ALBERT

Description: Solar Radiation Management (SRM) has been proposed as a means to partly counteract global warming. The Geoengineering Model Intercomparison Project (GeoMIP) has simulated the climate consequences of a number of SRM techniques. Thus far, the effects on vegetation have not yet been thoroughly analyzed. Here, the vegetation response to the idealized GeoMIP G1 experiment from eight fully coupled earth system models (ESMs) is analyzed, in which a reduction of the solar constant counterbalances the radiative effects of quadrupled atmospheric CO 2 concentrations (abrupt4xCO2). For most models and regions, changes in net primary productivity (NPP) are dominated by the increase in CO 2 , via the CO 2 fertilization effect. As SRM will reduce temperatures relative to abrupt4xCO2, in high latitudes this will offset increases in NPP. In low latitudes, this cooling relative to the abrupt4xCO2 simulation decreases plant respiration while having little effect on gross primary productivity, thus increasing NPP. In Central America and the Mediterranean, generally dry regions which are expected to experience increased water stress with global warming, NPP is highest in the G1 experiment for all models due to the easing of water limitations from increased water-use efficiency at high-CO 2 concentrations and the reduced evaporative demand in a geoengineered climate. The largest differences in the vegetation response are between models with and without a nitrogen-cycle, with a much smaller CO 2 fertilization effect for the former. These results suggest that until key vegetation processes are integrated into ESM predictions, the vegetation response to SRM will remain highly uncertain.

Description: Aerosol growth dynamics may have implications for the steerability of stratospheric solar radiation management via sulfur particles. This paper derives a set of critical initial growth conditions that are analyzed as a function of two key parameters: the initial concentration of the injected sulfuric acid and its dilution rate with the surrounding air. Based upon this analysis, early aerosol growth dynamical regimes may be defined and classified in terms of their likelihood to serve as candidates for the controlled generation of a radiatively effective aerosol. Our results indicate that the regime that fulfills all critical conditions would require that airplane turbines be used to provide sufficient turbulence. The regime's parameter space is narrow and related to steep gradients, thus pointing to potential fine tuning requirements. More research, development and testing would be required to refine our findings and determine their global scale implications.

Description: Atmospheric science: Asia under a high-level brown cloud Nature Geoscience 4, 352 (2011). doi:10.1038/ngeo1166 Author: Mark G. Lawrence Gaseous pollutants such as ozone and carbon monoxide from Asia are lifted to altitudes of more than 10 km during the summer monsoon season. Satellite observations show that aerosol particles, too, can rise high and spread across thousands of kilometres.

Description: The Indo-Gangetic Plain (IGP) region is one of the most densely populated regions in the World, but ground-based observations of air pollutants are highly limited in this region. Here, surface ozone observations made during March 2009–June 2011 at a semi-urban site (Pantnagar; 29.0°N, 79.5°E, 231 m amsl) in the IGP region are presented. Ozone mixing ratios show a daytime photochemical buildup with ozone levels sometimes as high as 100 ppbv. Seasonal variation in 24-h average ozone shows a distinct spring maximum (39.3 ± 18.9 ppbv in May) while daytime (1130–1630 h) average ozone shows an additional peak during autumn (48.7 ± 13.8 ppbv in November). The daytime, but not daily average, observed ozone seasonality is in agreement with the space-borne observations of OMI tropospheric column NO2, TES CO (681 hPa), surface ozone observations at a nearby high altitude site (Nainital) in the central Himalayas and to an extent with results from a global chemistry transport model (MATCH-MPIC). It is suggested that spring and autumn ozone maximum are mainly due to photochemistry, involving local pollutants and small-scale dynamical processes. Biomass burning activity over the northern Indian region could act as an additional source of ozone precursors during spring. The seasonal ozone photochemical buildup is estimated to be 32–41 ppbv during spring and autumn and 9–14 ppbv during August–September. A correlation analysis between ozone levels at Pantnagar and Nainital along with the mixing depth data suggests that emissions and photochemical processes in the IGP region influence the air quality of pristine Himalayan region, particularly during midday hours of spring. The evening rate of change (8.5 ppbv hr−1) is higher than the morning rate of change, which is dissimilar to those at other urban or rural sites. Ozone seasonality over the IGP region is different than that over southern India. Results from the MATCH-MPIC model capture observed ozone seasonality but overestimate ozone levels. Model simulated daytime ratios of H2O2/HNO3 are higher and suggesting that this region is in a NOx-limited regime. A chemical box model (NACR Master Mechanism) is used to further corroborate this using a set of sensitivity simulations, and to estimate the integrated net ozone production in a day (72.9 ppbv) at this site.

Description: Hydroxyl radical buffered by isoprene oxidation over tropical forests Nature Geoscience 5, 190 (2012). doi:10.1038/ngeo1405 Authors: D. Taraborrelli, M. G. Lawrence, J. N. Crowley, T. J. Dillon, S. Gromov, C. B. M. Groß, L. Vereecken & J. Lelieveld The hydroxyl radical is a key oxidant in the Earth’s atmosphere. This short-lived highly reactive molecule plays an important role in the degradation of volatile organic compounds, leading to the production of ozone and the formation and growth of aerosol particles. In this way, hydroxyl radicals influence air quality and regional climate. Measurements over tropical forests suggest that hydroxyl radicals are recycled following reaction with the volatile organic compound isoprene, but the chemistry underpinning this observation is uncertain. Here, we propose a detailed chemical mechanism for the oxidation of isoprene by hydroxyl radicals. The photo-oxidation of unsaturated hydroperoxy-aldehydes—a product of isoprene oxidation—is a central part of the mechanism; their photolysis initiates a hydroxyl radical production cascade that is limited by the reaction of hydroperoxy-aldehydes with the hydroxyl radical itself. We incorporate this mechanism into a global atmospheric chemistry model and find that measurements of hydroxyl radical concentrations over a pristine region of the Amazon, and in moderately polluted conditions, are captured well. On the basis of this agreement, we suggest that isoprene oxidation can buffer hydroxyl radical concentrations, by serving as both a sink and source for these radicals.

Description: [1]  The Geoengineering Model Intercomparison Project (GeoMIP) was designed to determine robust climate system model responses to solar geoengineering. GeoMIP currently consists of four standardized simulations involving reduction of insolation or increased amounts of stratospheric sulfate aerosols. Three more experiments involving marine cloud brightening are planned. This project has improved confidence in the expected climate effects of geoengineering in several key areas, such as the effects of geoengineering on spatial patterns of temperature and the spatial distribution of precipitation, especially extreme precipitation events. However, GeoMIP has also highlighted several important research gaps, such as the effects on terrestrial net primary productivity and the importance of the CO 2 physiological effect in determining the hydrologic cycle response to geoengineering. Future efforts will endeavor to address these gaps, as well as encourage cooperation with the chemistry modeling communities, the impact assessment communities, and other groups interested in model output.

Description: Hydroxyl radical buffered by isoprene oxidation over tropical forests Nature Geoscience 5, 300 (2012). doi:10.1038/ngeo1433 Authors: D. Taraborrelli, M. G. Lawrence, J. N. Crowley, T. J. Dillon, S. Gromov, C. B. M. Groß, L. Vereecken & J. Lelieveld

Description: Large point sources such as major population centers (MPCs) emit pollutants which can be deposited nearby or transported over long distances before deposition. We have used tracer simulations of aerosols emitted from MPCs worldwide to assess the fractions which are deposited at various distances away from their source location. Considering only source location, prevailing meteorology, and the aerosol size and solubility, we show that fine aerosol particles have a high potential to pollute remote regions. About half of the emitted mass of aerosol tracers with an ambient diameter ≤1.0 μm is typically deposited in regions more than 1000 km away from the source. Furthermore, using the Köppen-Geiger climate classification to categorize the sources into various climate classes we find substantial differences in the deposition potential between these classes. Tracers originating in arid regions show the largest remote deposition potentials, with values more than doubled compared to the smallest potentials from tracers in tropical regions. Seasonal changes in atmospheric conditions lead to variations in the remote deposition potentials. On average the remote deposition potentials in summer correspond to about 70–80% of the values in winter, with a large spread among the climate classes. For tracers from tropical regions the summer remote deposition values are only about 31% of the winter values, while they are about 95% for tracers from arid regions.

Description: Nature Climate Change 4 842 doi: 10.1038/nclimate2360

Description: Despite a growing literature on the climate response to solar geoengineering – proposals to cool the planet by increasing the planetary albedo – there has been little published on the impacts of solar geoengineering on natural and human systems such as agriculture, health, water resources, and ecosystems. An understanding of the impacts of different scenarios of solar geoengineering deployment will be crucial for informing decisions on whether and how to deploy it. Here we review the current state of knowledge about impacts of a solar geoengineered climate and identify major research gaps. We suggest that a thorough assessment of the climate impacts of a range of scenarios of solar geoengineering deployment is needed and can build upon existing frameworks. However, solar geoengineering poses a novel challenge for climate impacts research as the manner of deployment could be tailored to pursue different objectives making possible a wide range of climate outcomes. We present a number of ideas for approaches to extend the survey of climate impacts beyond standard scenarios of solar geoengineering deployment to address this challenge. Reducing the impacts of climate change is the fundamental motivator for emissions reductions and for considering whether and how to deploy solar geoengineering. This means that the active engagement of the climate impacts research community will be important for improving the overall understanding of the opportunities, challenges and risks presented by solar geoengineering.