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 defines "climate change" as anthropogenic  (http://www.grida.no/climate/ipcc_tar/wg1/518.htm) and uses "climate variation" to mean what is used by most other sources to be "climate change". 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 kyr 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 kyr, 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.
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. Other factors include forest alterations, and agricultural or other changes that affect the Earth's albedo, the carbon cycle, or methane production. 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 North America and 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 conbustion (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. Radiative forcing
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.