1.4 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 cycleThe solar cycle is the eleven-year cycle of solar activity of the sun. At periods of highest activity, known as solar maximum or solar max sunspots appear. Periods of lowest activity are known as solar minimum. The last solar maximum was in 2001. The solas and on century to millennial timescales because of cyclic changes in Earth's orbit (see Milankovitch_cyclesMilankovitch cycles is the name given to the collective effect of changes in the Earth's movements upon its climate. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of th ). On much longer (hundreds of millions of years) timescales, the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt is getting hotter.

1.5 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 PeriodThe Medieval Warm Period (MWP or Medieval Climate Optimum was an unusually warm period in history lasting from about the 10th century to about the 14th century. During this time wine grapes were grown in Europe up to 300 miles north of their present north) 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).

1.6 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[1].

1.7 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.

2 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 [2] [3] [4] 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.

2.1 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.

3 See also

• Climate model
• Paleoclimatology
• Committee on Climate Change Science and Technology Integration
• global warming
• global warming controversy
• Kyoto Protocol
• Copenhagen Consensus
• Sea level change
• United Kingdom Climate Change Programme
• UNFCCC

4 External links

• Global Change Master Directory: A directory of Earth science data
• GreenFacts 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
• Tyndall Centre for Climate Change Research, Norwich, UK
• Carbon Activism for Beginners
• Paleoclimateology from a 1995 Report from the American Geophysical Union
• A Brief Introduction to History of Climate, an excellent overview by Prof. Richard A Muller of UC Berkeley.

Climate change Climatology Ecology Environment

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