Date: 22 Nov This article is largely based on "Terrestrial Ecosystem Feedbacks to Global Climate Change" by Daniel A. Lashof and Benjamin J. DeAngelo of the Natural Resources Defense Council and John Harte and Scott R. Saleska of the University of California at Berkeley, submitted for publication to the Annual Review of Energy and the Environment, 1997 edition, volume 22.
What are feedbacks and why are they important? Current predictions of global warming and other climatic changes due to human activities are primarily based on sophisticated climate models, the present generation of which do not account for important interactions between the climate and marine and terrestrial ecosystems. But scientists have recognized that changes in these interactions have the potential to produce so-called "feedbacks" to the climate system, which could either amplify or dampen expected rates of global warming and climatic change.
How do feedbacks work? The simple answer is in much the same manner that a microphone, amplifier and speaker interact to pick up an initial sound, amplify it, project it, and then cycle it though this loop again and again until the initial sound becomes an unbearably loud tone. This is a classic example of a positive feedback, so-called due to this self-reinforcing effect. Both positive and negative (i.e., dampening) feedbacks are possible in the global climate system. Rising atmospheric concentrations of greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), cause the initial climatic disturbance in the form of global warming. Both the increase in greenhouse gases and associated warming can affect the structure and function of ecosystems, which in turn may accelerate or diminish the warming. The end result is global warming and climatic change of a different magnitude compared to the initial disturbance.
What does historical evidence say about feedbacks? Historical evidence suggests that feedback mechanisms may have played a significant role in past climatic fluctuations, such as the apparent cyclical nature of the Earth's ice ages. Data spanning the past 220,000 years, for example, indicate that variations in temperature and CO2 concentrations were strongly correlated, with CO2 changes lagging behind temperature changes. Thus, it is a fair assumption that past warming resulted in the release of greater amounts of CO2, which in turn could have led to further warming -- a positive feedback.
Model simulations of past climatic changes reveal that other types of positive feedbacks may have been at work as well. Such studies suggest that long-term changes in the Earth's orbit -- to which much of the historical climatic changes are attributed -- were insufficient to account for the full magnitude of past temperature fluctuations. Another key factor, then, seems to have been the poleward shift of forests into high-latitude grasslands induced by warming; this would have reduced the reflectivity (i.e., albedo) of polar regions, increased absorption of sunlight, and thus led to greater warming.
Which feedbacks may be most important in the coming "greenhouse" century and beyond that are not accounted for in current models?
* In similar manner to past climatic and large-scale ecosystem changes, human-induced global warming is expected to cause a poleward shift of forest zones, and thus decrease the reflectivity of the Earth's surface, increase absorption of sunlight, and enhance rates of warming -- a positive feedback.
* Warming in these high-latitude regions may also result in increased rates of methane production from moist bogs or peatlands. Methane is the second most important greenhouse gas. However, likely changes in soil moisture from global climatic change will also affect rates of methane emissions, but in less certain ways. Future changes in topography in these regions may also increase rates of methane release -- a positive feedback.
* Warming and associated decreases in soil moisture may bring about an increased frequency of natural fires. The burning vegetation would pump even more CO2 into the atmosphere -- a positive feedback.
* Elevated concentrations of CO2 have been shown to cause stunted plant transpiration, the process by which plants release water to the atmosphere. Transpiration normally acts to cool the surface; thus, the result could be even higher regional temperatures at the surface -- a positive feedback (although the global implications of this are not entirely clear).
* In soils, the CO2 "enrichment" could lead to changing ratios among important plant nutrients and in the process lead to decreased nitrogen availability. In this case, any stimulatory effect that increased CO2 may have on plant growth could be constrained -- a positive feedback.
* Global climatic change is expected to aggravate rates of land degradation and desertification, which in turn would result in the emission of more disturbed, windblown dust. These particles act to cool the surface, on a regional scale, by increasing atmospheric reflectivity (when the underlying surface is relatively dark) -- a negative feedback.
* As warming penetrates the ocean sediment layers it could result in the release of large amounts of methane, billions of tons of which are locked away in an icy mixture, called gas hydrate, that is only stable under specific conditions of high pressure and low temperatures -- a positive feedback.
* Oceanic temperature increases as a result of global warming could lead to decreased solubility of CO2, and thus turn some regional oceanic CO2 "sinks" into sources -- a positive feedback.
What conclusions should we draw about potential feedbacks? The possible feedbacks mentioned here are some of the most important not adequately incorporated into current climate models, and it appears the net effect of these unaccounted-for processes will be to amplify global warming, perhaps substantially, compared with current predictions. Based largely on historical evidence, for instance, it appears the reflectivity changes associated with poleward shifts of forest zones could greatly amplify a warming trend initially induced by human activities. The feedbacks involved in the release of methane also have a large potential to accelerate global warming.
Even some of the feedbacks which are included in Intergovernmental Panel on Climate Change (IPCC) projections may not be adequately represented. For example, the negative feedback involving the "fertilization effect" of CO2 on plants -- whereby plant growth is stimulated and thus more CO2 taken up -- may be largely overestimated due to a range of unaccounted-for ecological interactions and responses to climatic change that are likely to curtail or even reverse the fertilization effect.
In short, there remain many factors which could potentially drive rates of global warming and climatic change beyond the current projections made by the IPCC. The scientific community is working to improve our understanding and representation of such important processes. In the meantime, the policymakers will have to take the potential impact of these feedbacks into account when formulating strategies that seek to minimize the risks of global warming.