Attribution of recent climate change

Further information: Global warming, Climate change, and Climate change denial

Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent changes observed in the Earth's climate. The effort has focused on changes observed during the period of instrumental temperature record, when records are most reliable; particularly on the last 50 years, when human activity has grown fastest and observations of the troposphere have become available. The dominant mechanisms (to which recent climate change has been attributed) are the result of human activity. They are:[1]

There are also natural mechanisms for variation including climate oscillations, changes in solar activity, variations in the Earth's orbit, and volcanic activity.

Attribution of recent change to anthropogenic forcing is based on the following facts:

Recent reports from the Intergovernmental Panel on Climate Change (IPCC) have concluded that:

The panel defines "very likely," "extremely likely," and "virtually certain" as indicating probabilities greater than 90%, 95%, and 99%, respectively.[1] The IPCC's attribution of recent global warming to human activities is a view shared by most scientists,[4][5]:2 and is also supported by a number of scientific organizations (see scientific opinion on climate change).

Contents

Background

Main article: climate change#Causes

This section introduces some concepts in climate science that are used in the following sections:

Factors affecting Earth's climate can be broken down into feedbacks and forcings.[5]:7 A forcing is something that is imposed externally on the climate system. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output.[6] Human activities can also impose forcings, for example, through changing the composition of the atmosphere.

Radiative forcing is a measure of how various factors alter the energy balance of the Earth's atmosphere.[7] A positive radiative forcing will tend to increase the energy of the Earth-atmosphere system, leading to a warming of the system. Between the start of the Industrial Revolution in 1750, and the year 2005, the increase in the atmospheric concentration of carbon dioxide (chemical formula: CO2) lead to a positive radiative forcing, averaged over the Earth's surface area, of about 1.66 watts per square metre (abbreviated W m-2). [8]

Climate feedbacks can either amplify or dampen the response of the climate to a given forcing.[5]:7 There are many feedback mechanisms in the climate system that can either amplify (a positive feedback) or diminish (a negative feedback) the effects of a change in climate forcing.

Aspects of the climate system will show variation in response to changes in forcings.[9] In the absence of forcings imposed on it, the climate system will still show internal variability. This internal variability is a result of complex interactions between components of the climate system, such as the coupling between the atmosphere and ocean.[10] An example of internal variability is the El Niño-Southern Oscillation.

Key attributions

Greenhouse gases

Scientific consensus has identified carbon dioxide as the dominant greenhouse gas forcing. (The dominant greenhouse gas overall is water vapor. Water vapor, however, has a very short atmospheric lifetime (about 10 days) and is very nearly in a dynamic equilibrium in the atmosphere, so it is not a forcing gas in the context of global warming.[11]) Methane and nitrous oxide are also major forcing contributors to the greenhouse effect. The Kyoto Protocol lists these together with hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6),[12] which are entirely artificial (i.e. anthropogenic) gases which also contribute to radiative forcing in the atmosphere. The chart at right attributes anthropogenic greenhouse gas emissions to eight main economic sectors, of which the largest contributors are power stations (many of which burn coal or other fossil fuels), industrial processes (among which cement production is a dominant contributor[13]), transportation fuels (generally fossil fuels), and agricultural by-products (mainly methane from enteric fermentation and nitrous oxide from fertilizer use).

Land use

Climate change is attributed to land use for two main reasons. While 66% of anthropogenic CO2 emissions over the last 250 years have resulted from burning fossil fuels, 33% have resulted from changes in land use, primarily deforestation.[13] Deforestation both reduces the amount of carbon dioxide absorbed by deforested regions and releases greenhouse gases directly, together with aerosols, through biomass burning that frequently accompanies it. A second reason that climate change has been attributed to land use is that the terrestrial albedo is often altered by use, which leads to radiative forcing. This effect is more significant locally than globally.[13]

Livestock and land use

Worldwide, livestock production occupies 70% of all land used for agriculture, or 30% of the ice-free land surface of the Earth.[14] More than 18% of anthropogenic greenhouse gas emissions are attributed to livestock and livestock-related activities such as deforestation and increasingly fuel-intensive farming practices.[14] Specific attributions to the livestock sector include:

Aerosols

With virtual certainty, scientific consensus has attributed various forms of climate change, chiefly cooling effects, to aerosols, which are small particles or droplets suspended in the atmosphere.[15] Key sources to which anthropogenic aerosols are attributed[16] include:

Attribution of 20th century climate change

IPCC
Assessment reports:
First (1990)
1992 sup.
Second (1995)
Third (2001)
Fourth (2007)
Fifth (2014)
UNFCCC | WMO | UNEP

Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. This has led to increases in mean global temperature, or global warming. Other human effects are relevant—for example, sulphate aerosols are believed to have a cooling effect. Natural factors also contribute. According to the historical temperature record of the last century, the Earth's near-surface air temperature has risen around 0.74 ± 0.18 °Celsius (1.3 ± 0.32 °Fahrenheit).

A historically important question in climate change research has regarded the relative importance of human activity and non-anthropogenic causes during the period of instrumental record. In the 1995 Second Assessment Report (SAR), the IPCC made the widely quoted statement that "The balance of evidence suggests a discernible human influence on global climate". The phrase "balance of evidence" suggested the (English) common-law standard of proof required in civil as opposed to criminal courts: not as high as "beyond reasonable doubt". In 2001 the Third Assessment Report (TAR) refined this, saying "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities".[17] The 2007 Fourth Assessment Report (AR4) strengthened this finding:

Over the past five decades there has been a global warming of approximately 0.65 °C (1.17 °F) at the Earth's surface (see historical temperature record). Among the possible factors that could produce changes in global mean temperature are internal variability of the climate system, external forcing, an increase in concentration of greenhouse gases, or any combination of these. Current studies indicate that the increase in greenhouse gases, most notably CO2, is mostly responsible for the observed warming. Evidence for this conclusion includes:

Detection vs. attribution

Detection and attribution of climate signals, as well as its common-sense meaning, has a more precise definition within the climate change literature, as expressed by the IPCC.[21]

Detection of a signal requires demonstrating that an observed change is statistically significantly different from that which can be explained by natural internal variability.

Attribution requires demonstrating that a signal is:

Detection does not imply attribution, and is easier to show than attribution. Unequivocal attribution would require controlled experiments with multiple copies of the climate system, which is not possible. Therefore, attribution, as described above, can only be done within some margin of error. For example, the IPCC's Fourth Assessment Report says "it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750," where "extremely likely" indicates a probability greater than 95%.[1]

Following the publication of the Third Assessment Report (TAR) in 2001, "detection and attribution" of climate change has remained an active area of research. Some important results include:

Scientific literature and opinion

Main article: Scientific opinion on climate change

There are a number of examples of published and informal support for the consensus view. The IPCC's Assessment Reports have informed the United Nations Framework Convention on Climate Change,[27] and each assessment has considered the role of human activities on the climate.[28] The IPCC's First Assessment Report, published in 1990, was unable to distinguish a human influence on the climate. The later Second Assessment Report, published in 1996, concluded that human influences had had a "discernable" effect on the climate. More evidence of human-induced climate change was found in the third and fourth Assessment Reports, published in 2001 and 2007, respectively.

The IPCC's conclusions are consistent with those of several reports produced by the US National Research Council.[4][29][30] A report published in 2009 by the U.S. Global Change Research Program concluded that "[global] warming is unequivocal and primarily human-induced."[31] A number of scientific organizations have issued statements that support the consensus view. Two examples include:

Individual papers

A small minority of scientists do disagree with the consensus: see list of scientists opposing global warming consensus. For example Willie Soon and Richard Lindzen[37] say that there is insufficient proof for anthropogenic attribution. Generally this position requires new physical mechanisms to explain the observed warming.[38]

Difficulties in attribution

At the time of the IPCC Fourth Assessment Report, attribution was possible for a number of observed changes in the climate (see effects of global warming). However, attribution was found to be more difficult when assessing changes over smaller regions (less than continental scale) and over short time periods (less than 50 years).[39] Over larger regions, averaging reduces natural variability of the climate, making detection and attribution easier.

Solar activity

The role of the sun in recent climate change has been looked at by climate scientists. Since 1978, output from the Sun has been precisely measured by satellites.[5]:6 These measurements indicate that the Sun's output has not increased since 1978, so the warming during the past 30 years cannot be attributed to an increase in solar energy reaching the Earth (see the graph opposite). In the three decades since 1978, the combination of solar and volcanic activity probably had a slight cooling influence on the climate.[40]

Climate models have been used to examine the role of the sun in recent climate change.[41] Models are unable to reproduce the rapid warming observed in recent decades when they only take into account variations in solar output and volcanic activity. Models are, however, able to simulate the observed 20th century changes in temperature when they include all of the most important external forcings, including human influences and natural forcings. As has already been stated, Hegerl et al. (2007) concluded that greenhouse gas forcing had "very likely" caused most of the observed global warming since the mid-20th century. In making this conclusion, Hegerl et al. (2007) allowed for the possibility that climate models had been underestimated the effect of solar forcing.[42]

The role of solar activity in climate change has also been calculated over longer time periods using "proxy" datasets, such as tree rings.[43] Models indicate that solar and volcanic forcings can explain periods of relative warmth and cold between A.D. 1000 and 1900, but human-induced forcings are needed to reproduce the late-20th century warming.[44]

Another line of evidence against the sun having caused recent climate change comes from looking at how temperatures at different levels in the Earth's atmosphere have changed.[45] Models and observations show that greenhouse warming results in warming of the lower atmosphere (called the troposphere) but cooling of the upper atmosphere (called the stratosphere).[46] Depletion of the ozone layer by chemical refrigerants has also resulted in a strong cooling effect in the stratosphere. If the sun was responsible for observed warming, warming of both the troposphere and stratosphere would be expected.[47]

Non-consensus views

See also: extraterrestrial atmospheres

Habibullo Abdussamatov (2004), head of space research at St. Petersburg's Pulkovo Astronomical Observatory in Russia, has argued that the sun is responsible for recently observed climate change.[48] Journalists for news sources canada.com (Solomon, 2007b),[49] National Geographic News (Ravillious, 2007),[50] and LiveScience (Than, 2007)[51] reported on the story of warming on Mars. In these articles, Abdussamatov was quoted. He stated that warming on Mars was evidence that global warming on Earth was being caused by changes in the sun.

Ravillious (2007)[50] quoted two scientists who disagreed with Abdussamatov: Amato Evan, a climate scientist at the University of Wisconsin-Madison, in the US, and Colin Wilson, a planetary physicist at Oxford University in the UK. According to Wilson, "Wobbles in the orbit of Mars are the main cause of its climate change in the current era" (see also orbital forcing).[52] Than (2007) quoted Charles Long, a climate physicist at Pacific Northwest National Laboratories in the US, who disagreed with Abdussamatov.[51]

Than (2007) pointed to the view of Benny Peiser, a social anthropologist at Liverpool John Moores University in the UK.[51]

In his newsletter, Peiser had cited a blog which had commented on warming observed on several planetary bodies in the Solar system. These included Neptune's moon Triton,[53] Jupiter,[54] Pluto[55] and Mars. In an e-mail interview with Than (2007), Peiser stated that:

“I think it is an intriguing coincidence that warming trends have been observed on a number of very diverse planetary bodies in our solar system, (...) Perhaps this is just a fluke.”

Than (2007) provided alternative explanations of why warming had occurred on Triton, Pluto, Jupiter and Mars.

The US Environmental Protection Agency (US EPA, 2009) responded to public comments on climate change attribution.[56] A number of commenters had argued that recent climate change could be attributed to changes in solar irradiance. According to the US EPA (2009), this attribution was not supported by the bulk of the scientific literature. Citing the work of the IPCC (2007), the US EPA pointed to the low contribution of solar irradiance to radiative forcing since the start of the Industrial Revolution in 1750. Over this time period (1750 to 2005),[57] the estimated contribution of solar irradiance to radiative forcing was 5% the value of the combined radiative forcing due to increases in the atmospheric concentrations of carbon dioxide, methane and nitrous oxide.

Earlier climate changes

Factors other than increased CO2 concentrations can initiate warming or cooling episodes (see, e.g., orbital forcing). The ice core record shows that on some occasions temperature starts rising hundreds of years before CO2 increases.[58][59] Such results confirm that the relationship between CO2 and climate can go in both directions: changes in CO2 concentrations affect climate, while changes in climate can affect CO2 concentrations. One proposed mechanism for this effect is increased release of sequestered CO2 from oceans as circulation patterns shift, perhaps abruptly, in response to climate change.[60][61]

A more speculative and polemical inference sometimes drawn is that the causal relationship between temperature rises and global CO2 concentrations is only one-way, so that historical increases in CO2 have been nothing more than the product of independently rising temperatures.[62] However, a strictly "one-way" view of the relationship between CO2 and temperature contradicts basic results in physics, specifically the fact that the absorption and emission of infrared radiation by CO2 increases as its atmospheric concentration increases.[63][64]

First principles as well as empirical observation suggest that positive feedbacks from CO2 concentrations amplify warming initially caused by other factors:

Close analysis of the relationship between the two curves [i.e., temperature and CO2] shows that, within the uncertainties of matching their timescales, the temperature led by a few centuries. This is expected, since it was changes in the Earth’s orbital parameters (including the shape of its orbit around the Sun, and the tilt of Earth’s axis) that caused the small initial temperature rise. This then raised atmospheric CO2 levels, in part by outgassing from the oceans, causing the temperature to rise further. By amplifying each other’s response, this "positive feedback" can turn a small initial perturbation into a large climate change. There is therefore no surprise that the temperature and CO2 rose in parallel, with the temperature initially in advance. In the current case, the situation is different, because human actions are raising the CO2 level, and we are starting to observe the temperature response.[65]

Present CO2 levels greatly exceed the range found in the ice core data. Isotopic analysis of atmospheric CO2 confirms that fossil fuel burning is the source of most of the CO2 increase, unlike during prior interglacial periods.[66] As noted above, models that include increased CO2 levels when simulating recent climate match the observed data far better than those that do not.[67]

See also

Global warming portal
Energy portal


Notes

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  2. ^ IPCC, Summary for Policymakers, Section ?? in IPCC AR4 WG1 2007.
  3. ^ Hegerl et al., Chapter 9, Section ?? in IPCC AR4 WG1 2007.
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  6. ^ Le Treut et al., Chapter 1: Historical Overview of Climate Change Science, FAQ 1.1, What Factors Determine Earth’s Climate?, in IPCC AR4 WG1 2007.
  7. ^ Forster et al., Chapter 2: Changes in Atmospheric Constituents and Radiative Forcing, FAQ 2.1, How do Human Activities Contribute to Climate Change and How do They Compare with Natural Influences? in IPCC AR4 WG1 2007.
  8. ^ IPCC, Summary for Policymakers, Human and Natural Drivers of Climate Change, Figure SPM.2, in IPCC AR4 WG1 2007.
  9. ^ Committee on the Science of Climate Change, US National Research Council (2001). "2. Natural Climatic Variations". Climate Change Science: An Analysis of Some Key Questions. Washington, D.C., USA: National Academy Press. p. 8. ISBN 0-309-07574-2. http://www.nap.edu/openbook.php?record_id=10139&page=8. Retrieved 2011-05-20. 
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  14. ^ a b c Food and Agriculture Organization of the United Nations. Henning Steinfeld .... (2006) (PDF). Livestock's Long Shadow. Food and Agricultural Organization of the U.N.. ISBN 9251055718. http://www.virtualcentre.org/en/library/key_pub/longshad/A0701E00.pdf. 
  15. ^  ?? in IPCC AR4 WG1 1007.
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  17. ^ a b IPCC, Summary for Policymakers. in IPCC TAR WG1 2001.
  18. ^ Solomon et al., Technical Summary, p. ?. in IPCC AR4 WG1 2007.
  19. ^ Mitchell et al., Chapter 12: Detection of Climate Change and Attribution of Causes Section 12.4.3, Optimal Fingerprint Methods, in IPCC TAR WG1 2001.
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  27. ^ Houghton 2002.
  28. ^ Cubasch et al., Chapter 9: Projections of Future Climate Change, Section 9.1.3: The Basis from which We Begin, in IPCC AR4 WG1 2007.
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  31. ^ Karl & others 2009, Excutive Summary, p. 12.
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  40. ^ Global Climate Change, in Karl & others 2009, pp. 15-16.
  41. ^ Hegerl, et al., Chapter 9: Understanding and Attributing Climate Change, Frequently Asked Question 9.2: Can the Warming of the 20th century be Explained by Natural Variability?, in IPCC AR4 WG1 2007.
  42. ^ Hegerl, et al., Chapter 9: Understanding and Attributing Climate Change, Executive Summary, in IPCC AR4 WG1 2007.
  43. ^ Simmon, R. and D. Herring (November 2009). "Notes for slide number 5 titled "Over 100 years of total solar irradiance data," in presentation, "Human contributions to global climate change"". Presentation library on the U.S. National Oceanic and Atmospheric Administration's Climate Services website. http://www.climate.gov/#understandingClimate/presentationLibrary. Retrieved 2011-06-23. 
  44. ^ Committee on Surface Temperature Reconstructions for the Last 2,000 Years, US National Research Council (2006). "10. Climate Forcings and Climate Models". Surface Temperature Reconstructions for the Last 2,000 Years. Washington, D.C., USA: National Academy Press. p. 109. ISBN 0-309-66144-7. http://books.nap.edu/openbook.php?record_id=11676&page=109. Retrieved 2011-06-23. 
  45. ^ Simmon, R. and D. Herring (November 2009). "Notes for slide number 7, titled "Satellite evidence also suggests greenhouse gas warming," in presentation, "Human contributions to global climate change"". Presentation library on the U.S. National Oceanic and Atmospheric Administration's Climate Services website. http://www.climate.gov/#understandingClimate/presentationLibrary. Retrieved 2011-06-23. 
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  47. ^ Karl & others 2009, p. 20.
  48. ^ Abdussamatov, Habibullo I. (June 2004). "About the long-term coordinated variations of the activity, radius, total irradiance of the Sun and the Earth's climate". Proceedings of the International Astronomical Union 223: 541–542. doi:10.1017/S1743921304006775. http://journals.cambridge.org/action/displayFulltext?type=6&fid=395857&jid=IAU&volumeId=2004&issueId=IAUS223&aid=288608&bodyId=&membershipNumber=&societyETOCSession=&fulltextType=RA&fileId=S1743921304006775.  "The main cause of climate change during the last millennia is the corresponding cyclic variation of the 80- and 200-year component of irradiance correlated with activity. That is why, the contemporary (climate change) is not anomalous but is ordinary secular global warming."
  49. ^ Solomon, Lawrence (2 February 2007b). "Look to Mars for the truth on global warming". National Post. http://www.canada.com/nationalpost/story.html?id=edae9952-3c3e-47ba-913f-7359a5c7f723&k=0. Retrieved 2007-03-02. 
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  67. ^ IPCC AR4 WG1 2007

References

Further reading

External links