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Global warming

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Environment

   Global mean surface temperatures 1850 to 2006
   Global mean surface temperatures 1850 to 2006
   Mean surface temperature anomalies during the period 1995 to 2004 with
   respect to the average temperatures from 1940 to 1980
   Mean surface temperature anomalies during the period 1995 to 2004 with
   respect to the average temperatures from 1940 to 1980

   Global warming is the observed increase in the average temperature of
   the Earth's near surface air and oceans in recent decades and its
   projected continuation. Models referenced by the Intergovernmental
   Panel on Climate Change (IPCC) predict that global temperatures are
   likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and
   2100. The uncertainty in this range results from two factors: differing
   future greenhouse gas emission scenarios, and uncertainties regarding
   climate sensitivity.

   Global average near-surface atmospheric temperature rose 0.74 ± 0.18 °
   Celsius (1.3 ± 0.32 ° Fahrenheit) in the last century. The prevailing
   scientific opinion on climate change is that "most of the observed
   increase in globally averaged temperatures since the mid-20th century
   is very likely due to the observed increase in anthropogenic greenhouse
   gas concentrations," which leads to warming of the surface and lower
   atmosphere by increasing the greenhouse effect. Greenhouse gases are
   released by activities such as the burning of fossil fuels, land
   clearing, and agriculture. Other phenomena such as solar variation and
   volcanoes have had smaller but non-negligible effects on global mean
   temperature since 1950. A few scientists disagree about the primary
   causes of the observed warming.

   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, glacier retreat, reduced summer streamflows,
   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, even if
   no further greenhouse gases were released after this date warming (and
   sea level) would be expected to continue to rise for more than a
   millennium, since carbon dioxide (CO[2]) has a long average atmospheric
   lifetime.

   Remaining scientific uncertainties include the exact degree of climate
   change expected in the future, and especially how changes will vary
   from region to region across the globe. A hotly contested political and
   public debate also has yet to be resolved, regarding whether anything
   should be done, and what could be cost-effectively done to reduce or
   reverse future warming, or to deal with the expected consequences. Most
   national governments have signed and ratified the Kyoto Protocol aimed
   at combating global warming. (See: List of Kyoto Protocol signatories.)

Terminology

   The term global warming is a specific example of the broader term
   climate change, which can also refer to global cooling. In principle,
   global warming is neutral as to the period or causes, but in both
   common and scientific usage the term generally refers to recent warming
   and implies a human influence. The UNFCCC uses the term "climate
   change" for human-caused change, and "climate variability" for other
   changes. Some organizations use the term "anthropogenic climate change"
   for human-induced changes.

History of 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.
   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; the urban heat island is not believed to be
   significant. 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 (0.22 and 0.4 °F) per decade since 1979,
   according to satellite temperature measurements. Over the one or two
   thousand years before 1850, 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. Estimates prepared
   by the World Meteorological Organization and the UK Climatic Research
   Unit concluded that 2005 was the second warmest year, behind 1998.

   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.

   Note that the anthropogenic emissions of other pollutants—notably
   sulphate 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.

Causes

   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.
   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 through natural, internal processes and in
   response to variations in external "forcing" from both human and
   natural causes. These forcing factors include solar activity, volcanic
   emissions, variations in the earth's orbit ( orbital forcing) and
   greenhouse gases. The detailed causes of the recent warming remain an
   active field of research, but the scientific consensus identifies
   greenhouse gases as the main influence.

   Contrasting with this consensus view, other hypotheses have been
   proposed to explain all or most of the observed increase in global
   temperatures, including: 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; and the warming is primarily a
   result of variances in solar radiation.

   Adding carbon dioxide (CO[2]) or methane (CH[4]) 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, so
   that Earth would be uninhabitable. It is therefore not correct to say
   that there is a debate between those who "believe in" and "oppose" the
   greenhouse effect as such. Rather, the debate concerns the net effect
   of the addition of greenhouse gases when allowing for compounding or
   mitigating factors.

   One example of an important feedback process is ice-albedo feedback.
   The increased CO[2] 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 on average less
   reflective than ice, and thus absorb more solar radiation. This causes
   more warming, which in turn causes more melting, and this 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 about 0.5 °C
   (0.9 °F) would still occur.

Greenhouse gases in the atmosphere

   Recent increases in atmospheric CO2. 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.
   Recent increases in atmospheric CO2. The monthly CO[2] 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 CO[2] from the atmosphere.

   The greenhouse effect, first discovered by Joseph Fourier in 1824, and
   first investigated quantitatively by Svante Arrhenius in 1896, is the
   process in which the emission of infrared radiation by atmospheric
   gasses warms a planet's surface. On Earth, the major natural greenhouse
   gases are water vapor, which causes about 36-70% of the greenhouse
   effect ( not including clouds); carbon dioxide, which causes 9-26%;
   methane, which causes 4-9%, and ozone, which causes 3-7%.

   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 24
   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, especially deforestation.

   Future carbon dioxide levels are expected to rise due to ongoing
   burning of fossil fuels. The rate of rise will depend on uncertain
   economic, sociological, technological, natural developments, but may be
   ultimately limited by the availability of fossil fuels. 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, tar sands or Methane
   clathrates are extensively used.

   Carbon sink ecosystems (forests and oceans) are being degraded by
   pollutants. Degradation of major carbon sinks results in higher
   atmospheric carbon dioxide levels.
   Anthropogenic emission of greenhouse gases broken down by sector for
   the year 2000.
   Anthropogenic emission of greenhouse gases broken down by sector for
   the year 2000.

   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 not included in IPCC's climate models.

   The measure of the temperature response to increased greenhouse gas
   concentrations and other anthropogenic and natural climate forcings is
   climate sensitivity. It is found by observational and model studies.
   This sensitivity is usually expressed in terms of the temperature
   response expected from a doubling of CO[2] in the atmosphere. The
   current literature estimates sensitivity in the range of 1.5 to 4.5 °C
   (2.7 to 8.1 °F).

Solar variation

   Modeling studies reported in the IPCC Third Assessment Report (TAR)
   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. The IPCC Fourth
   Assessment Report (AR4) gives a best estimate for radiative forcing
   from changes in solar activity of +0.12 watts per square meter. This is
   less than half of the estimate given in the TAR. For comparison, the
   combined effects of all human activity are estimated to be an order of
   magnitude greater at +1.6 watts per square meter.

   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.

   Some scientists assert that a warming of the stratosphere, which has
   not been observed, would be expected if there were a significant
   increase in solar activity.

   Some 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 IPCC describes the level of scientific
   understanding of the contribution of variations in solar irradiance to
   historical climate changes as "low."
   400 year history of sunspot numbers.
   400 year history of sunspot numbers.

   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. Additionally, in 2005,
   researchers at Duke University have found that 10–30% of the warming
   over the last two decades may be due to increased solar output.

Attributed and expected effects

   Global glacial mass balance in the last 50 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 glacial mass balance in the last 50 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.

   Some effects on both the natural environment and human life are, at
   least in part, already being attributed to global warming. Glacier
   retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level
   rise, changes in rainfall patterns, increased intensity and frequency
   of hurricanes and extreme weather events, are being attributed at least
   in part to global warming. While changes are expected for overall
   patterns, intensity, and frequencies, it is difficult or impossible to
   attribute specific events (such as Hurricane Katrina) to global
   warming.

   Some anticipated effects include sea level rise of 110 to 770 mm (0.36
   to 2.5 feet) by 2100, repercussions to agriculture, possible slowing of
   the thermohaline circulation, reductions in the ozone layer, increased
   intensity and frequency of hurricanes and extreme weather events,
   lowering of ocean pH, the spread of diseases such as malaria and dengue
   fever, and mass extinction events.

   Increasing extreme weather catastrophes are due to increasing severe
   weather and an increase in population densities. The World
   Meteorological Organization and the U.S. Environmental Protection
   Agency have linked increasing extreme weather events to global warming,
   as have Hoyos et al. (2006), writing that the increasing number of
   category 4 and 5 hurricanes is directly linked to increasing
   temperatures. Similarly, Kerry Emmanuel in Nature writes that hurricane
   power dissipation is highly correlated with temperature, reflecting
   global warming. Hurricane modeling has produced similar results,
   finding that hurricanes, simulated under warmer, high-CO[2] conditions,
   are more intense than under present-day conditions. NOAA claims that
   warming induced by greenhouse gas may lead to increasing occurrence of
   highly destructive category-5 storms.

Mitigation

   The broad agreement among climate scientists that global temperatures
   will continue to increase has led nations, states, corporations and
   individuals to implement actions to try to curtail global warming. Some
   of the strategies that have been proposed for mitigation of global
   warming include development of new technologies; carbon offsets;
   renewable energy such as wind power, and solar power; nuclear power;
   electric or plug-in hybrid electric vehicles; non-fossil fuel cells;
   energy conservation; carbon taxes; improving natural carbon dioxide
   sinks; deliberate production of sulfate aerosols, which produce a
   cooling effect on the Earth; population control; carbon capture and
   storage; and nanotechnology. Many environmental groups encourage
   individual action against global warming, often aimed at the consumer,
   and there has been business action on climate change.

Kyoto Protocol

   The world's primary international agreement on combating global warming
   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. Developing countries are
   exempt from meeting emission standards in Kyoto. This includes China
   and India, the second and third largest emitters of CO[2], behind the
   United States.

Climate models

   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.
   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 (5.4 °F)
   The geographic distribution of surface warming during the 21^st 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 (5.4 °F)

   Scientists have studied global warming with computer models of the
   climate. These models predict that the net effect of adding greenhouse
   gases will be a warmer climate in the future. However, even when the
   same assumptions of fossil fuel consumption and CO[2] emission are
   used, the amount of predicted warming varies between models and there
   still remains a considerable range of climate sensitivity.

   Including model and future greenhouse gas uncertainty, the IPCC
   anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °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.

   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.

   Most global climate models, when run to predict future climate, are
   forced by imposed greenhouse gas scenarios, generally one from the IPCC
   Special Report on Emissions Scenarios (SRES). Less commonly, models may
   be run by adding a simulation of the carbon cycle; this 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
   CO[2]). Some observational studies also show a positive feedback.

   The representation of clouds is one of the main sources of uncertainty
   in present-generation models, though progress is being made on this
   problem. There is also an ongoing discussion as to whether climate
   models are neglecting important indirect and feedback effects of solar
   variability.

Other related issues

Ocean acidification

   Increased atmospheric carbon dioxide increases the amount of CO[2]
   dissolved in the oceans. Carbon dioxide gas dissolved in the ocean
   reacts with water to form carbonic acid resulting in ocean
   acidification. Since biosystems are adapted to a narrow range of pH,
   this is a serious concern directly driven by increased atmospheric
   CO[2]and not global warming.

Relationship to ozone depletion

   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 holes.

   Radiative forcing from various greenhouse gases and other sources
   Radiative forcing from various greenhouse gases and other sources
     * Conversely, ozone depletion represents a radiative forcing of the
       climate system. There are two opposed effects: Reduced ozone allows
       more solar radiation to penetrate, thus warming the troposphere
       instead of the stratosphere; 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 O[3] losses over the past
       two decades have caused a negative forcing of the
       surface-troposphere system" of about −0.15 ± 0.10 W/m^2.

     * 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 National Oceanic and
       Atmospheric Administration's Geophysical Fluid Dynamics Laboratory
       show that above 20 km (12.4 miles), the greenhouse gases dominate
       the cooling.

     * Ozone depleting chemicals are also greenhouse gases, representing
       0.34 ± 0.03 W/m^2, or about 14% of the total radiative forcing from
       well-mixed greenhouse gases.

Relationship to global dimming

   Scientists have stated with 66-90% confidence that the effects of
   volcanic and human-caused aerosols have offset some of global warming,
   and that greenhouse gases would have resulted in more warming than
   observed if not for this effect.

          For comparison of the relative significance of these factors:

     * The best estimate for the magnitude of radiative forcing from the
       long-lived greenhouse gases CO2, CH4, and N2O alone is +2.3
       watts/m^2.
     * Radiative forcing from the halocarbon class of long-lived
       greenhouse gases is about +0.34 watts/m^2.
     * The cooling effects of aerosols are estimated to be:
          + Direct cooling effects of -0.5 watts/m^2
          + Cloud albedo cooling effects of -0.7 watts/m^2
     * Total warming effects from post-industrial human activity including
       the above and other cooling and warming factors are estimated at
       +1.6 watts/m^2.

Pre-human global warming

   Curves of reconstructed temperature at two locations in Antarctica and
   a global record of variations in glacial ice volume. Today's date is on
   the left side of the graph
   Curves of reconstructed temperature at two locations in Antarctica and
   a global record of variations in glacial ice volume. Today's date is on
   the left side of the graph

   The earth has experienced natural global warming and cooling many times
   in the past. The recent Antarctic EPICA ice core spans 800,000 years,
   including eight glacial cycles with interglacial warming periods much
   hotter than current temperatures. The chart also shows the time of the
   last glacial maximum about 20,000 years ago.

   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 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.

   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 (1.35 °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. argue that Shaviv and Veizer arbitrarily
   tuned their data, and that their conclusions are unreliable.

Pre-industrial global warming

   Paleoclimatologist William Ruddiman has argued 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 eleven 22,000-year cycles.
   Ruddiman argues that without these effects, the Earth would be nearly
   2 °C (3.6 °F) cooler and "well on the way" to a new ice age. Ruddiman's
   interpretation of the historical record, with respect to the methane
   data, has been disputed.
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