to reach 400ppm by 2016,
450ppm by 2041, and 550ppm by around 2091.
Other related issues
Increased atmospheric carbon dioxide increases
the amount of CO2 dissolved in the oceans. This ameliorates the
greenhouse effect by removing the greenhouse gas from the atmosphere.
Unfortunately, carbon dioxide gas dissolved in the ocean reacts
with water to form carbonic acid. The net effect, even accounting
for warming of the oceans, is an observed and accelerating ocean
acidification. Since biosystems are adapted to a narrow range of
pH this is a very serious concern directly driven by increased atmospheric
CO2 and not global warming. Ocean acidification would not be created
by any other postulated climate change mechanisms. In fact, the
result of these other forcings would be to decrease the acidity
of the oceans as the sea temperature warmed. This is exactly contrary
to what has been observed. Roger Revelle and Hans Suess first described
the chemistry of the process.
Although they are often interlinked in the mass media, the connection
between global warming and ozone depletion is not strong. There
are five areas of linkage:
The same carbon dioxide radiative forcing that
produces near-surface global warming is expected (perhaps somewhat
surprisingly) to cool the stratosphere. This, in turn, would lead
to a relative increase in ozone depletion and the frequency of ozone
Radiative forcing from various greenhouse gases and other sourcesConversely,
ozone depletion represents a radiative forcing of the climate system.
There are two opposed effects: 1) reduced ozone allows more solar
radiation to penetrate, thus warming the troposphere instead of
the stratosphere. 2) The resulting colder stratosphere emits less
long-wave radiation down to the troposphere, thus having a cooling
effect. Overall, the cooling dominates: the IPCC concludes that
observed stratospheric O3 losses over the past two decades have
caused a negative forcing of the surface-troposphere system 
of about -0.15 ± 0.10 W/m² .
One of the strongest predictions of the greenhouse effect theory
is that the stratosphere will cool. Although this cooling has been
observed, it is not trivial to separate the effects of changes in
the concentration of greenhouse gases and ozone depletion since
both will lead to cooling. However, this can be done by numerical
stratospheric modeling. Results from the NOAA Geophysical Fluid
Dynamics Laboratory show that above 20 km, the greenhouse gases
dominate the cooling. 
Ozone depleting chemicals are also greenhouse gases, representing
0.34 ±0.03 W/m², or about 14% of the total radiative forcing from
well-mixed greenhouse gases .
Decreased ozone leads to an increase in ultraviolet levels. Ultraviolet
radiation may be responsible for the death of ocean algae, which
operate as a carbon dioxide sink in the ocean. Increased UV, therefore,
may lead to a decrease in carbon dioxide uptake, thereby raising
global carbon dioxide levels. 
Some scientists now consider that the effects of global dimming
(the reduction in sunlight reaching the surface of the planet, possibly
due to aerosols) may have masked some of the effect of global warming.
If this is so, the indirect aerosol effect is stronger than previously
believed, which would imply that the climate sensitivity to greenhouse
gases is also stronger. Concerns about the effect of aerosol on
the global climate were first researched as part of concerns over
global cooling in the 1970s.
Pre-human global warming
The Earth has experienced natural global warming and cooling many
times in the past, and can offer useful insights into present processes.
It is thought by some geologists that a rapid buildup of greenhouse
gases caused the Earth to experience global warming in the early
Jurassic period, with average temperatures rising by 5 °C (9.0 °F).
Research by the Open University published in Geology (32: 157–160,
2004 ) indicates that this caused the rate of rock weathering
to increase by 400%. As such weathering locks away carbon in calcite
and dolomite, carbon dioxide levels dropped back to normal over
roughly the next 150,000 years.
Sudden releases of methane from clathrate compounds
(the Clathrate Gun Hypothesis) have been hypothesized as a cause
for other past global warming events, including the Permian-Triassic
extinction event and the Paleocene-Eocene Thermal Maximum. However,
warming at the end of the last glacial period is thought not to
be due to methane release . Instead, natural variations in the
Earth's orbit (Milankovitch cycles) are believed to have triggered
the retreat of ice sheets by changing the amount of solar radiation
received at high latitude and led to deglaciation.
The greenhouse effect is also invoked to explain
how the Earth made it out of the Snowball Earth period 600 million
years ago. During this period all silicate rocks were covered by
ice, thereby preventing them from combining with atmospheric carbon
dioxide. The atmospheric carbon dioxide level gradually increased
until it reached a level that could have been as much as 350 times
the current level. At this point temperatures were raised enough
to melt the ice, even though the reflective ice surfaces had been
reflecting most sunlight back into space. Increased amounts of rainfall
would quickly wash the carbon dioxide out of the atmosphere, and
thick layers of abiotic carbonate sediment have been found on top
of the glacial rocks from this period.
Using paleoclimate data for the last 500 million
years Veizer et al. (2000, Nature 408, pp. 698–701) concluded that
long-term temperature variations are only weakly related to carbon
dioxide variations. Most paleoclimatologists believe this is because
other factors, such as continental drift and mountain building have
larger effects in determining very long term climate. However, Shaviv
and Veizer (2003, ) proposed that the biggest long-term influence
on temperature is actually the solar system's motion around the
galaxy, and the ways in which this influences the atmosphere by
altering the flux of cosmic rays received by the Earth. Afterwards,
they argued that over geologic times a change in carbon dioxide
concentrations comparable to doubling pre-industrial levels, only
results in about 0.75 °C (1.3 °F) warming rather than the usual
1.5–4.5 °C (2.7–8.1 °F) reported by climate models . They acknowledge
(Shaviv and Veizer 2004) however that this conclusion may only be
valid on multi-million year time scales when glacial and geological
feedback have had a chance to establish themselves. Rahmstorf et
al. 2004  argue that S+V have highly and arbitrarily tuned
their data, and that their conclusions are unreliable.
Pre-industrial global warming
Paleoclimatologist William Ruddiman has argued (e.g., Scientific
American, March 2005) that human influence on the global climate
began around 8,000 years ago with the start of forest clearing to
provide land for agriculture and 5,000 years ago with the start
of Asian rice irrigation. He contends that forest clearing explains
the rise in carbon dioxide levels in the current interglacial that
started 8,000 years ago, contrasting with the decline in carbon
dioxide levels seen in the previous three interglacials. He further
contends that the spread of rice irrigation explains the breakdown
in the last 5,000 years of the correlation between the Northern
Hemisphere solar radiation and global methane levels, which had
been maintained over at least the last 11 22,000-year cycles. Ruddiman
argues that without these effects, the Earth would be nearly 2 °C
cooler and "well on the way" to a new ice age. Ruddimann's
interpretation of the historical record, with respect to the methane
data, has been disputed.
Climate changes reflect variations within the
Earth's environment, natural processes going on around it, and the
impact of human activity. The external factors which can shape climate
are often called climate forcings and include such processes as
variations in solar radiation, the Earth's orbit, and greenhouse
Variations within the Earth's climate
Weather, in and of itself, is a chaotic non-linear dynamical system,
but in many cases, it is observed that the climate (i.e., the average
state of weather) is fairly stable and predictable. This includes
the average temperature, amount of precipitation, days of sunlight,
and many other variables that might be measured at any given site.
However, there are also changes within the Earth's environment that
can affect the climate.
Glaciers are recognized as one of the most sensitive indicators
of climate change, advancing substantially during climate cooling
(e.g., the Little Ice Age) and retreating during climate warming
on moderate time scales. Glaciers grow and collapse, both contributing
to natural variability and greatly amplifying external forces. For
the last century, however, glaciers have been unable to regenerate
enough ice during the winters to make up for the ice lost during
the summer months (see glacier retreat).
The most important climate processes of the last
several million years are the glacial and interglacial cycles of
the present ice age. Though shaped by orbital variations, the internal
responses involving continental ice sheets and 130 m sea-level change
certainly played a key role in deciding what climate response would
be observed in most regions. Other changes, including Heinrich events,
Dansgaard–Oeschger events and the Younger Dryas show the potential
for glacial variations to influence climate even in the absence
of specific orbital changes.
A schematic of modern thermohaline circulationOn the scale of mere
decades, climate changes can also result from changes within the
ocean/atmosphere systems. Many climate states, most obviously El
Niño Southern oscillation, but also including the Pacific decadal
oscillation, the North Atlantic oscillation, and the Arctic oscillation,
have been recognized as modes within the climate system, owing their
existence at least in part to different ways that heat can be stored
in the oceans and move between different reservoirs. On longer time
scales, ocean processes such as thermohaline circulation play a
key role in redistributing heat, and could, if changed, dramatically
The memory of climate
More generally, most forms of internal variability in the climate
system can be recognized as a form of hysteresis, meaning that the
current state of climate reflects not only the inputs, but also
the history of how it got there. For example, a decade of dry conditions
may cause lakes to shrink, plains to dry up and deserts to expand.
In turn, these conditions may lead to less rainfall in the following
years. In short, climate change can be a self-perpetuating process
because different aspects of the environment respond at different
rates and in different ways to the fluctuations that inevitably
Non-climate factors driving climate change
Carbon dioxide variations during the last 500 million yearsCurrent
studies indicate that radiative forcing by greenhouse gases is the
primary cause of global warming. Greenhouse gases are also important
in understanding Earth's climate history. According to these studies,
the greenhouse effect, which is the warming produced as greenhouse
gases trap heat, plays a key role in regulating Earth's temperature.
Over the last 600 million years, carbon dioxide
concentrations have varied from perhaps >5000 ppm to less than
200 ppm, due primarily to the impact of geological processes and
biological innovations. Curiously, it has been argued (Veizer et
al. 1999) that variations in greenhouse gas concentrations over
tens of millions of years have not been well correlated to climate
change, with plate tectonics perhaps playing a more dominant role.
However, there are several examples of rapid changes in the concentrations
of greenhouse gases in the Earth's atmosphere that do appear to
correlate to strong warming, including the Paleocene–Eocene thermal
maximum, the Permian–Triassic extinction event, and the end of the
Varangian snowball earth event.During the modern era, rising carbon
dioxide levels are implicated as the primary cause to global warming
On the longest time scales, plate tectonics will reposition continents,
shape oceans, build and tear down mountains and generally serve
to define the stage upon which climate exists. More recently, plate
motions have been implicated in the intensification of the present
ice age when, approximately 3 million years ago, the North and South
American plates collided to form the Isthmus of Panama and shut
off direct mixing between the Atlantic and Pacific Oceans.
Variations in solar activity during the last several centuries based
on observations of sunspots and beryllium isotopes.The sun is the
ultimate source of essentially all heat in the climate system. The
energy output of the sun, which is converted to heat at the earth's
surface, is an integral part of shaping the Earth's climate. On
the longest time scales, the sun itself is getting brighter with
higher energy output; as it continues its main sequence, this slow
change or evolution effects the earths atmosphere. Early in Earth's
history, it is thought to have been too cold to support liquid water
at the Earth's surface, leading to what is known as the Faint young
On more modern time scales, there are also a variety
of forms of solar variation, including the 11–year solar cycle and
longer-term modulations. However, the 11–year sunspot cycle does
not manifest itself clearly in the climatological data. Solar intensity
variations are considered to have been influential in triggering
the Little Ice Age, and for some of the warming observed from 1900
to 1950. The cyclical nature of the sun's energy output is not yet
fully understood; it differs from the very slow change that is occurring
to the sun as it ages and evolves.
In their impact on climate, orbital variations are in some sense
an extension of solar variability, because slight variations in
the Earth's orbit lead to changes in the distribution and abundance
of sunlight reaching the Earth's surface. Such orbital variations,
known as Milankovitch cycles, are a highly predictable consequence
of basic physics due to the mutual interactions of the Earth, its
moon, and the other planets. These variations are considered the
driving factors underlying the glacial and interglacial cycles of
the present ice age. Subtler variations are also present, such as
the repeated advance and retreat of the Sahara desert in response
to orbital precession.
A single eruption of the kind that occurs several times per century
can impact climate, causing cooling for a period of a few years.
For example, the eruption of Mount Pinatubo in 1991 is barely visible
on the global temperature profile. Huge eruptions, known as large
igneous provinces, occur only a few times every hundred million
years, but can reshape climate for millions of years and cause mass
extinctions. Initially, scientists thought that the dust emitted
into the atmosphere from large volcanic eruptions was responsible
for the cooling by partially blocking the transmission of solar
radiation to the Earth's surface. However, measurements indicate
that most of the dust thrown in the atmosphere returns to the Earth's
surface within six months.
Attribution of recent climate change
Human influences on climate
Anthropogenic factors are acts by humans that change the environment
and influence climate. The biggest factor of present concern is
the increase in CO2 levels due to emissions from fossil fuel combustion,
followed by aerosols (particulate matter in the atmosphere) which
exerts a cooling effect. Other factors, including land use, ozone
depletion, and deforestation also impact climate.
Carbon dioxide variations over the last 400,000 years, showing a
rise since the industrial revolution.Beginning with the industrial
revolution in the 1850s and accelerating ever since, the human consumption
of fossil fuels has elevated CO2 levels from a concentration of
~280 ppm to more than 370 ppm today. These increases are projected
to reach more than 560 ppm before the end of the 21st century. Along
with rising methane levels, these changes are anticipated to cause
an increase of 1.4–5.6 °C between 1990 and 2100 (see global warming).
Anthropogenic aerosols, particularly sulphate aerosols from fossil
fuel combustion, are believed to exert a cooling influence; see
graph. This, together with natural variability, is believed to
account for the relative "plateau" in the graph of 20th
century temperatures in the middle of the century.
Prior to widespread fossil fuel use, humanity's largest impact on
local climate is likely to have resulted from land use. Irrigation,
deforestation, and agriculture fundamentally change the environment.
For example, they change the amount of water going into and out
of a given locale. They also may change the local albedo by influencing
the ground cover and altering the amount of sunlight which is absorbed.
For example, there is evidence to suggest that the climate of Greece
and other Mediterranean countries was permanently changed by widespread
deforestation between 700 BC and 0 BC (the wood being used for ship-building,
construction and fuel purposes), with the result that the modern
climate in the region is significantly hotter and drier and the
species of trees which were used for ship-building in the ancient
world can no longer be found in the area.
A controversial hypothesis by William Ruddiman
 suggests that the rise of agriculture and the accompanying deforestation
led to the increases in carbon dioxide and methane during the period
5000–8000 years ago. These increases, which reversed previous declines,
may have been responsible for delaying the onset of the next glacial
period, according to Ruddimann's hypothesis.
Interplay of factors
If a certain forcing (for example, solar variation) acts to change
the climate, then there may be mechanisms which act to amplify or
reduce the effects. These are called positive and negative feedbacks.
As far as is known, the climate system is generally stable with
respect to these feedbacks: positive feedbacks do not "run
away". Part of the reason for this is the existence of a powerful
negative feedback between temperature and emitted radiation: radiation
increases as the fourth power of absolute temperature.
However, a number of important positive feedbacks
do exist. The glacial and interglacial cycles of the present ice
age provide an important example. It is believed that orbital variations
provide the timing for the growth and retreat of ice sheets. However,
the ice sheets themselves reflect sunlight back into space and hence
promote cooling and their own growth, known as the ice–albedo feedback.
Further, falling sea levels and expanding ice decrease plant growth
and indirectly lead to declines in carbon dioxide and methane. This
leads to further cooling.
Similarly, rising temperatures caused, for example,
by anthropogenic emissions of greenhouse gases could lead to retreating
snow lines, revealing darker ground underneath, and consequently
result in more absorption of sunlight.
Water vapor, methane, and carbon dioxide can also
act as significant positive feedbacks, their levels rising in response
to a warming trend, thereby accelerating that trend. Water vapor
acts strictly as a feedback (excepting small amounts in the stratosphere),
unlike the other major greenhouse gases, which can also act as forcings.
More complex feedbacks involve the possibility
of changing circulation patterns in the ocean or atmosphere. For
example, a significant concern in the modern case is that melting
glacial ice from Greenland will interfere with sinking waters in
the North Atlantic and inhibit thermohaline circulation. This could
affect the Gulf Stream and the distribution of heat to Europe and
the east coast of the United States.
Other potential feedbacks are not well understood
and may either inhibit or promote warming. For example, it is unclear
whether rising temperatures promote or inhibit vegetative growth,
which could in turn draw down either more or less carbon dioxide.
Similarly, increasing temperatures may lead to either more or less
cloud cover. Since on balance cloud cover has a strong cooling
effect, any change to the abundance of clouds also impacts climate.
In all, it seems likely that overall climate feedbacks
are negative, as systems with overall positive feedback are highly
Monitoring the current status of climate
Scientists use "Indicator time series" that represent
the many aspects of climate and ecosystem status. The time history
provides an historical context. Current status of the climate is
also monitored with climate indices.
Evidence for Climatic Change
Evidence for climatic change is taken from a variety of sources
which can be used to reconstruct past climates. Most of the evidence
is indirect—climatic changes are inferred from changes in indicators
that reflect climate, such as vegetation, dendrochronology, ice
cores, sea level change, glacial retreat.
Species have particular climatic requirements which influence their
geographical distributions. Each plant species has a distinctively
shaped pollen grain and if these fall into oxygen-free environments,
such as peat bogs, they resist decay. Changes in the pollen found
in different levels of the bog indicate, by implication, changes
in climate.One limitation of this method is the fact that pollen
can be transported considerable distances by wind or sometimes by
Remains of Coleoptera beetles are common in freshwater and land
sediments. Different species of this beetle tend to be found under
different climatic conditions. Knowledge of the present climatic
range of the different species, and the age of the sediments in
which remains are found, allows past climatic conditions to be worked
Advancing glaciers leave behind moraines and other features which
often have datable material in them, recording the time when a glacier
advance and deposited a feature. Similarly the lack of glacier cover
can be identified by the presence of datable soil or volcanic tephra
horizons. Glaciers are considered one of the most sensitive climate
indicators by the IPCC, and their recent observed variations provide
a global signal of climate change, see Retreat of glaciers since
Historical records include cave paintings, depth of grave digging
in Greenland, diaries, documentary evidence of events (such as 'frost
fairs' on the Thames) and evidence of areas of vine cultivation.
Since 1873 daily weather reports have been documented, and the Royal
Society has encouraged the collection of data since the seventeenth
century. Parish records are often a good source of climate data.
Examples of climate change
Climate change has continued throughout the entire history of Earth.
The field of paleoclimatology has provided information of climate
change in the ancient past, supplementing modern observations of
climate. Obviously, these prehistoric changes are solely the result
of natural factors.