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Will Global Warming Cause Earth's Forests to Lose Carbon?
Volume 7, Number 38: 22 September 2004

Will rising temperatures induce forest ecosystems to begin transferring carbon from the land to the air? Many people say they will, especially people with a vested interest in seeing mandatory reductions in anthropogenic CO2 emissions become binding on the nations of the world. Scientists who carefully study the subject, however, are not so sure.

In introducing their report on what is perhaps the longest experimental investigation of the topic ever to be conducted, Melillo et al. (2002) report that both the magnitude and sign of the postulated phenomenon "are still highly uncertain because of gaps in basic understanding of terrestrial ecosystem processes." But instead of wringing their hands and lamenting the bleakness of the situation, they proceed to cautiously fill in a portion of those knowledge gaps with important observations obtained from some impressive real-world experiments.

The scientists began their landmark study in 1991 in an even-aged mixed hardwood stand of the Harvard Forest in central Massachusetts, USA, where they installed buried heating cables at 10-cm depths, spaced 20 cm apart, in a number of 6-m by 6-m plots, half of which were not operated to produce any heat (their "disturbance control" plots) and half of which were operated to maintain the average soil temperature approximately 5C above ambient. Then, at monthly intervals from April through November of each year of the 10-year study, they measured CO2 evolution rates from the soils of both sets of plots.

Over the first six years of the study, Melillo et al. observed an approximate 28% increase in CO2 emissions from the heated plots relative to the non-heated plots; but, as they describe it, "over the last four years of the study, the 'stimulatory' effect of warming on soil respiration markedly decreased," so much so, in fact, that from 1998 through 2000, there was "only about a 5% increase in soil respiration in the warmed versus disturbance control plots," and by the tenth year of the study "soil respiration showed no significant response to warming."

This exemplary study shows just how important it is to do experiments ? and to do long-term experiments. Most prior theoretical studies, for example, had tended to predict what the scientists observed over the first six years of their measurement program, i.e., a large warming-induced increase in respiratory carbon loss from the forest soil, which would likely have been "confirmed" as the truth if they had terminated their work at any time during that period. Continuing their measurements for four more years, however, the scientists discovered what is likely the true long-term response, i.e., no significant change in soil respiratory carbon loss. Yet that was but the beginning of what they learned; and here, as Paul Harvey would say, is "the rest of the story."

Concurrent with their soil CO2 evolution measurements, Melillo et al. had measured rates of net nitrogen mineralization, finding that "over the entire 10-year study period, warming resulted in a cumulative increase in net nitrogen mineralization." Also, partially overlapping Melillo et al.'s long-term warming experiment was a long-term nitrogen fertilization study of a similar hardwood stand in the Harvard Forest that is described by Magill et al. (2000), where after nine years it was determined that "12.7% of the total amount of nitrogen fertilizer added ended up in the woody tissue of the stand's trees."

Assuming that an identical portion of the increased nitrogen made available by the controlled warming of their experiment ended up in the woody tissues of the trees they studied, Melillo et al. calculated that this phenomenon "would result in an additional 1560 g m-2 of carbon storage in the vegetation over the decade of warming." Hence, since they had previously determined that the soil carbon loss stimulated by their applied warming for the entire 10-year period was 944 g m-2, the net result of the two competing phenomena for the complete forest ecosystem was a net carbon gain on the order of 600 g m-2.

In discussing the significance of this unexpected but extremely positive finding, Melillo et al. note there is other independent "direct field evidence that soil warming enhances carbon storage in trees." Citing the work of Bergh et al. (1999) and Jarvis and Linder (2000), they report that in a similar long-term study these investigators also found "there was a significant (more than 50%) increase in stem-wood growth of the trees on the heated plots relative to the controls."

In light of these similar independent experimental observations, it would appear that worn-out climate-alarmist claims to the contrary are simply just that: worn-out claims that have no backing in empirical science. Both the aerial fertilization effect of atmospheric CO2 enrichment and the soil fertilization effect of the increase in nitrogen mineralization induced by global warming have a tendency to increase carbon sequestration in forest ecosystems, thereby providing a strong, double-barreled, negative-feedback brake on the impetus for warming created by the enhanced greenhouse effect of the ongoing rise in the air's CO2 content. The threats to nature that climate alarmists see in these global changes (rising atmospheric CO2 concentration and temperature) are in reality blessings in thinly-veiled disguise that truly give power to the biosphere.

Sherwood, Keith and Craig Idso

Bergh, J., Linder, S., Lundmark, T. and Elfving, B. 1999. The effect of water and nutrient availability on the productivity of Norway spruce in northern and southern Sweden. Forest Ecology and Management 119: 51-62.

Jarvis, P. and Linder, S. 2000. Constraints to growth of boreal forests. Nature 405: 904-905.

Magill, A.H., Aber, J.D., Berntson, G.M., McDowell, W.H., Nadelhoffer, K.J., Melillo, J.M. and Steudler, P. 2000. Long-term nitrogen additions and nitrogen saturation in two temperate forests. Ecosystems 3: 238-253.

Melillo, J.M., Steudler, P.A., Aber, J.D., Newkirk, K., Lux, H., Bowles, F.P., Catricala, C., Magill, A., Ahrens, T. and Morrisseau, S. 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298: 2173-2176.