Response of isoprene emission to ambient CO₂ changes and implications for global budgets

Abstract

We explore the potential role of atmospheric carbon dioxide (CO 2 ) on isoprene emissions using a global coupled land-atmosphere model [Community Atmospheric Model-Community Land Model (CAM-CLM)] for recent (year 2000, 365 ppm CO 2 ) and future (year 2100, 717 ppm CO 2 ) conditions. We incorporate an empirical model of observed isoprene emissions response to both ambient CO 2 concentrations in the long-term growth environment and short-term changes in intercellular CO 2 concentrations into the MEGAN biogenic emission model embedded within the CLM. Accounting for CO 2 inhibition has little impact on predictions of present-day global isoprene emission (increase from 508 to 523 Tg C yr À1 ). However, the large increases in future isoprene emissions typically predicted in models, which are due to a projected warmer climate, are entirely offset by including the CO 2 effects. Projected global isoprene emissions in 2100 drop from 696 to 479 Tg C yr À1 when this effect is included, maintaining future isoprene sources at levels similar to present day. The isoprene emission response to CO 2 is dominated by the long-term growth environment effect, with modulations of 10% or less due to the variability in intercellular CO 2 concentration. As a result, perturbations to isoprene emissions associated with changes in ambient CO 2 are largely aseasonal, with little diurnal variability. Future isoprene emissions increase by more than a factor of two in 2100 (to 1242 Tg C yr À1 ) when projected changes in vegetation distribution and leaf area density are included. Changing land cover and the role of nutrient limitation on CO 2 fertilization therefore remain the largest source of uncertainty in isoprene emission prediction. Although future projections suggest a compensatory balance between the effects of temperature and CO 2 on isoprene emission, the enhancement of isoprene emission due to lower ambient CO 2 concentrations did not compensate for the effect of cooler temperatures over the last 400 thousand years of the geologic record (including the Last Glacial Maximum).

Climate Feedback on Aerosol Emission and Atmospheric Concentrations

“…Carslaw et al [3] summarize several earlier publications that show that projections of future changes in isoprene emissions from climate effects (i.e., increasing temperatures) from year 2000 to 2100 range from an increase by 20 to 55%, increasing up to 90% when dynamic vegetation is included [30]. The response to land use changes ranged in different studies from − 20 to + 30%, while a decrease of 8% was predicted in response to increasing CO 2 concentrations [31]. Monoterpene emissions were projected to increase from 20 to 60% considering the effect of changed climates [32][33][34].…”

Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years

“…Global modelling studies simulating future isoprene emissions have shown that including an algorithm that accounts for inhibition of the isoprene emissions from plants due to elevated CO 2 concentration can significantly diminish the predicted isoprene increase (e.g. Heald et al, 2009). However, recent studied suggest that this inhibition may not occur in warmer climate (Sun et al, 2013) and a better understanding of this process is needed.…”

“…The γ CO 2 factor is a new MEGANv2.1 model feature, which accounts for the impact of concentration of carbon dioxide (CO 2 ) on isoprene emission. Following the parameterization of Heald et al (2009), isoprene emissions decrease when ambient CO 2 concentration increases above the level of 400 ppmv (parts per million by volume). In view of the lack of clear experimental evidence of an effect, γ CO 2 is set to 1 for all other species.…”

Aerosol loading in the Southeastern United States: reconciling surface and satellite observations

“…However, given limitations in both our measurement and understanding of BVOC emissions and SOA formation (Hallquist et al, 2009), it is unclear whether biogenic emissions, and the aerosols produced upon oxidation of these emissions, have also changed over this same time period. The evolution of these biogenic emissions is difficult to predict (e.g., Heald et al, 2009), representing a significant hurdle for future air quality management efforts and the prediction of climate forcing.…”