CO2 inhibition of global terrestrial isoprene emissions: Potential implications for atmospheric chemistry



[1] Isoprene is the dominant volatile organic compound produced by the terrestrial biosphere and fundamental for atmospheric composition and climate. It constrains the concentration of tropospheric oxidants, affecting the lifetime of other reduced species such as methane and contributing to ozone production. Oxidation products of isoprene contribute to aerosol growth. Recent consensus holds that emissions were low during glacial periods (helping to explain low methane concentrations), while high emissions (contributing to high ozone concentrations) can be expected in a greenhouse world, due to positive relationships with temperature and terrestrial productivity. However, this response is offset when the recently demonstrated inhibition of leaf isoprene emissions by increasing atmospheric CO2 concentration is accounted for in a process-based model. Thus, isoprene may play a small role in determining pre-industrial tropospheric OH concentration and glacial-interglacial methane trends, while predictions of high future tropospheric O3 concentrations partly driven by isoprene emissions may need to be revised.

1. Introduction

[2] Isoprene is the dominant volatile organic compound (VOC) produced by the terrestrial biosphere, recent global estimates have been converging around 0.5 Pg C emitted annually by plants [Guenther et al., 1995, 2006; Lathière et al., 2006]. Reactions of isoprene play a major role in determining the concentration of oxidants in the troposphere, thus contributing to the regulation of the lifetime of other reduced species such as methane and, in the presence of nitrogen oxides, to production of tropospheric ozone, a greenhouse gas and toxic pollutant [Jenkin and Clemitshaw, 2000; Collins et al., 2002]. Condensed oxidation products of isoprene also contribute to biogenic aerosol growth [Claeys et al., 2004; Henze and Seinfeld, 2006]. Understanding the temporal and spatial patterns of isoprene emissions is therefore fundamental for explaining variability in atmospheric composition and climate and is important for the development of effective and sustainable air quality regulations.

[3] The short-term environmental responses of leaf isoprene emission are firmly established experimentally. Isoprene emission follows a hyperbolic relationship with light intensity and a modified Arrhenius relationship with temperature. These findings are the basis for widely used, empirical algorithms that underlie the current global emission estimates [Guenther et al., 1995; Guenther et al., 2006].

[4] The light and temperature responses of leaf isoprene emission have also been used to project global responses to climate and atmospheric CO2 concentration changes in the past and future. Model experiments have indicated emissions substantially below present under glacial maximum (cool, low-CO2) conditions, due to the isoprene-temperature response and reduced vegetation productivity [Adams et al., 2001; Lathière et al., 2005; Valdes et al., 2005; Kaplan et al., 2006], and correspondingly up to twice as high as present in a warm, high-CO2 world [Sanderson et al., 2003; Lathière et al., 2005; Guenther et al., 2006]. However, the direct effect of CO2 on leaf emissions has been neglected in these global analyses. Most experimental studies have shown a several-fold increase of leaf emissions from plants grown at sub-ambient CO2, and a decline at above-ambient CO2 concentration [Arneth et al., 2007]. This response has been observed in woody as well as herbaceous growth forms and for species from tropical to boreal environments [Arneth et al., 2007]. The biochemical basis for these observations is not fully resolved, though one compelling suggestion relates isoprene production to CO2-mediated variations in the cellular sink distribution for key substrates such as pyruvate, which would render the observed CO2 response as general among all isoprene emitting plants [Rosenstiel et al., 2004].

[5] Here we examine the hypothesis that the direct CO2 effect on isoprene emissions might substantially offset the changes due to changing temperature and terrestrial productivity. We apply a recently developed model that couples a process-based leaf isoprene emission algorithm to the dynamic global vegetation model framework LPJ-GUESS [Smith et al., 2001; Sitch et al., 2003; Arneth et al., 2007]. We calculate emissions for the last glacial maximum (LGM), the early and late 20th century, and a range of climatic futures.

2. Modeling Protocol

[6] Our model analysis differs in essential aspects from the empirical algorithms used in previous work [Guenther et al., 1995, 2006; Lathière et al., 2006]. Here, isoprene production in the leaf is linked to carbon assimilation, specifically electron transport, which supplies ATP and redox equivalents (NADPH) required for the synthesis of the chief isoprene precursors [Niinemets et al., 1999]. Isoprene inhibition by increasing atmospheric CO2 concentration is represented by a response to changing CO2 at the cellular level, mimicking the incomplete coupling of assimilation to isoprene due to substrate supply [Rosenstiel et al., 2004; Arneth et al., 2007]. The resulting response of modeled leaf isoprene emissions was indistinguishable from an empirical curve through measured emissions at a range of growth CO2 levels although considerable uncertainties remain caused by the available data being limited. Simulated canopy emissions were evaluated against observations under present-day conditions and agreed well with data-based estimates [Arneth et al., 2007]. The vegetation model has been shown to correctly reproduce the CO2-response of ecosystem net primary production on multi-annual time scales [Gerten et al., 2005].

[7] LPJ-GUESS was used in the global mode that represents potential vegetation by 10 plant functional types (PFT) [Smith et al., 2001; Sitch et al., 2003]. Leaf isoprene emissions are normally reported as emission factors Is, for a temperature of 30°C and quantum flux density of 1000 μmol m−2 s−1 and a wide range of observations have been used to recommend average PFT values of Is [Guenther et al., 1995]. Our model does not use Is directly, the link between leaf isoprene synthesis, assimilation rate and vegetation composition is provided by ɛ, the fraction of electrons used for isoprene production, and the electron transport rate J [Niinemets et al., 1999]. In an approach that takes advantage of the published Is we instead assign PFT-specific values of ɛ such that I = Is under standard environmental conditions and CO2 concentration = 370 ppm, based on the calculated value of J under these conditions [Arneth et al., 2007] (Table S1 in the online auxiliary material).

[8] Climate input is provided as monthly values of temperature, diurnal temperature range, precipitation, and fractional sunshine hours on 0.5° horizontal resolution. For the 20th century, the model was driven by CO2 concentrations derived from ice-core measurements and (later) atmospheric observations, and by the gridded climate data set from the Climatic Research Unit (CRU) for 1901–2000 [Mitchell and Jones, 2005]. Future projections were based on simulations with the UK Met Office HadCM3 and NCAR PCM1 climate models under the SRES A2, B1, and “committed” (i.e., atmospheric CO2 levels held constant after the year 2000) greenhouse-gas emissions scenarios [Scholze et al., 2006]. Climate anomalies and CO2 inputs for LGM were obtained from UK Met Office UM-AGCM (4.4) using HadAM3 coupled to a slab ocean ( Future and LGM climate model anomalies were interpolated from their original resolutions, and applied to a baseline climatology from the CRU dataset. Here we compare two model experiments: a “traditional” experiment in which the direct isoprene-CO2 effect is disregarded, and an experiment with this direct CO2 effect included. Photosynthesis and vegetation productivity respond to CO2 in both experiments. In calculations using the committed scenario the distinction is of no consequence since these apply a constant CO2 concentration of 370 ppm from 2000 onwards.

3. Results

[9] Modeled global total emissions from the terrestrial biosphere average 0.41 PgC a−1 over the period 1981–2000 with a regional distribution similar to previous modeling experiments (Figure 1) [Naik et al., 2004; Guenther et al., 2006; Lathière et al., 2006]. The modeled global total is about 20% lower than recent estimates for emissions from potential natural vegetation based on the standard algorithms (0.45–0.56 PgC a−1 [Adams et al., 2001; Sanderson et al., 2003; Naik et al., 2004]). A lower estimate is consistent with the fact that many (though not all) atmospheric chemistry and transport models can only match observational constraints on isoprene, or concentrations of trace gases that are linked to its chemistry (such as carbon monoxide), when emissions are adjusted downward [Guenther et al., 2006; Stevenson et al., 2006].

Figure 1.

Average isoprene emissions from potential natural vegetation (gC m−2 a−1) for 1981–2000.

[10] Simulations that disregard the direct effect atmospheric CO2 concentration has on leaf isoprene metabolism yield substantially reduced isoprene emission in the glacial world, and strongly increased emission in future projections of climate and atmospheric CO2 levels (Table 1 and Figure 2), in agreement with previous model results [Adams et al., 2001; Sanderson et al., 2003; Lathière et al., 2005; Valdes et al., 2005; Guenther et al., 2006; Kaplan et al., 2006]. Isoprene synthase has a high temperature optimum (Topt), ≈45°C in vitro and leaf emissions therefore respond strongly to temperature [Monson et al., 1992]. Plants may acclimate their isoprene emission potential to changes in their growth environment but the relative effects of short-term variability vs. long-term temperature trends are unknown. Our model accounts for the higher Topt of isoprene emission compared to photosynthesis [Arneth et al., 2007] but a relative Topt difference remains between the PFTs that is related to their respective photosynthesis-temperature response [Sitch et al., 2003]. Attempts to allow for effects of recent weather history introduced empirical tuning relationships [Guenther et al., 2006], but since understanding of the underlying cellular control is still lacking, and since the model is driven by monthly climate we did not consider such an approach to be appropriate and emphasize instead the need for further research on that question.

Figure 2.

Isoprene emissions from potential natural vegetation (gC m−2) for the last glacial maximum (20 year average), and average for 2081–2100 using climate data for the HadCM3 and PCM1 models forced with SRES A2. Numbers are anomalies relative to ambient (gC m−2). Panels on the left are for ‘traditional’ calculations without direct CO2-isoprene interactions, panels on the right include the direct CO2-isoprene effect.

Table 1. Total Global Leaf Area, Gross Primary Productivity and Isoprene Emissions From Potential Natural Vegetation, for the Last Glacial Maximum, Early 20th Century and Late 21st Centurya
 Leaf AreaGPPIsoprene Emissions
CO2 Affecting Only GPP, not IsoprenebCO2 Affecting GPP and Isoprene
  • a

    GPP is gross primary productivity; LGM is Last Glacial Maximum (20-year-average); early 20th century is 1901–20; and late 20th century is 2081–00, SRES A2, B1 & committed. Numbers are relative to the average value simulated for the period 1981–2000 (leaf area: 5.5 1014 m2; GPP: 127 PgC a−1; Isoprene emissions: 0.41 PgC a−1).

  • b

    “Traditional” experiment.

Early 20th century0.980.90.881.09

[11] Emissions are influenced by vegetation and leaf area distribution. For this reason, global emissions simulated under the HadCM3 and PCM1 generated climates are almost the same despite the 40% greater warming projected the former [Scholze et al., 2006]. HadCM3 projects not only a warmer but also a drier world, with smaller increases in LAI and GPP compared to PCM1 (Table 1) and with a shift towards non-forest vegetation (with lower isoprene emission potential) in parts of Amazonia (Figure 2). In these simulations climate is a primary control on isoprene emission, acting both directly on leaves and via the responses of plant growth and biome shifts to rainfall reduction.

[12] In contrast, a quite different result is obtained when the direct CO2 effect on isoprene emissions is included. On the scale of the leaf, isoprene emissions more than double in a 200 ppm CO2 growth environment, and are halved in a 1000 ppm environment [Possell et al., 2005; Arneth et al., 2007]. This effect largely counteracts the combined temperature and primary productivity effect, and, when the isoprene-CO2 interaction is accounted for in the model, maintains global isoprene emissions within ± 15% of present values (Table 1).

4. Discussion and conclusions

[13] Scientists have struggled to explain the glacial-interglacial methane difference preserved in ice cores. Low methane concentrations at the LGM were once attributed to lower prevalence of wetlands compared to present day, but although continental areas were dryer on average, this was offset by the presence of wetlands on the exposed continental shelves. More recent studies have invoked a concomitant change in the atmospheric methane sink strength, methane levels being highly sensitive to variations in OH [Valdes et al., 2005; Kaplan et al., 2006]. For pre-industrial tropospheric conditions, lower levels of VOCs would lessen the competition for oxidants and could reduce the atmospheric residence time of methane by several years [Poisson et al., 2000; Collins et al., 2002; Kaplan et al., 2006]. A decreasing ratio of simulated CH4 to total non-methane reactive carbon (RCC) from 0.25 to 0.19 from the LGM to pre-industrial conditions was accompanied by a 22% decline in tropospheric OH concentration [Kaplan et al., 2006]. In these calculations, isoprene and monoterpene emissions at the LGM were c. 65% of pre-industrial values, respectively.

[14] Our results do not support this scenario. Given that isoprene is by far the dominant biogenic VOC, assuming that the observed isoprene-CO2 response is indeed general for all plants, and that emission of monoterpenes also displays CO2 inhibition [Loreto et al., 2001; Rapparini et al., 2004], we suggest that total VOC emissions have varied much less between the LGM and today than previously thought. Monoterpenes are mostly produced in the chloroplast along a similar pathway as isoprene, and emitted either directly (like isoprene) or from storage organs. A reduction in isoprene and monoterpene emissions of only approximately 15% at LGM as suggested by our model (Table 1) results in a ratio of CH4 to RCC of 0.2 at the LGM, or 0.21 when only isoprene is reduced by 15%. These ratios are nearly identical to the pre-industrial values and imply a rather more stable OH concentration over the Holocene. The observed glacial-interglacial change in methane concentration thus remains enigmatic.

[15] Isoprene is an important precursor for photochemical production of tropospheric ozone, a pollutant and greenhouse gas, in the presence of hydrogen-containing free radicals and NOx [Jenkin and Clemitshaw, 2000]. The production of ozone at ground level is of particular concern because of its adverse effects on human and plant health. The O3 burden in urban environments is believed to have risen by several tenths of ppb over the last one to two centuries[Prather et al., 2001], and is projected to further increase considerably [Liao et al., 2006; Stevenson et al., 2006]. Poor constraints on isoprene emissions are a major uncertainty in simulations of pre-industrial-to-present changes in radiative forcing of O3 [Shindell et al., 2003]. Depending on the ratio of NOx to isoprene emission and the distance of isoprene sources from NOx-containing plumes, the contribution of biogenic isoprene to ozone peaks can be well above 30%, even in urban areas, where VOC production is dominated by fossil-fuel sources [Cortinovis et al., 2005]. The tropospheric background concentration of ozone is also affected by changes in isoprene emissions [Wang and Shallcross, 2000; Collins et al., 2002]. One study [Sanderson et al., 2003] predicted increases of up to 20–30 ppbv in regional ground-level ozone concentrations for a relatively moderate climate warming scenario, partly driven by a global isoprene emission increase of 25%. Our results suggest that such a large isoprene emission increase is unlikely.

[16] Accounting for isoprene as a source of secondary organic aerosol (SOA) [Claeys et al., 2004] can have substantial effect on tropospheric SOA concentration with the potential of increasing free tropospheric levels by up to 170% of the base value [Henze and Seinfeld, 2006]. This effect was attributed to the source strength of isoprene that results in oxidation products being transported to higher altitudes, and the gas-particle partitioning of the isoprene oxidation products that require lower temperatures to be shifted toward the particle phase. In addition, the isoprene-related SOA also enhanced the formation from other sources. The increase was particularly strong in remote marine environments where aerosol concentrations are thought to represent clean conditions with relatively little human impact. The centennial to millennial trends in isoprene emissions have important implications for attempts to assess the aerosol conditions and concentration of cloud condensation nuclei in the pristine atmosphere as it is believed that without anthropogenic pollution cloud microphysical properties over the continents would be similar to marine conditions [Andreae, 2007].

[17] We conclude that disregarding the direct CO2-isoprene interaction in global change simulations likely leads to unrealistic responses in modeled tropospheric OH, ozone or secondary aerosol formation, but our results plainly also indicate the need for a larger number of experiments on CO2-isoprene interactions. There is considerable scope, for isoprene (and oxidant and aerosol) concentrations to be further modified, up or down, by vegetation cover change. For example, tropical deforestation is estimated to reduce isoprene emission [Lathière et al., 2006] while some widely-planted tree crops, including Populus spp. (poplar/aspen), Eucalyptus ssp. or oil palm are among the highest isoprene emitters known. This observation suggests that realistic future projections may also have to consider land use, rather than simply temperature and productivity, as a potential factor in controlling VOC emissions, air quality and reactive chemistry.


[18] A.A. acknowledges support from a European Union FP6 Marie Curie Excellence grant, from Vetenskapsrådet, and from VOCBAS. We acknowledge the international modeling groups for providing their data for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the JSC/CLIVAR Working Group on Coupled Modeling (WGCM) and their Coupled Model Intercomparison Project (CMIP) and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC Working Group 1 Technical Services Unit for technical support. Paul Valdes at the Bristol Research Initiative for the Dynamic Global Environment (BRIDGE) kindly supplied the LGM climate data.