Aerosols and Climate

The ever-diversifying field of aerosol effects on climate is comprehensively presented here, describing the strong connection between fundamental research and model applications in a way that will allow both experienced researchers and those new to the field to gain an understanding of a wide range of topics. The material is consistently presented at three levels for each topic: (i) an accessible "quick read" of the essentials, (ii) a more detailed description, and (iii) a section dedicated to how the processes are handled in models. The modelling section in each chapter summarizes the current level of knowledge and what the gaps in this understanding mean for the effects of aerosols on climate, enabling readers to quickly understand how new research fits into established knowledge. Definitions, case studies, reference data, and examples are included throughout.

Aerosols and Climate is a vital resource for graduate students, postdoctoral researchers, senior researchers, and lecturers in departments of atmospheric science, meteorology, engineering, and environment. It will also be of interest to those working in operational centers and policy-facing organizations, providing strong reference material on the current state of knowledge.

• Includes a section in each chapter that focuses on the treatment of relevant aerosol processes in climate models
• Provides clear exposition of the challenges in understanding and reducing persistent gaps in knowledge and uncertainties in the field of aerosol-climate interaction, going beyond the fundamentals and existing knowledge
• Authored by experts in modeling and aerosol processes, analysis or observations to ensure accessibility and balance

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 19, 2022
• ISBN: 9780128231722

Book preview

Acronyms and abbreviations

ACI    Aerosol–cloud interaction

ACSM    Aerosol Chemical Speciation Monitor

ACTRIS    Aerosols, Clouds and Trace gases Research InfraStructure Network

AerChemMIP    Aerosol Chemistry Model Intercomparison Project

AeroCom    An open international initiative of scientists interested in the advancement of the understanding of the global aerosol and its impact on climate

AERONET    Aerosol Robotic Network

AGCM    Atmospheric general circulation model

AI    Aerosol index

AIRS    Atmospheric InfraRed Sounder

AMIP    Atmospheric Model Intercomparison Project

AMOC    Atlantic meridional overturning circulation

AMS    Aerosol Mass Spectrometer

AMV    Atlantic multidecadal variability

AOD    Aerosol optical depth

AOGCM    Atmosphere-ocean general circulation model

AR4, AR5, AR6    Assessment reports of the Intergovernmental Panel on Climate Change

ARI    Aerosol–radiation interaction

ATOFMS    Aerosol Time-of-Flight Mass Spectrometer

ATSR    Along Track Scanning Radiometer

AVHRR    Advanced Very High Resolution Radiometer

BB    Biomass burning

BC    Black carbon

BVOC    Biogenic volatile organic compound

CALIOP    Cloud-Aerosol Lidar with Orthogonal Polarization

CALIPSO    Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations

CAMS    Copernicus Atmosphere Monitoring Service

CAPE    Convective available potential energy

CATS    Cloud-Aerosol Transport System

CCN    Cloud condensation nucleus/nuclei

CEDS    Community Emissions Data System

CFMIP    Cloud Feedback Model Intercomparison Project

CLAW    Charlson, Lovelock Andreae, Warren (authors of a publication)

CLE    Current legislation

CLRTAP    Convention on Long-Range Transboundary Air Pollution

CMIP    Coupled Model Intercomparison Project

CPC (CNC)    Condensation Particle Counter (Condensation Nucleus Counter)

CPM    Cloud-permitting model or convection-permitting model

CPU    Central processing unit

CRM    Cloud-resolving model

CS    Condensation sink

CTM    Chemical transport model

DCC    Deep convective cloud

DJF    December January February

DMA    Differential Mobility Analyzer

DMPS    Differential Mobility Particle Sizer

DMS    Dimethyl sulfide

DNS    Direct numerical simulation (atmospheric model)

DSCOVR    Deep Space Climate Observatory

EARLINet    European Aerosol Research Lidar Network

EBAS    A database hosting observation data of atmospheric chemical composition and physical properties

EBC    Equivalent black carbon (based on optical absorption)

ECS    Equilibrium climate sensitivity

ELVOC    Extremely low volatility organic compound

EMIC    Earth system model of intermediate complexity

ENSO    El Niño Southern Oscillation

ENVISAT    Environmental satellite operated by the European Space Agency

EOF    Empirical orthogonal function

EOS    Earth Observing System (NASA)

ERF    Effective radiative forcing

ESA    European Space Agency

ESM    Earth system model

EUMETSAT    European Organisation for the Exploitation of Meteorological Satellites

FAR    First Assessment Report (of the Intergovernmental Panel on Climate Change)

FAIR    Finite Amplitude Impulse Response simple climate model/emulator

FF    Fossil fuel

FT    Free troposphere

GAINS    Greenhouse gas Air pollution INteractions and Synergies

GAW    Global Atmosphere Watch

GCCN    Giant cloud condensation nucleus/nuclei

GCM    General circulation model (also global climate model)

GDE    General dynamic equation of aerosol

GEOS    Goddard Earth Observing System

GOES    Geostationary Environmental Satellites

GOME    Global Ozone Monitoring Experiment

GOMOS    Global Ozone Monitoring by Occultation of Stars

GTP    Global temperature potential

GWP    Global warming potential

HNLC    High nutrient-low chlorophyll

HS    Apparent hydrological sensitivity

HSRL    High Spectral Resolution Lidar

IAM    Integrated assessment model

IASI    Infrared Atmospheric Sounding Interferometer

IMPROVE    Interagency Monitoring of Protected Visual Environments

INP    Ice-nucleating particle

IPCC    Intergovernmental Panel on Climate Change

IR    Infrared

ITCZ    Intertropical Convergence Zone

IUPAC    International Union of Pure and Applied Chemistry

JJA    June July August

LES    Large-eddy simulation (atmospheric model)

LVOC    Low volatility organic compound

LW    Longwave

M/R    Maximum/random (overlap)

MACC    Monitoring Atmospheric Composition and Climate

MAGICC    Model for the Assessment of Greenhouse Gas Induced Climate Change

MAM    March April May

MCB    Marine cloud brightening

MERRA    Modern-Era Retrospective Analysis for Research and Applications

MIP    Model Intercomparison Project

MISR    Multi-angle Imaging SpectroRadiometer

MME    Multi-model ensemble

MOA    Marine organic aerosol

MODIS    MODerate Resolution Imaging Spectroradiometer

MS    Mass spectrometer

MXD    Maximum latewood density

NDC    Nationally determined contribution

N:P    Nitrogen:Phosphorus (nutrient ratio)

NIR    Near-infrared

NIST    National Institute of Standards

NMVOC    Non-methane volatile organic compound

NOAA    National Oceanic and Atmospheric Administration

NOx    Nitrogen oxides

NWP    Numerical weather prediction

OA    Organic aerosol

OMI    Ozone Mapping Instrument

OPC (OPS, OPSS)    Optical particle counter (spectrometer, size spectrometer)

OSIRIS    Optical Spectrograph and InfraRed Imaging System

PD    Present day

PDF    Probability density function

PDO    Pacific decadal oscillation

PI    Preindustrial

POA    Primary organic aerosol

POLDER    POLarization and Directionality of the Earth's Reflectances

POM    Particulate organic matter

PPE    Perturbed parameter ensemble

RCP    Representative concentration pathway

RF    Radiative forcing (usually defined as instantaneous radiative forcing)

RFMIP    Radiative Forcing Model Intercomparison Project

RGCM    Regional general circulation model

RH    Relative humidity

SAGE    Stratospheric Aerosol and Gas Experiment

SAR    Second Assessment Report (of the Intergovernmental Panel on Climate Change)

SCIAMACHY    SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY

SDG    Sustainable development goal

SEMS    Scanning Electrical Mobility Spectrometer

SEVIRI    Spinning Enhanced Visible and Infrared Imager

SIP    Secondary ice production

SLCF    Short-lived climate forcer

SLSTR    Sea and Land Surface Temperature Radiometer

SMPS    Scanning Mobility Particle Sizer

SOA    Secondary organic aerosol

SON    September October November

SP2    Soot Particle Photometer

SRES    Special Report on Emissions Scenarios

SSA    Single-scattering albedo

SSP    Shared socio-economic pathway

SST    Sea surface temperature

STP    Standard temperature and pressure

SVOC    Semi-volatile organic compound

SW    Shortwave

TAR    Third Assessment Report (of the Intergovernmental Panel on Climate Change)

TIR    Thermal infrared

TOA    Top of the atmosphere

TOMS    Total Ozone Mapping Spectrometer

TRW    Tree ring width

UAP    Ultrafine aerosol particle

UT    Upper troposphere

UV    Ultraviolet

UVAI    Ultraviolet Aerosol index

VEI    Volcanic explosivity index

VIIRS    Visible Infrared Imaging Radiometer Suite

VNIR    Visible-near-infrared

VOC    Volatile organic compound

WBF    Wegener-Bergeron-Findeisen

WMGHG    Well-mixed greenhouse gas

WHO    World Health Organization

WMO    World Meteorological Organization

Symbols

a These are the units typically used to report quantities in atmospheric aerosol science.

Chapter 1: Introduction

Ken S. Carslaw    School of Earth and Environment, University of Leeds, Leeds, United Kingdom

Abstract

This chapter summarizes the importance of aerosol in the climate system and provides an overview of the book Aerosols and Climate.

Keywords

Aerosol; Radiative forcing; Climate change; Global warming; Precipitation; Climate model

1.1: What is aerosol and why is it important for climate?

Earth's atmosphere from the surface to the top of the stratosphere at about 40 km altitude contains particles ranging in size from molecular clusters of nanometers in diameter up to particles of several micrometers near the surface. Aerosol is the term used to describe this suspension of liquid and solid particles in the air. This book is called Aerosols and Climate (plural) because there are many different types of aerosol with varying effects on climate. Aerosol is important for climate because the particles scatter and absorb solar and terrestrial radiation and because they are the nuclei upon which cloud droplets and ice particles form, which dominate Earth's albedo.

Particles enter the atmosphere mainly at the surface by direct emission of material like wind-blown sea spray and mineral dust, and from natural and human (anthropogenic) combustion sources that produce particles composed of complex mixtures of organic carbon, soot and other material. There is also a tiny input of particles at the top of the atmosphere from cosmic dust. Particles also form directly within the atmosphere from gas-to-particle conversion (nucleation) of a wide range of inorganic and organic gas-phase compounds derived from natural and anthropogenic sources. These particles start life as molecular clusters that can eventually grow up to several tens of nanometers in diameter through coagulation and partitioning of a wide range of gas-phase compounds into the particles. Particles subsequently undergo many physical and chemical changes during transport through the atmosphere, involving interaction of particles with each other, further condensation of material like sulfate and organic material, and interaction with clouds. These processes mix particles together, creating a hugely diverse array of particle sizes, chemical compositions, and number concentrations.

In almost any part of the troposphere submicron-sized particles are a complex mixture of varying amounts of inorganic salt and acidic species (sodium, ammonium, sulfate, nitrate, chloride), organic compounds in widely varying states of oxidation, as well as insoluble material such as soot (black carbon), mineral material, and other inclusions. Almost all particles have at least some water associated with them, in amounts that depend on the chemical composition of the particle and ambient relative humidity. Some of the chemical components were present when the particles were emitted and some enter the particles sometime later by condensation of gas-phase compounds potentially thousands of kilometers from where the particles or gases were emitted. This spatial variability of sources combined with the relatively short lifetime of particles and gases due to wet and dry deposition on the Earth's surface creates a highly heterogeneous distribution of particles in the troposphere that is far more challenging to simulate in a global model than long-lived greenhouse gases.

In the cleanest parts of the atmosphere remote from sources of particles or gases that can nucleate to form new particles, the particle number concentration can be lower than 1 cm− 3 and the mass concentration less than a fraction of a microgram per cubic meter. In the most polluted regions with strong sources or slow removal processes, number concentrations can exceed 10⁵ cm− 3 and mass concentrations can reach several hundred micrograms per cubic meter. In the majority of environments, most of the particle mass resides in the largest super-micron particles and most of the number lies in particles smaller than about 100 nm diameter. The particles most relevant for climate are those at intermediate sizes of several tens of nanometers up to about a micrometer, which readily form cloud droplets and scatter and absorb solar radiation efficiently.

Human activities have profoundly altered aerosol emissions to the atmosphere primarily through combustion of fossil fuels, but also through agriculture, changes in land use, and alteration of natural processes like wildfires. Anthropogenic emissions of some important aerosol species like black carbon and sulfur dioxide (which forms sulfate aerosol) exceed natural emissions, resulting in substantial increases in aerosol particle mass and number concentrations over large parts of the planet with implications for Earth's radiative energy balance and climate. As we show in Chapter 2 and in a later chapter on satellite observations, continental-scale plumes of aerosol from human activities are visible in satellite images thousands of kilometers downwind of sources.

The effects of aerosol on Earth's climate are diverse and complex mainly because they depend on many more properties of the particles than just the number or mass concentration. The scattering and absorption of radiation, which alter Earth's radiative energy balance (Fig. 1.1), depend on the size of the particles, their chemical composition, the refractive index of the material, and the particle shape, as well as several environmental factors such as the humidity of the air (which controls the amount of water absorbed by the particles), the nature of the surfaces that they overly, and their location relative to clouds. When we refer to ‘changes in aerosol’ from human activities as a driver of climate change we are referring to changes in any or all of these particle properties as well as changes in their horizontal and vertical distribution in the atmosphere.

Fig. 1.1

Fig. 1.1 The main ways in which an increase in aerosol from anthropogenic activities affects the climate. Scattering and absorption of solar radiation by aerosol and modification of cloud properties results in a net loss of radiative energy from the planet, and hence a cooling effect on climate. Other effects (not shown) include changes in the temperature structure and stability of the atmosphere, changes in the absorption and emission of radiation in the terrestrial (infrared) spectrum, darkening of snow and ice surfaces, changes in ice particle formation in clouds, and effects on the carbon cycle from deposition of nutrients required by biota.

Aerosol particles are also the nuclei upon which cloud droplets and ice particles form. Cloud droplets are typically several tens of micrometers in diameter – 2–3 orders of magnitude larger than the aerosol particles upon which they form, so globally they reflect considerably more solar radiation than aerosol particles. Changes in aerosol alter the number concentration and surface area of cloud droplets as well as several other climatically important cloud properties like water content, thickness, and areal coverage. These aerosol-induced changes to cloud properties alter the reflection and absorption of solar radiation and the absorption and emission of terrestrial radiation by clouds, with consequent effects on Earth's energy balance and climate. A tiny fraction of aerosol particles with concentrations as low as 10− 4 cm− 3 possess special properties that enable them to initiate the formation of ice in clouds, which can profoundly alter the behavior of clouds below 0°C.

Changes in aerosol caused by human activities and natural variability affect climate by altering energy flows within the atmosphere and between the surface, atmosphere, and space. Scattering of solar radiation back to space and the absorption of radiation within the atmosphere alter the net amount of radiative energy in the climate system (Fig. 1.1). The first-order response to this change in radiative energy is a change in Earth's global average temperature, although the thermal inertia of the ocean means that it takes many decades to centuries to re-establish a new equilibrium global temperature after aerosol has been perturbed. Regional- and hemispheric-scale changes in atmospheric circulation and land temperatures can occur on much shorter timescales, with regionally important impacts on climate.

Changes in aerosol also affect precipitation. The distribution of precipitation can be affected locally and regionally on the timescale of hours to days by changes in cloud microphysical processes triggered by changes in droplet and ice particle concentrations. The total amount of precipitation can also be affected on regional and global scales on the timescale of days to months by changes in the radiative energy deposited in the atmosphere, which determines the amount of water that can condense. Ultimately, on the timescale of decades to centuries aerosol-induced changes in surface temperature further alter the hydrological cycle and precipitation.

It has proven extremely challenging to accurately quantify the magnitude of aerosol effects on climate driven by anthropogenic emissions. It is known that the net effect of anthropogenic aerosol is an enhancement of reflection of solar radiation from the atmosphere and clouds and therefore an increase in planetary albedo. This radiative forcing has caused a cooling effect on climate over the industrial period that is commensurate with the warming effect of anthropogenic greenhouse gases. The fact that Earth has warmed over the industrial period indicates that the aerosol cooling effect is smaller than the greenhouse gas warming (unless other major forcings have not been accounted for). The radiative forcing relative to preindustrial conditions has been persistently uncertain in climate model simulations and observations, and there is even less confidence in how the forcing has affected global and regional temperatures and weather patterns. The difficulty stems from the many ways in which aerosol affects climate, the huge diversity of aerosol properties that matter, and the enormous variations in aerosol properties around the planet. Subsequent effects on clouds, weather patterns, and regional climate driven by changes in aerosol, radiation, and temperature add further complications that challenge our understanding of meteorology and climate dynamics.

Aside from aerosol effects on the physical climate (radiation, clouds, precipitation, temperature), there are also numerous effects involving changes in Earth's biota in the ocean and on land caused by altered deposition of nutrient species like iron, phosphorus, and nitrogen. Changes in biota have the potential to affect the carbon cycle and hence carbon dioxide concentrations.

1.2: Aims and scope of the book

This book has been written for scientists entering what has become a hugely diverse field of science. When Sean Twomey's seminal book Atmospheric Aerosols (Twomey, 1977) was published, the role of aerosol in climate change deserved one short chapter in a book focused on aerosol dynamics, optics, and electrical properties. At that time, when the potential role of anthropogenic aerosol in climate change was beginning to be studied (see Chapter 2), a knowledge of physics and some basic chemistry was sufficient to investigate the processes that were considered to be important at the time. In the intervening 40 years or so, aerosol-climate science has grown to encompass many aspects of meteorology, climate dynamics, cloud and radiation physics, biogeochemistry, and other disciplines. It also stimulated and now relies heavily on a vast array of measurements from satellites and in situ instrumentation as well as numerical modeling from the scale of individual clouds to global Earth system models. The challenge for new entrants to the field of aerosol-climate science is that many of the big questions need to be tackled in collaborations that span many or all of these disciplines. As a consequence, most atmospheric aerosol scientists now find themselves needing to understand at least the basics of a wide range of topics outside their immediate specialism.

A further challenge for new entrants is that diversification of aerosol science has also led to the emergence of several subdisciplines, which has inevitably led to the development of specialized terminology, concepts, definitions, and nomenclature. Such specialization makes aerosol-climate science increasingly inaccessible to new entrants as well as to scientists whose research diversifies. At the very least it makes it challenging to switch between parallel sessions at a major aerosol conference or to participate actively in a collaborative project. The aim of this book is to connect these diverse areas of aerosol science and present the state of knowledge in a consistent way.

One of the main motivations for pursuing a research career in aerosol-climate science is that the climatic effects of aerosol are highly uncertain, a situation that has persisted since the first attempts in the 1990s to evaluate these effects in a consistent way. However, new entrants to this field soon appreciate that an understanding of aerosol fundamentals alone is insufficient to understand and tackle the uncertainty. This is because the uncertainty stems from how the fundamental processes interact, how they are applied in models, and what assumptions are made because of a lack of process-level understanding or limited computational power to run large-scale models. Indeed, perhaps the greatest separation of subdisciplines in aerosol science is between investigations of aerosol and cloud processes in ever-increasing detail and the development of large-scale models that are ultimately used to define our overall level of understanding and to inform climate policy. To address this disconnection, the book includes a dedicated chapter on modeling and each of the subsequent topic chapters includes a section on models with the aim to ‘lift the lid’ on how various processes are handled in models and what is neglected or treated inadequately.

This is not a book about aerosol physical and chemical fundamentals. For that there are several other excellent textbooks aimed at scientists working within subdisciplines of aerosol-climate science. Some fundamental concepts are introduced in each chapter, especially where we think the concepts may be difficult to assemble in a coherent way from other sources or where the terminology has begun to deviate from what has been used in the subdisciplines. Rather, the book is about how fundamental concepts in aerosol science are being applied in climate science and how the concepts are ultimately translated into better climate models.

The topics covered in the book reflect the major challenges in the field of aerosol-climate science. In designing the chapters, we were also aware of the need to provide a primer on topics that have been vital to the development of our understanding of global aerosol, including the development of emission inventories, ambient and remote aerosol measurements, and modeling. We have tried to avoid structuring the chapters around aerosol chemical species that are important research foci, such as dust, black carbon, and organic aerosols. Instead, these are incorporated in the relevant chapters on processes, emissions, radiation, and observations. Chapters on volcanic and Arctic aerosol might appear to be the exceptions, but these are also associated with particular regions of the atmosphere, so we describe the processes in the context of the environments in which they occur.

Following this introduction, the book is organized as follows:

Chapter 2 provides an overview of the effects of aerosol on climate, including changes in radiative energy fluxes, temperature, and precipitation on a global scale. It introduces the important concept of aerosol radiative forcing (the radiative energy imbalance driving climate change), atmospheric and radiative adjustments to the forcing that have proved difficult to quantify, long-term climate response, and the importance of aerosol in climate sensitivity. This chapter also provides a brief history of our understanding of aerosol effects on climate.

Chapter 3 extends the discussion of aerosol climatic effects to the role of aerosol in the Earth system, in particular the ways in which aerosol alters biogeochemical processes on land and in the ocean leading to changes in the carbon cycle that alter climate. We also describe how biological processes on land and in the ocean generate aerosols and precursor gases that affect climate and are, in turn, affected by climate change.

Chapter 4 describes the properties and distribution of aerosol. The chapter defines the fundamental properties of aerosol particles that determine the effects on climate – the particle size distribution and chemical composition – and how these properties vary spatially on the scale of meters to thousands of kilometers and temporally on the timescale of hours to years.

Chapter 5 describes the aerosol physical processes that shape the aerosol physical and chemical properties, covering particle formation from gas-phase precursors, particle growth, coagulation, exchange of chemical species with the gas phase, water uptake, and dry and wet deposition. We emphasize the timescales of these processes and how they shape particle properties during their short residence time in the atmosphere.

Chapter 6 summarizes the representation of aerosol in climate modeling. Our aim in this chapter is to ‘lift the lid’ on how aerosols, clouds, and radiation are handled in the wide range of models used in climate science. This is a vast topic, so we focus on highlighting the many assumptions that are made, why they are made, and how they affect the realism and reliability of models. Modeling is often seen as a downstream activity, which ought to place it at the end of the book. However, the anatomy of a model described here sets the scene for the more-detailed descriptions of model processes in each of the subsequent chapters.

Chapter 7 describes the historical variations in aerosol as recorded in ice cores and from direct and indirect measurements since the 1960s. It is these changes in aerosol that have caused changes in climate. The measurement records are patchy and often difficult to interpret directly in terms of atmospheric aerosol abundance, but they paint a consistent picture of long-term climatically important variations.

Chapter 8 follows directly from Chapters 6 and 7 by describing aerosol and precursor gas emissions, which are a vital input to climate models and key to understanding long-term aerosol trends. We explain how emission rates are defined and how they are developed for input into models. Emissions derive from human activities (often referred to as anthropogenic) and from natural sources like sea spray, fires, and the biosphere. We address how emissions are estimated from data on human activity and from measurements and models of natural processes.

Chapter 9 describes the measurement of ambient particle properties. Aerosol science makes extensive use of ambient (in situ) measurements to understand processes and to constrain the state and behavior of models. In this chapter we describe measurements of particle number and mass concentration, size distribution, water uptake, optical properties, chemical composition, and interaction with clouds.

Chapter 10 describes satellite measurements of aerosol. Satellites provide the only way to view the distribution of aerosol on a global scale and they have been a critical part of assessments of the global radiative effects of aerosol as well as the effects on clouds. In this chapter we describe the key instruments, measurement techniques, and methods of retrieving aerosol properties from radiative measurements.

Chapter 11 focuses on aerosol-radiation interactions, including the scattering and absorption of solar and terrestrial radiation by particles, the net effect aerosol on Earth's radiative energy budget, and the effect that anthropogenic emissions have had on these processes. After defining some fundamental processes, estimates of the magnitude of radiative forcing from satellite measurements and models are presented, with further detail on how satellite measurements are made in Chapter 10.

Chapter 12 focuses on aerosol-cloud interactions in shallow liquid clouds. The chapter introduces some fundamental aspects of cloud physics required to understand how cloud processes and properties are perturbed by changes in aerosol. The chapter focuses on shallow liquid clouds like stratus and stratocumulus that dominate the cloud radiative effect on climate. We outline how perturbation of cloud properties by changes in aerosol is quantified and address some of the reasons why the magnitude of radiative effects remains very challenging to quantify.

Chapter 13 addresses large-scale dynamical responses to aerosol. This chapter explains how nonuniform cooling and heating in the atmosphere and at the surface caused by aerosol affect regional-scale energy flows, resulting in changes in the atmospheric circulation on scales ranging from subcontinental to hemispheric-wide patterns, with subsequent impact on temperature and precipitation.

Chapter 14 describes our understanding of aerosol effects on deep convective clouds. The processes and climatic effects in deep clouds reaching 10–15 km in the troposphere are more varied and complex than in shallow clouds owing to the cascade of water and ice-phase processes that occur within a complex dynamical environment. The effect of changes in aerosol on cloud vigor and the creation of extensive, radiatively important anvil clouds is consequently incompletely understood and regionally variable.

Chapter 15 addresses aerosol effects on ice formation in clouds, which is another important way that aerosol can affect clouds and climate. The chapter covers the properties of ice-nucleating particles, how ice is formed in clouds, and our current understanding from models and observations of the effects of these particles on shallow mixed-phase clouds in the lower troposphere and cirrus clouds in the upper troposphere.

Chapter 16 describes the aerosol processes in polar and high-latitude regions. Aerosol effects at high latitudes deserve special treatment because of their potential role in the strongly amplified rate of climate change in the Arctic in particular. We describe some of the unique aerosol properties and processes in this region, including the interactions with clouds and the ways in which these differ from other regions.

Chapter 17 explores the effects of volcanic aerosol on climate. Volcanic eruptions stand out in the climate record as substantial perturbations to surface solar radiation, global temperature, and precipitation patterns caused by substantial increases in mainly sulfuric acid aerosol in the stratosphere. Degassing volcanoes and effusive eruptions are also an important source of aerosol to the global troposphere. We describe how these different eruption styles affect aerosol in the troposphere and stratosphere.

Chapter 18 addresses the role of aerosol in climate engineering. Two of the most prominent proposals to deliberately modify the climate involve aerosol – injection of particles into the stratosphere to mimic the effects of volcanic eruptions, and injection of sea spray particles into shallow marine clouds to increase their reflection of solar radiation. This chapter describes the principles of these two methods of climate engineering, including potential inadvertent effects on aspects of regional climate.

Chapter 19 outlines the role of aerosol in climate and air quality policy. Many of the changes in aerosol abundance over the industrial period have occurred as a result of policies related to improving air quality and preventing environmental degradation. We describe these policies as well as ways in which future climate change policies such as the Paris 1.5 degree target account for aerosol effects. We also outline how air quality and climate are coupled problems in which changes in climate can also affect aerosol and air quality.

1.3: Terminology, symbols, and units

Aerosol science is a very broad discipline with roots in fundamental sciences like physics and chemistry, established sciences like meteorology and instrument technology (metrology), and sciences that have existed for only a few decades, such as climate modeling, air pollution, and policy. This breadth brings with it a vast range of terminologies, acronyms, and symbols to describe physical quantities, as well as a diverse range of units. We have aimed for consistency through the book, but there are a few areas of divergence.

Aerosol is itself inconsistently used in aerosol science. In this book we refer to aerosol as a suspension of particles in air, so we mostly refer to it in the singular. The plural is used when referring specifically to multiple types of aerosol, so dust and sea spray are aerosols. The word aerosol is increasingly used as a synonym of particle such as aerosol growth or the aerosols, meaning the particles. We avoid this usage.

Diameters and radii are used often interchangeably in aerosol science. Particle measurements are almost always reported in terms of diameter (although less common in the stratosphere), while the equations of particle microphysics (growth, coagulation, etc.) almost always use radius: nobody learned in school that the volume of a sphere is si1_e . We have not attempted to unify the use of radius and diameter through the book.

Acronyms and symbols. Aerosol science is rife with acronyms. We have tried to keep their use to a minimum, but many are now so deeply ingrained that they are often not defined in publications. A list of acronyms is provided.

It has become common to use acronyms and symbols for physical quantities, often with a symbol used in an equation but an acronym for the same quantity in the text. Prominent examples are AOD (aerosol optical depth) and radiative forcing (RF) in phrases such as RF = 2 W m− 2. We avoid this usage, which is inconsistent with IUPAC guidelines (Cohen et al., 2006). In keeping with IUPAC, we use single letters for physical quantities, such as τa for aerosol optical depth and △ F for radiative forcing, where △ indicates that it is a change in radiative flux F. Likewise, although CCN is an appropriate acronym for cloud condensation nuclei, we use NCCN for the associated physical quantity of CCN number concentration. The only case where we diverge from this usage is with PM (particulate matter concentration), which is deeply embedded in our field in relation to air quality regulations. We recognize that our approach may make some of the chapters a little unfamiliar to specialists who are used to their acronyms, but we felt that consistency (or something approaching it) was the more important consideration.

References

Cohen et al., 2006 Cohen E.R., Cvitaš T., Frey J.G., Holmström B., Kuchitsu K., Marquardt R., Mills I., Pavese F., Quack M., Stohner J., Strauss H.L., Takami M., Thor A.J. Quantities, Units and Symbols in Physical Chemistry. In: International Union of Pure and Applied Chemistry. 3rd ed. 2006.

Twomey, 1977 Twomey S. Atmospheric Aerosols, Developments in Atmospheric Science. Amsterdam: Elsevier Scientific Publishing Company; 1977.

Chapter 2: Aerosol in the climate system

Ken S. Carslaw    School of Earth and Environment, University of Leeds, Leeds, United Kingdom

Abstract

Aerosol particles affect Earth's climate through processes of scattering and absorption of radiation in the atmosphere, through interaction with clouds, and by altering the albedo of snow and ice. This chapter describes how these processes affect Earth's energy balance and how changes in energy balance affect temperature and precipitation. Following a short history of our understanding of aerosol effects on climate, the chapter defines the energy flows within the climate system and the ways in which aerosol affects these flows. It then defines the concept of radiative forcing of climate and summarizes current estimates of the magnitude of aerosol radiative forcing over the industrial period. The chapter concludes by describing the response of Earth's temperature and precipitation to radiative perturbations in terms of climate sensitivity and hydrological sensitivity.

Keywords

Radiative forcing; Energy balance; Precipitation; Radiative adjustment; Forcing efficacy; Aerosol–radiation interaction; Aerosol–cloud interaction; Climate sensitivity; Hydrological sensitivity

Summary. Aerosol particles affect Earth's climate through processes of scattering and absorption of radiation in the atmosphere, through interaction with clouds, and by altering the albedo of snow and ice. This chapter describes how these processes affect the energy balance of the planet and how changes in energy balance affect temperature and precipitation. The wider effects on the terrestrial and marine biosphere, including the carbon cycle, are the subject of Chapter 3.

The effects of aerosol on the atmosphere are readily observable with the naked eye as a reduction in visibility (or visual range) and a change in the color of the sky (a whitening that diminishes the blue color) – see Fig. 2.1. Satellite measurements (Chapter 10) often show extensive plumes of aerosol pollution emanating from continental areas, and ship tracks and degassing volcanoes show up as modifications of cloud brightness (Chapter 12).

Fig. 2.1

Fig. 2.1 Visible effects of atmospheric aerosol particles. (a) Moderate Resolution Imaging Spectroradiometer (MODIS) image from the Aqua satellite on March 31, 2003 showing pollution over the Atlantic Ocean off the west coast of France (NASA Visible Earth, Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC). (b) Ship tracks off the California coast caused by particles emitted in ship effluent altering clouds (NASA Visible Earth, Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC). (c) Effect of aerosol pollution on visual range. Two images taken at the same time on two different days. The particulate matter concentration is given on each image.

In the atmosphere, the main effect of aerosol particles is to scatter and absorb solar radiation, leading to reflection of a small fraction of incoming solar energy back to space, absorption of solar energy within the atmosphere, and reduction in solar energy at the Earth's surface. In addition, terrestrial radiation (infrared radiation emitted by the Earth and atmosphere) is affected by large dust particles in the troposphere and by large sulfate aerosol particles in the stratosphere after volcanic eruptions. The net effect of aerosol is to reduce the radiative energy in the climate system – a climatic cooling effect. In addition, the absorption of some radiative energy in the atmosphere and at surface affects evaporation, latent heat release, and precipitation. In the solar spectrum atmospheric aerosol (i.e., the global atmosphere with aerosol minus aerosol-free air) directly reflects about 2 W m− 2 at the top of the atmosphere (TOA), compared to net incoming solar radiation of 340 W m− 2 and a net reflection by the atmosphere and clouds of 75 W m− 2. This is a small radiative effect of aerosol compared to the effect of all well-mixed greenhouse gases (carbon dioxide, methane, and nitrous oxide), which warm the planet by about 35 W m− 2 by reducing emission of terrestrial radiation to space. However, human activities have altered these two radiative effects by similar amounts.

The change in abundance of aerosol caused by human activities has altered the natural aerosol radiative effect, resulting in a radiative forcing of climate and additional energy loss from the climate system. The effect on radiation of additional aerosol in the air (called aerosol–radiation interaction) is estimated to have caused a radiative forcing of − 0.14 to − 0.71 W m− 2 (90% confidence) globally averaged at the top of the atmosphere since 1850. Changes in aerosol abundance have also increased the reflection of solar radiation by clouds by increasing the number of cloud droplets and by triggering an array of subsequent changes in precipitation, cloud water content, coverage, and depth (called aerosol–cloud interaction). These effects have caused a radiative forcing of − 0.07 to − 2.65 W m− 2 (90% confidence) since 1850.

The net global mean aerosol radiative forcing due to aerosol–radiation and aerosol–cloud interactions in the atmosphere is estimated to be in the range − 0.35 to − 2.0 W m− 2 with 90% confidence. This range is tighter than the sum of aerosol–radiation and aerosol–cloud effects because the magnitude can be further constrained by considering the radiative energy balance of the climate system in relation to changes in global mean temperature. Therefore, despite natural greenhouse gases having a much larger effect on Earth's climate than natural aerosol, human perturbation of these radiative effects over the industrial period are much more comparable (about 2.2–3.4 W m− 2 for greenhouse gases versus − 0.35 to − 2.0 W m− 2 for aerosol). The wide range of aerosol forcing estimates has barely altered since the first assessments in the early 1990s, due to the complexity of the aerosol-climate system, which is the subject of this book.

Increases in aerosol affect global precipitation as well as the distribution of precipitation between land and ocean. On short timescales of a few decades the main driver of changes in global precipitation is the absorption of solar radiation by particles in the atmosphere containing black carbon (BC), which increases atmospheric stability and reduces turbulent fluxes of moisture from the surface, and therefore reduces precipitation. On the timescale of many decades, the net effect of an increase in aerosol abundance is a reduction in global mean surface temperature, which reduces evaporation and hence precipitation. Over the industrial period over land, anthropogenic aerosol is estimated to have reduced global precipitation by about 10 mm year− 1, which has more than counteracted the increase due to rising temperature from the increase in well-mixed greenhouse gases.

The spatial patchiness of aerosol abundance in the atmosphere, caused by the short residence time of aerosol particles, is important for regional climate. Changes in aerosol over the industrial period have occurred mostly over the Northern Hemisphere close to anthropogenic emissions, which has led to a contrast between Northern and Southern Hemisphere energy balance and a small shift in the latitude of the intertropical convergence zone that is important for regional rainfall patterns. On smaller scales, regional differences in the absorption of solar radiation in the atmosphere and at the surface have altered monsoon circulations. Such regional adjustments in energy balance, weather patterns and precipitation have influenced the long-term (multidecadal) response of global mean temperature and precipitation to anthropogenic aerosol, making a global analysis difficult (see Chapter 13).

We begin this chapter with a short history of our understanding of aerosol effects on climate starting around the 1960s when aerosol pollution was first linked to potential effects on climate. In Section 2.2 we then outline evidence for aerosol effects on climate. Section 2.3 covers fundamental aspects of Earth's energy flows and how the climate responds to radiative imbalances such as caused by changes in aerosol. Section 2.4 focuses on aerosol radiative forcing of climate including its magnitude and uncertainty, which is expanded in Chapters 11 and 12. Finally, in Section 2.5 we describe the response of the climate to changes in aerosol, including temperature and precipitation.

2.1: The historical development of our understanding of aerosol effects on climate

Our understanding of the effects of anthropogenic aerosol on climate has evolved quite differently to our understanding of greenhouse gases. While the effect of anthropogenic greenhouse gases was understood from the earliest studies to be unequivocally one of global warming, several factors have prevented development of such a clear picture of aerosol effects. These factors include the regional distribution of aerosol pollution (in contrast to the more homogeneous distribution of long-lived well-mixed greenhouse gases), the lack of adequate observations (exacerbated by the spatial heterogeneity), and the need to consider more-complicated aerosol physical processes and properties (including complex changes to clouds) to estimate the radiative effects. Many of the challenges that slowed the development of aerosol-climate science remain challenges today, and some still contribute to the large uncertainties in our estimates of aerosol climatic effects.

The effects of volcanic eruptions on climate were the main motivation for studying atmospheric aerosol up to the 1960s, initially stimulated by the eruption of Krakatau in 1883, although as early as 1780s Benjamin Franklin had attributed a change in climate to the Laki eruption in Iceland. The effect of volcanic dust (now understood to be mainly sulfate aerosol) on climate had already been explored in a textbook on atmospheric physics in 1920 (Humphreys, 1920). The main components of Earth's albedo were known in the 1950s (Fritz, 1949) and changes in atmospheric turbidity (reduction in transparency) through volcanic eruptions were speculated to affect temperature, atmospheric circulation, and evaporation (Wexler, 1956), all of which are now confirmed by modern observations and models. Anders Knutsson Ångström's work developed the fundamental equations for the effect of changes in turbidity on albedo (Ångström, 1962) in terms of a turbidity coefficient (related to what is now called optical depth), developing his fundamental work on scattering and absorption by particles (Ångström, 1929). Based on Ångström's estimate of albedo of 0.333 (now measured to be 0.30), he estimated that a change of 10% in turbidity would alter the albedo by about 1.5%, or about 0.8% of the Earth's incoming energy at the surface.

Trends in air pollution and atmospheric turbidity were of major interest in the 1960s, with air quality very poor over parts of the United States and Europe. Studies began to show that the turbidity of the atmosphere was increasing and that this might be a cause of the observed global cooling from the 1940s until the 1960s (see Fig. 2.2), which was opposite to what was expected from rising CO2 concentrations. A key study (McCormick and Ludwig, 1967) based on observations at just two locations showed that atmospheric turbidity had increased by more than 50% since the early 1900s, suggesting a worldwide build-up of atmospheric aerosol that may be leading to the decrease in worldwide air temperature. Mikhail Budyko showed that variations in direct surface solar radiation and global temperature between 1880 and 1960 were similar and suggested that the decrease in temperature after 1940 could depend on the increase in aerosol due to human activity (Budyko, 1969). Bryson and Wendland (1970) also stated that since 1940, the rapid rise of turbidity appeared to have exceeded the warming effect of rising CO2, resulting in a rapid downward trend of temperature. The competing effect of rising CO2 and aerosol pollution was projected, with large uncertainty, to lead to either future warming or cooling, although it was not known whether the cooling since the 1940s had a natural (volcanic) or anthropogenic cause (Mitchell, 1970, 1975).

Fig. 2.2

Fig. 2.2 Variations in surface air temperature of the Northern Hemisphere and direct solar radiation over Europe and America. From Budyko (1969).

Observations of atmospheric conductivity starting in the early 1900s indicated decadal increases in aerosol pollution in the North Atlantic (Gunn, 1964; Wait, 1946) supported by direct measurements using nuclei counters (aerosol scavenges ions from the air and thereby reduces conductivity). A decrease in conductivity by 20% over the North Atlantic in the first half of the twentieth century was equated to an increase by a factor of two in aerosol concentration (Cobb and Wells, 1970), a rare observation that became well cited in later aerosol-climate studies. The conductivity data showed most oceanic regions of the world had natural aerosol levels, but aerosol pollution could be detected in the outflow from the United States over the North Atlantic, from Japan over the North Pacific, and from Asia over the Indian Ocean (Cobb, 1973).

Potential global cooling from a change in anthropogenic aerosol was first calculated in a global-average model by Rasool and Schneider (1971), which followed the earliest model estimates of the importance of the solar constant and greenhouse gases in controlling global climate (Sellers, 1969). Rasool and Schneider's results, although underestimating the effect of rising CO2, suggested that increases in aerosol could substantially alter future climate, supported by a doubling of the amount of aerosol in the last few decades (Cobb and Wells, 1970). However, anthropogenic aerosol barely figured in a later study (Schneider and Mass, 1975), with attention shifting to the effect of solar radiation variations (although their model did not reproduce the observed Northern Hemisphere cooling since the 1940s). This cooling became a key feature of the historical record that would keep resurfacing in the development of aerosol-climate science up to the present day. It has often been stated that global aerosol cooling and triggering of the next Ice Age (or glacial period) was the consensus in the early 1970s, but this is not the case (Peterson et al., 2008), and the problem of CO2-induced warming was already more prominent (Damon and Kunen, 1976; SMIC (Study of Man's Impact on Climate), 1971). Nevertheless, the Rasool and Schneider (1971) paper, later supported quantitatively by other early climate models (e.g., Weare and Snell, 1974), marked the beginning of a period of intense interest in anthropogenic aerosol effects on global climate.

Does anthropogenic aerosol cool or warm the climate? A key limitation in our knowledge in the 1970s was whether aerosol particles from air pollution warmed or cooled the climate (Charlson et al., 1972; Charlson and Pilat, 1969). It was understood that a lot of light-absorbing material was emitted with other material and could warm the climate and affect atmospheric stability and cloudiness (Charlson et al., 1972). Optical calculations, global radiative estimates (Atwater, 1970; Chýlek and Coakley, 1974; Ensor et al., 1971), and model simulations (Yamamoto and Tanaka, 1972) showed that aerosol could either cool or warm the climate. Other uncertainties were the relative location of aerosol with respect to clouds (Weare et al., 1974) and the distribution of aerosol absorption relative to the albedo of the underlying surface (Reck, 1976), both challenges that remain today (Chapter 11). Given the uncertainties, a review of global air pollution and climate change (Bach, 1976) concluded that the effect of increasing aerosol loading could not be assessed reliably, but the effect would probably be small or one of warming.

The global distribution of aerosol was another missing but vital piece of information. In the late 1960s, the background aerosol state was not well defined, so it was difficult to determine the effect of air pollution (Porch et al., 1970). But by the mid-1970s there were sufficient observations of particle size distributions, chemical composition, and optical properties to produce a self-consistent global average description of atmospheric aerosol (Toon and Pollack, 1976). However, measurement gaps meant that miscellaneous materials like burning debris, nitrates, pollutants and regions with large optical depth (i.e., anthropogenic aerosol) were neglected. Information on global aerosol enabled a wide range of model estimates of aerosol effects on climate (Charlock and Sellers, 1980; Coakley et al., 1983; Ohring, 1979). These suggested a global cooling effect at the surface of between 1 and 3 K, but they did not address the anthropogenic increment in aerosol for which the amount of absorption was still an open question.

The regional cooling effect of anthropogenic aerosol was estimated in the late 1970s. For example Bolin and Charlson (1976) estimated a 5%–10% reduction in solar radiation at the surface caused by sulfate aerosol over the most polluted regions of the United States and Europe, and a global mean reduction in temperature of 0.1 K, both of which are consistent with more recent estimates in polluted locations (e.g., Figs. 2.3 and 2.4). Haze associated with anthropogenic emissions in the central United States was observed to be increasing at 18% per decade (Peterson et al., 1981). The effect of this haze on regional surface temperature (known later as the warming holea (Pan et al., 2004)) became a key line of evidence to support aerosol effects on climate (Leibensperger et al., 2012) – see Section 2.2. However, interest in the effect of anthropogenic aerosol waned in the 1980s as temperatures began to rise again such that a decade later the case for an anthropogenic aerosol effect on global climate was still weak. Hansen et al.’s (1981) pioneering radiative-convective model analysis of historical temperature changes from 1880 to 1980 attributed changes to CO2, solar, and volcanic inputs, with the dip in temperatures from the 1940s attributed to volcanic effects. A substantial review of trace gases and other potential perturbations to global climate (Wang et al., 1986) highlighted the regional aspects of anthropogenic aerosol and concluded that aerosols of anthropogenic origins may have substantial effects on regional climate, while the volcanic aerosols may have an effect on large-scale climate for up to a few years after injection.

Fig. 2.3

Fig. 2.3 Variation in surface solar radiation over Europe. The values are shown as relative deviations (in percent) from the 1971 to 2012 mean. Adapted from Sanchez-Lorenzo et al. (2015) with permission. © American Geophysical Union.

Fig. 2.4

Fig. 2.4 Global and regional temperature changes illustrating the effects of anthropogenic aerosol. (a) Temperature change over the Northern and Southern Hemispheres. The shading shows 95% confidence intervals of the smoothed data. (b) Temperature change over the United States from 1930 to 1990. (a) Data are from the HadCRUT4 dataset (https://www.metoffice.gov.uk/hadobs/hadcrut4/ extended from Morice et al. (2012). (b) From Leibensperger et al. (2012).

The effects of changes in aerosol on clouds became better understood in the 1970s. Much was known about cloud droplet formation as early as the 1940s (Howell, 1949) and the transmission, reflection, and absorption of radiation in clouds and the scattering by droplets had been understood in principle for some time (Fritz, 1954; Hewson, 1943; Houghton and Chalker, 1949). Cloud chamber experiments in the mid-1950s had even shown some remarkable insights regarding droplet concentrations and precipitation that were to be relevant to climate 40 years later (Gunn and Phillips, 1957). This study was largely ignored for 30 years after 1970 (Rosenfeld, 1999) while climate science focused on aerosol-driven effects on cloud radiative properties rather than precipitation, although changes in precipitation are now understood to be an important cloud adjustment to changes aerosol (Chapter 12). The first primitive global 3-D model simulation of precipitation response to aerosol came in 1975 (Koenig, 1975), showing patterns of response that are recognizable in today's climate model simulations, although such global climate model simulations of aerosol effects on precipitation and circulation were not pursued again for many years.

In the early 1970s the concept of dirty clouds suggested that pollution might enhance absorption and warm the climate, a prelude to what was later called the semidirect effect (Ackerman and Baker, 1977; Venkatram and Viskanta, 1977). The enhancement of droplet concentrations and hence cloud reflectivity by anthropogenic aerosol was first proposed in 1971 (SMIC (Study of Man's Impact on Climate), 1971) and then in 1974 (Twomey, 1974) (Twomey had been a participant in SMIC), but still with an open question about absorption (Twomey, 1977). The proposal for pollution modification of cloud reflectivity was strongly grounded in extensive observations of increases in cloud nuclei due to pollution (Braham, 1974; Hobbs et al., 1970; Warner and Twomey, 1967). However, the first estimate of a potential effect on global cloud albedo was made only much later (Twomey et al., 1984), and again the question of absorption was raised (Heintzenberg and Ogren, 1985). It was estimated that a doubling of cloud nucleus concentration would result in a change in cloud albedo large enough to counter the warming effect of rising CO2, but reliance on Cobb and Wells' (1970) conductivity trends highlights the extent to which these estimates were severely limited by observational evidence. Therefore observations of enhanced cloud reflectivity caused by particle emissions from ships (first observed in the 1960s (Conover, 1966)) became a defining observational demonstration of what became known as the Twomey effect (Coakley et al., 1987). It was another decade before the equations of cloud susceptibility to changes in aerosol (Twomey, 1991) were eventually tested against observations (Ackerman et al., 2000). Other changes in clouds have subsequently been observed, starting with changes in cloud fraction hypothesized to be due to aerosol (Albrecht, 1989) – see Chapter 12. Many of these cloud processes are still not well handled in global models (Chapters 11 and 12).

Studies of the influence of anthropogenic aerosol on global climate were not stimulated by improved measurements or models but more in response to a proposal for climate modulation of clouds by marine phytoplankton (Chapter 3) (Charlson et al., 1987). Crucially, this paper quantified a link between the emission of an aerosol precursor gas (dimethyl sulfide), a change in cloud condensation nuclei, and a global radiative effect, which was a connection that had been lacking for anthropogenic aerosol. On that basis Schwartz (1988) hypothesized that very large changes in Northern Hemisphere anthropogenic sulfur dioxide emissions should show up as different changes in historical temperature and cloud albedo in the Northern and Southern Hemispheres. The paper concluded that the lack of an observed hemispheric temperature asymmetry did not support the case for sulfur dioxide-induced aerosol cooling (although in the analysis of the temperature record Jones et al. (1986) had stated that the relatively steady temperatures in