How does rising atmospheric CO2 affect marine organisms?

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No Pain, No Gain
Volume 3, Number 23: 20 September 2000

For many years the story out of the Alaskan Arctic tundra was that global warming would change the land from a carbon sink to a carbon source, further exacerbating the cause of the ecosystem change, i.e., regional warming, by adding to the burden of atmospheric greenhouse gases and hastening that portion of the biosphere's inevitable degradation (Oechel et al., 1993, 1995).  Now, almost a decade later, the same researchers are singing another song (Oechel et al., 2000).  To the scientists' credit, however, the reason for their change of tune is to be found in a repository of data that they and others diligently collected over many years, some of it going back in time almost four decades.

In the early to mid-1970s, when the first carbon balance studies of Alaskan Arctic ecosystems were conducted under the aegis of the International Biological Program, both wet-sedge communities and moist-tussock tundra were observed to be net sinks of carbon.  By the mid-1980s and the early 1990s, however, following significant increases in air temperature and surface water deficit, both ecosystems had become net sources of carbon.   Then, between 1992 and 1996, in response to further warming and drying - resulting, in the words of the authors, in "the highest average summer temperature and surface water deficit observed for the entire 39-year period" - both ecosystems' net summer releases of CO2 to the atmosphere declined, and they eventually became CO2 sinks once again.

So how did it happen?   In the words of the scientists, whose song is now melodious indeed, their observations indicate "a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate."  All right!

But how did that happen?  Was there help along the way from the contemporaneous rise in the air's CO2 content and its aerial fertilization and anti-transpiration effects?  Although these well-documented consequences of atmospheric CO2 enrichment are known to help plants respond to the environmental challenges of both warming and drying - see the materials posted under our Subject Index headings of Growth Response to CO2 with Other Variables (Temperature, Water Stress) - these effects are not mentioned.  Instead, the researchers note some other possibilities that are, indeed, quite plausible.

First, there is the likelihood that, during the initial stages of warming and soil drying, younger and more labile carbon would be rapidly decomposed, shifting the net summer carbon balance of the ecosystems from one of carbon sequestration to one of carbon evolution.  After this initial perturbation, however, the authors suggest that "enhanced rates of net nitrogen-mineralization should eventually stimulate rates of gross primary production and atmospheric CO2 sequestration."

Another possibility is a gradual shift in plant species towards more productive types that would further reduce the large initial carbon losses over time.  And in this regard, the researchers note "there is evidence that the relative abundance of deciduous shrubs has increased in response to climate change over the past 1-2 decades in Alaskan moist-tusssock tundra ecosystems," which is also something that is expected to occur as a consequence of the ongoing rise in the air's CO2 content (see materials in our Subject Index under the heading Trees (Range Expansions).

The bottom line of this discussion is that there are several reasons to expect a long-term increase in the carbon-sequestering potential of Alaskan Arctic ecosystems in response to the simultaneous increases in that region's air temperature and CO2 concentration that have been experienced over the past few decades.   As with the "no pain, no gain" approach to muscle development in the human body, however, there is the initial pain of ecosystem carbon loss that precedes the ultimate gain of ecosystem carbon acquisition.  The important paper of Oechel et al. (2000) lifts that concept as applied to ecosystems from the level of theory to the pinnacle of reality

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

Oechel, W.C., Hastings, S.J., Vourlitis, G., Jenkins, M., Riechers, G. and Grulke, N.  1993.  Recent change of Arctic tundra ecosystems from a net carbon dioxide sink to a source.  Nature 361: 520-523.

Oechel, W.C., Vourlitis, G.L., Hastings, S.J. and Bochkarev, S.A.  1995.  Change in Arctic CO2 flux over two decades: Effects of climate change at Barrow, Alaska.  Ecological Applications 5: 846-855.

Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D.  2000.  Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response to decadal climate warming.  Nature 406: 978-981.