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Natural Forcing - Coggle Diagram
Natural Forcing
Milankovitch Cycles
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These cyclical changes in the Earth’s circumnavigation of the sun create alterations in the seasonality of solar radiation reaching the earth’s surface and these times of increased or decreased solar radiation directly influence the earth’s climate system, impacting the advance and retreat of Earth’s glaciers.
Orbital Eccentricity
The earth’s orbit around the sun follows an elliptical path with the eccentricity of the orbit varying from near circular to markedly elliptical over a 100,000 year period. Changes distance the sun’s short wave radiation must travel to the earth
At maximum eccentricity 30% difference in solar radiation between when earth is closest to the sun (perihelion) and when earth is further from the sun (aphelion). Ice ages occur when it is at maximum orbital eccentricity.
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Milankovitch cycles help to explain the fluctuating nature of glacial and where the cycles all reach their maximums – we have inter-glacial (hotter phases) however where these cycles all reach their lowest we have glacials (cold periods).
Solar Output
Solar output varies over time at timescale which can be measured in millennia or centuries and these can contribute to climate change.
Sunspot activity can be used as a proxy for solar output. There is a positive correlation between sunspots and solar energy output.
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On longer timescales solar output is more variable – near the end of the 17th century, sunspot activity declined to almost 0 for several decades – corresponded to the severe winters in Europe we know as the ‘Little Ice Age. In the past 50 years, sunspot activity has been relatively high coinciding with warming climate.
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Volcanic Eruptions
Huge amounts of volcanic ash and sulphur dioxide are pumped into the stratosphere by volcanic eruptions and can lead to change in global climate, at least in the short term.
Sulphur dioxide has a cooling effect – it is converted to sulphuric acid which forms sulphate aerosols which reflect solar radiation back into space, lowering temperatures.
Mount Pinatubo (15th June 1991) was one of the largest eruptions of the 20th century – injected 20 million tonnes of sulphur dioxide into the stratosphere – cooled the earth’s temperature by 1.3oC over a 3 year period.
The 1815 Tambora eruption emitted 60 to 80 million tonnes of SO2 to the stratosphere
World temperature reduced by 3oC for several years
Natural greenhouse gases
Periods of icehouse Earth such as the past 3 million years correspond with low levels of CO2 in the atmosphere which reduce the Earth's natural greenhouse level
During the past 800,000 years when the Earth's climate has been dominated by numerous glacial periods CO2 levels have fluctuated between just 170 ppm and 300 ppm
Fifty million years ago CO2 levels reached 1000 ppm;average global temperatures were 10°C higher,