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Climate change

The term climate change is used to refer to changes in the Earth's climate. Generally, this is taken to mean changes in the temperature, though 'climate' encompasses many other variables (precipitation, clouds, etc). 'Climate change' includes natural and anthropogenic forcing; 'global warming' is usually used to mean changes with predominantly anthropogenic forcing.

For information on climate change over various periods, and the data sources available, see historical temperature record.

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Climate change factors

Climate changes occur due to internal and external, natural and anthropogenic factors. Internal factors are associated with the complexity of the climate system. External factors can be natural factors (such as increased solar radiation) and anthropogenic factors (due to human actions).

Internal factors

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.

Natural Factors

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. On much longer (hundreds of millions of years) timescales, the Sun is getting hotter.

It is clear that natural external factors have caused significant climate changes in the past, and it is probable that internal factors have too.

The Medieval Warm Period was caused by internal factors and natural external factors only. The subsequent Little Ice Age is usually attributed to the reduction of solar activity. Some scientists have claimed that the observed warming since 1860 is a natural climate recovery from the Little Ice Age. (Source: The Skeptical Environmentalist) but this is not supported by the temperature record (http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm).

Human Factors

Anthropogenic factors are acts by humans (Homo sapiens) that change the environment and influence the climate.

The main anthropogenic factors are the emission of greenhouse gases which are thought to contribute to the greenhouse effect, the depletion of stratospheric ozone, 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 mainly produced by cattle and by emissions of landfills. It is thought that the reduction in tropical forested area has also played a role, as old forests store large amounts of carbon. However, growing forests 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???).

Not all the CO2 emitted to the atmosphere accumulates there; half of it is absorbed by oceans and forests.

Evaluation of the relative importance of various factors

The relative importance of each of the proposed causes can only be established through the quantification of the factors involved. Internal factors and the response to external factors can be quantified 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. Anthropogenic emissions of greenhouse gases, stratospheric ozone depletion, and solar radiation have positive radiative forcing, whereas aerosols and land use changes have negative radiative forcing. (Source: IPCC)

See also: Climate model; global warming

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