Are global cloud albedo and climate controlled by marine phytoplankton?

A study of the thermodynamics and mechanisms of the atmospherically relevant reaction dimethyl sulphide (DMS) with atomic chlorine (Cl) in the absence and presence of water, using electronic structure methods

The thermodynamics and mechanisms of the atmospherically relevant reaction dimethyl sulphide (DMS) + atomic chlorine (Cl) were investigated in the absence and presence of a single water molecule, using electronic structure methods. Stationary points on each reaction surface were located using density functional theory (DFT) with the M06-2X functional with aug-cc-pVDZ (aVDZ) and aug-cc-pVTZ (aVTZ) basis sets. Then fixed point calculations were carried out using the UM06-2X/aVTZ optimised stationary point geometries, with aug-cc-pVnZ basis sets (n = T and Q), using the coupled cluster method [CCSD(T)], as well as the domain-based local pair natural orbitals coupled cluster [DLPNO-UCCSD(T)] approach. Four reaction channels are possible, formation of (A) CH₃SCH₂ + HCl, (B) CH₃S + CH₃Cl, (C) CH₃SCl + CH₃, and (C') CH₃S(Cl)CH₃. The results show that, in the absence of water, channels A and C' are the dominant channels. In the presence of water, the calculations show that the reaction mechanisms for A and C formation change significantly. Channel A occurs via submerged TSs and is expected to be rapid. Channel B occurs via TSs which present significant energy barriers indicating that this channel is not significant in the presence of water relative to CH₃SCH₂ + HCl and DMS·Cl adduct formation, as is the case in the absence of water. Channel C was not considered as it is endothermic in the absence of water. In the presence of water, pathways which proceed via (a) DMS·H₂O + Cl, (b) Cl·H₂O + DMS and (c) DMS·Cl + H₂O were considered. It was found that under tropospheric conditions, reactions via pathway (b) are of minor importance relative to those that proceed via pathways (a) and (c). This study has shown that water changes the mechanisms of the DMS + Cl reactions significantly but the presence of water is not expected to affect the overall reaction rate coefficient under atmospheric conditions as the DMS + Cl reaction has a rate coefficient at room temperature close to the collisional limit.

Review of Land Surface Albedo: Variance Characteristics, Climate Effect and Management Strategy

Surface albedo plays a controlling role in the surface energy budget, and albedo-induced radiative forcing has a significant impact on climate and environmental change (e.g., global warming, snow and ice melt, soil and vegetation degradation, and urban heat islands (UHIs)). Several existing review papers have summarized the algorithms and products of surface albedo as well as climate feedback at certain surfaces, while an overall understanding of various land types remains insufficient, especially with increasing studies on albedo management methods regarding mitigating global warming in recent years. In this paper, we present a comprehensive literature review on the variance pattern of surface albedo, the subsequent climate impact, and albedo management strategies. The results show that using the more specific term “surface albedo” is recommended instead of “albedo” to avoid confusion with similar terms (e.g., planetary albedo), and spatiotemporal changes in surface albedo can indicate subtle changes in the energy budget, land cover, and even the specific surface structure. In addition, the close relationships between surface albedo change and climate feedback emphasize the important role of albedo in climate simulation and forecasting, and many albedo management strategies (e.g., the use of retroreflective materials (RRMs)) have been demonstrated to be effective for climate mitigation by offsetting CO2 emissions. In future work, climate effects and management strategies regarding surface albedo at a multitude of spatiotemporal resolutions need to be systematically evaluated to promote its application in climate mitigation, where a life cycle assessment (LCA) method considering both climate benefits and side effects (e.g., thermal comfort) should be followed.

Opportunistic experiments to constrain aerosol effective radiative forcing

Aerosol-cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The nonlinearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well-defined sources provide "opportunistic experiments" (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

Approaches to Observe Anthropogenic Aerosol-Cloud Interactions

Anthropogenic aerosol particles exert an-quantitatively very uncertain-effective radiative forcing due to aerosol-cloud interactions via an immediate altering of cloud albedo on the one hand and via rapid adjustments by alteration of cloud processes and by changes in thermodynamic profiles on the other hand. Large variability in cloud cover and properties and the therefore low signal-to-noise ratio for aerosol-induced perturbations hamper the identification of effects in observations. Six approaches are discussed as a means to isolate the impact of anthropogenic aerosol on clouds from natural cloud variability to estimate or constrain the effective forcing. These are (i) intentional cloud modification, (ii) ship tracks, (iii) differences between the hemispheres, (iv) trace gases, (v) weekly cycles and (vi) trends. Ship track analysis is recommendable for detailed process understanding, and the analysis of weekly cycles and long-term trends is most promising to derive estimates or constraints on the effective radiative forcing.

From Conventional to Phase-Sensitive Vibrational Sum Frequency Generation Spectroscopy: Probing Water Organization at Aqueous Interfaces

Elucidation of water organization at aqueous interfaces has remained a challenging problem. Conventional vibrational sum frequency generation (VSFG) spectroscopy and its most recent extension, phase-sensitive VSFG (PS-VSFG), have emerged as powerful experimental methods for unraveling structural information at various aqueous interfaces. In this Perspective, we briefly describe the two possible VSFG detection modes, and we point out features that make these methods highly suited to address questions about water organization at air/aqueous interfaces. Several important aqueous interfacial systems are discussed to illustrate the versatility of these methods. Remaining challenges and exciting prospective directions are also presented.

Organization of water and atmospherically relevant ions and solutes: vibrational sum frequency spectroscopy at the vapor/liquid and liquid/solid interfaces

The nature of water's hydrogen-bonding network is a vital influence on the chemistry that occurs at interfaces, but a complete understanding of interfacial water has proven elusive. Even-order nonlinear optical spectroscopies, such as vibrational sum frequency generation (VSFG) spectroscopy and heterodyne detected phase-sensitive sum frequency generation (PS-SFG) spectroscopy, are inherently surface specific. With the advent of advances in these spectroscopic techniques, researchers can now explore many long-standing questions about the dynamics and structures present at the vapor-water and water-solid interfaces. Of special interest to the atmospheric chemistry community is the accommodation of ions and solutes by water's hydrogen-bonding network. A better understanding of how ions and solutes behave in hydrogen-bonded water has afforded a fresh perspective of aqueous aerosols, because the interactions involved therein drive phenomena such as the hydrolysis of atmospheric chemical species. In this Account, we present work from our laboratory focusing on applying VSFG and the recently developed PS-SFG techniques to probe the perturbation of water's hydrogen-bonding network at the vapor-water interface by a variety of ions and solutes. We also present very recent results from our laboratory on the direct observation of the adsorption of ions at the water-CaF(2) interface. We begin by discussing the influence of ions and solutes on interfacial water structure. Results for halide salts and the acid analogs on interfacial water structure are shown to be quite different, as would be expected from differences in surface tension measurements that have been known for a long time. Also examined are systems with the largely polarizable molecular anions nitrate (NO(3)(-)), sulfate (SO(4)(2-)), carbonate (CO(3)(2-)), and bicarbonate (HCO(3)(-)).These systems feature more complicated influences on interfacial water structure than halide-containing solutions; however, our conventional VSFG results for both nitrate and sulfate solutions are in agreement with recent PS-SFG results and molecular dynamics simulations. We also discuss recent PS-SFG work on carbonate and bicarbonate systems in which the accommodation of the bicarbonate ion at the vapor-water interface is in stark contrast to the carbonate results. Perturbation of interfacial water by solutes is examined for solutions of dimethyl sulfoxide and methylsulfonic acid. PS-SFG results for these systems are striking: they illustrate the dramatic changes that interfacial water molecules undergo in the presence of solutes that are not observed with conventional VSFG. Finally, we discuss direct sulfate ion adsorption for the aqueous sodium sulfate-CaF(2) interface, with the goal of elucidating water behavior at this surface.

Environmental chemistry at vapor/water interfaces: insights from vibrational sum frequency generation spectroscopy

The chemistry that occurs at surfaces has been an intense area of study for many years owing to its complexity and importance in describing a wide range of physical phenomena. The vapor/water interface is particularly interesting from an environmental chemistry perspective as this surface plays host to a wide range of chemistries that influence atmospheric and geochemical interactions. The application of vibrational sum frequency generation (VSFG), an inherently surface-specific, even-order nonlinear optical spectroscopy, enables the direct interrogation of various vapor/aqueous interfaces to elucidate the behavior and reaction of chemical species within the surface regime. In this review we discuss the application of VSFG to the study of a variety of atmospherically important systems at the vapor/aqueous interface. Chemical systems presented include inorganic ionic solutions prevalent in aqueous marine aerosols, small molecular solutes, and long-chain fatty acids relevant to fat-coated aerosols. The ability of VSFG to probe both the organization and reactions that may occur for these systems is highlighted. A future perspective toward the application of VSFG to the study of environmental interfaces is also provided.

Influence of anthropogenic aerosol on cloud optical depth and albedo shown by satellite measurements and chemical transport modeling

The Twomey effect of enhanced cloud droplet concentration, optical depth, and albedo caused by anthropogenic aerosols is thought to contribute substantially to radiative forcing of climate change over the industrial period. However, present model-based estimates of this indirect forcing are highly uncertain. Satellite-based measurements would provide global or near-global coverage of this effect, but previous efforts to identify and quantify enhancement of cloud albedo caused by anthropogenic aerosols in satellite observations have been limited, largely because of strong dependence of albedo on cloud liquid water path (LWP), which is inherently highly variable. Here we examine satellite-derived cloud radiative properties over two 1-week episodes for which a chemical transport and transformation model indicates substantial influx of sulfate aerosol from industrial regions of Europe or North America to remote areas of the North Atlantic. Despite absence of discernible dependence of optical depth or albedo on modeled sulfate loading, examination of the dependence of these quantities on LWP readily permits detection and quantification of increases correlated with sulfate loading, which are otherwise masked by variability of LWP, demonstrating brightening of clouds because of the Twomey effect on a synoptic scale. Median cloud-top spherical albedo was enhanced over these episodes, relative to the unperturbed base case for the same LWP distribution, by 0.02 to 0.15.

Utilization of dimethyl sulfide as a sulfur source with the aid of light by Marinobacterium sp. strain DMS-S1

Strain DMS-S1 isolated from seawater was able to utilize dimethyl sulfide (DMS) as a sulfur source only in the presence of light in a sulfur-lacking medium. Phylogenetic analysis based on 16S ribosomal DNA genes indicated that the strain was closely related to Marinobacterium georgiense. The strain produced dimethyl sulfoxide (DMSO), which was a main metabolite, and small amounts of formate and formaldehyde when grown on DMS as the sole sulfur source. The cells of the strain grown with succinate as a carbon source were able to use methyl mercaptan or methanesulfonate besides DMS but not DMSO or dimethyl sulfone as a sole sulfur source. DMS was transformed to DMSO primarily at wavelengths between 380 and 480 nm by heat-stable photosensitizers released by the strain. DMS was also degraded to formaldehyde in the presence of light by unidentified heat-stable factors released by the strain, and it appeared that strain DMS-S1 used the degradation products, which should be sulfite, sulfate, or methanesulfonate, as sulfur sources.

Marine biological controls on climate via the carbon and sulphur geochemical cycles

We review aspects of the influence of the marine biota on climate, focusing particularly on their role in mediating surface temperatures via their influence on atmospheric carbon dioxide (CO 2 ) and dimethyl sulphide (DMS) concentrations. Variation in natural CO 2 concentrations occurring over 10 3 to 10 5 years are set by oceanic processes, and in particular by conditions in the Southern Ocean, so it is to this region that we must look to understand the glacial–interglacial changes in CO 2 concentrations. It seems likely that marine productivity in the Southern Ocean is limited by a combination of restricted iron supply to the region and insufficient light.

Plankton–produced DMS is thought to influence climate by changing the numbers of cloud condensation nuclei available in remote regions; the efficiency of this mechanism is still unknown, but calculations suggest it may be a powerful influence on climate. It has a much shorter time–scale than the CO 2 effect, and as a consequence may well be a player on the ‘global change’ timescale. The direction of both the CO 2 and the DMS mechanisms is such that more marine productivity would lead to lower global temperatures, and we speculate that the overall effect of the marine biota today is to cool the planet by ca. 6°C as a result of these two mechanisms, with one–third of this figure being due to CO 2 effects and two–thirds due to DMS.

While the marine biota influence climate, climate also influences the marine biota, chiefly via changing atmospheric circulation. This in turn alters ocean circulation patterns, responsible for mixing up sub–surface nutrients, and also influences the transport of nutrients, such as iron, in atmospheric dust. A more vigorous atmospheric circulation would be expected to increase the productivity of the marine biota on both counts. Thus during glacial time, the colder and drier climate might be expected to stimulate greater marine productivity than occurs today. Since more production leads to greater cooling by reduction in CO 2 and increase in DMS, the marine biota–climate system appears to have been in positive feedback in the glacial–interglacial transition, with the changes in the climate system being reinforced by changes in the marine biota. In the context of anthropogenic change, we cannot at present say what sign the feedback on climate will have, because we have no clear idea whether circulation will become more or less vigorous in the future.

Ecosystem feedbacks on climate at the landscape scale

Vegetation controls aspects of climate at all scales. These controls operate through fluxes of mass (water vapour, particulates, trace gases, condensation nuclei, and ice nuclei) and energy (latent and sensible heat, radiative exchanges, and momentum dissipation) between the biosphere and the atmosphere. The role these fluxes play in controlling minimum and maximum temperature, temperature range, rainfall, and precipitation processes are detailed. On the hemispheric scale, the importance of evapotranspiration, vegetation surface roughness, and vegetation albedo in the current generation of atmospheric general circulation models is reviewed. Finally, I assess at the planetary scale the global climate effects of biogenic emissions that are well mixed throughout the troposphere. I show that daily maximum and minimum temperatures are, in part, controlled by the emission of non–methane hydrocarbons and transpired water vapour. In many regions, a substantial fraction of the rainfall arises from upstream evapotranspiration rather than from oceanic evaporation. Biosphere evapotranspiration, surface roughness, and albedo are key controls in the general circulation of the atmosphere: climate models that lack adequate specifications for these biosphere attributes fail. The biosphere modulates climate at all scales.

Field measurements of dimethyl sulphide and its oxidation products in the atmosphere

Dimethyl sulphide (DMS) is released into ocean waters by phytoplankton and may then cross the water–air interface into the atmosphere. Various models have been formulated to describe its behaviour in the atmosphere. Here, measurements of its concentrations and those of its major oxidation products, methane sulphonate, sulphur dioxide, non sea–salt sulphate and demethyl sulphoxide, in both the gas and aerosol phases, in Atlandtic air, are used to validate these qualitative descriptions of its oxidation. Behaviour consistent with day–time oxidation by the hydroxyl radical, with the yield of methane sulphonic acid being both temperature dependent and under the influence of the nitrogen dioxide mixing ratio, is seen. The rapid production of new particles also seems likely under certain conditions but it is not clear whether or not they enhance the concentrations of cloud condensation nuclei in the maritime troposphere. In maritime air a substantial fraction of the sulphate formed appears to be neutralized by reaction with ammonia to form ammonium aerosol.

Detecting Climatic Change Signals: Are There Any "Fingerprints"?

Projected changes in the Earth's climate can be driven from a combined set of forcing factors consisting of regionally heterogeneous anthropogenic and natural aerosols and land use changes, as well as global-scale influences from solar variability and transient increases in human-produced greenhouse gases. Thus, validation of climate model projections that are driven only by increases in greenhouse gases can be inconsistent when one attempts the validation by looking for a regional or time-evolving "fingerprint" of such projected changes in real climatic data. Until climate models are driven by time-evolving, combined, multiple, and heterogeneous forcing factors, the best global climatic change "fingerprint" will probably remain a many-decades average of hemi-spheric- to global-scale trends in surface air temperatures. Century-long global warming (or cooling) trends of 0.5 degrees C appear to have occurred infrequently over the past several thousand years-perhaps only once or twice a millennium, as proxy records suggest. This implies an 80 to 90 percent heuristic likelihood that the 20th-century 0.5 +/- 0.2 degrees C warming trend is not a wholly natural climatic fluctuation.

Global climate change in the instrumental period

The instrumental period of climate history began in the 18th century with the commencement of routine weather observations at fixed sites. Estimates of global-mean climate (e.g. temperature and precipitation) were not possible, however, until the establishment of extensive observing networks midway through the 19th century. This paper reviews our knowledge of global climate change in the instrumental period. Time series of global-mean temperature and precipitation are examined and a comparison is made between two independent 30-year climatologies: 1931-1960 and 1961-1990. Examples are also provided of regional-scale climate changes. Such assessments are important for two reasons. First, they establish the variability of climate on the time-scale of decades, time-scales upon which it is reasonable to plan economic and socio-political activities. Second, and more specifically, they enable us to quantify the magnitude of global-mean climate change which has occurred over this period. Such detailed diagnostic climate information is a necessary, although not sufficient, prerequisite for the detection of global-scale warming which may have occurred due to the enhanced greenhouse effect. Some attention is given to explanations of the observed changes in global-mean climate.

Climate of the earth: an overview

Climate has varied over a large range in the recent history of the Earth, with extremes represented by equable environments of the Cretaceous and Eocene and the comparatively frigid conditions of the ice ages that punctuated the past few million years. It is suggested that major shifts in climate are controlled largely by variations in CO(2) with related fluctuations in modes of ocean circulation. Changes in climate can proceed rapidly, on time scales as short as centuries or even decades, as indicated by data for the Younger Dryas (a period of globally cold conditions interrupting recovery of the Earth from the last ice age) and the Little Ice Age (a cold snap extending from about 1250 to about 1850 ad). Rapid fluctuations in climate appear to be linked to changes in production of deep water in the North Atlantic, possibly also to variations in circulation of intermediate waters in the Pacific. Mechanisms are discussed whereby changes in ocean circulation can result in shifts of climate on a global scale. A 10 000 year record of climate from Norway is used to provide context for a discussion of possible changes in climate today arising as a result of the build-up of industrially related greenhouse gases. Brief, somewhat pessimistic, comments are offered concerning the prospects for meaningful near-term predictions of the response of climate to increased concentrations of greenhouse gases. Studies of past climates, by drawing attention to important processes and feedbacks, can play a valuable role in the development of credible models for the future.

Effects of Aerosol from Biomass Burning on the Global Radiation Budget

An analysis is made of the likely contribution of smoke particles from biomass burning to the global radiation balance. These particles act to reflect solar radiation directly; they also can act as cloud condensation nuclei, increasing the reflectivity of clouds. Together these effects, although uncertain, may add up globally to a cooling effect as large as 2 watts per square meter, comparable to the estimated contribution of sulfate aerosols. Anthropogenic increases of smoke emission thus may have helped weaken the net greenhouse warming from anthropogenic trace gases.

Climate Forcing by Anthropogenic Aerosols

Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol in particular has imposed a major perturbation to this forcing. Both the direct scattering of shortwavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.