[Marxism] land use and global warming (was: Freeman Dyson ...)

Les Schaffer schaffer at optonline.net
Wed Aug 15 14:12:00 MDT 2007


Bob Hopson wrote:

> He also claims there's a real dearth of ground-level measurement of CO2
> soil-to-air transfer that is only now being addressed, which is  one 
> of the main reasons he's skeptical of over-reliance on computer
> modelling.
[snip]
> That being said, the only reference you actually posted was an
> article about global warming causing rainforest growth to
> slow rather than accelerate, which doesn't seem to actually have
> much to do with Dyson's points about land use as a "cure" for
> atmospheric CO2 


Bob:

here is another article hot off the presses on the relationship between
land-carbon sinks and global warming. see what you think. a nice
combination of modeling and experimental data analysis on the sink dynamics.

it seems that any "land use" cure will need to take into account
multiple coupled factors.

full available offlist on request.

Les



Nature 448, 791-794 (16 August 2007) | doi:10.1038/nature06059; Received
9 September 2006; Accepted 3 July 2007; Published online 25 July 2007

Indirect radiative forcing of climate change through ozone effects on
the land-carbon sink

Abstract

The evolution of the Earth's climate over the twenty-first century
depends on the rate at which anthropogenic carbon dioxide emissions are
removed from the atmosphere by the ocean and land carbon cycles[1].
Coupled climate–carbon cycle models suggest that global warming will act
to limit the land-carbon sink[2], but these first generation models
neglected the impacts of changing atmospheric chemistry. Emissions
associated with fossil fuel and biomass burning have acted to
approximately double the global mean tropospheric ozone
concentration[3], and further increases are expected over the
twenty-first century[4]. Tropospheric ozone is known to damage plants,
reducing plant primary productivity and crop yields[5], yet increasing
atmospheric carbon dioxide concentrations are thought to stimulate plant
primary productivity[6]. Increased carbon dioxide and ozone levels can
both lead to stomatal closure, which reduces the uptake of either gas,
and in turn limits the damaging effect of ozone and the carbon dioxide
fertilization of photosynthesis[6]. Here we estimate the impact of
projected changes in ozone levels on the land-carbon sink, using a
global land carbon cycle model modified to include the effect of ozone
deposition on photosynthesis and to account for interactions between
ozone and carbon dioxide through stomatal closure[7]. For a range of
sensitivity parameters based on manipulative field experiments, we find
a significant suppression of the global land-carbon sink as increases in
ozone concentrations affect plant productivity. In consequence, more
carbon dioxide accumulates in the atmosphere. We suggest that the
resulting indirect radiative forcing by ozone effects on plants could
contribute more to global warming than the direct radiative forcing due
to tropospheric ozone increases.

[snip]

Ozone causes cellular damage inside leaves that adversely affects plant
production, reduces photosynthetic rates and requires increased resource
allocation to detoxify and repair leaves[5]. There have been few global
modelling studies of the impact of tropospheric ozone on plant
production and global land-carbon storage11, and no study has estimated
the indirect radiative forcing of tropospheric ozone through feedbacks
on the global carbon cycle. Here, we are concerned with the possible
impacts of future tropospheric ozone on global-scale plant primary
production, land-carbon storage, and its implications for
twenty-first-century climate change.

Future elevated [CO2] may itself lead to reductions in stomatal
conductance[12] at levels that act to alleviate future O3 plant damage.
Hence, future O3 effects on plants are defined by the interplay of
ambient [O3], [CO2] and climate change on stomatal conductance and plant
production, with important ramifications for global land-carbon and
hydrological cycles[7, 11]. Free air CO2 enrichment (FACE) experiments
and other ambient air experiments indicate a nonlinear interaction
between plant responses to CO2 and O3 (refs 6, 13–15). To account for
these interactions, we use a flux-gradient approach to modelling ozone
damage[16], rather than the more usual empirical approach based on the
accumulated ozone exposure above 40 p.p.b. (ref. 11).

[snip to conclusion]

Suppression of the land-carbon sink results in additional anthropogenic
CO2 emissions accumulating in the atmosphere, and therefore an indirect
radiative forcing of climate change by O3 effects on the terrestrial
biosphere. The indirect radiative forcing due to O3 is diagnosed for
comparison to the direct forcing due to tropospheric O3 (Fig. 2b),
assuming 50% of the extra CO2 from the land is sequestered by the
oceans[26]. The indirect forcing by 2100 is estimated at 0.62 W m-2 and
1.09 W m-2 for the 'low' and 'high' plant ozone sensitivity runs,
respectively, which compares with a mean direct radiative forcing from
11 atmospheric chemistry models of 0.89 W m-2 (refs 3, 4, 27, 28).
Although the absolute values of radiative forcing are dependent on our
choice of emissions scenario, the relative importance of direct and
indirect radiative forcing is much less sensitive to the uncertainty in
emissions. As such, these results suggest that ozone effects on
vegetation could double the effective radiative forcing due to increases
in tropospheric ozone, significantly increasing the importance of
changes in atmospheric chemistry as a driver of twenty-first-century
climate change.





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