Abstract
Abstract. Atmospheric oxidation of isoprene emission from land plants affects radiative forcing of global climate change. There is an urgent need to understand the factors that control isoprene emission variability on large spatiotemporal scales but such direct observations of isoprene emission do not exist. Two readily available global-scale long-term observation-based data sets hold information about surface isoprene activity: gross primary productivity (GPP) and tropospheric formaldehyde column variability (HCHOv). We analyze multi-year seasonal linear correlations between observed GPP and HCHOv. The observed GPP–HCHOv correlation patterns are used to evaluate a global Earth system model that embeds three alternative leaf-level isoprene emission algorithms. GPP and HCHOv are decoupled in the summertime in the southeast US (r=−0.03). In the Amazon, GPP and HCHOv are weakly correlated in March-April-May (MAM), correlated in June-July-August (JJA) and weakly anticorrelated in September-October-November (SON). Isoprene emission algorithms that include soil moisture dependence demonstrate greater skill in reproducing the observed interannual seasonal GPP–HCHOv correlations in the southeast US and the Amazon. In isoprene emission models that include soil moisture dependence, isoprene emission is correlated with photosynthesis and anticorrelated with HCHOv. In an isoprene emission model without soil moisture dependence, isoprene emission is anticorrelated with photosynthesis and correlated with HCHOv. Long-term monitoring of isoprene emission, soil moisture and meteorology is required in water-limited ecosystems to improve understanding of the factors controlling isoprene emission and its representation in global Earth system models.
Highlights
Isoprene emission, a by-product of photosynthesis, is fundamental in global chemistry–climate interactions
The direct comparison results are included in the Supplement for reference: simulated and FLUXNET-derived gross primary productivity (GPP) are of comparable absolute amounts (Fig. S1 in the Supplement), while simulated tropospheric HCHO columns are considerately higher than those obtained from the Ozone Monitoring Instrument (OMI) retrieval by about a factor of 2 (Fig. S2), which is likely due to the large uncertainties in the models as well as the satellite retrieval
We find that all three models reproduce the observed Northern Hemisphere (NH) midlatitude GPP–HCHO column variability (HCHOv) strong correlation in spring and fall, but predict anticorrelation in summer when the observations suggest decoupling
Summary
A by-product of photosynthesis, is fundamental in global chemistry–climate interactions. The global annual source strength is estimated at 0.5 Pg C year−1 (Guenther et al, 2006), which is of comparable magnitude to the present day total (anthropogenic and biogenic) annual source of methane (CH4) (Kirschke et al, 2013), and the net carbon dioxide (CO2) emission from land use change (Ciais et al, 2013). Isoprene emission rate depends upon ecosystem type, photosynthesis, temperature, and atmospheric CO2, and is sensitive to changes in land cover and climate (Monson et al, 2007). Largescale perturbations to isoprene emission influence global climate change (Scott et al, 2014; Unger, 2014a). In Earth’s history, plant isoprene emission is recognized as an important terrestrial biogeochemical feedback that influences the global climate sensitivity (Beerling et al, 2007, 2011; Unger and Yue, 2014). While shortterm (hours to days) weather-related fluctuations in isoprene emission in the temperate zone are well understood (Guenther et al, 1995, 1991), many open questions remain as to the long-term (months to years) factors controlling isoprene
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