Planetary Ruin

Global warming is the observed increase in the average temperature of the Earth’s atmosphere and oceans in recent decades and its projected continuation into the future.

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 warming observed over the last 50 years is attributable to human activities.” The main cause of the human-induced component of warming is the increased atmospheric concentration of greenhouse gases (GHGs) such as carbon dioxide (CO2), 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.

Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures may increase by 1.4 to 5.8 °C (2.5 to 10.5 °F) between 1990 and 2100. The uncertainty in this range results from both the difficulty of predicting the volume of future greenhouse gas emissions and uncertainty about climate sensitivity.

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, warming (and sea level rise) is expected to continue past then, 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 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 combatting global warming. (See List of Kyoto Protocol signatories.)

Terminology
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.
Historical warming of the Earth
See also: Temperature record of the past 1000 years
 
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. 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 based on different data sets. The longest 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. An approximately global instrumental record of temperature near the earth’s surface begins in about 1860. Global observations of the atmosphere well above the earth’s surface using data from radiosondes began shortly after World War II. Satellite temperature measurements of the tropospheric temperature date from 1979. 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.
Causes
Main articles: Attribution of recent climate change and Scientific opinion on climate change
 
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 be controversial.

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 on average less reflective than ice, and thus 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 in the atmosphere
 
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. 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 1.5-4.5 °C (2.7-8.1 °F).

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, especially deforestation.

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. The rate of rise 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.

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.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.
Other hypotheses
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 the 928 refereed scientific articles in the ISI citation database identified with the keywords “global climate change” and published from 1993–2003. 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 accepted the hypothesis of human-induced warming, but none of the 928 articles surveyed accepted any other hypothesis.

Contrasting with the consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures. Some of these hypotheses 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 nuclei.
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
 
30 years of solar variability.Main article: 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.
Attributed and Expected effects
 
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.Main article: Effects of global warming
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 of 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 up to 14 metres, 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, and the spread of diseases such as Malaria and Dengue Fever.

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 .
Mitigation
Main articles: Mitigation of global warming and adaptation to global warming
 
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 significantly increase has led to measures to mitigate global warming being introduced by many nations, states, corporations and individuals with proposals for further measures. 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
Carbon sequestration
Planetary engineering to cool the earth, including screening out sunlight and increasing the reflectivity of the earth.
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; increased use of sulfate aerosols, which exhibit a cooling effect on the Earth; 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 some debate about how effective the Kyoto protocol has been and debate about what should follow Kyoto.

While globally the Stern report is evidence that the future cost of global warming is too high to adopt a “do nothing” or “business as usual” approach, some politicians, including President of the United States George W. Bush and Prime Minister of Australia John Howard have argued that the cost of mitigating global warming via the Kyoto protocol is too high for their countries to be economic or politically acceptable.

Some signatories of the Kyoto protocol, including Europe and Japan, are currently struggling to meet their targets. After only five years, Canada has given up entirely. On the other hand, the New York Times reports (18 Jan 2007) that ten major companies with operations across the US economy — utilities, manufacturing, petroleum, chemicals and financial services — have banded together with leading environmental groups to call for a firm nationwide limit on carbon dioxide emissions that would lead to reductions of 10 to 30 percent over the next 15 years. These include General Electric, DuPont, Alcoa, and also Caterpillar (manufacturing) Duke Energy, PG&E, FPL Group,PNM Resources (four energy utilities) BP (multinational oil company) Lehman Brothers (finance).

Of the 163 countries that have signed and ratified Kyoto, only 31 (those with the highest greenhouse gas emissions per capita) are actually required to lower greenhouse emissions. Notable among those countries that have signed and ratified Kyoto but are not required to reduce greenhouse gas emissions are China and India with their per capita greenhouse gas emissions of only 16% and 6% of those of the USA respectively but with huge populations and rapidly growing economies.

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.
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.
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)Main article: Global climate model
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.” (See , page 78.) 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 climate change.

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.

Other related issues

Ocean acidification
Main article: Ocean acidification
Increased atmospheric carbon dioxide increases the amount of CO2 dissolved in the oceans. Unfortunately, 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 CO2 and not global warming.
Relationship to ozone depletion
Main article: 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 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. 

Relationship to global dimming
Main article: Global dimming
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 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 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.
Notes
^ a b Climate Change 2001: Working Group I: The Scientific Basis, Part 7. Intergovernmental Panel on Climate Change (2001). Retrieved on 2007-01-18.
^ United Nations Framework Convention on Climate Change, Article I. United Nations. Retrieved on 2007-01-15.
^ Goddard Institute for Space Studies, GISS Surface Temperature Analysis. NASA Goddard Institute for Space Studies (2006-01-12). Retrieved on 2007-01-17.
^ Real Climate, 2005 temperatures. RealClimate (2007-12-15). Retrieved on 2007-01-17.
^ Gregory, J. M.; R. J. Stouffer, S. C. B. Raper (2002-11-15). “An Observationally Based Estimate of the Climate Sensitivity” (PDF). Journal of Climate 15 (22): 3117-21. Retrieved on 2007-01-18.
^ Climate Change 2001: Working Group I: The Scientific Basis, Part 6. Intergovernmental Panel on Climate Change (2001). Retrieved on 2007-01-18.
^ Earth System Research Laboratory, Trends in Atmospheric Carbon Dioxide. National Oceanic & Atmospheric Administration. Retrieved on 2007-01-18.
^ Earth System Research Laboratory, NOAA/ESRL Global Monitoring Division. National Oceanic & Atmospheric Administration. Retrieved on 2007-01-18.
^ Global warming – the blame is not with the plants
^ RealClimate, “Scientists baffled!”
^ Hirsch, Tim. “Plants revealed as methane source”, BBC News, 2006-01-11. Retrieved on 2007-01-18.
^ Climate Change 2001: Working Group I: The Scientific Basis, “Estimates of the global methane budget (in Tg(CH4)/yr) from different sources compared with the values adopted for this report (TAR).”
^ Physorg.com, “Level of important greenhouse gas has stopped growing”
^ Climate Change 2001: Working Group I: The Scientific Basis, 3.7.3.3 SRES scenarios and their implications for future CO2 concentration
^ OceanOutfall Community Website, Information
^ OceanOutfall Community Website, Los Angeles Times: Ocean Report
^ UNFCC, Greenhouse Gas Inventory Data
^ Sample, Ian. “Warming Hits ‘Tipping Point’”, The Guardian, 2005-08-11. Retrieved on 2007-01-18.
^ Climate Change 2001: Working Group I: The Scientific Basis, Chapter 12
^ VandeHei, Jim. “President Holds Firm As G-8 Summit Opens”, Washington Post, 2005-07-07. Retrieved on 2007-01-18.
^ Holmes, Jonathan. “John Howard Interview – Energy”, Australian Broadcasting Corporation, 2006-12-09. Retrieved on 2007-01-18.

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Naomi Oreskes, 2004 Beyond the Ivory Tower: The Scientific Consensus on Climate Change – Retrieved December 8, 2004. Also available as a 1 page PDF file Reviewed 928 refereed scientific articles identified with the keywords “global climate change” and published 1993-2003—concluded that 75% of the articles explicitly or implicitly accepted the scientific consensus. The remainder of the articles did not take any stance on recent climate change.
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RealClimate Scientists Baffled
Revkin, Andrew C (2005). “Rise in Gases Unmatched by a History in Ancient Ice”. New York Times. “Shafts of ancient ice pulled from Antarctica’s frozen depths show that for at least 650,000 years three important heat-trapping greenhouse gases never reached recent atmospheric levels caused by human activities, scientists are reporting today.” (November 25, 2005) 
Ruddiman, William F. (2001). Earth’s Climate Past and Future. New York: Princeton University Press. ISBN 0-7167-3741-8.  
Ruddiman, William F. (2005). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. New Jersey: Princeton University Press. ISBN 0-691-12164-8. 
Shaviv and Veizer (2004). “Forum: Comment”. Eos 85 (48): 510–511. 
Smith, T.M. and R.W. Reynolds, 2005: A global merged land and sea surface temperature reconstruction based on historical observations (1880–1997). J. Climate, 18, 2021–2036.
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S. K. Solanki, I. G. Usoskin, B. Kromer, M. Schüssler and J. Beer (2005). “Climate: How unusual is today’s solar activity? (Reply)”. Nature 436: E4-E5. DOI:10.1038/nature04046.
Sowers T. (2006). “Late Quaternary Atmospheric CH4 Isotope Record Suggests Marine Clathrates Are Stable”. Science 311 (5762): 838–840. DOI:10.1126/science.1121235.
Svensmark, Henrik; Jens Olaf P. Pedersen, Nigel D. Marsh, Martin B. Enghoff, and Ulrik I. Uuggerhøj (2006). “Experimental evidence for the role of ions in particle nucleation under atmospheric conditions”. Proceedings of the Royal Society A 462. DOI:10.1098/rspa.2006.1773.(online version requires registration)
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K. M. Walter, S. A. Zimov, J. P. Chanton, D. Verbyla and F. S. Chapin (2006). “Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming”. Nature 443: 71-75. DOI:10.1038/nature05040.
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