Global Warming

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Global warming is defined by the IPCC as the unequivocal increase in the average temperature of the Earth's atmosphere, land and oceans due to an increase in Greenhouse gases since the Industrial Revolution and its projected exponential continuation unless mediated.

Mediation to maintain present concentations between 450 and 550 ppm is expected by the Intergovernmental Panel on Climate Change (IPCC) to cost $1,800 trillion dollars worldwide by the end of the century.

Models referenced by the IPCC predict that average global temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. [1] and could go as high as 11°C in the Northern Hemisphere where the concentration of emmissions and resultant warming is highest.

The resultant dangers to 80% of the Worlds cities due to rising sea levels is only now being recognized because of the damage to New Orleans during Hurricane Katrina. Much of the coastal United States is vulnerable to similar levels of damage over the next few decades and will almost certainly experience synergysic effects due to worsening storms and damage to barrier islands over the next century. 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 0.6 ± 0.2 °Celsius (1.1 ± 0.4 °Fahrenheit) in the 20th 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,"[1] 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. There is also an effect from the release of methane as the frozen tundras permafrost melts. Energy exploration for methane in the artic may affect this release of methane adversly.

Other phenomena such as solar variation have had smaller but non-negligible effects on global temperature trends since 1950.[2] A small number of notable scientists disagree with regard to the nature of the observed warming. CO2-induced global warming was first predicted in 1896 by Svante Arrhenius.

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 CO2 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.)

Plots of atmospheric Carbon dioxide and global temperature during the last 650,000 years.

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 measure of the response to increased GHGs, and other anthropogenic and natural climate forcings is climate sensitivity. It is found by observational and model studies.[10] This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere. The current literature estimates sensitivity in the range of 1.5 to 4.5 °C (2.7 to 8.1 °F). See an Illustration of the Greenhouse Effect:

The major natural greenhouse gases are water vapor, which causes about 36-70% of the greenhouse effect on Earth (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 40 million years ago.[citation needed] 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.[11]

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.[12][13] 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,[14][15][16] but others have been increased or created by agricultural activities such as the cultivation of rice paddies.[17] Recent evidence indicates that methane concentrations have begun to stabilize, perhaps due to reductions in leakage from fuel transmission and storage facilities.[18]

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 availibility of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future carbon dioxide scenarios,[19] 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.[citation needed]Carbon sink ecosystems (forests and oceans)[20] are being degraded by pollutants.[21] 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.

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)[verification needed] , emissions from industrial processes (excluding fuel combustion), and agriculture: these contributed 5.8%, 5.2% and 3.3% respectively in 1990.[citation needed] Current figures are broadly comparable.[22] 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.[citation needed]

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[23] not included in IPCC's climate models.[citation needed]

Solar variation
30 years of solar variability.

Some researchers (e.g. Stott et al. 2003)[27] 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)[28] 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."[1]
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.[29] 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.[30]

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.

Some effects on both the natural environment and human life are already being attributed at least in part 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 by 2100,[31] 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.

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.[3]


The Energy Information Administration predicts world energy and fossil fuel usage will rise in the next decades.

The consensus 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 biodiesel, wind power, and solar power; nuclear power; electric or hybrid automobiles; 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 CO2, 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 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)

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, models with sufficiently high resolution are able to successfully simulate summer and winter differences, the North Atlantic Oscillation,[4] and El Niño.[5] All validated current 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 CO2 emission are used, the amount of predicted warming varies between models and there still remains a considerable range of climate sensitivity predicted by the models which survive these tests. 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." (See [6], page 78.) An example of a study which aims to do this is the project; their methodology is to investigate the range of climate sensitivities predicted for the 21st century by those models which have first been shown to give a reasonable simulation of late 20th century climate change.

As noted above, climate models have been used by the IPCC to anticipate 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. 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.[7] 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.[8]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[9] 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 and hurricanes). However, despite these and other limitations, the IPCC considered climate models "to be suitable tools to provide useful projections of future climates."[10]

In December, 2005, Bellouin et al. suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected. These results imply that as air quality improves additional global warming may be unmasked [11].


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The Greenhouse Effect: Interactive: