The term climate change is used to refer to changes in the Earth's climate. In the most general sense, it can be taken to mean changes over all timescales and in all of the components of climate, including precipitation and clouds as well as temperature. Climate changes can be caused both by natural forces and by human activities.
However in recent usage, especially in the context of environmental policy, it refers more specifically to changes being studied in the present, including an average rise in surface temperature, or global warming. International efforts to study and address climate change are coordinated through the United Nations Framework Convention on Climate Change. Note, however, that the UNFCCC uses "climate change" for human caused change and “climate variability” for non-human caused change  (http://www.grida.no/climate/ipcc_tar/wg1/518.htm). Sometimes the term "anthropogenic climate change" is used to indicate the presumption of human influence.
For information on climate measurements over various periods, and the data sources available, see historical temperature record. For attribution of climate change over the past century, see attribution of recent climate change. For global warming episodes in the geological record, see Permian-Triassic extinction event and Paleocene_Eocene Thermal Maximum.
Climate change factors
Climate changes due to internal factors and external factors. Internal factors are those due to interactions within the earth's climate system. External factors, called climate forcings, are divided into natural factors, such as variations in solar radiation, and anthropogenic factors (those attributed to human activities).
There is general agreement among scientists (as revealed by the scientific literature) that:
- The 100,000 year ice age cycles are controlled by orbital forcing - variations in the seasonal and geographical distribution of insolation; and in the total insolation.
- The rapid temperature changes seen in ice cores during the last glacial were probably caused by events associated with the Laurentide ice sheet and thus count as "internal variability".
- The Little Ice Age was probably caused by solar variation or volcanic activity or a combination of the two.
It is known that the weather is a chaotic non-linear dynamical system. It is not clear that the climate (the average of weather) is such a system. Restricting ourselves to the last 400,000 years, the ice core record shows that the largest swings in climate are periodic, with the same periodicity as various orbital variations. These are thus non-chaotic. However, there are large short-term changes which do seem to be best explained as chaotic. Those variations do not seem to occur in the current climate state. Thus, it is possible that the climate system varies between chaotic and non-chaotic, depending on the state of the external forcing.
It is clear that natural external factors have caused significant climate changes in the past, and it is probable that internal factors have too. For example, Earth's atmosphere today contains about 370 ppm CO2 (0.037%). Over the past 600 million years, the atmospheric concentration of CO2 has varied between 6000 and 400 ppm, with values less than 300 over the last 400,000 years up to pre-industrial times  (http://www.grida.no/climate/ipcc_tar/wg1/fig3-2.htm). To see a graph of the world's average temperature and and the CO2 levels throughout the Phanerozoic, Click this Link (http://www.geocraft.com/WVFossils/PageMill_Images/image277.gif) Compared to former geologic times, our present atmosphere is significantly lower in CO2. In the last 600 million years of Earth's history only the Carboniferous Period and our present age, the Quaternary Period, have witnessed CO2 levels less than 400 ppm (see "Climate and the Carboniferous Period" (http://www.geocraft.com/WVFossils/Carboniferous_climate.html|)). See also the discussion at carbon dioxide.
Anthropogenic factors are acts by humans (Homo sapiens) that change the environment and influence the climate. The major factor is CO2 emission from fossil fuel combustion. Factors include forest alterations, and agricultural or other changes that affect the Earth's albedo, the carbon cycle, or methane production.
If a certain forcing (for example, solar variation) acts to change the climate, then there may be mechanisms which act to amplify or reduce the effects. These are called positive feedback or negative feedback. As far as is known, the climate system is stable with respect to these feedbacks: positive feedbacks do not "runaway". Part of the reason for this is the existence of a powerful negative feedback between temperature and emitted radiation, which increases as the fourth power of absolute temperature. The IPCC TAR chapter 7 discusses feedbacks in more detail  (http://www.grida.no/climate/ipcc_tar/wg1/260.htm).
An example of a positive feedback is the role of CO2 in the 100,000 year ice age cycles. The forcing for these cycles is believed to be orbital variation but the imposed forcing appears to be too small to account for the effects; thus is it generally believed that variations in CO2 act to amplify the signal. Another is the ice-albedo feedback, whereby a small reduction in (highly reflective) snow covered areas leads to the dark underlying earth absorbing more sunshive and thus heating more, melting more snow  (http://www.grida.no/climate/ipcc_tar/wg1/294.htm). Positive feedback mechanisms, due to their dramatic potential, have been popularized in mainstream media such as Al Gore's book Earth in the Balance.
Negative feedback mechanisms typically fall into two categories. The first category involves "carbon dioxide sinks", which are natural repositories which absorb carbon dioxide from the atmosphere, such as the ground, ocean, or biomass. This class of feedback mechanisms simply predicts that only some of the human emitted carbon dioxide will reside in the atmosphere, but makes no claims about the variation of temperature with relation to atmospheric carbon dioxide levels. Note that this class is not by definition negative, and may indeed be positive: for example, rising temperature may lead to loss of soil carbon; changing climate may reduce tropical forest area. The second category of negative feedback mechanisms involves the relationship between human_controlled greenhouse gases and temperature. One such theory is the Infrared Iris model, which proposes that the water vapor levels actually act as a negative feedback mechanism powerful enough to over_time stabilize increases in temperature due to carbon dioxide  (http://earthobservatory.nasa.gov/Study/Iris/iris.html) although this has been disputed  (http://www.atmos.washington.edu/~dennis/IRIS-BAMS.pdf). The third category invloves basic physics, as in the temperature_radiation feedback mentioned earlier.
For some systems _ clouds are the most obvious example _ it is not known with confidence whether they fall into the negative of positive feedback category  (http://www.grida.no/climate/ipcc-tar/wg1/271.htm).
Solar radiation variability
The main natural external factor is the variability in the amount, and geographic and temporal distribution of, solar radiation that reaches Earth. The solar radiation can change on short (yearly to century) timescales because of solar cycles and on century to millennial timescales because of cyclic changes in Earth's orbit (see Milankovitch_cycles ). On much longer (hundreds of millions of years) timescales, the Sun is getting hotter.
Examples of change due to natural factors
The 100,000 year ice age cycles are due to natural causes. Within the last 1000 years, there are two extensive periods where temperatures were relatively warmer (the Medieval Warm Period) or cooler (the Little Ice Age). Since anthropogenic forcing is believed to have been small then, it is assumed that these changes were due to natural factors. The Little Ice Age is usually attributed to the reduction of solar activity or increase in volcanoes; the causes of the MWP are even less clear.
A few scientists have claimed that the observed warming since 1860 is a natural climate recovery from the Little Ice Age. (Source: The Skeptical Environmentalist).
Anthropogenic greenhouse gases
The main anthropogenic factors are greenhouse gases, whose increased emissions add to the greenhouse effect; changes in land use; and the emission of aerosols such as sulphates. (Source: IPCC) Large amounts of anthropogenic greenhouse gases have been emitted to the atmosphere since the beginning of the industrial revolution. Since 1750: the carbon dioxide concentration has increased by 31%, methane has increased 151%, nitrous oxide has increased 17% and tropospheric ozone has increased 36%. (Source: IPCC). The majority of the anthropogenic carbon dioxide is produced by the combustion of fossil fuels. Methane is produced by cattle, energy, and rice production in similar amounts, each of which emit about 66% of the amount produced by the major natural source, wetlands (http://www.grida.no/climate/ipcc-tar/wg1/134.htm#4211).
Carbon sources and sinks
Forests which are regrowing in Russia contribute to absorbing carbon dioxide (they act as CO2 sinks), and since 1990, the amount of carbon absorbed may be larger than the amount released by deforestation (source???). Conversely, deforestation largely in tropical countries is a source of CO2 to the atmosphere. CO2 releases from deforestation are probably about 1/6 of sources from fossil fuel combustion (source???).
Not all the CO2 emitted to the atmosphere accumulates there; half of it is absorbed, presumably by oceans and forests, as a modification to the natural carbon cycle.
Evaluation of the relative importance of various factors
The relative importance of each of the proposed causes varies according to the period of interest: for example, anthropogenic factors are presumed to be negligibly small for climate change before, say, 1750. However, this has recently been challenged by Ruddiman  (http://www.nature.com/nsu/031208/031208_7.html)  (http://www.newscientist.com/news/news.jsp?id=ns99994464)  (http://mac01.eps.pitt.edu/Courses/GEOL1055/Ruddiman-2003.pdf) who claims that ancient forest clearance and rice paddies increased CO2 and methane levels starting 8000 years ago (The anthropogenic greenhouse era began thousands of years ago, Ruddiman WF; CLIMATIC CHANGE, 61 (3): 261_293 DEC 2003).
Otherwise, their importance can be established through the quantification of the factors involved. Internal factors and the response to external factors can be estimated by the analysis of climate simulations based on the best climate models.
The influence of external factors can be compared using the concept of radiative forcing. A positive radiative forcing warms the planet, and negative radiative forcing cools the planet.
- Global Change Master Directory: (http://gcmd.nasa.gov/Data/portals/gcmd/param_search/PALEOCLIMATE.html) A directory of Earth science data
- GreenFacts (http://www.greenfacts.org/studies/climate_change/index.htm) A faithful summary of the "Third Assessment Report", a leading scientific consensus report published by the Intergovernmental Panel on Climate Change (IPCC).
- Potsdam Institute for Climate Impact Research (PIK), Potsdam, Germany (http://www.pik-potsdam.de/)
- Tyndall Centre for Climate Change Research, Norwich, UK (http://www.tyndall.ac.uk/)
- Carbon Activism for Beginners (http://www.activistmagazine.com/index.php?option=content&task=view&id=120)
- Paleoclimateology from a 1995 Report from the American Geophysical Union (http://www.agu.org/revgeophys/overpe00/overpe00.html)
- A Brief Introduction to History of Climate (http://muller.lbl.gov/pages/IceAgeBook/history_of_climate.html), an excellent overview by Prof. Richard A Muller of UC Berkeley.
- Climate Change (http://www.climatechange.com.au/) The latest news and information on climate change, alternative energy, global warming and energy efficiency from around the internet
- Skeptical view about the concept of anthropogenic climate change (http://www.greeningearthsociety.org/)
- Intergovernmental Panel on Climate Change (http://www.ipcc.ch)
- Methane, a "greenhouse" gas, is 10 times more effective than carbon dioxide in causing climate warming. (http://marine.usgs.gov/fact-sheets/gas-hydrates/title.html)"Methane bound in hydrates amounts to approximately 3,000 times the volume of methane in the atmosphere. There is insufficient information to judge what geological processes might most affect the stability of hydrates in sediments and the possible release of methane into the atmosphere. Methane released as a result of landslides caused by a sea-level fall would warm the Earth, as would methane released from gas hydrates in Arctic sediments as they become warmed during a sea-level rise. This global warming might counteract cooling trends and thereby stabilize climatic fluctuation, or it could exacerbate climatic warming and thereby destabilize the climate."