Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change - Slow feedbacks

When the authoors1 averaged climate change over several thousand years, a period of time long enough to make sure that the energy balance was assured and the effects of ocean thermal response time and climate change leads and lags between the Northern and Southern Hemisphere [22] was minmised. The variations in temperature are global at such temporal resolution, with amplification at high latitudes being present in polar ice cores and sediment cores from the ocean that indicated ocean surface temperature.

The mechanisms that cause large global climate changes are changes in the greenhouse gas and surface albedo, though they do not initiate these swings of climate. Temperature change occurs several hundred years after there are changes in the greenhouse gas and sea level (a measure of ice sheet size).

Changes of greenhouse gas and surface albedo are positive climate feedbacks. It is actually the slow changes in the orbit of the Earth that initiate major climate swings from glacial to interglacial and back again, this applying especially to the tilt of the spin-axis of the Earth relative to its orbital plane, and the intensity of insolation during summer that are influenced by the equinoxes [25, 26]. The orbital change produced small changes in global radiative forcing. Changes of geographical and seasonal insolation affects ice sheet size, as when ice melts at both poles as the spin-axis tilt increases and ice melts at only 1 pole when the Sun is at perihelion, the closest it reaches to the Earth, that occurs in late spring [7]. When the climate warms it results in a net release of greenhouse gases, and the release of CO2 by the ocean is the most effective greenhouse gas feedback.  This is in part because of the temperature dependence of the solubility of CO2, but mostly from an increase of ocean mixing that occurs in a warmer climate, that the effect of flushing CO2 from the deep ocean and the alteration of the ocean biological productivity [27].

Temperature changes caused by any climate forcing, whether natural or human activity, leads to greenhouse gas and surface albedo feedbacks responding and contributing to the temperature change, if the length of time is sufficient. During the Late Pleistocene the greenhouse gas feedback was close to linear in global temperature [6, 28]]. As the Earth cools surface albedo feedback increases, as does the area covered by ice.

The timescales of the Pleistocene include slow feedbacks and is larger than the Charney sensitivity as the dominant slow feedbacks are positive. Weathering increases as CO2 increases and is an important negative feedback on longer timescales.

Evaluation of long-term sensitivity to specified greenhouse gas change is made possible by palaeoclimate data. The only assumption of the authors¹ is that the area of ice is a function of global temperature. The global climate sensitivity, including the slow surface global feedback, is shown when greenhouse gas forcing [7] when plotted against temperature is 1.5°C per W/m² or 6° C for doubled CO2, twice as large as the Charney fast feedback sensitivity. The authors¹ say they assume the area covered by ice and snow on the Earth's surface to be predominantly dependent on the global temperature, though some changes of regional ice sheet properties result from the orbital climate forcing of the Earth (see Supplement).

For doubled CO2 the equilibrium sensitivity of 6°C is valid for a specified amount of greenhouse gases, as in studies employing emission scenarios and coupled carbon cycle/climate models for determining the amount of greenhouse gases. According to the authors¹ if solar radiance is used as forcing, and greenhouse gases are included as a fast feedback, sensitivity is even larger on Pleistocene time scales (see supplement), but negative feedbacks on geological timescales may reduce sensitivity [29, 30].

For greenhouse gases remaining airborne for centuries to millennia this climate sensitivity that is long-term becomes relevant. The increase in greenhouse gases in the atmosphere produced by human activities will slowly decline if anthropogenic emissions are reduced enough. This is easily illustrated by the use of a simplified carbon cycle model. A positive greenhouse gas feedback on scale times of from centuries to millennia if the globe warms much further it is predicted by carbon cycle models [2] and empirical data [6, 28]. If the warming is kept to within the temperature range of recent interglacial periods [6] amplification of greenhouse gas amount is moderate, but higher levels released of CH4 and CO2 from methane hydrates in permafrost in tundra and ocean sediments [29] would be risked by greater warming.

Weathering of rocks on even longer geological time scales causes negative feedback on amounts of atmospheric CO2, though this feedback is too slow to prevent the change of climate that would impact on humans.

Sources & Further readingSources & Further reading

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


Author: M. H. Monroe
Last updated 17/05/2012

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