Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30
Lecture 5: Recent history of the Earth climate.
We have been able to measure the concentrations of carbon dioxide and methane in the atmosphere in the past by examining ice cores drilled from glaciers. Annual layers of precipitated snow can be detected and dated in these cores. Small air bubbles that are trapped within the ice preserve a record of the composition of the atmosphere when the snow fell on the ice glacier. The longest record obtained so far was drilled from Vostok, in Antarctica, it went down to 3600 meters of depth, all the way from the top of the glacier to bedrock. The record goes back to 420000 years into the past (the scientific paper describing these findings is in Petit et al. 1999, Nature, 399, 429).
Several quantities could be obtained from this ice core, among them the carbon dioxide and methane concentration (obtained from the air bubbles), and the temperature over Antarctica. The temperature was inferred by measuring the proportion of deuterium that is in the water. Deuterium is an isotope of hydrogen (hydrogen usually has a nucleus consisting of one proton, but some hydrogen also has a proton and a neutron in the nucleus, which is deuterium). The fraction of the hydrogen in the snow that condenses from water vapor in the air that is deuterium depends on the air temperature. By measuring the fraction of deuterium at each annual layer in the glacier, we can infer how the temperature compared to the present one.
What was the climate like here in Ohio 20000 years ago? Much colder. It was the peak of the ice age!
Results from the Vostok ice core analysis:
During the ice age, carbon dioxide concentrations were much lower than during the present interglacial period. Methane concentration was also much lower during the ice age.
The most striking feature of the Vostok ice core is that we can
see four ice ages and interglacial periods that have occurred in the
past, and that the curves for carbon dioxide, methane and temperature
very much parallel each other in their variations.
Ice ages seem to occur periodically every 100000 years. At the end of an ice age, both temperature and carbon dioxide rise rapidly. Moreover, there seem to be well-defined limits to the maximum and minimum concentrations of greenhouse gases. During warm periods like the present one, carbon dioxide is usually near a maximum concentration of 270 ppmv, and methane is near 0.7 ppmv. At the coldest times during ice ages, carbon dioxide drops to 190 ppmv and methane to 0.35 ppmv.
Can we interpret that to mean that greenhouse gases cause ice ages? Although it is true that increased greenhouse gases should cause warming and therefore help end an ice age, if the greenhouse gases were really the cause for starting and ending ice ages we would expect to see that, at the end of an ice age, greenhouse gases rise first, and then ice gradually melts. It turns out that it takes thousands of years for the ice glaciers to melt after the radiative forcing increases, favoring melting. Instead, what the data show is that at the end of the ice age, greenhouse concentrations seem to had risen at the same time as ice was melting and temperatures were warming, with no significant lag.
One can also compute that the radiative forcing produced by the change in greenhouse gases from the ice age to the interglacial period accounts for only about one quarter of what is needed to explain the change in temperature over the whole planet. So something else must be helping to cause the ice ages.
Much evidence has been obtained that the ultimate cause of ice ages are subtle variations of insolation at high latitudes (where the ice glaciers can form) that are due to changes in the orbit of the Earth and its spin axis. Milankovitch was one of the first scientists to study this possibility and compute the variations of insolation that we expect, from astronomical calculations of the orbital and spin axis changes. There are three effects that cause variations in insolation which can result in climate change:
The Milankovitch theory is quite successful at explaining many recorded climate variations in the past, from monsoons near the tropics to the advance and retreat of ice glaciers at high latitudes. The theory predicts strong periodicities at the precession and tilt angle periods. These are observed in other records of ice ages measured from the abundance of oxygen isotopes in sea-floor sediments, which show that from 2.5 to 0.9 million years ago ice ages were varying with the periods of 21500 and 41000 years. You can also see in the Vostok ice core some variations with a period near 22000 years, particularly for methane, which are driven by precession. However, after 0.9 million years and until the present, a much stronger amplitude of variation in the ice ages has been at the period of 100000 years. This periodicity is not very well explained: the insolation variations caused by eccentricity changes are too small to account for it. But eccentricity also makes the precession variations to increase in amplitude, and it has been suggested that the end of an ice age is caused by particularly strong peaks of northern summer insolation which occur when the eccentricy is high, which might explain the 100000 year periodicity.
Even though a lot of the observed periodicity in ice ages can be explained by variations in insolation, we also know that the global climate of the planet cooled by as much as 4 degrees Celsius during the peak of the ice age compared to the present temperature. Even the tropics were colder than today. When averaged over the entire planet, there are no variations of insolation (the changes in the North and South cancel out), except from those due to eccentricity variations which are very small.
So the global climate change in the ice ages must have other causes as well. These are related to feedback effect. A positive feedback effect is a change in the climate that tends to magnify its own cause. Examples:
A clear answer has not yet emerged to this question. However, there are certain things that can be said about the answer if we remember some things about the carbon cycle from the previous lecture.
During the ice age, the carbon reservoir of the atmosphere is reduced. The carbon reservoir of the surface ocean is in equilibrium with the atmosphere and is also reduced. There is less vegetation in land because of the land surface covered by glaciers, and a drier climate (colder global climate means less evaporation from the ocean globally which means less precipitation). So the carbon reservoir of vegetation and soil is also reduced. So where does all this carbon go during the ice age?
Most scientists think it is probably in the deep ocean, which is the only big reservoir that can rapidly exchange carbon with the system of the atmosphere-vegetation-soil-surface ocean. It is known that a colder ocean holds more carbon dioxide when in equilibrium with the atmosphere, but this explains only a small fraction of the reduction of the atmospheric carbon dioxide in the ice ages. An explanation that has been proposed is that increased ocean circulation brings an increased supply of nutrients (nitrogen, phosphorus and iron compounds) for plankton in the sea surface, and this causes a larger number of marine animals to be dropping to the ocean floor when they die, increasing the flux of carbon to the deep ocean.