• increasing atmospheric concentrations of greenhouse gases
• global changes to land surface, such as deforestation
• increasing atmospheric concentrations of aerosols. Recent reports from the Intergovernmental Panel on Climate Change (IPCC) report 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."
• "From new estimates of the combined anthropogenic forcing due to greenhouse gases, aerosols, and land surface changes, it's extremely likely that human activities have exerted a substantial net warming influence on climate since 1750.") 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 contribute to climate change primarily by interfering with atmospheric ozone concentrations. 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 byproducts (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. 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. 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

The attribution of climate change is discussed extensively, with references to peer-reviewed research, in chapter 12 of the IPCC TAR, which discusses The Meaning of Detection and Attribution, Quantitative Comparison of Observed and Modelled Climate Change, Pattern Correlation Methods and Optimal Fingerprint Methods.

An essay (External Link) in Science surveyed 928 abstracts related to climate change, and concluded that most journal reports accepted the consensus. This is discussed further in scientific opinion on climate change.

A recent paper (Estimation of natural and anthropogenic contributions to twentieth century temperature change, Tett SFB et al., JGR 2002), says that "Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming." (External Link)

Some scientists noted for their somewhat skeptical view of global warming accept that recent climate change is mostly anthropogenic. John Christy has said that he supports the American Geophysical Union (AGU) declaration, and is convinced that human activities are the major cause of the global warming that has been measured. (External Link) Some scientists do disagree with the consensus: see list of scientists opposing global warming consensus. For example Willie Soon and Richard Lindzen say that there's insufficient proof for anthropogenic attribution. Generally this position requires new physical mechanisms to explain the observed warming; for example "Climate hypersensitivity to solar forcing?", Soon W et al., 2000, Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences 18(5).

Findings that complicate attribution to CO₂

Warming sometimes leads CO₂ increases

Factors other than increased CO₂ concentrations can initiate warming or cooling episodes (see for example, orbital forcing). The ice core record shows that on some occasions temperature starts rising hundreds of years before CO₂ increases. 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.
   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.(External Link) 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.
   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 [for example,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're starting to observe the temperature response. (External Link)

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. As noted above, models that include increased CO₂ levels when simulating recent climate match the observed data far better than those that do not. Pluto, Jupiter, and Neptune's largest moon Triton. It has sometimes been asserted in the popular press that this points to a solar explanation for the recent warming on Earth. Physicist Khabibullo Abdusamatov claims that solar variation has caused global warming on Earth, and that the coincident warmings "can only be a straightline consequence of the effect of the one same factor: a long-time change in solar irradiance." This view isn't accepted by other scientists. Planetary physicist Colin Wilson responded, "His views are completely at odds with the mainstream scientific opinion," and climate scientist Amato Evan stated, "the idea just isn't supported by the theory or by the observations." Jay Pasachoff, an astronomy professor at Williams College, said that Pluto's global warming was "likely not connected with that of the Earth. The major way they could be connected is if the warming was caused by a large increase in sunlight. But the solar constant—the amount of sunlight received each second—is carefully monitored by spacecraft, and we know the Sun's output is much too steady to be changing the temperature of Pluto.".

Footnotes

Further Information

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