Australia: The Land Where Time Began

A biography of the Australian continent 

Pleistocene Climate

Fast-feedback(Charney) climate sensitivity can be assessed accurately because the composition of the atmosphere and surface properties of the Late Pleistocene are known Well enough. The Holocene, that is pre-industrial, is compared with the last glacial maximum [LGM, 20 Ka]. As the Earth was within a small fraction of 1W/m² in both the Holocene and the LGM, the Earth was in energy balance, as an imbalance of 1 W/m² that was maintained for a few thousand years would cause all the ice on the Earth to melt or change the temperature of the oceans well outside the variations that have been measured. For most of the history of the Earth the approximate equilibrium is unlike the current situation, with atmospheric concentrations of greenhouse gases rising much faster than the coupled climate system can respond to.

In the LGM equilibrium state climate forcing resulting from the surface properties of the ice age, such as increased areas of ice, a different distribution of vegetation and the amount of continental shelf exposed as dry land was -3.5 W/m² (Hansen, J., Lacis, A., Rind, D., et al.1984) compared to that of the Holocene. As the amounts of long-lived greenhouse gases were reduced, CO₂, CH₄ , and N₂O, including the effects on the tropospheric ozone and stratospheric water vapour, that were indirect, that resulted in additional forcing, was -3 ± 0.5 W/m². Slight changes in the Earth's orbit caused global forcing that was a negligible fraction of 1 W/m². The total forcing of 6.5 W/m² and change of global surface temperature of 5 ± 1^o C, when compared to the Holocene (Braconnot, P., Otto-Bliesner, B.L., Harrison, S., et al., 2007; Farrer, I., Harrison, S.P., Prentice, I.C., et al., 1999) result in an empirical sensitivity of about ¾ ± ¼° C per W/m² forcing of double CO₂. All fast feedbacks that exist in the real world, water vapour, sea ice, clouds, etc., are allowed by the empirical fast-feedback climate sensitivity to respond naturally as global climate changes.

As the Earth becomes warmer or cooler climate sensitivity varies. When colder extremes are approached the sea-ice area expands, and the Earth approaches a runaway snowball Earth condition. At high temperatures the Earth can approach a runaway greenhouse state. The Earth is on the flat portion of its fast-feedback climate sensitivity curve. The empirical sensitivity, though strictly the mean fast-feedback sensitivity for climate states that range from the ice age to the interglacial of the present, is also the fast-feedback sensitivity of the present.

Sources & Further reading

1. Hansen, James et al., 2008, Target Atmospheric CO²: Where Should Humanity Aim? The Open Atmospheric Science Journal, 2008, 2, 217-231.