Why are estimates of global terrestrial isoprene emissions so similar (and why is this not so for monoterpenes)?

Abstract

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-tomodel 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 Correspondence to: A. Arneth (almut.arneth@nateko.lu.se) "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.

Isoprene emissions from a tundra ecosystem

“…Our results lend support to the hypothesis of Arneth et al (2008). Arneth et al (2008) start with the observation that different groups modelling global isoprene emissions often converge on the same estimate (Table 1, Arneth et al, 2008).…”

“…Our results lend support to the hypothesis of Arneth et al (2008). Arneth et al (2008) start with the observation that different groups modelling global isoprene emissions often converge on the same estimate (Table 1, Arneth et al, 2008). But they find this convergence "is in stark contrast with our lack of process understanding and the small number of observations for model parameterisation and evaluation" (abstract, Arneth et al, 2008).…”

“…Arneth et al (2008) start with the observation that different groups modelling global isoprene emissions often converge on the same estimate (Table 1, Arneth et al, 2008). But they find this convergence "is in stark contrast with our lack of process understanding and the small number of observations for model parameterisation and evaluation" (abstract, Arneth et al, 2008). While our measured whole-system fluxes are in reasonable agreement with predictions from global models, this agreement was based on offsetting errors in two important parameters.…”

“…More generally, errors such as these demonstrate that much more experimental data are necessary to improve the parameters used by these models. Moving past improved parameterization and more to the point of Arneth et al (2008), there are indications that the underlying algorithms used in MEGAN are not optimal for the tundra ecosystem. Adjusting two parameters in the algorithm improved the fit of the simple big-leaf model.…”

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…However, Guenther et al (1995) do not consider the CO 2 inhibition effect on isoprene emissions, implying that the future isoprene emissions (as projected in the RC2 simulations) may be too strong. Guenther et al (2006) estimated the annual total emissions of isoprene from biogenic origin to be 440 to 660 Tg (C) a −1 (see also Arneth et al, 2008, and references therein). All our simulations are within this range, however, these total emissions are further scaled with a factor of 0.6 to yield realistic mixing ratios of isoprene in the boundary layer (see Jöckel et al, 2006).…”