Estimates of global biogenic isoprene emissions from the terrestrial biosphere with varying levels of CO2

David J. Wilton1,2*, Kirsti Ashworth2, Juliette Lathière3, C. Nick Hewitt2 and David J. Beerling1

1 Department of Animal and Plant Sciences, University of Sheffield, UK; 2 Lancaster Environment Centre, Lancaster University, UK; 3 Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France

* Email : d.j.wilton@shef.ac.uk

Total Annual VOC Emissions (Tg C yr-1)

Calculated without (and with) CO2 suppression on isoprene

The Eocene climate produces higher emissions

0

1000

2000

3000

4000

5000

6000

Pre-Ind 2xPre-Ind 4xPre-Ind 6xPre-Ind

CO2 level

Isop

rene

Em

issi

ons

(Tg

C)

Effect of CO2 level on isoprene emissions, Eocene 4xPI climate

Isoprene emissions for the Eocene remain high unless we include the effect of CO2 suppression and have a much higher CO2 concentration.

References

[1] Guenther, A. et al, Atmos. Chem. Phys, 6, 3181-3210, 2006

[2] Guenther, A. et al, J. Geophys. Res. Atmos, 100, 8873-8892, 1995

[3] Beerling, D.J. & Woodward, F.I. (2001) Vegetation and the terrestrial carbon cycle. Cambridge University Press.

[4] Possell, M. et a, Global Change Biology, 1, 60-69, 2005

AcknowledgementsWe thank the Natural Environment Research Council for funding, Andy Fox and Paul Valdes (BRIDGE) for data.

Climate Model

PI Eocene 2xPI CO2 Eocene 4xPI CO2

Isoprene 823 2780 (1386) 5226 (1286)

Monoterpenes 80 277 457

Introduction

A variety of volatile organic compounds (VOCs) are produced and emitted by terrestrial ecosystems at the global scale that have a substantial impact on atmospheric chemistry. Estimates of above canopy fluxes of isoprene and monoterpenes are required for quantitative Earth system studies, and particularly atmospheric chemistry models used to assess changes in tropospheric ozone and secondary aerosol formation with past and future atmospheric CO2 regimes and climates. Here, we focus on the Earth system during the ‘greenhouse’ world of the Eocene, 50 million years ago and report a CO2 sensitivity analysis and comparison of global annual isoprene and monoterpene emissions for a pre-industrial (PI).

Methodology

We use an emission model for isoprene based on MEGAN (Model of Emissions of Gases and Aerosols from Nature) [1] adapted to take account of the suppression of isoprene emissions by atmospheric CO2 and a model for monoterpene emissions based on [2]. The Sheffield Dynamic Global Vegetation Model and the Hadley Centre ocean-atmosphere generation circulation model provide the inputs [3]. We compare results from a pre-industrial climate (CO2 = 280 ppm) with early Eocene climates at 2x and 4x PI CO2. Isoprene and monoterpene emissions are calculated, varying CO2 for the former based on the equation from [4]. The CO2 concentration in the Eocene is uncertain, but was much higher than modern levels and high CO2 is known to suppress isoprene emissions. We consider the effect of CO2 and climate on total annual emissions of the VOCs as well as the distribution of plant functional types.

Conclusions

The higher CO2, warmer Eocene climate enhances emissions of VOCs, even when the effect of CO2 suppression is taken into account. In particular, we can report substantially higher emissions of monoterpenes in the Eocene from far northerly and far southerly latitudes, as the climate allowed significant plant cover in these regions. These emissions can be used to derive estimates of secondary aerosol production which can in turn feed back in to the climate model. Work on this is in progress.

Comparison of emissions and vegetation calculated for different climates

Pre-Industrial Eocene 4xPI CO2

Sea

Bare

C3 grass

Evergreen Needle leaf

Deciduous Broad leaf

Evergreen Broad leaf

Deciduous Needle leaf

Isoprene Emissions / Tg yr-1

Monoterpene Emissions / Tg yr-1

Distribution of Major Plant Functional Types