The sequence of ice ages followed by warm interglacials has been the dominant force in creating, extinguishing and changing nature and life on Earth. For the past two million years, there have been many cycles of ice ages followed by short warming periods, typically of 40,000 to 120,000 years. One of the findings revealed by the analysis of Antarctic ice cores is that every glacial maximum has been characterized by the same low atmospheric CO2 concentration and low surface temperature. The repeated occurrence of the same low CO2 concentration and surface temperature prior to deglaciations is suggestive of a feedback mechanism that kicks-in whenever climactic conditions approach a threshold. Extensive research has shown that photosynthesis is inhibited by low atmospheric CO2. A recent study argues that a biological feedback mechanism may be responsible for the rapid increase in atmospheric CO2 and temperature leading to deglaciation at the end of ice ages.
There is evidence that atmospheric CO2 concentration has never dropped below 190 parts per million (ppm). Analyses of ice cores have revealed that a minimum of 190 ppm was reached at the end of eight ice ages over the past 800,000 years. Although atmospheric CO2 concentration has varied widely, even reaching more than 1,000 parts per million (ppm), there is little evidence for values lower than 190 ppm. Even during the last prolonged ‘icehouse’ of the Carboniferous period (360 to 300 million years ago) the minima that the CO2 concentrations repeatedly reached during Carboniferous glaciations are identical to the minima recorded in ice cores reflecting the past 800,000 years.
Analysis of ice cores has shown that surface temperature as measured by a temperature proxy called deuterium depletion or delta-D in ice cores has reached the same minimum (?440 ‰) multiple times over the past 800,000 years. A well-researched aspect of the Earth’s glacial/deglacial cycles is the strong correlation over millennia between atmospheric CO2 concentration and surface temperature and the ice core record shows that the CO2 and surface temperature minima coincide.
Biological feedback mechanisms and the end of ice ages
There is strong evidence that orbital forcing, small variations in the Earth’s orbit about the Sun called Milankovitch cycles, are implicated in the glacial/deglacial cycles but are not sufficient by themselves to explain the glacial/deglacial cycles. The recurrence of the same CO2 and surface temperature minima at glacial maxima suggests that there is a feedback mechanism operating on the scale of millennia that prevents atmospheric CO2 level and temperature from dropping below a threshold.
A possible biological feedback mechanism results from the dependence of photosynthetic organisms on atmospheric CO2. Carbon fixation by photosynthesis in land plants is inhibited by a low concentration of CO2 in the atmosphere. Studies have shown that within a single generation of exposure to low CO2, modern plants that rely on photosynthesis show an average reduction in photosynthesis of 50% when grown at low (180–220 ppm) vs current (350–380 ppm) CO2 concentrations. When CO2 is reduced even lower to 150 ppm, research has shown that biomass production may be reduced by over 90%. When applied to the entire Earth ecosystem, these physiological responses imply large reductions in Net Primary Productivity or NPP (the net carbon uptake by plants after accounting for plant respiration) and carbon storage during glacial periods.
Several possible biological mechanisms have been suggested. Two that have been proposed recently are discussed below.
Reduced nitrogen fixing in the ocean
One possibility is that low CO2 slows down the oceanic soft-tissue pump thus reversing the process by which the oceans remove up to 90% of CO2 from the atmosphere. In the oceans, where over 90% of CO2 is accumulated, CO2 is fixed by photo-synthesis in surface waters creating organic material. Ultimately the organic material produced in surface waters sinks and ends up as sediments on the ocean floor. Biologically available nitrogen is the primary limiting nutrient for the production of organic material by the ocean ‘soft tissue’ carbon pump.
Laboratory experiments have found that marine nitrogen fixing bacteria (cyanobacteria) are especially sensitive to low CO2, with growth rates strongly decreased when CO2 concentration drops below 200 ppm. Marine sediment records have found that nitrogen fixation rates were much slower during the last ice age than during the present warm period. If nitrogen fixation had been progressively handicapped as CO2 fell because nitrogen fixers were having a hard time growing, it could have inhibited the soft tissue pump, shifting carbon from the ocean back to the atmosphere. In support of this mechanism, at the end of the last ice age, there is evidence of CO2 degassing from the Southern Ocean.
Reduced photosynthesis causing dust and reduced albedo
Another mechanism that has recently been suggested is a feedback system involving low CO2 and temperature resulting in increased atmospheric dust and greater absorption of solar radiation. During the initial glacial period, the high reflectivity of the northern ice sheets reflects most of the solar radiation resulting in cooling. As the oceans and atmosphere cool, more atmospheric CO2 is absorbed by the oceans. Atmospheric CO2 concentrations eventually reach a critical minimum of about 190 ppm, which combined with cool arid conditions, cause a die-back of temperate and boreal forests and grasslands, especially at high latitudes. The ensuing soil erosion generates dust storms, resulting in increased dust deposition on the northern ice sheets and greater absorption of solar radiation. As northern hemisphere solar radiation increases during the next Milankovitch cycle, the dust-laden ice-sheets absorb more solar radiation and undergo rapid melting, which forces the climate into an interglacial period. In support of this mechanism, Antarctic ice cores provide evidence of increasing atmospheric dust at the end of all ice ages over the past 800,000 years.
It is known that orbital forcing is involved in glacial/deglacial cycles but is insufficient by itself to explain the glacial/deglacial cycles. Some other mechanism is required to explain when and how deglaciations occur. The ice core record reveals that glacial maxima are characterized by the same recurring atmospheric CO2 and surface temperature minima. This study argues that a biological feedback mechanism kicks in when a threshold is reached and together with orbital forcing results in the sustained and rapidly increasing CO2 and surface temperature of a deglaciation. Possible mechanisms are based on the observation that low CO2 inhibits photosynthesis on land and in the oceans. Reduced biomass could result in increased dust, reduced reflection of solar radiation by dust-laden ice sheets, and the reversal of the oceanic soft-tissue pump resulting in CO2 degassing from the oceans.