from Climate Alert Volume 10, No. 4 September-October 1997

Feedbacks Could Amplify Global Warming Beyond Current Predictions

By Benjamiin DeAngelo, EPA's Stratospheric Protection Division *

Current predictions of global warming and associated climatic changes resulting from human activities are based on sophisticated climate models, which do not, however, in every case include important interactions between the climate and marine and terrestrial ecosystems. But scientists recognize that changes in these interactions could produce "feedbacks" which could either amplify or dampen expected rates of global warming and climatic change.

Feedbacks can be either positive or negative (i.e., dampening). As rising atmospheric concentrations of greenhouse gases, including carbon dioxide (CO2) and methane (CH4), lead to global warming, both the increase in greenhouse gases and the warming can affect the structure and function of ecosystems. Such effects may accelerate or diminish the warming and precipitate change of a different magnitude from the initial disturbance.

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 indicate that variations in temperature and CO2 concentrations were strongly correlated, with CO2 changes lagging behind temperature changes. 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 &emdash; a positive feedback.

Other Positive Feedbacks

• Model simulations of past climatic changes reveal that other types of positive feedbacks may have been at work as well. Long-term changes in the Earth's orbit &emdash; the apparent source of historical climatic changes &emdash; were, according to the models, insufficient to account for the full magnitude of past temperature fluctuations. An additional key driving factor seems to have been the poleward shift of forests into high-latitude grasslands, reducing the reflectivity (albedo) of polar regions, increasing absorption of sunlight, and producing greater warming compared to what was initially induced by orbital changes alone.

• Like these past nautral changes, human-induced global warming over the coming century is expected to cause a poleward shift of forests, decreasing the reflectivity of the Earth's surface, increasing absorption of sunlight, and enhancing rates of warming.

• Warming in the high-latitude regions may bring expanded release of methane from moist bogs or peatlands. Methane is the second most important anthropogenic greenhouse gas in terms of its warming effect. Likely changes in soil moisture from global climatic change will also affect rates of methane emissions, but in less certain ways.

• Warming and associated decreases in soil moisture may bring about a rise in the number of natural fires. The burning vegetation would pump even more CO2 into the atmosphere.

• Elevated concentrations of CO2 have been shown to stunt plant transpiration, the process by which plants release water to the atmosphere. Transpiration normally cools the surface; its reduction could bring even higher regional temperatures at the surface (although the global implications of this are not entirely clear).

• In soils, 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 of increased CO2 on plant growth could be constrained.

• As warming penetrates the ocean sediment layers, billions of tons of methane locked away in an icy mixture called gas hydrate (which is only stable under specific conditions of high pressure and low temperatures) could be released.

• Oceanic temperature rise from global warming could lower the solubility of CO2 and turn some regional oceanic CO2 "sinks" into sources.

Negative Feedback

Global climatic change is expected to aggravate rates of land degradation and desertification, stirring up windblown dust. Such particles cool the surface regionally, increasing atmospheric reflectivity (especially when the underlying surface is relatively dark).

Conclusions

These possible feedbacks are among the most important not adequately incorporated into current climate models. It appears their effect will be to amplify global warming, perhaps substantially, compared with current predictions. Based largely on historical evidence, for instance, it seems likely the reflectivity changes of poleward shifts of forests could greatly amplify a human-induced warming trend. The feedbacks from methane release also have a large potential to accelerate global warming.

Even some of the feedbacks included in Intergovernmental Panel on Climate Change (IPCC) projections may not be adequately represented. The negative feedback of the "fertilization effect" of CO2 on plants&emdash; in which plant growth is stimulated and takes up more CO2 &emdash; may be largely overestimated. A range of unpredicted ecological interactions and responses to climatic change are likely to curtail or even reverse the fertilization effect.

In short, many factors remain which could drive rates of global warming and climatic change beyond the current IPCC projections. The scientific community is working to improve our understanding and representation of such important processes. In the meantime, policy makers will have to consider the potential impact of these feedbacks when formulating strategies to minimize the risks of global warming.

* The views of the author do not necessarily represent those of the EPA.

For more information on this subject, see "Terrestrial Ecosystem Feedbacks to Global Climate Change," by Lashof et al inAnnual Review of Energy and the Environment, 22: 75-118, 1997

 

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