Runaway Climate Change - Examples

Examples

There are known examples of the Earth's climate producing a large response to small forcings; most obviously CO₂ feedback effect is believed to be part of the transition between glacial and interglacial periods, with the Milankovitch cycle providing the initial trigger. This is generally not considered to be a runaway climate change. Another example would be Dansgaard-Oeschger events.

Potentially unstable methane deposits exists in permafrost regions, which are expected to retreat as a result of global warming, and also clathrates, with the clathrate effect probably taking millennia to fully act. The potential role of methane from clathrates in near-future runaway scenarios is not certain, as studies show a slow release of methane, which may not be regarded as 'runaway' by all commentators. The clathrate gun runaway effect may be used to describe more rapid methane releases. Methane in the atmosphere has a high global warming potential, but breaks down relatively quickly to form CO₂, which is also a greenhouse gas. Therefore, slow methane release will have the long-term effect of adding CO₂ to the atmosphere.

In order to model clathrates and other reservoirs of greenhouse gases and their precursors, global climate models would have to be 'coupled' to a carbon cycle model. Some current global climate models do not include such modelling of methane deposits.

A 2006 book chapter by Cox et al. considers the possibility of a future runaway climate feedback due to changes in the land carbon cycle:

Here we use a simple land carbon balance model to analyse the conditions required for a land sink-to-source transition, and address the question; could the land carbon cycle lead to a runaway climate feedback? The simple land carbon balance model has effective parameters representing the sensitivities of climate and photosynthesis to CO₂, and the sensitivities of soil respiration and photosynthesis to temperature. This model is used to show that (a) a carbon sink-to-source transition is inevitable beyond some finite critical CO₂ concentration provided a few simple conditions are satisfied, (b) the value of the critical CO₂ concentration is poorly known due to uncertainties in land carbon cycle parameters and especially in the climate sensitivity to CO₂, and (c) that a true runaway land carbon-climate feedback (or linear instability) in the future is unlikely given that the land masses are currently acting as a carbon sink.

Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability

More recently there has been a suggestion that the release of heat associated with soil decomposition, which is neglected in the vast majority of large-scale models, may be critically important under certain circumstances. Models with and without the extra self-heating from microbial respiration have been shown to yield significantly different results. The present paper presents a mathematical analysis of a tipping point or runaway feedback that can arise when the heat from microbial respiration is generated more rapidly than it can escape from the soil to the atmosphere. This ‘compost-bomb instability’ is most likely to occur in drying organic soils with high porosity covered by an insulating lichen or moss layer. However, the instability is also found to be strongly dependent on the rate of global warming. This paper derives the conditions required to trigger the compost-bomb instability, and discusses the relevance of these to the concept of dangerous rates of climate change. On the basis of simple numerical experiments, rates of long-term warming equivalent to 10°C per century could be sufficient to trigger compost-bomb instability in drying organic soils.