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Biospheric Carbon Sequestration: Does It Really Work?
Volume 2, Number 23: 1 December 1999

Part I: A Matter of Incredibly Simple Logic

According to Fred Pearce, consultant to Britain's New Scientist magazine, the thought that the planting of trees could sequester some of the carbon that is emitted to the air by anthropogenic CO2 emissions is "based on a dangerous delusion" (Pearce, 1999). He cites as the basis for this statement a recent report (which we have not seen) of the UN's Intergovernmental Panel on Climate Change (IPCC), claiming that its scientists say that "planned new forests, called 'carbon sinks,' will swiftly become saturated with carbon and begin returning most of their carbon to the atmosphere." He then quotes Peter Cox of the British Meteteorological Office's Hadley Centre as stating that "this is not something that may or may not happen," but something that is "more or less inevitable." Pearce then adds that "the suggestion that planting trees means less atmospheric CO2 ignores simple logic." And in derisive denigration of the concept of biospheric carbon sequestration, he mockingly asks: "How did researchers get it so wrong?"

Let's see if we can help Mr. Pearce figure it out. According to "simple logic," if there are X trees on the planet today, and if we plant Y trees tomorrow, won't the sum of X + Y trees tomorrow remove more carbon from the air than the original X trees of today do? Absolutely. So how did Mr. Pearce "get it so wrong?" And how can he possibly believe that newly-planted trees will swiftly return "most of their carbon to the atmosphere"?

The only thing we could think of when confronted with the conundrum was evaporation. Maybe the new trees, possibly altered in some strange way by exposure to higher CO2 concentrations than those to which their ancestors were accustomed, somehow just vaporize when they reach a certain age. When we consulted Dr. Dennis Clark of our Scientific Advisory Board, however, he said we were way off base on that one, and that the real reason newly-planted trees would swiftly lose the majority of the carbon comprising their tissues was spontaneous combustion.

We hope you forgive us for the levity; but it's hard to be serious about so preposterous a story as the one told by Mr. Pearce. Normally, we would not have even bothered to respond to it. Indeed, we actually waited a while before doing so, hoping that someone else - a functionary of the IPCC, perhaps - would step forward to debunk the outrageous tale; but when no one did, we felt compelled to say something, because of the disturbing possibility that some New Scientist readers might actually believe Mr. Pearce, especially when the fiction he writes about is presented as fact, and when he cites various scientists as his sources of information.

Consider, for example, the purported thoughts of South Africa's Bob Scholes, who Pearce describes as "a leading light in the International Geosphere-Biosphere Programme's Global Carbon Project." According to Scholes - according to Pearce - because increasing CO2 concentrations have an ever-smaller effect on plant growth as they rise higher and higher, and because respiration increases with temperature (which Pearce assumes will rise as a consequence of increases in the air's CO2 content), CO2 fertilization rates "will flatten out while respiration rates soar," so that by 2050 "forests will have released much of what they have absorbed."

There it is again, that incredibly irresponsible and ridiculous claim, that much of what forests have absorbed (which is much of what they are) will be released to the atmosphere - and within a mere 50 years. As we asked earlier, will soon-to-be-planted forests merely evaporate into thin air? Will they spontaneously burst into flame and go up in smoke? Of course not. As long as the wood of their trunks and branches continues to exist, either in living forests or in a host of forest products, the wood of the new trees that we plant today will continue to have locked within it tomorrow untold tons upon tons of carbon that would otherwise have remained in the atmosphere.

Is not this what "simple logic" teaches us? We defy Mr. Pearce to find a solitary soul who believes otherwise.

Part II: Setting the Record Straight: The Edinburgh Group Weighs In

After completing Part I of this editorial at approximately midnight on 26 November 1999, we awoke the next morning to find a subsequent issue of New Scientist in the mail, which arrives at our post box rather circuitously and, consequently, somewhat erratically; and there, leading off the Letters section of the magazine, was a short note from a group of researchers at the University of Edinburgh for whom we have much respect (Tipper et al. 1999). In a brief but effective rebuttal of Pearce's denunciation of biospheric carbon sequestration (which we dearly hope is not truly to be credited to the IPCC), they set much of his distorted record straight.

The "swift" saturation of new-forest carbon sinks, they note, is not anticipated for about another 150 years; and if these forests saturate - which Tipper et al. rightly acknowledge is highly uncertain and a matter of much debate - they will not release, in the parlance of Pearce, "much of what they have absorbed," but continue to keep it sequestered. Indeed, Tipper and colleagues correctly state that it is an "inescapable fact that even if sink saturation does occur on the global scale, young growing trees will still accumulate carbon, and eventually store more carbon than other types of land use."

Thanks to the scientists at the University of Edinburgh, the public now has a more accurate portrayal of the subject of terrestrial carbon sinks. Biospheric carbon sequestration, particularly by forests, really does work. But what is this "sink saturation" that both Pearce and Tipper et al. talk about? Basically, it's the idea - which is indeed logical - that there must be an upper limit to the amount of carbon that an ecosystem can sequester, plus the subsidiary and more practically-important idea that that limit will be reached well before mankind has either (1) depleted all of Earth's fossil fuel reserves or (2) discovered a more economical non-fossil-fuel energy source for powering the engines of industry and transportation, which would mean, of course, that biospheric carbon sequestration would ultimately fail to function as needed to forestall any global warming that might be occurring (for whatever reason) at that future time.

Clearly, the latter eventuality is indeed possible; but is it likely? Let's take a look at some of the recent peer-reviewed scientific literature on the subject to see what people working on these ideas think of them. And let's see how the ongoing rise in the air's CO2 concentration acts to modify biospheric carbon sequestration processes to make them even more powerful and efficient than they are currently.

Part III: Atmospheric CO2 Enrichment: Priming the Biospheric Carbon Pump

Consider, first of all, the indisputable fact that more CO2 in the air stimulates greater vegetative productivity in almost all plants under almost all conditions in almost all situations (Idso and Idso, 1994). As the air's CO2 content rises, this phenomenon causes ever more carbon to be stored away each year in the woody tissues of long-lived bushes, shrubs and trees, both above- and below-ground, where it eventually becomes part of the organic matrix of the soil. But how much more carbon can be biologically "pumped" into earth's forests and soils? And what about very long time periods and short-lived plants? Also, what happens when plants eventually die and decompose? And what happens if the globe does continue to warm for some reason, hastening the decomposition of dead organic matter and the return of its carbon to the atmosphere? Will these forces overpower the increased ability of plants to remove CO2 from the air and sequester its carbon in their tissues or in recalcitrant soil organic matter?

In an experiment designed to broach certain aspects of the first of these questions, Lutze and Gifford (1998) grew microcosms of the C3 grass Danthonia richardsonii for four consecutive years at CO2 concentrations of 360 and 720 ppm at three different levels of soil nitrogen, finding that the elevated CO2 increased total microcosm carbon gain by 15 to 34%. In analyzing the smallest of these gains (15% at the lowest level of soil nitrogen availability), they concluded that if all terrestrial ecosystems responded to atmospheric CO2 enrichment in a similar fashion, this phenomenon - biospheric carbon pumping into biospheric carbon sinks - would account for all of the so-called missing carbon that annually leaves the atmosphere and is sequestered in some terrestrial sink that cannot currently be identified without acknowledging the great sequestrative prowess of the already-significantly-CO2-enriched terrestrial biosphere.

Taking the experimental investigation of this concept out of the laboratory and into the field, i.e., the world of nature, DeLucia et al. (1999) observed a CO2-induced 25% increase in the productivity of 13-year-old loblolly pine trees in North Carolina, USA, after the first two years of a long-term Free-Air CO2 Enrichment (FACE) experiment, where the air's CO2 concentration was maintained at 350 and 560 ppm. If applicable to all of earth's forests, this enhanced rate of carbon sequestration, they stated, would be capable of absorbing fully half of the anthropogenic CO2 emissions of the next century. In addition, in an assessment of the current CO2-absorbing capacity of the biosphere, Phillips et al. (1998) found that tropical forests in Central and South America are yearly sequestering sufficient carbon to account for fully 40% of the missing carbon of the world; and in a complementary study, Fan et al. (1998) reported measurements suggesting that North American vegetation between latitudes 15 and 51N yearly sequesters all of the carbon that is annually discharged to the atmosphere by all fossil fuel burning in both the United States and Canada.

In another investigation of the prowess of the terrestrial biosphere's carbon pump, Feng (1999) evaluated the intrinsic water-use efficiencies of trees growing in natural forests scattered across western North America via carbon isotope analyses of numerous tree-ring chronologies running back in time as much as two centuries. The results indicated that from 1750 to 1970, over which time period the CO2 content of the air rose by approximately 16%, tree water-use efficiency rose by 10 to 25%, leading the author to conclude that "in arid environments where moisture limits the tree growth, biomass may have increased with increasing transpiration efficiency," indicative of the likelihood that the trees in these arid environments may have operated as a major carbon sink over this period. Likewise, and in confirmation of the findings of Feng, Fernandez et al. (1998) studied the physiological responses of plants growing in the vicinity of natural CO2 springs in Venezuela, finding that an increase in atmospheric CO2 concentration from a base level of 350 ppm to an enriched level of 1000 ppm boosted the water-use efficiency of certain native trees by a factor of two in the rainy season and by a factor of nineteen in the dry season.

Finally, in a review of the scientific literature pertaining to the effects of atmospheric CO2 enrichment on plant respiration, which returns CO2 to the atmosphere, Drake et al. (1999) discovered that a doubling of the air's CO2 content reduces plant respiration rates by an average of 17%. This observation led the authors to conclude that reductions in respiratory carbon losses, due to the ongoing rise in the atmosphere's CO2 concentration, will likewise "enhance the quantity of carbon stored by forests."

With respect to the second question, there is much evidence to suggest that the terrestrial biospheric carbon pump, and its associated carbon sink, will indeed perform wonderfully well for a very long time, particularly in the world's forests. In a recent review of the many experiments that have studied the effects of atmospheric CO2 enrichment on the growth of trees, Idso (1999) observed that the data that have been obtained to date suggest that even after a century of exposure to elevated levels of atmospheric CO2, trees will still be removing more carbon from the air than they would be able to do if the air's CO2 content remained constant. Furthermore, in the report of their study of tree size and age conducted near Manaus, Brazil, Chambers et al. (1998) note that about half of the above-ground biomass of tropical rainforests is contained in less than the largest 10% of their trees. They also observe that some of the larger old trees continue to grow and sequester carbon for over 1400 years. Hence, since the life span of these massive long-lived trees is considerably greater than the projected life span of what we could call the Age of Fossil Fuels, their preservation - and the planting of more such trees - represents an essentially permanent (from the human perspective) partial solution to the perceived problem of the long-term sequestration of carbon.

So what about shorter-lived non-woody plants and the effects of global warming? In response to this question, we note, first of all, that atmospheric CO2 enrichment often reduces the rate of plant residue decomposition (Henning et al. 1996; Van Ginkel et al., 1996; Hirschel et al. 1997; Torbert et al. 1998). In a field study of clover (Trifolium repens) at the Swiss Federal Institute of Technology near Zurich, for example, a 71% increase in atmospheric CO2 concentration increased the above-ground growth of the clover by 146%, while it increased the pumping of newly-fixed carbon into the soil of the CO2-enriched plots by approximately 50% (Nitschelm et al. 1997). In addition, root decomposition in the CO2-enriched plots was 24% less than in the ambient-treatment plots. Consequently, the authors concluded that "the occurrence at elevated CO2 of both greater plant material input, through higher yields, and reduced residue decomposition rates would be expected to impact soil carbon storage significantly." And in a similar study of the effects of a doubling of the air's CO2 concentration on three different grass species, Cotrufo and Gorissen (1997) concluded that "elevated CO2 could result in greater soil carbon stores due to increased carbon-input into soils ... thus counteracting increased decomposition under higher temperatures," and, we would add, thereby overpowering the negative influence of global warming.

In a related study that actually included temperature as a variable, Casella and Soussana (1997) grew perennial ryegrass (Lolium perenne) in ambient and elevated (700 ppm) CO2 at two different levels of soil nitrogen and at ambient and elevated (+3C) temperature for a period of two years, finding that "a relatively large part of the additional photosynthetic carbon is stored below-ground during the two first growing seasons after exposure to elevated CO2, thereby increasing significantly the below-ground carbon pool." At the low and high levels of soil nitrogen supply, for example, the elevated CO2 increased soil carbon storage by 32 and 96%, respectively, "with no significant increased temperature effect." The authors thus concluded that in spite of predicted increases in temperature, "this stimulation of the below-ground carbon sequestration in temperate grassland soils could exert a negative feed-back on the current rise of the atmospheric CO2 concentration."

Finally, based on a large body of prior experimental work that has established the growth and decomposition responses of perennial ryegrass to both atmospheric CO2 enrichment and increased temperatures, Van Ginkel et al. (1999) were able to assess the validity of the oft-stated hypothesis - trumpeted as fact by Mr. Pearce - that CO2-induced global warming will be self-amplifying, as a consequence of a temperature-induced increase in plant residue decomposition that will release greater quantities of CO2 into the air and increase the rate of rise of atmospheric CO2. The bottom line of their study was that, at both low and high soil nitrogen contents, CO2-induced increases in plant growth and CO2-induced decreases in plant decomposition rates "are more than sufficient to counteract the positive feedback caused by the increase in temperature." Furthermore, in a three-year FACE study of cotton, Leavitt et al. (1994) found that about 10% of the organic carbon present in the soil below the CO2-enriched plants (which received but an extra 180 ppm CO2) came from the extra CO2 supplied to the FACE plants. And it was determined that some of this carbon had made its way into a recalcitrant portion of the soil organic matter that displayed an average soil residence time of 2200 years.

Part IV: The Bottom Line: Turning Fiction into Fact

It is amazing what fictions certain people will attempt to pass off as fact. In our last issue's editorial - Vol. 2, No. 22: "Noticed the Weather Lately?" - we revealed the disingenuousness of the National Environmental Trust in bankrolling HOTEARTH ads in major newspapers and magazines that blame global warming (CO2-induced, of course) for imaginary increases in extreme weather events. In this issue we expose a hopefully-less-sinister failure of a reputable popular science magazine to detect and expunge what can only be termed a "scientific fairy tale" from its normal fare of sprightly science news reporting.

We do not enjoy producing such essays; but what are we to do when confronted with such misinformation? Look the other way and ignore the truth? We know not what course others may take, but as for us, we are compelled to do our job and bring you the most honest analyses we are capable of making of these several issues. We hope you appreciate our efforts.

Dr. Craig D. Idso
President
Dr. Keith E. Idso
Vice President

References
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