Ecosystem effects of CO<sub>2</sub> concentration: evidence from past climates

Abstract

Abstract. Atmospheric CO 2 concentration has varied from minima of 170-200 ppm in glacials to maxima of 280-300 ppm in the recent interglacials. Photosynthesis by C 3 plants is highly sensitive to CO 2 concentration variations in this range. Physiological consequences of the CO 2 changes should therefore be discernible in palaeodata. Several lines of evidence support this expectation. Reduced terrestrial carbon storage during glacials, indicated by the shift in stable isotope composition of dissolved inorganic carbon in the ocean, cannot be explained by climate or sea-level changes. It is however consistent with predictions of current processbased models that propagate known physiological CO 2 effects into net primary production at the ecosystem scale. Restricted forest cover during glacial periods, indicated by pollen assemblages dominated by non-arboreal taxa, cannot be reproduced accurately by palaeoclimate models unless CO 2 effects on C 3 -C 4 plant competition are also modelled. It follows that methods to reconstruct climate from palaeodata should account for CO 2 concentration changes. When they do so, they yield results more consistent with palaeoclimate models. In conclusion, the palaeorecord of the Late Quaternary, interpreted with the help of climate and ecosystem models, provides evidence that CO 2 effects at the ecosystem scale are neither trivial nor transient.

Pollen-based continental climate reconstructions at 6 and 21 ka: a global synthesis

“…6, 7), even if the reconstructions for individual sites can differ. We suggest that the way forward is to use inverse modelling in preference to analogue or response-surface techniques, because inverse modelling (a) provides sound logic to cope with the no-analogue problem, (b) makes the wrong-analogue problem explicit, (c) allows the consequences of changing atmospheric CO 2 concentration to be taken into account Prentice and Harrison 2009) and (d) allows the consequences of changing seasonality of climate forcing or climate responses to be taken into account . However there are many detailed methodological choices to be made in this approach, such as the assignment of taxa to PFTs, choice of model, and the choice of model variable to compare with PFT abundance and more work is required before this technique will entirely supersede other methods.…”

“…A further strength of this approach is that it provides a way to include effects of CO 2 concentration on competition between PFTs and therefore also the relative abundances of taxa Prentice and Harrison 2009). All other techniques are likely to give biased results for CO 2 concentrations unlike those of the Late Holocene.…”

“…lake level changes (Cheddadi et al 1997). Inversion of biogeographic models can also be adapted in a natural way because process-based models predict ancillary variables such as runoff and plant d 13 C (Hatté and Guiot 2005), the latter being a powerful additional indicator of C 3 plant water status and C 3 /C 4 plant competition Prentice and Harrison 2009). Hatté et al (2009) showed that the use of d 13 C as an additional constraint considerably reduces the uncertainties of climatic reconstructions obtained from inverse modelling compared to using pollen alone.…”

A few prospective ideas on climate reconstruction: from a statistical single proxy approach towards a multi-proxy and dynamical approach

“…The pollen PFT's are sometimes more precise and pollen information is sufficient to recognize several varieties of the same model PFT, for example pollen is able to separate warm and cool ts. The use of such models in the paleoclimatological context and the simulation of the CO 2 effect on ecosystems are particularly well reviewed in Prentice and Harrison (2009)…”

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…The extent of this retreat cannot be explained by the cooler, drier climates alone. DGVM modelling indicates that low CO 2 , which favours C4 grasses but inhibits tree growth, was a major contributor [45][46][47]. Thus, in the 2.6 Myr of the Pleistocene, grassy biomes would have been the most extensive vegetation cover of Africa, the 'norm' for the continent, not forests.…”