The Sun and Global Warming
Of the many trends that appear to cause fluctuations in the Sun’s energy, those that last decades to centuries are the most likely to have a measurable impact on the Earth’s climate in the foreseeable future. Many researchers believe the steady rise in sunspots and faculae since the late seventeenth century may be responsible for as much as half of the 0.6o of global warming over the last 110 years (IPCC, 2001).
The effects of orbital mechanics on global warming - the Milankovic cycle
The variation of insolation due to orbital mechanics was solved by Milankovich in the 1930's.
There are three relevant cycles and four factors that contribute: precession of the orbit, meaning that the location of the perihelion (point of closest approach of the Earth to the Sun) rotates around the Sun; precession of the Earth's spin axis relative to the orbital axis (like a top that is spinning with its axis of rotation at a tilt); change in eccentricity of the orbit between a perfect circle and an ellipse that is slightly different from a circle; and variation of the tilt of the Earth's axis between about 23o and 24.5o.
Thecombination of the variation of the orbit's eccentricity and the Earth's spin-axis precession creates climate change on about a twenty-thousand-year cycle. The oscillation of the spin-axis relative to the orbital plane between 23o and 24.5o has a 40,000 year cycle. The eccenticity change by itself causes small variations in the insolation with a cycle of about 100,000 years, but its effect is only about 0.1 % of the others. These factors are known as Milankovich forcing.
These cycles were correlated with the the timing of the ice ages and hypothesized to actually cause them. Currently, these variations should have been cooling the Earth's mean temperature over the past 150 years, but the global mean temperature has in fact been rising.
Relationship between CO2 emissions and rise in temperature
The relationship between CO2 emissions and rise in temperature, one tonne (one tonne = 1000 kilograms = 2200 lb = one long ton) of added carbon will cause 1.5 x 10-12 degree rise in temperature, is based on an article in Nature. In the USA, one tonne of carbon is emitted per person annually. Assuming that worldwide emissions per person are one tenth of a tonne per person, then for just the additional carbon emitted each year and assuming that this rate is steady (It is not, it is increasing), then each year the temperature rise will be 9 x 10-4 oC. However, this also assumes that the temperature has reached equilibrium with the current level of CO2 in the atmosphere, but it hasn't.
This chart shows the normal lag of CO2 concentration to temperature rise and that the relationship has reversed in the present, i.e., the CO2 concentration has continued to rise even as the temperature has been relatively stable. Based on the past record from the ice cores, one would expect that the temperature would be dropping with the CO2 concentration either stable or decreasing as well:
When the The Milankovich cycles cause an increase in solar energy striking the Earth, this will initiate a melting of some of the glaciers and at least one of the polar ice caps.(depending on the season). As the article states, 18,000 years ago, this increase in the intensity of the sunlight occurred during the spring in the Southern Hemisphere. This led to the Antarctic ice cap melting more than it had been. This decrease in the area of ice decreased the amount of sunlight reflected and a corresponding increase in the amount of sunlight absorbed. This initiated a positive feedback loop where reduced reflection and ocean warming lead to more temperature rise than would be expected from Milankovitch forcing. Currently, if Milankovitch forcing were the primary factor, global temperature should be falling.
As the temperature of the ocean increases, it can hold less carbon dioxide, so the carbon dioxide level in the atmosphere increases. This increased CO2 level creates a second positive feedback loop that also causes more warming than can be explained by Milankovitch forcing, oceanic warming, and reduced reflection alone. The lag in CO2 concentration from temperature rise has normally been about 600 +/- 400 years for the last 450,000 years based on several different sources (primarily ice cores). For about the last one hundred years, the rate of increase of CO2 has been leading the rise in temperature.
Another point is that with the melting of the permafrost, the risk of large methane releases increases. It isn't a matter of "if" but "when" if current trends continue. Methane has 25 times the warming effect of carbon dioxide. This is being studied in Alaska where new shallow summer lakes are forming on top of the permafrost and permafrost is meting more deeply than in recent history and increasing amounts of methane are being released. It is uncertain where the threshold is for large releases to occur.
For a more detailed explanation and the history of the theory that links CO2 to temperature rise, see http://www.aip.org/history/climate/co2.htm.
Of the many trends that appear to cause fluctuations in the Sun’s energy, those that last decades to centuries are the most likely to have a measurable impact on the Earth’s climate in the foreseeable future. Many researchers believe the steady rise in sunspots and faculae since the late seventeenth century may be responsible for as much as half of the 0.6o of global warming over the last 110 years (IPCC, 2001).The effects of orbital mechanics on global warming - the Milankovic cycleThe variation of insolation due to orbital mechanics was solved by Milankovich in the 1930's.
There are three relevant cycles and four factors that contribute: precession of the orbit, meaning that the location of the perihelion (point of closest approach of the Earth to the Sun) rotates around the Sun; precession of the Earth's spin axis relative to the orbital axis (like a top that is spinning with its axis of rotation at a tilt); change in eccentricity of the orbit between a perfect circle and an ellipse that is slightly different from a circle; and variation of the tilt of the Earth's axis between about 23o and 24.5o.
The
combination of the variation of the orbit's eccentricity and the Earth's spin-axis precession creates climate change on about a twenty-thousand-year cycle. The oscillation of the spin-axis relative to the orbital plane between 23o and 24.5o has a 40,000 year cycle. The eccenticity change by itself causes small variations in the insolation with a cycle of about 100,000 years, but its effect is only about 0.1 % of the others. These factors are known as Milankovich forcing.These cycles were correlated with the the timing of the ice ages and hypothesized to actually cause them. Currently, these variations should have been cooling the Earth's mean temperature over the past 150 years, but the global mean temperature has in fact been rising.
Relationship between CO2 emissions and rise in temperature
The relationship between CO2 emissions and rise in temperature, one tonne (one tonne = 1000 kilograms = 2200 lb = one long ton) of added carbon will cause 1.5 x 10-12 degree rise in temperature, is based on an article in Nature. In the USA, one tonne of carbon is emitted per person annually. Assuming that worldwide emissions per person are one tenth of a tonne per person, then for just the additional carbon emitted each year and assuming that this rate is steady (It is not, it is increasing), then each year the temperature rise will be 9 x 10-4 oC. However, this also assumes that the temperature has reached equilibrium with the current level of CO2 in the atmosphere, but it hasn't.This chart shows the normal lag of CO2 concentration to temperature rise and that the relationship has reversed in the present, i.e., the CO2 concentration has continued to rise even as the temperature has been relatively stable. Based on the past record from the ice cores, one would expect that the temperature would be dropping with the CO2 concentration either stable or decreasing as well:
When the The Milankovich cycles cause an increase in solar energy striking the Earth, this will initiate a melting of some of the glaciers and at least one of the polar ice caps.(depending on the season). As the article states, 18,000 years ago, this increase in the intensity of the sunlight occurred during the spring in the Southern Hemisphere. This led to the Antarctic ice cap melting more than it had been. This decrease in the area of ice decreased the amount of sunlight reflected and a corresponding increase in the amount of sunlight absorbed. This initiated a positive feedback loop where reduced reflection and ocean warming lead to more temperature rise than would be expected from Milankovitch forcing. Currently, if Milankovitch forcing were the primary factor, global temperature should be falling.
As the temperature of the ocean increases, it can hold less carbon dioxide, so the carbon dioxide level in the atmosphere increases. This increased CO2 level creates a second positive feedback loop that also causes more warming than can be explained by Milankovitch forcing, oceanic warming, and reduced reflection alone. The lag in CO2 concentration from temperature rise has normally been about 600 +/- 400 years for the last 450,000 years based on several different sources (primarily ice cores). For about the last one hundred years, the rate of increase of CO2 has been leading the rise in temperature.
Another point is that with the melting of the permafrost, the risk of large methane releases increases. It isn't a matter of "if" but "when" if current trends continue. Methane has 25 times the warming effect of carbon dioxide. This is being studied in Alaska where new shallow summer lakes are forming on top of the permafrost and permafrost is meting more deeply than in recent history and increasing amounts of methane are being released. It is uncertain where the threshold is for large releases to occur.For a more detailed explanation and the history of the theory that links CO2 to temperature rise, see http://www.aip.org/history/climate/co2.htm.
