Global Warming Greenhouse Fingerprinting

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

Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for relatively 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 upper atmosphere have become available. The dominant mechanisms to which recent climate change has been attributed all result from human activity. They are:

• increasing atmospheric concentrations of greenhouse gases
• global changes to land surface, such as deforestation
• increasing atmospheric concentrations of aerosols.

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

• The observed change is not consistent with natural variability.
• Known natural forcings would, if anything, be negative over this period.
• Known anthropogenic forcings are consistent with the observed response.
• The pattern of the observed change is consistent with the anthropogenic forcing.

Recent reports from the Intergovernmental Panel on Climate Change (IPCC) have concluded 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."; It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing (i.e., it is inconsistent with being the result of internal variability), and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling.
• "From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols, and land surface changes, it is extremely likely that human activities have exerted a substantial net warming influence on climate since 1750."
• "It is virtually certain that anthropogenic aerosols produce a net negative radiative forcing (cooling influence) with a greater magnitude in the Northern Hemisphere than in the Southern Hemisphere.

The panel, which represents consensus in the scientific community, defines "very likely," "extremely likely," and "virtually certain" as indicating probabilities greater than 90%, 95%, and 99%, respectively.

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.) 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), 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), 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 CO ₂ emissions over the last 250 years have resulted from burning fossil fuels, 33% have resulted from changes in land use, primarily deforestation. 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.

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. Scientists attribute more than 18% of anthropogenic greenhouse gas emissions to livestock and livestock-related activities such as deforestation and increasingly fuel-intensive farming practices. Specific attributions to the livestock sector include:

• 9% of global carbon dioxide emissions
• 35-40% of global methane emissions (chiefly due to enteric fermentation and manure)
• 64% of global nitrous oxide emissions, chiefly due to fertilizer use.

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. Key sources to which anthropogenic aerosols are attributed include:

• biomass burning such as slash and burn deforestation. Aerosols produced are primarily black carbon.
• industrial air pollution, which produces soot and airborne sulfates, nitrates, and ammonium
• dust produced by land use effects such as desertification

Attribution of 20th century climate change

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 lead to cooling—and 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". The 2007 fourth assessment report (WG1 AR4) strengthened this finding:

• "Anthropogenic warming of the climate system is widespread and can be detected in temperature observations taken at the surface, in the free atmosphere and in the oceans. Evidence of the effect of external influences, both anthropogenic and natural, on the climate system has continued to accumulate since the TAR."

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 CO ₂ , is mostly responsible for the observed warming. Evidence for this conclusion includes:

• Estimates of internal variability from climate models, and reconstructions of past temperatures, indicate that the warming is unlikely to be entirely natural.
• Climate models forced by natural factors and increased greenhouse gases and aerosols reproduce the observed global temperature changes; those forced by natural factors alone do not.
• "Fingerprint" methods indicate that the pattern of change is closer to that expected from greenhouse gas-forced change than from natural change.
• The plateau in warming from the 1940s to 1960s can be attributed largely to sulphate aerosol cooling.

In 2001, the U.S. National Academy of Sciences released a report supporting the IPCC’s conclusions regarding the causes of recent climate change. It stated, "Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes are also a reflection of natural variability."

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