Why are estimates of global terrestrial isoprene emissions so similar (and why is this not so for monoterpenes)? A. Arneth1, R. K. Monson2, G. Schurgers1, Ü. Niinemets3, and P. I. Palmer4 1Dept. of Physical Geography and Ecosystems Analysis, Geobiosphere Science Centre, Lund University, Lund, Sweden 2Dept. of Ecology and Evolutionary Biology, and Coop. Inst. for Environ. Sci., University of Colorado, Boulder, CO, USA 3Inst. of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Tartu, Estonia 4School of GeoSciences, University of Edinburgh, King's Buildings, Edinburgh, UK
Abstract. Emissions of biogenic volatile organic compounds (BVOC) are a chief
uncertainty in calculating the burdens of important atmospheric compounds
like tropospheric ozone or secondary organic aerosol, reflecting either
imperfect chemical oxidation mechanisms or unreliable emission estimates, or
both. To provide a starting point for a more systematic discussion we review
here global isoprene and monoterpene emission estimates to-date. We note a
surprisingly small variation in the predictions of global isoprene emission
rate that is in stark contrast with our lack of process understanding and
the small number of observations for model parameterisation and evaluation.
Most of the models are based on similar emission algorithms, using fixed
values for the emission capacity of various plant functional types. In some
cases, these values are very similar but differ substantially in other
models. The similarities with regard to the global isoprene emission rate
would suggest that the dominant parameters driving the ultimate global
estimate, and thus the dominant determinant of model sensitivity, are the
specific emission algorithm and isoprene emission capacity. But the models
also differ broadly with regard to their representation of net primary
productivity, method of biome coverage determination and climate data.
Contrary to isoprene, monoterpene estimates show significantly larger
model-to-model variation although variation in terms of leaf algorithm,
emission capacities, the way of model upscaling, vegetation cover or
climatology used in terpene models are comparable to those used for
isoprene. From our summary of published studies there appears to be no
evidence that the terrestrial modelling community has been any more
successful in "resolving unknowns" in the mechanisms that control global
isoprene emissions, compared to global monoterpene emissions. Rather, the
proliferation of common parameterization schemes within a large variety of
model platforms lends the illusion of convergence towards a common estimate
of global isoprene emissions. This convergence might be used to provide
optimism that the community has reached the "relief phase", the phase when
sufficient process understanding and data for evaluation allows models'
projections to converge, when applying a recently proposed concept. We argue
that there is no basis for this apparent relief phase. Rather, we urge
modellers to be bolder in their analysis, and to draw attention to the fact
that terrestrial emissions, particularly in the area of biome-specific
emission capacities, are unknown rather than uncertain.
Citation: Arneth, A., Monson, R. K., Schurgers, G., Niinemets, Ü., and Palmer, P. I.: Why are estimates of global terrestrial isoprene emissions so similar (and why is this not so for monoterpenes)?, Atmos. Chem. Phys., 8, 4605-4620, doi:10.5194/acp-8-4605-2008, 2008.