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Chapter 5:
Atmospheric Composition
The global and regional composition of the atmosphere -- its gases and
particles -- is at the intersection of global and regional changes and their
relation to humankind:
The atmosphere is shared by all. It links the other components of the
Earth system, including the oceans, land, terrestrial and marine plants and
animals, and the frozen regions. Because of these linkages, the atmosphere
is a conduit of change. For example, natural events and human
activities that change atmospheric composition will change the Earth's
radiative (energy) balance. Subsequent responses by the stratospheric ozone
layer, the climate system, and regional chemical composition (air quality)
create multiple environmental effects that influence the well being of human
and natural systems.
Atmospheric composition changes are indicators of many potential
environmental issues. Observations of trends in atmospheric
composition are among the very earliest harbingers of global changes, such
as the growth rates of carbon dioxide (CO2) concentrations in the
atmosphere. Similarly, the decline of the concentrations of ozone-depleting
substances, such as the chlorofluorocarbons (CFCs), has been the first
measure of the effectiveness of international agreements to end production
and use of these compounds.
The atmosphere can be a forcing-agent "reservoir" for long-term changes.
The long removal times of some compounds, such as CO2 (>100
years) and perfluorocarbons (>1000 years), may imply virtually irreversible
global changes over decades, centuries, and millennia -- for all countries and
populations, not just the pollutant emitters.
An effective program of scientific inquiry relating to managed or unmanaged
changes in atmospheric composition must address two major foci:
A focus on Earth system interactions: How do changes in atmospheric
composition alter and respond to the energy balance of the climate system?
What are the interactions between the climate system and ozone layer? What
are the effects of regional pollution on the global atmosphere and the
effects of global climate and chemical change on regional air quality?
A focus on Earth system and human system linkages: How is the
composition of the global atmosphere, as it relates to climate, ozone
depletion, ultraviolet radiation, and pollutant exposure, altered by human
activities and natural phenomena? How do such composition changes influence
human well being and ecosystem health?
The overall research approach is integrated application of long-term
systematic observations, laboratory and field studies, and modeling, with
periodic assessments of understanding and significance to decisionmaking.
Specific emphasis will also be placed on national and international
partnerships, recognizing that such partnerships are necessitated by the
breadth and complexity of current issues and because the atmosphere links all
nations.
In looking ahead at what the specific information needs associated with
atmospheric composition will be, five broad challenges are apparent, with goals
and examples of key research objectives outlined below.
Question 1: What are the climate-relevant chemical
and radiative properties, and spatial and temporal distributions, of
human-caused and naturally occurring aerosols?
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State of Knowledge
Research has demonstrated that certain atmospheric particles (aerosols) cause
cooling of the climate system (e.g., sulfate), while others result in warming
(e.g., black carbon or soot). When climate models incorporate this knowledge,
they simulate the observed trends much better. However, one of the largest
uncertainties about the impact of aerosols on climate is the diverse warming and
cooling influences of the very complex mixture of aerosol types and their
spatial distributions. Further, the poorly understood impact of aerosols on the
formation of both water droplets and ice crystals in clouds also results in
large uncertainties in the ability to predict climate changes.
Illustrative Research
Questions
What are the sources of atmospheric aerosols, and what are their
magnitudes and variability?
What are the global distributions and radiative characteristics of
aerosols?
What are the processes that control the spatial and temporal
distributions and variability of aerosols and that modify their chemical and
radiative properties during transport, and how well can these processes and
resulting spatial distributions currently be simulated?
How do aerosols affect a cloud's radiative properties and ability to
generate precipitation?
Research Needs
A series of research activities are focusing on these questions.
Remote-sensing instruments paired with correlative in situ observations
will provide better data on global distributions of aerosols, their temporal
variabilities, and resulting changes in radiative balance. Emission estimates
and supportive direct measurements are critical for assessing the balance of
human and natural influences on aerosol distributions. The exploration of
critical aerosol and chemical processes will involve field experimentation, some
laboratory studies, and model development and testing. Diagnostic model
estimates, assessed against observations, will characterize aerosol-determined
temperature change and its uncertainties. Measurements and models will form the
basis for describing the interactions of various types of aerosols and their
impact on the radiative effect of clouds.
Products and Payoffs
Improved description of the global distributions of aerosols (2-4 years).
Empirically tested assessment of the capabilities of current models to
link emissions to (i) global distributions and (ii) chemical and
warming/cooling properties (and their uncertainties) of atmospheric aerosols
(2-4 years).
These capabilities will support the scenarios planned as decision
support resources by providing better estimates of the uncertainties
associated with those simulations.
Because of the relatively short atmospheric residence times of
aerosols, this assessment will yield potential options for changing
radiative forcing within a few decades, in contrast to the longer response
times associated with CO2.
An improved estimate of the indirect climate effects (e.g., on clouds) of
aerosols, compared to the benchmark of the Intergovernmental Panel on
Climate Change (IPCC, 2001) (2-4 years).
More accurate detection and attribution of temperature changes and more
accurate analysis of climate model projections (4-6 years).
Better understanding and description of uncertainties about the physical
and chemical processes that form, transform, and remove aerosols during
long-range atmospheric transport (4-6 years).
Characterization of the impact of human activities and natural sources on
global aerosol distributions (4-6 years).
Question 2: What is the current quantitative skill for
simulating the atmospheric budgets of the growing suite of chemically active
greenhouse gases and their implications for the Earth's energy balance?
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State of Knowledge
The increasing concentrations of atmospheric constituents that absorb
infrared radiation, such as CO2 (see Chapter 8), methane (CH4),
tropospheric ozone, nitrous oxide (N2O), and the chlorofluorocarbons
(CFCs) are the primary gases that are forcing agents of global climate change.
The anthropogenic emission sources leading to the observed growth rates of CH4
(the second-most influential anthropogenic greenhouse gas) and N2O
are qualitatively understood but poorly quantified (e.g., CH4 emitted
by rice agriculture). Trends in tropospheric ozone (the third-most influential
anthropogenic greenhouse gas) are not well determined and are driven by a mix of
emissions, including regional pollutants and CH4. The atmospheric
concentrations and sources of the CFCs are well studied because of their role in
stratospheric ozone layer depletion. In addition to these gases, water vapor
plays a strong role in amplifying greenhouse warming (see Chapter 6).
Observations and trends of this highly variable constituent are problematic.
Illustrative Research
Questions
Driven by the need to have a predictive understanding of the relationship
between the emission sources of these gases and their global distributions and
radiative forcing, several question face the research community. These include:
What are global anthropogenic and natural (biospheric -- see
Chapter 10)
sources of CH4 and N2O?
What are the causes of the observed large variations in their growth
rate?
What are the global anthropogenic and natural sources (both biogenic and
lightning-related) of nitrogen oxides?
What are the trends in mid-tropospheric ozone, particularly in the
Northern Hemisphere, and how well can the variations be attributed to
causes?
What water vapor observations will best test and improve the
understanding of the water vapor feedback?
Research Needs
Field and laboratory studies, satellite observations, and diagnostic
transport/chemical modeling are focusing on these questions. Examples of
activities are:
Global monitoring sites to continue recording the growth rate of CH4
and its variations.
Satellite observations to provide estimates of the global distributions
of tropospheric ozone and some of its precursors (e.g., nitrogen dioxide).
Planned satellite (Aura) measurements and focused field studies to better
characterize water vapor in the climate-critical area of the tropical
tropopause (the boundary between the troposphere and the stratosphere).
Model studies to simulate past trends in tropospheric ozone to improve
the understanding of its contribution to radiative forcing over the past ~50
years.
Field studies to characterize the regional- and continental-scale changes
occurring between emission areas and global tropospheric ozone
distributions, thereby providing tests of and improvement in the
ozone-related process representation of models.
Products and Payoffs
Observationally-assessed and improved uncertainty ranges for future
scenarios of the radiative forcing of the chemically-active greenhouse
gases, which will be part of the 2006 Climate Change Research Initiative
(CCRI) suite of climate change scenarios.
As a result, there will be a broader suite of options (i.e., in
addition to CO2) for potential choices to influence radiative
forcing, particularly in coming decades (4 years).
Better understanding of the processes that control water vapor in the
upper troposphere and lower stratosphere, resulting in improved input to the
planned evaluation of the knowledge of water vapor feedback in climate
models (4-6 years).
Question 3: What are the
effects of regional pollution on the global atmosphere and the effects of
global climate and chemical change on regional air quality and atmospheric
chemical inputs to ecosystems?
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State of Knowledge
Increased development in rapidly industrializing regions of the world has the
potential to impact air quality and ecosystem health in regions far from the
sources. Paleo-chemical data from ice cores and snow document past perturbations
and demonstrate that even pristine areas, such as Greenland, are influenced by
worldwide emissions.
Illustrative Research
Questions
This emerging picture is shaping several policy-relevant questions, which
include the following examples:
What are the chemical exposures experienced by food-producing areas that
are in proximity to large urban areas?
How do the primary and secondary pollutants from the world's megacities
contribute to global atmospheric composition?
What are, and what contributes to, North American "background" levels of
air quality -- that is, what levels of pollution are beyond national control?
Research Needs
These questions are being addressed by measurements of key tropospheric
constituents, including both global mapping by satellites and intensive local
observations from surface sites or airborne platforms, supported by analyses and
model simulations. The near-term goals include the following:
Characterize the outflow from polluted regions around the world, with an
initial emphasis on North American impact;
Understand the balance between long-range transport and transformation of
pollutants;
Establish baseline observations of atmospheric composition over North
America and globally;
Quantify the inflow-outflow atmospheric composition budget of North
America and project future changes; and
Carry out the first global survey of vertically-resolved distributions of
tropospheric ozone and its key precursor species.
Products and Payoffs
Description of the changes in the impacts of global tropospheric ozone on
radiative forcing over the past decade brought about by clean air
regulations (2-4 years).
A 21st century chemical baseline for the Pacific region,
against which future changes can be assessed (2-4 years).
An assessment of the vulnerability of ecosystems to urban growth, with an
emphasis on food production (4-6 years).
Question 4: What are the
time scale and other characteristics of the recovery of the stratospheric
ozone layer in response to declining abundances of ozone-depleting gases and
increasing abundances of greenhouse gases?
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State of Knowledge
The primary cause of the stratospheric ozone depletion observed over the last
two decades is an increase in the concentrations of industrially-produced
ozone-depleting chemicals. The depletion has been significant, ranging from a
few percent per decade at mid-latitudes to greater than fifty percent seasonal
losses at high latitudes. Notable is the annually recurring Antarctic ozone
hole, as well as smaller, but still large, winter/spring ozone losses recently
observed in the Arctic. Reductions in atmospheric ozone levels lead to increased
fluxes of ultraviolet radiation at the surface, with harmful effects on plant
and animal life, including human health. In response to these findings, the
nations of the world ratified the Montreal Protocol on Substances That
Deplete the Ozone Layer and agreed to phase out the production of most
ozone-depleting chemicals. Ground-based in situ and satellite
measurements show that concentrations of many of these compounds are now
beginning to decrease in the lower atmosphere. In the absence of other
atmospheric change, as the atmospheric burden of ozone-depleting chemicals falls
in response to international efforts, stratospheric ozone concentrations should
begin to recover.
Illustrative Research Questions
How will changes in the atmospheric composition of greenhouse gases, such
as CO2 and N2O, and the resulting changes in the
radiation and temperature balance (e.g., stratospheric cooling), alter
ozone-related processes?
How will changes in the physical climate affect the distributions of
ozone (e.g., unusually cold Arctic winters and particle-enhanced ozone-loss
processes)?
What are the ozone-depleting and radiative forcing properties of new
chemicals, such as the substitutes for the now-banned ozone-depleting
substances?
Research Needs
Improving our understanding of this complex and interactive ozone
layer-climate system calls for detailed investigation of the relationships
between the distributions of ozone, water vapor, aerosols, temperature, and
relevant trace constituents, notably chlorine and bromine compounds and nitrogen
oxides. Research needs include the following:
Continue global monitoring of the changes in ozone-depleting substances
and their substitutes and assessing compliance with the Montreal Protocol.
Test the "ozone and climate friendliness" of proposed substitutes with
laboratory chemistry and atmospheric models to provide early information to
industry prior to large plant investments.
Carry out focused aircraft, balloon, and ground-based campaigns, and
chemical transport modeling activities with emphases on:
Cross-tropopause processes to better understand the ozone-depleting
role of the newly proposed, very short-lived (days to months) substances;
The role of particles in accelerating ozone-loss chemistry; and
Stratospheric transport to better understand ozone-layer responses to
climate change.
Extend interagency and international satellite observations of ozone
trends, with an emphasis on detecting and attributing recovery.
Continue monitoring of the trends in ultraviolet radiation, particularly
in regions of high radiation exposure and high biological sensitivity.
Products and Payoffs
In 2006, the international ozone research community will provide
decisionmakers an updated assessment of the state of the ozone layer,
including new ozone and ultraviolet radiation trends, analysis of
compliance, and forecasts of recovery. This sixth in the series of
"operational" products of the ozone science community is a key to
accountability in this issue; namely, is the outcome expected from
international actions being observed?
Question 5: What are the couplings among climate change,
air pollution, and ozone layer depletion, which were once considered as
separate issues?
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State of Knowledge
The atmosphere does not segregate atmospheric composition phenomena by
scientific discipline or societal issue. For example, research has demonstrated
that stratospheric ozone depletion not only causes increased exposure to
ultraviolet radiation at the surface, but also exerts a cooling influence
on the global climate. Conversely, climate-related changes may cool the lower
stratosphere and increase the depletion of the ozone layer at high latitudes.
Formation of tropospheric ozone, previously of concern primarily as a component
of smog, is not only a local health risk, but also exerts a warming influence on
the global climate. Emissions of sulfur dioxide from fossil-fuel combustion not
only lead to the formation of regional acid rain, but also contribute to the
hemispheric sulfate aerosol haze, which exerts a cooling influence on the global
climate system. It is now clear that multiple issues that have been treated
separately by scientists and policymakers alike are indeed coupled phenomena.
Illustrative Research
Questions
How do actions taken or considered with regard to one issue influence
other issues, positively or negatively?
What are the multiple stresses that climate change, ozone layer
depletion, and regional air quality exert on humans and ecosystems?
Research needs
Build and evaluate diagnostic/prognostic models of the coupled climate,
chemistry, transport, and ecological systems (in collaboration with other
elements of the program).
Synthesize the understanding of the impacts of multiple stresses on
humans (e.g., heat and air quality) and ecosystems (e.g., soil moisture and
chemical exposure).
Build and evaluate models that couple the biogeochemical systems with the
decisionmaking frameworks.
Carry out multiple issue state-of-understanding assessments, in
partnership with the spectrum of stakeholders, with the aim of
characterizing integrated "If..., then..." options.
Products and Payoffs
A policy-relevant assessment of the issues related to intercontinental
transport and the climatic effects of air pollutants, in order to provide
scientifically sound information to policymakers for consideration in
developing integrated control strategies to benefit both regional air
quality and global climate change, and to assess local attainment of air
quality standards (2-4 years).
A State of the Atmosphere: 2006 report that describes and
interprets to the Nation the annual status of atmospheric phenomena such as
atmospheric composition, ozone layer depletion, temperature, rainfall, and
ecosystem exposure (see Chapter 3).
Diagnostic/prognostic models of the coupled climate, chemistry/transport,
and ecological systems (in collaboration with other elements of the
program).
A process that bridges various issues and stakeholders in order to
conduct multiple-issue integrated assessments.
The Atmospheric Composition research focus is linked via co-planning and
joint execution to several national and international planning and coordinating
activities. A few examples are:
USGCRP/CCRI:
Interaction with the US Global Change Research Program (USGCRP) Climate
Variability and Change (Chapter 6) and Water Cycle (Chapter 7) components,
including radiative forcing input to climate model simulations, as well as
characterization of other composition -- climate processes (e.g., impact of
aerosols on cloud formation and precipitation).
Interaction with the CCRI Scenarios near-term focus (Chapter 4),
providing explicit simulations of emissions, atmospheric composition, and
radiative forcing changes.
Interactions with the USGCRP Carbon Cycle component (Chapter 9) for CH4
changes, Ecosystems (Chapter 10) for assessing chemical impacts, and Human
Contributions (Chapter 11) for health impacts.
Interagency Programs: Joint planning, such as the
National
Aerosol -- Climate Interactions Program (NACIP) is a major vehicle for carrying
out USGCRP/CCRI objectives.
Committee on Environment and Natural Resources:
Air Quality Research
Subcommittee (AQRS): Joint research on the global/continental scales of
the USGCRP and on the regional/local scales of the AQRS (global influences
on the "natural background" of air pollutants and linkages with the
stakeholders via the AQRS).
International Global Atmospheric Chemistry
(IGAC): IGAC, a
Core Project of the International Geosphere-Biosphere Programme, coordinates
several international projects focused on the chemistry of the global
troposphere and its impact on the radiative balance, such as the new
Intercontinental Transport and Chemical Transformation project, which
involves Asian, North American, and European researchers.
References:
IPCC, 2001.
Intergovernmental Panel on Climate Change, Climate Change 2001.
Third Assessment Report of the IPCC. (Cambridge, United
Kingdom, and New York: Cambridge
University Press). Includes:
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IPCC, 2001a. The Scientific Basis,
a contribution of Working Group I.
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IPCC, 2001b. Impacts,
Adaptation, and Vulnerability, a contribution of Working Group
II.
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IPCC, 2001c. Mitigation,
a contribution of Working Group III.
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IPCC, 2001d. Synthesis
Report. A Contribution of Working Groups I, II, and III
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