Table of contents

We use CMIP3 multi-model simulations to show how individual hydroclimatic changes will concur to determine even greater alterations of 21st century Mediterranean water cycle characteristics, with contrasting behavior over land and sea. By 2070–2099, the average of the models predicts a 20% decrease in land surface water availability and a 24% increase in the loss of fresh water over the Mediterranean Sea due to precipitation reduction and warming-enhanced evaporation, with a remarkably high consensus among analyzed models. The projected decrease in river runoff from the surrounding land will further exacerbate the increase in Mediterranean Sea fresh water deficit.

20th century simulations indicate that the 'transition' toward drier conditions has already started to occur and has accelerated around the turn of the century towards the larger rates projected for the 21st century. These tendencies are supported by observational evidence of century-long negative trends in regionally averaged precipitation, PDSI and discharge from numerous rivers; and are consistent with reported increases in Mediterranean sea water salinity.

Global policy targets for greenhouse gas emissions reductions are being negotiated. The amount of emitted carbon dioxide remaining in the atmosphere is controlled by carbon cycle processes in the ocean and on land. These processes are themselves affected by climate. The resulting 'climate–carbon cycle feedback' has recently been quantified, but the policy implications have not. Using a scheme to emulate the range of state-of-the-art model results for climate feedback strength, including the modelled range of climate sensitivity and other key uncertainties, we analyse recent global targets. The G8 target of a 50% cut in emissions by 2050 leaves CO₂ concentrations rising rapidly, approaching 1000 ppm by 2300. The Stern Review's proposed 25% cut in emissions by 2050, continuing to an 80% cut, does in fact approach stabilization of CO₂ concentration on a policy-relevant (century) timescale, with most models projecting concentrations between 500 and 600 ppm by 2100. However concentrations continue to rise gradually. Long-term stabilization at 550 ppm CO₂ requires cuts in emissions of 81 to 90% by 2300, and more beyond as a portion of the CO₂ emitted persists for centuries to millennia. Reductions of other greenhouse gases cannot compensate for the long-term effects of emitting CO₂.

Previously we identified associations between the mother's air pollution exposure and birth weight for births in Connecticut and Massachusetts from 1999–2002. Other studies also found effects, though results are inconsistent. We explored potential uncertainties in earlier work and further explored associations between air pollution and birth weight for PM₁₀, PM_2.5, CO, NO₂, and SO₂. Specifically we investigated: (1) whether infants of younger (≤24 years) and older (≥40 years) mothers are particularly susceptible to air pollution's effects on birth weight; (2) whether the relationship between air pollution and birth weight differed by infant sex; (3) confounding by co-pollutants and differences in pollutants' measurement frequencies; and (4) whether observed associations were influenced by inclusion of pre-term births. Findings did not indicate higher susceptibility to the relationship between air pollution and birth weight based on the mother's age or the infant's sex. Results were robust to exclusion of pre-term infants and co-pollutant adjustment, although sample size decreased for some pollutant pairs. These findings provide additional evidence for the relationship between air pollution and birth weight, and do not identify susceptible sub-populations based on infant sex or mother's age. We conclude with discussion of key challenges in research on air pollution and pregnancy outcomes.

We use root exclusion plots, subsurface gas sampling and in situ diffusivity measurements to quantify in situ soil organic carbon (SOC) decomposition dynamics within separate depth-dependent soil pools (0 and 35 cm). We contrast these measurements with observations of temperature–decomposition potentials, generated from laboratory incubations of the same soils at optimal moisture levels and native temperatures. The decomposition–temperature response was similar at different depths in the field, but every gram of soil C at 35 cm was more than 100 times less active in decomposition than surface soil. These depth-related variations were not evident in decomposition potentials generated from aerobic laboratory incubations, highlighting the importance of environmental physical factors in constraining soil organic carbon decomposition. At depth, physical protection of SOC could match or even override the importance of quality and temperature in determining the future stability of deeper, recalcitrant pools.

Daily rainfall and temperature data of 158 weather stations in eight European countries and Iceland are investigated to set up a weekly cycle. The time series are divided into five time slices that are analyzed separately. As they depend strongly on the data availability, the significance of weekly cycles is generally higher for the past three time slices of 1931–1960, 1961–1990, and 1991–2005 compared to the two earlier analyzed time slices of 1871–1900 and 1901–1930.

Precipitation does not follow any distinct significant weekly cycle. For temperature, however, significant weekly cycles exist in all analyzed countries. The weekly periodicities cannot be explained by random effects. A clear weekly signal is detected by means of a stationary block bootstrap approach. The cycles of temperature vary with the region and the time slice. However, they are found to be more stable for the last two time slices. For the dominant pattern of the weekly cycle in Germany, a coinciding significant weekly cycle of the large-scale circulation is detected for the time slice 1991–2005.

In Germany, persistence can be observed for the weekday holding the minimum value of the temperature variables. The minimum is observed to occur on Saturday for the past two time slices. When judging from significant results exclusively, most other countries also show persistence for the past two time slices, except for the weekday with the maximum value of the temperature variables. This weekday either is Tuesday for Iceland and the UK or Wednesday for Sweden and Norway.

Policies for climate mitigation on land rarely acknowledge biophysical factors, such as reflectivity, evaporation, and surface roughness. Yet such factors can alter temperatures much more than carbon sequestration does, and often in a conflicting way. We outline a framework for examining biophysical factors in mitigation policies and provide some best-practice recommendations based on that framework. Tropical projects—avoided deforestation, forest restoration, and afforestation—provide the greatest climate value, because carbon storage and biophysics align to cool the Earth. In contrast, the climate benefits of carbon storage are often counteracted in boreal and other snow-covered regions, where darker trees trap more heat than snow does. Managers can increase the climate benefit of some forest projects by using more reflective and deciduous species and through urban forestry projects that reduce energy use. Ignoring biophysical interactions could result in millions of dollars being invested in some mitigation projects that provide little climate benefit or, worse, are counter-productive.

It has been conjectured that global warming will increase the prevalence of insect pests in many agro-ecosystems. In this paper, we quantitatively assess four of the key pests of maize, one of the most important systems in North American grain production. Using empirically generated estimates of pest overwintering thresholds and degree-day requirements, along with climate change projections from a high-resolution climate model, we project potential future ranges for each of these pests in the United States. Our analysis suggests the possibility of increased winter survival and greater degree-day accumulations for each of the pests surveyed. We find that relaxed cold limitation could expand the range of all four pest taxa, including a substantial range expansion in the case of corn earworm (H. zea), a migratory, cold-intolerant pest. Because the corn earworm is a cosmopolitan pest that has shown resistance to insecticides, our results suggest that this expansion could also threaten other crops, including those in high-value areas of the western United States. Because managing significant additional pressure from this suite of established pests would require additional pest management inputs, the projected decreases in cold limitation and increases in heat accumulation have the potential to significantly alter the pest management landscape for North American maize production. Further, these range expansions could have substantial economic impacts through increased seed and insecticide costs, decreased yields, and the downstream effects of changes in crop yield variability.

Climate–environment variability affects the rates of incidence of vector-borne and zoonotic diseases and is possibly associated with epidemics outbreaks. Over southernmost South America the joint spatio-temporal evolution of climate–environment is analyzed for the 1982–2004 period. Detailed mapping of normalized difference vegetation index (NDVI) and rainfall variability are then compared to zones with preliminary epidemiological reports. A significant quasi-biennial signal (2.2- to 2.4-year periods, or QB) for joint NDVI–rainfall variability is revealed. From rotated EOFs, dominant NDVI patterns are partitioned according to their lead frequencies: (1) the 'QB group' (2.1-to 3-year periods) includes six modes over southern Brazil, Uruguay, northern-central Argentina (two modes), the southern Paraguay–northern Argentina border, and the Santa Cruz Province; (2) the QB1 (2.4- to 3-year periods) + quasi-quadrennial (QQ) mode over the Misiones Province; and (3) the QB2 (2.1- to 2.5-year periods) + QQ + inter-annual (IA) (3- to 7-year periods) two modes over south-eastern Argentina. Modes within the 'QB group' are positively correlated with global climate signals and SST. The Uruguayan mode is correlated with global ENSO (8-month lag) whilst the southern Entre-Rios/northern Buenos Aires provinces are correlated with central equatorial Pacific SSTs (3-month lag). The Santa Cruz (Patagonia) Province is most correlated with the Pacific South America (PSA) index and SST patterns (3-month lag) along the Antarctica circumpolar current. The spatial distribution of lead NDVI modes includes the Formosa, Misiones, Chaco and Buenos Aires provinces among others, known for being prone to vector-borne epidemics such as dengue fever, malaria, leishmaniasis (American cutaneous leishmaniasis or ACL), hantivirus, chagas and Argentine hemorrhagic fever (AHF). Some provinces also correspond to regions where lead NDVI PCs' modes are associated with high-frequency climate signals such as the quasi-biennial oscillation in northwest Argentina. The joint preliminary results (climate–environment–public health reports) presented here for the first time are meant: (1) to contribute to a better understanding of climate–environment–epidemics process-based and modeling studies and (2) to facilitate, in the long run, the implementation of local and regional health early warning systems (HEWS) over southernmost South America. The latter is becoming crucial with ever-increasing migration, urban sprawl (re-emergence of dengue fever epidemics since the late 1990s), all embedded in a climate change context.

Reducing the environmental impact of supplying electricity is a key to China's sustainable development, and a focus of both domestic and international concerns with greenhouse gas emissions. The environmental performance of the electricity sector is strongly affected by its institutional arrangements: regulatory frameworks, wholesale markets, pricing mechanisms, planning and coordination, and enforcement and incentive mechanisms. These arrangements are set to change as electricity reforms inaugurated in 2002, but sidetracked by several years of supply shortages, are being resumed. In this paper we examine the impact of electricity reform on environmental sustainability by analyzing case studies of four environmental initiatives in the electricity sector: retirement of inefficient generators, installation of pollution control equipment, renewable energy development, and efforts to promote energy efficiency. We find that implementation of these policies falls short of objectives for two main underlying reasons: conflicting priorities between central and provincial governments, and ineffective regulation. Sustainability will be best served not by redoubling short-term supply-oriented, market-based reforms, but by better aligning central and provincial government incentives, and by developing competent, independent regulation at the provincial level. China's central government and sub-national governments in industrialized countries can both contribute to the latter goal.

Salt marshes are among the most productive ecosystems on Earth, and play an important role in the global carbon cycle. Net carbon dioxide (CO₂) ecosystem exchanges in coastal salt marshes remain poorly investigated. In Spartina alterniflora dominated North American Atlantic coast marshes, the lack of a clear understanding of how Spartina alterniflora responds to flooding limits our current ability to understand and predict salt marsh response to sea-level rise. Here we investigate the processes influencing ecosystem-level carbon exchanges between a S. alterniflora dominated salt marsh on the eastern shore of Virginia and the atmosphere. We examined the impacts of tidal inundation on the marsh–atmosphere carbon exchanges through a combination of eddy covariance measurements and in situ photosynthetic measurements. Maximum daytime carbon fluxes were observed during the middle of the growing season (July and August) and amounted to −10 μmol CO₂ m⁻² s⁻¹, and the marsh assimilated 130 gC m⁻² during the 2007 growing season. Our study is the first to quantify the effects of tidal inundation on marsh plants, which caused anywhere from 3% to 91% reductions in atmospheric carbon fluxes, with a mean reduction of 46 ± 26%, when compared to non-flooded conditions.

A new parameterization of the Wegener–Bergeron–Findeisen (WBF) process has been developed, and implemented in the general circulation model CAM-Oslo. The new parameterization scheme has important implications for the process of phase transition in mixed-phase clouds. The new treatment of the WBF process replaces a previous formulation, in which the onset of the WBF effect depended on a threshold value of the mixing ratio of cloud ice. As no observational guidance for such a threshold value exists, the previous treatment added uncertainty to estimates of aerosol effects on mixed-phase clouds. The new scheme takes subgrid variability into account when simulating the WBF process, allowing for smoother phase transitions in mixed-phase clouds compared to the previous approach. The new parameterization yields a model state which gives reasonable agreement with observed quantities, allowing for calculations of aerosol effects on mixed-phase clouds involving a reduced number of tunable parameters. Furthermore, we find a significant sensitivity to perturbations in ice nuclei concentrations with the new parameterization, which leads to a reversal of the traditional cloud lifetime effect.

The composition of cloud droplet nuclei was compared to the composition of ambient aerosol particles in non-precipitating clouds in different regions. Single-particle electron microscope techniques were used to identify particle types. The smallest particles (<0.2 µm diameter) exhibited a dependence of composition on nucleating ability, with salts (chlorides and sulfates of Na, K, Ca and Mg) being preferred nuclei types, followed by ammonium sulfates and organics. Crustal and industrial metals were less likely to be incorporated into cloud droplets. However, for experiments where only larger particles were sampled, a weaker dependence on composition was found. This suggests that larger particles have sufficient soluble material to nucleate despite their primary type. Both size and composition seem to be important, with composition becoming increasingly important as smaller particles activate at higher supersaturations in the cloudy environment.

Light extinction by atmospheric particles is strongly dependent on the size, chemical composition, and water content of the aerosol. Since light extinction by particles directly impacts climate and visibility, measurements of the extinction at various relative humidities (RHs) are needed. In this work, the optical growth factors, fRH_ext (80%RH, Dry) have been measured using cavity ring-down aerosol extinction spectroscopy at 532 nm for particles of varying organic/sulfate compositions. Specifically, slightly soluble, multifunctional aromatic compounds resulting from biomass burning have been investigated. In general, the organic compounds studied exhibit much smaller optical growth than inorganic compounds such as ammonium sulfate. Also, a linear relationship between mass fraction organic and optical growth has been observed for most organic compounds studied, in agreement with previous studies of more water-soluble organics. The role of particle density for mixtures that do not follow a linear relationship is also explored.

Ice in the environment, whether in the form of ice particles in clouds or sea ice and snow at the Earth's surface, has a profound influence on atmospheric composition and climate. The interaction of trace atmospheric gases with snow and sea ice surfaces largely controls atmospheric composition in polar regions. The heterogeneous chemistry of ice particles in clouds also plays critical roles in polar stratospheric ozone depletion and in tropospheric chemistry. A quantitative physical understanding of the interactions of snow and ice with trace gases is critical for predicting the effects of climate change on atmospheric composition, for the interpretation of ice core chemical records, and for modeling atmospheric chemistry.

The motivation behind this focus issue of Environmental Research Letters (ERL), and the special session at the Fall 2007 meeting of the American Geophysical Union that generated it, was to enhance communication and interactions among field and laboratory scientists and modelers working in this area. Members of these three groups are each working toward a mutual goal of understanding and quantifying the connections between the chemistry of snow and ice in the environment and atmospheric composition, and communication and collaboration across these traditional disciplinary boundaries pose a challenge for the community.

We are pleased to present new work from several current leaders in the field and laboratory communities in this focus issue. Topics include the interaction of organics and mercury with snow and ice surfaces, halogen activation from halide ice, and the emissions of reactive nitrogen oxides from snow. Novel experimental techniques are presented that make progress towards overcoming the experimental challenges of quantifying the chemistry of realistic snow samples and ice chemistry at temperatures relevant to the polar boundary layer. Several of the papers in this issue also touch on one of the significant gaps in our current understanding of the atmospheric chemistry of ice: the role of a quasi-liquid layer (QLL) or quasi-brine layer (QBL) at the ice surface.

The studies presented here advance our understanding of the complex interactions of snow and ice with important reactive components in our atmosphere. It has become clear in recent years that the polar regions do not act as an ultimate sink for many compounds—the release of halogens and reactive nitrogen oxides from ice and snow are examples of this. Two notable implications arise from these findings (i) the impact of anthropogenic pollutants in our environment may extend further than we fully appreciate with current global atmospheric chemistry models and (ii) our interpretation of chemical records in ice cores requires that we fundamentally understand and quantify air–snow and air–ice interactions. Additionally, laboratory studies are elucidating the details of heterogeneous reactions that are prevalent on ice and snow surfaces throughout the troposphere, and we are poised to make significant strides in the near future quantifying these effects on regional and global scales. We look forward to continued progress in this field in the coming years, and we will continue to work to connect those conducting modeling, field and laboratory studies.

Focus on Connections between Atmospheric Chemistry and Snow and Ice Contents

HONO emissions from snow surfacesHarry Beine, Agustín J Colussi, Antonio Amoroso, Giulio Esposito, Mauro Montagnoli and Michael R Hoffmann

Heterogeneous ozonation kinetics of phenanthrene at the air–ice interface T F Kahan and D J Donaldson

Release of gas-phase halogens from sodium halide substrates: heterogeneous oxidation of frozen solutions and desiccated salts by hydroxyl radicals S J Sjostedt and J P D Abbatt

Uptake of acetone, ethanol and benzene to snow and ice: effects of surface area and temperature J P D Abbatt, T Bartels-Rausch, M Ullerstam and T J Ye

Interaction of gaseous elemental mercury with snow surfaces: laboratory investigation Thorsten Bartels-Rausch, Thomas Huthwelker, Martin Jöri, Heinz W Gäggeler and Markus Ammann

Major solutes, metals, and alkylated aromatic compounds in high-latitude maritime snowpacks near the trans-Alaska pipeline terminal, Valdez, Alaska Jonathan P Bower, Eran Hood and Lisa A Hoferkamp

The fate of volatile chemicals during wet growth of a hailstone Ryan Michael and Amy L Stuart

Photochemical production of NO_x and HONO from surface snow can significantly impact the NO_x, OH, and O₃ budgets in the overlying atmosphere. NO_x production is driven by the solar photolysis of NO₃⁻ within or at the surface of snowpacks. HONO, however, is a secondary species that involves H-atom transfer between natural donors and photogenerated NO₂. Here we investigate the mechanism of HONO generation in snowpacks by exploring how its emissions respond to on-and-off illumination and temperature cycles, and to the addition of various snow dopants. The presence of humic substances within or at the surface of the snowpack significantly enhances, and may be an essential requisite for HONO production.

Emission fluxes of NO, NO₂, and HONO from snow surfaces were measured under controlled temperature, ozone mixing ratio and actinic flux conditions. We used natural mid-latitude surface snow as the snow substrate. Their combined peak emission fluxes reached up to ∼3 × 10¹⁰ molecules cm⁻² s⁻¹, ∼10³ times larger than typical emissions from polar snowpacks. Less than 1% of available N was released in these experiments. We report significant post-irradiation HONO emissions from the snow. Present results indicate a strong, direct correlation between HONO emissions and the HULIS (humic-like substances) content of the snow surface.

Phenanthrene ozonation kinetics were measured on ice at −30 and −10 °C, and on a water surface at 22 °C using glancing angle laser-induced fluorescence. A Langmuir–Hinshelwood type kinetic mechanism was observed on ice. The maximum ozonation rates were a factor of ten higher on ice than on water. No temperature dependence to the kinetics was observed between −30 and −10 °C, suggesting that the differences in the rates on ice and water are due to different physical properties at the two surfaces. Fluorescence spectra of phenanthrene show significant self-association on ice that is not observed on water, adding further evidence for the hypothesis that the quasi-liquid layer at the air–ice interface presents a very different environment to liquid water.

Motivated by the need to determine the mechanism of the initial release of halogens from sea ice and marine aerosol substrates, a study of the interactions of OH radicals with a variety of halide-containing surfaces has been performed in a coated-wall flow tube using chemical ionization mass spectrometry for gas-phase analysis. The salts studied were NaCl with 0.01% and 0.002% impurities of Br⁻ and I⁻ respectively, and NaCl/NaBr mixtures with Cl⁻/Br⁻ seawater ratios. The surfaces were desiccated salts, desiccated salts exposed to elevated relative humidity, and frozen solutions. In all cases, gas-phase Br₂ and BrCl were formed, with the Br₂ yield (defined as a molar ratio of halogen produced to OH lost) larger than 0.2 and the BrCl yield as roughly 0.01. For the first time, an observation of heterogeneous release of iodine-containing halogens (in particular IBr) was made with yields comparable to those of BrCl. We note that pH neutral frozen solutions demonstrated halogen release, although the yields were higher for acidic frozen solutions.

The interactions of gas-phase acetone, ethanol and benzene with smooth ice films and artificial snow have been studied. In one technique, the snow is packed into a cylindrical column and inserted into a low-pressure flow reactor coupled to a chemical-ionization mass spectrometer for gas-phase analysis. At 214 and 228 K, it is found for acetone and ethanol that the adsorbed amounts per surface area match those for adsorption to thin films of ice formed by freezing liquid water, when the specific surface area of the snow (as determined from Kr adsorption at 77 K) and the geometric surface area of the ice films are used. This indicates that freezing thin films of water leads to surfaces that are smooth at the molecular level. Experiments performed to test the effect of film growth on ethanol uptake indicate that uptake is independent of ice growth rate, up to 2.4 µm min⁻¹. In addition, traditional Brunauer–Emmett–Teller (BET) experiments were performed with these gases on artificial snow from 238 to 266.5 K. A transition from a BET type I isotherm indicative of monolayer formation to a BET type II isotherm indicative of multilayer uptake is observed for acetone at T≥263 K and ethanol at T≥255 K, arising from solution formation on the ice. When multilayer formation does not occur, as was the case for benzene at T≤263 K and for acetone at T≤255 K, the saturated surface coverage increased with increasing temperature, consistent with the quasi-liquid layer affecting adsorption prior to full dissolution/multilayer formation.

The interaction of elemental mercury with ice surfaces based on the migration behaviour at sub-ppt concentrations in a packed bed flow tube is discussed. Analysis shows that elemental mercury interacts only weakly with ice surfaces and suggests an adsorption enthalpy of −28 ± 2 kJ mol⁻¹. The experiments further reveal an adsorption equilibrium constant, which quantifies the partitioning of elemental mercury between the surface and the gas phase, with a value of 12 cm at 140 K. Extrapolation to environmental conditions gives a range for the equilibrium constant of 3 × 10⁻⁴ to 4 × 10⁻³ cm at 230 K.

The chemical constituents within a snowpack can provide information about the atmosphere through which the snow was deposited. Valdez is located in south-central Alaska and has a high-latitude maritime climate, with annual snowfall typically exceeding 8 m within the city limits. Valdez is also the termination point of the trans-Alaska pipeline system, where tankers are loaded with crude oil from the North Slope of Alaska. Integrated samples of the top 1 m of snow were collected at seven sites near Valdez and analyzed for major solutes, lead, and alkylated aromatic compounds, in particular benzene, toluene, ethylbenzene, and xylene (BTEX). For comparison, sites were also sampled near Juneau, Alaska, which has a similar climate but no petroleum transport infrastructure. Major solute chemistry at all sites was dominated by chloride and was consistent with a marine air mass source of ions in precipitation. Sulfate levels in Valdez were typically on the order of 10 µeq l⁻¹ and significantly higher than found in Juneau snow. Other major solute levels were low in Valdez and Juneau. Lead levels were below detection limits for all sites, with the exception of trace concentrations (<0.4 µg l⁻¹) reported at two Valdez locations. Alkylated organics were present at all Valdez locations, at levels similar to those documented previously in urban locations. No alkylated organics were detected in Juneau snowpacks.

Observations from the moderate resolution imaging spectroradiometer (MODIS) were used in combination with a large data set of field measurements to map woody above-ground biomass (AGB) across tropical Africa. We generated a best-quality cloud-free mosaic of MODIS satellite reflectance observations for the period 2000–2003 and used a regression tree model to predict AGB at 1 km resolution. Results based on a cross-validation approach show that the model explained 82% of the variance in AGB, with a root mean square error of 50.5 Mg ha⁻¹ for a range of biomass between 0 and 454 Mg ha⁻¹. Analysis of lidar metrics from the Geoscience Laser Altimetry System (GLAS), which are sensitive to vegetation structure, indicate that the model successfully captured the regional distribution of AGB. The results showed a strong positive correlation (R² = 0.90) between the GLAS height metrics and predicted AGB.

Scenarios have become a standard tool in the portfolio of techniques that scientists and policy-makers use to envision and plan for the future. Defined as plausible, challenging and relevant stories about how the future might unfold that integrate quantitative models with qualitative assessments of social and political trends, scenarios are a central component in assessment processes for a range of global issues, including climate change, biodiversity, agriculture, and energy. Yet, despite their prevalence, systematic analysis of scenarios is in its beginning stages. Fundamental questions remain about both the epistemology and scientific credibility of scenarios and their roles in policymaking and social change.

Answers to these questions have the potential to determine the future of scenario analyses. Is scenario analysis moving in the direction of earth system governance informed by global scenarios generated through increasingly complex and comprehensive models integrating socio-economic and earth systems? Or will global environmental scenario analyses lose favour compared to more focused, policy-driven, regionally specific modelling? These questions come at an important time for the climate change issue, given that the scenario community, catalyzed by the Intergovernmental Panel on Climate Change (IPCC), is currently preparing to embark on a new round of scenario development processes aimed at coordinating research and assessment, and informing policy, over the next five to ten years.

These and related questions about where next to go with global environmental scenarios animated a workshop held at Brown University (Note1) that brought together leading practitioners and scholars of global environmental change scenarios from research, policy-making, advocacy, and business settings. The workshop aimed to provide an overview of current practices/best practices in scenario production and scenario use across a range of global environmental change arenas. Participants worked to bring the experience generated from over four decades of scenario development in other issue domains, including energy and security, to bear on environmental scenarios, and to bring into dialogue scenario practitioners, both producers and users, with social science scholars. The set of contributions to this focus issue of Environmental Research Letters arose out of this workshop and collectively examines key challenges facing the scenario community, synthesizes lessons, and offers recommendations for new research and practice in this field.

One theme that emerged in many of the discussions at the workshop revolved around the distinction between two broad perspectives on the goals of scenario exercises: scenarios as products and scenarios as processes. Most global environmental change scenario exercises are product-oriented; the content of the scenarios developed is the main goal of many participants and those who commission or organize the scenario development process. Typically, what is of most interest are the environmental outcomes produced, how they relate to the various factors driving them, and what the results tell us about the prospects for future environmental change, for impacts, and for mitigation. A product-oriented perspective assumes that once produced, scenario products have lives of their own, divorced from the processes that generated them and able to serve multiple, often unspecified purposes. Thus, it is often assumed that the scenario products can be 'taken up' by a variety of users in a variety of fora. A contrasting scenario approach is process-oriented and self-consciously privileges the process of scenario development as the primary goal, for example as a means to motivate organizational learning, find commonalities across different perspectives, achieve consensus on goals, or come to a shared understanding of challenges. Focusing on scenarios as processes highlights the social contexts in which scenarios are created and used. Process-oriented scenario exercises also generate scenario products, but such products are recognized as meaningful mostly (or only) in the social context in which they were developed. It should be noted that those seeking to understand the functions, implications and utility of scenarios can approach analysis of scenarios and their impacts from either perspective—focusing attention on product outcomes and influence or assessing procedural and contextual dynamics and implications.

Papers in this issue examine various aspects of scenario products, scenario processes and their interactions, with specific reference to global environmental change scenarios. Hulme and Dessai (2008) use the product–process distinction as a starting point for developing a framework to evaluate the success of scenario exercises. They identify 'prediction success', 'decision success' and 'learning success' as three evaluation metrics for scenarios, with the first two most relevant to scenario products and the last emphasizing procedural aspects of scenarios. They suggest that viewing scenarios primarily as products implies examining how closely actual outcomes have matched envisioned outcomes, while viewing them primarily as processes suggests evaluating the extent to which scenarios engaged participants and enabled their learning.

O'Neill and Nakicenovic (2008) focus on Hulme and Dessai's evaluation metric, learning. Based on a review of six scenario/assessment exercises, they ask if and how scenario products have incorporated comparative assessments of results in order to enable cumulative learning across scenario efforts. The authors conclude that, although participating modelling teams have benefited greatly from the process of scenario activities and applied that learning to other scenario exercises in which they engage, learning from comparative assessments of scenario products has been rather limited; the latter due to the limited time and resources invested in comparative analysis. Pitcher (2009) speaks to a similar audience, namely the emissions scenario communities that are organizing to undertake a new round of scenario development in the lead-up to the IPCC Fifth Assessment Report. His focus is primarily on a set of concerns that need to be addressed if the new set of socio-economic and emissions scenario products are to adequately support climate model runs, mitigation analyses, and impacts, adaptation and vulnerability research. Pitcher flags issues associated with assessment and measurement of economic growth, challenges associated with downscaling long-term, global scenarios to finer geographic and time scales, and possible ways to grapple with probability and uncertainty in scenario analyses.

Garb et al (2008) shift focus to the process aspects of scenarios, focusing on how scenarios simultaneously shape and embed their social contexts. They outline and give examples from a research agenda, drawing on concepts and methods from sociology, political science, and science and technology studies, aimed at redressing the growing imbalance between the increasing technical sophistication of the quantitative components of scenarios on the one hand, and the continued simplicity of our understandings of the social origins, linkages, and implications of the narratives to which they are coupled on the other. Focusing on the treatment of equity concerns in the IPCC Special Report on Emissions Scenarios, Baer (2009) offers a concrete example of how particular social assumptions and definitions of equity are built into scenarios which then create particular worldviews about rights and responsibilities. Baer argues that incorporating distributions of income within—and not only between—countries in quantitative scenario exercises makes visible questions regarding the assignment of rights and the distribution of costs and benefits; such equity considerations, he argues, are central to engendering the cooperation necessary to address the climate crisis.

For Parson (2008), the product–process distinction serves to highlight the unique characteristics and challenges of scenarios for global environmental change, including their use in large-scale official assessments, basis in biophysical modelling, weak connections to decision-makers, and roles as sites of public controversy. Parson argues that these characteristics of global environmental change scenarios prohibit process-oriented approaches, which rely on pre-identifying intended users and engaging them in the scenario development process. Instead, he proposes ways in which scenario products can be enhanced to support use by multiple, non-participant user communities. Wilkinson and Eidinow (2008) reach a different conclusion. They too identify the particular challenges of grappling with global environmental change. They examine approaches to past scenario efforts and categorize them into two groups that map loosely onto the product–process distinction: 'problem-focused' and 'actor-centric' approaches. They propose that progress in global environmental issues can best be made through a new, third type of approach ('reflexive interventionist or multi-agent based') that would combine elements of problem- and actor-focused approaches, creating scenario processes that can simultaneously support longer-term thinking as well as more immediate actions.

Collectively, the papers in this issue range widely across issues associated with contemporary scenario processes and products. We can discern in them the outlines of an important set of suggestions for improving scenario development in the future, including, among others, the following:

• Focus scenario exercises on more specific questions so that results from multiple models can be more illuminating (O'Neill and Nakicenovic; Garb et al 2008).

• Enhance scenario transparency so as to enable extensions by users, rather than further expanding representation in global scenarios themselves (Parson 2008).

• Incorporate relatively simple measures (such as sub-national disaggregation of income distributions and climate change impacts) in order to boost the equity sensitivity of scenarios (Baer 2009).

• Recognize topics where social science inputs are becoming important for improving modelling and model relevance, such as providing a logic for how societies manage to transition from historical paths to the various future development paths foreseen in the scenarios, or developing measures of well-being which are independent of income levels, and include in global environmental scenario teams more representatives of social science professionals (Pitcher 2009; Garb et al 2008).

• Invest greater resources in assessing scenario results, and in understanding and overcoming the barriers to carrying out such assessment (Hulme and Dessai 2008; O'Neill and Nakicenovic, 2008).

• Disaggregate the variety of global change decision makers targeted as audiences for scenarios (Parson 2008; Garb et al 2008).

• Develop an additional 'reflective interventionist' scenarios approach that involves different epistemologies for active learning in the public interest (Wilkinson and Eidinow 2008).

• Draw on the extensive toolkit of social science research methods to analyze the social work of scenarios (Garb et al 2008).

• Create new institutions and scenario activities that can adapt and extend global scenarios to specific, often local or regional decision contexts (Parson 2008).

• Create fora in which scenario practitioners, modellers, decision-makers, and social scientists of various kinds can discuss the process of scenario construction and use (Garb et al 2008).

We do not mean to imply a consensus among the participants in the Brown University workshop or of contributors to this collection of papers. At the same time, we believe that these and other insights and suggestions from these contributions do have a certain coherence, and collectively point to a deepening and reinvigoration of the environmental scenario-modelling enterprise—an enterprise now facing environmental change processes that are emerging as some of the most pressing challenges of our time.

Acknowledgements We would like to thank the Global Environment Program at the Watson Institute for International Studies at Brown University and the US Environmental Protection Agency for financially supporting publication of this focus issue.

Focus on Global Environmental Scenarios Contents

Predicting, deciding, learning: can one evaluate the 'success' of national climate scenarios?Mike Hulme and Suraje Dessai

Learning from global emissions scenariosBrian C O'Neill and Nebojsa Nakicenovic

Scenarios in society, society in scenarios: toward a social scientific analysis of storyline-driven environmental modelingYaakov Garb, Simone Pulver and Stacy D VanDeveer

Useful global-change scenarios: current issues and challengesE A Parson

Evolving practices in environmental scenarios: a new scenario typologyAngela Wilkinson and Esther Eidinow

Equity in climate–economy scenarios: the importance of subnational income distributionPaul Baer

The future of scenarios: issues in developing new climate change scenarios Hugh M Pitcher

NotesNote1 The workshop was held in March 2007, jointly sponsored by the Watson Institute for International Studies at Brown University, the International Institute for Applied Systems Analysis (IIASA) in Austria, and the US National Intelligence Council. See http://www.watsoninstitute.org/ge/scenarios/ for more information.

Scenarios may be understood as products and/or processes. Viewing scenario exercises as productive tends to emphasize their tangibility: scenario products may acquire value unrelated to the processes of their creation. Viewing scenario exercises as procedural tends to emphasize their modes of formation: the process of constructing scenarios may have benefits irrespective of the value of ensuing products. These two framings yield different expectations about how one might evaluate the 'success' or otherwise of scenario exercises. We illustrate three approaches to evaluating the success or otherwise of scenarios using the example of the series of national UK climate scenarios published between 1991 and 2002. These are: predictive success (has the future turned out as envisaged?), decision success (have 'good' decisions subsequently been made?) and learning success (have scenarios proved engaging and enabled learning?). We reflect on the different ways the 'success' of national climate scenarios might be evaluated and on the relationship between the productive and procedural dimensions of scenario exercises.

Scenarios of global greenhouse gas emissions have played a key role in climate change analysis for over twenty years. Currently, several research communities are organizing to undertake a new round of scenario development in the lead-up to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). To help inform this process, we assess a number of past efforts to develop and learn from sets of global greenhouse gas emissions scenarios. We conclude that while emissions scenario exercises have likely had substantial benefits for participating modeling teams and produced insights from individual models, learning from the exercises taken as a whole has been more limited. Model comparison exercises have typically focused on the production of large numbers of scenarios while investing little in assessing the results or the production process, perhaps on the assumption that later assessment efforts could play this role. However, much of this assessment potential remains untapped. Efforts such as scenario-related chapters of IPCC reports have been most informative when they have gone to extra lengths to carry out more specific comparison exercises, but in general these assessments do not have the remit or resources to carry out the kind of detailed analysis of scenario results necessary for drawing the most useful conclusions. We recommend that scenario comparison exercises build-in time and resources for assessing scenario results in more detail at the time when they are produced, that these exercises focus on more specific questions to improve the prospects for learning, and that additional scenario assessments are carried out separately from production exercises. We also discuss the obstacles to better assessment that might exist, and how they might be overcome. Finally, we recommend that future work include much greater emphasis on understanding how scenarios are actually used, as a guide to improving scenario production.

Scenario analysis, an approach to thinking about alternative futures based on storyline-driven modeling, has become increasingly common and important in attempts to understand and respond to the impacts of human activities on natural systems at a variety of scales. The construction of scenarios is a fundamentally social activity, yet social scientific perspectives have rarely been brought to bear on it. Indeed, there is a growing imbalance between the increasing technical sophistication of the modeling elements of scenarios and the continued simplicity of our understanding of the social origins, linkages, and implications of the narratives to which they are coupled. Drawing on conceptual and methodological tools from science and technology studies, sociology and political science, we offer an overview of what a social scientific analysis of scenarios might include. In particular, we explore both how scenarios intervene in social microscale and macroscale contexts and how aspects of such contexts are embedded in scenarios, often implicitly. Analyzing the social 'work' of scenarios (i) can enhance the understanding of scenario developers and modeling practitioners of the knowledge production processes in which they participate and (ii) can improve the utility of scenario products as decision-support tools to actual, rather than imagined, decision-makers.

Scenarios are increasingly used to inform global-change debates, but their connection to decisions has been weak and indirect. This reflects the greater number and variety of potential users and scenario needs, relative to other decision domains where scenario use is more established. Global-change scenario needs include common elements, e.g., model-generated projections of emissions and climate change, needed by many users but in different ways and with different assumptions. For these common elements, the limited ability to engage diverse global-change users in scenario development requires extreme transparency in communicating underlying reasoning and assumptions, including probability judgments. Other scenario needs are specific to users, requiring a decentralized network of scenario and assessment organizations to disseminate and interpret common elements and add elements requiring local context or expertise. Such an approach will make global-change scenarios more useful for decisions, but not less controversial. Despite predictable attacks, scenario-based reasoning is necessary for responsible global-change decisions because decision-relevant uncertainties cannot be specified scientifically. The purpose of scenarios is not to avoid speculation, but to make the required speculation more disciplined, more anchored in relevant scientific knowledge when available, and more transparent.

A new approach to scenarios focused on environmental concerns, changes and challenges, i.e. so-called 'environmental scenarios', is necessary if global environmental changes are to be more effectively appreciated and addressed through sustained and collaborative action.

On the basis of a comparison of previous approaches to global environmental scenarios and a review of existing scenario typologies, we propose a new scenario typology to help guide scenario-based interventions. This typology makes explicit the types of and/or the approaches to knowledge ('the epistemologies') which underpin a scenario approach.

Drawing on previous environmental scenario projects, we distinguish and describe two main types in this new typology: 'problem-focused' and 'actor-centric'. This leads in turn to our suggestion for a third type, which we call 'RIMA'—'reflexive interventionist or multi-agent based'. This approach to scenarios emphasizes the importance of the involvement of different epistemologies in a scenario-based process of action learning in the public interest. We suggest that, by combining the epistemologies apparent in the previous two types, this approach can create a more effective bridge between longer-term thinking and more immediate actions. Our description is aimed at scenario practitioners in general, as well as those who work with (environmental) scenarios that address global challenges.

The effect of aerosol on clouds and precipitation poses one of the largest uncertainties in the estimation of the anthropogenic contribution to climate change. Often the local effect of aerosol on the radiation field both directly and indirectly by changing cloud properties can be more than an order of magnitude larger than the effect of greenhouse gases. Recent works also suggest that some of the local aerosol effects such as the heating of the atmosphere from aerosol absorption can have a large-scale impact. However, due to the inhomogeneous distribution and short lifetime of aerosols, the inherent complexity of cloud microphysics and dynamics, and the strong coupling of aerosol-related processes with meteorology, it is still challenging to estimate the overall effect that aerosols exert on the radiation field and climate.

The climatic effect of aerosol–cloud interaction is not limited to the radiation field. By changing the cloud microphysical and radiative properties, aerosols may affect precipitation amount and patterns. Precipitation processes are located at the end of the 'food chain' of aerosol–cloud processes. Rain rates and patterns are the final result of many cloud processes and feedbacks, some of which are affected by aerosols. Changes in precipitation patterns could lead to serious hydrological consequences. For example if the same amount of rain precipitates in shorter time, e.g. heavier rain rates, the probability of floods increases, a larger portion of the water is drained by rapid surface run-off and the amount of water penetrating the subsurface and available as an underground reservoir of drinking water is reduced. Moreover, the subsurface water distribution depends heavily on topography and geomorphology. Therefore small changes in the timing or location of precipitation may dramatically alter the surface and subsurface water distribution and affect the water reservoir.

In studying aerosol–cloud–precipitation interactions, the largest challenge is determining how to separate the effect of meteorological processes from aerosol effects. Simple statistical correlations between observed or retrieved aerosol and cloud properties do not imply causality. With the help of sophisticated cloud numerical models where certain variables or processes can be controlled in sensitivity simulations, aerosol–precipitation casual relationships could be examined in specific conditions. However, it is not always clear whether such aerosol–precipitation relationships seen in the model could be applied to more general meteorological scenarios.

Aerosol–cloud–precipitation interaction is a highly complex problem involving processes and feedbacks that span the size range from an aerosol particle (10⁻⁷ m) to a cloud (10³ m), and all the way to synoptic scale systems (10⁶ m). These feedbacks determine whether a droplet, initiated at a size of few microns, could grow within the time scale of a cloud's lifetime to reach a raindrop size of a few mm, and whether this raindrop will fall all the way to the surface and be available as fresh water. These feedbacks include dynamic and thermodynamic processes of precipitating particles, which in conjunction with other processes determine the micro and macrophysical properties of the cloud and hence determine the cloud's effect on the regional radiation field and local climate. Thus, aerosol, cloud, precipitation and radiation interactions are inherently linked, and need to be addressed as a single problem when attempting to better understand human-induced changes in the climate system.

Focus on Aerosol Precipitation Contents

Drizzle rates versus cloud depths for marine stratocumuli A B Kostinski

Characteristics of vertical velocity in marine stratocumulus: comparison of large eddy simulations with observations Huan Guo, Yangang Liu, Peter H Daum, Gunnar I Senum and Wei-Kuo Tao

Dispersion bias, dispersion effect, and the aerosol–cloud conundrum Yangang Liu, Peter H Daum, Huan Guo and Yiran Peng

The impact of smoke from forest fires on the spectral dispersion of cloud droplet size distributions in the Amazonian region J A Martins and M A F Silva Dias

A conceptual model for the link between Central American biomass burning aerosols and severe weather over the south central United StatesJun Wang, Susan C van den Heever and Jeffrey S Reid

Simple approximations for condensational growth A B Kostinski

African aerosol and large-scale precipitation variability over West Africa Jingfeng Huang, Chidong Zhang and Joseph M Prospero

Temporal rainfall fluctuations in Israel and their possible link to urban and air pollution effectsNoam Halfon, Zev Levin and Pinhas Alpert

Marine stratocumuli make a major contribution to Earth's radiation budget. Drizzle in such clouds can greatly affect their albedo, lifetime and fractional coverage, so drizzle rate prediction is important. Here we examine a question: does a drizzle rate (R) depend on cloud depth (H) and/or drop number concentration n in a simple way? This question was raised empirically in several recent publications and an approximate H³/n dependence was observed. Here we suggest a simple explanation for H³ scaling from viewing the drizzle rate as a sedimenting volume fraction (f) of water drops (radius r) in air, i.e. R = fu(r), where u is the fall speed of droplets at the cloud base. Both R and u have units of speed. In our picture, drizzle drops begin from condensation growth on the way up and continue with accretion on the way down. The ascent contributes H () and the descent H² () to the drizzle rate. A more precise scaling formula is also derived and may serve as a guide for parameterization in global climate models. The number concentration dependence is also discussed and a plausibility argument is given for the observed n⁻¹ dependence of the drizzle rate. Our results suggest that deeper stratocumuli have shorter washout times.

We simulated a marine stratus deck sampled during the Marine Stratus/Stratocumulus Experiment (MASE) with a three-dimensional large eddy simulation (LES) model at different model resolutions. Various characteristics of the vertical velocity from the model simulations were evaluated against those derived from the corresponding aircraft in situ observations, focusing on standard deviation, skewness, kurtosis, probability density function (PDF), power spectrum, and structure function. Our results show that although the LES model captures reasonably well the lower-order moments (e.g., horizontal averages and standard deviations), it fails to simulate many aspects of the higher-order moments, such as kurtosis, especially near cloud base and cloud top. Further investigations of the PDFs, power spectra, and structure functions reveal that compared to the observations, the model generally underestimates relatively strong variations on small scales. The results also suggest that increasing the model resolutions improves the agreements between the model results and the observations in virtually all of the properties that we examined. Furthermore, the results indicate that a vertical grid size <10 m is necessary for accurately simulating even the standard-deviation profile, posing new challenges to computer resources.

This work examines the influences of relative dispersion (the ratio of the standard deviation to the mean radius of the cloud droplet size distribution) on cloud albedo and cloud radiative forcing, derives an analytical formulation that accounts explicitly for the contribution from droplet concentration and relative dispersion, and presents a new approach to parameterize relative dispersion in climate models. It is shown that inadequate representation of relative dispersion in climate models leads to an overestimation of cloud albedo, resulting in a negative bias of global mean shortwave cloud radiative forcing that can be comparable to the warming caused by doubling CO₂ in magnitude, and that this dispersion bias is likely near its maximum for ambient clouds. Relative dispersion is empirically expressed as a function of the quotient between cloud liquid water content and droplet concentration (i.e., water per droplet), yielding an analytical formulation for the first aerosol indirect effect. Further analysis of the new expression reveals that the dispersion effect not only offsets the cooling from the Twomey effect, but is also proportional to the Twomey effect in magnitude. These results suggest that unrealistic representation of relative dispersion in cloud parameterization in general, and evaluation of aerosol indirect effects in particular, is at least in part responsible for several outstanding puzzles of the aerosol–cloud conundrum: for example, overestimation of cloud radiative cooling by climate models compared to satellite observations; large uncertainty and discrepancy in estimates of the aerosol indirect effect; and the lack of interhemispheric difference in cloud albedo.

At temperatures below 238 K, cirrus clouds can form by homogeneous and heterogeneous ice nucleation mechanisms. ECHAM5 contains a two-moment cloud microphysics scheme and permits cirrus formation by homogeneous freezing of solution droplets and heterogeneous freezing on immersed dust nuclei. On changing the mass accommodation coefficient, α, of water vapor on ice crystals from 0.5 in the standard ECHAM5 simulation to 0.006 as suggested by previous laboratory experiments, the number of ice crystals increases by a factor of 14, as a result of the delayed relaxation of supersaturation. At the same time, the ice water path increases by only 29% in the global annual mean, indicating that the ice crystals are much smaller in the case of low α. As a consequence, the short wave and long wave cloud forcing at the top of the atmosphere increase by 15 and 18 W m⁻², respectively. Assuming heterogeneous freezing caused by immersed dust particles instead of homogeneous freezing, the effect is much weaker, decreasing the global annual mean short wave and long wave cloud forcing by 2.7 and 4.7 W m⁻². Overall, these results provide little support, if any, for kinetic growth limitation of ice particles (i.e. a very low α).