Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 107
Climatic and Hydrologic Systems
COORDINATOR: ROBERT E. D1CK1NSON
The climate system* consists of many linked components, involv-
ing the atmosphere and its interactions with the oceans, land surface,
cryosphere, and biosphere (see Figure 1~. Various aspects of the hy-
drological cycle are important to all these components. The climate
system and its manifestations in the hydrological cycle are central
to the description, understanding, and prediction of the processes of
global change.
Human activities are capable of producing large changes in the
global climate system through massive alteration of the concentra-
tions of radiatively active trace gases, especially CO2, CH4, and the
CFCs. The atmospheric concentrations of CO2, CH4, and other im-
portant trace gases are maintained by biogeochemical cycles. Warm-
ing from an increase in radiative forcing, promoted by human activ-
ities, will alter the global distribution of temperature and moisture
on time scales of at least decades to centuries and possibly over much
A draft of this paper was prepared by committee member Robert Dickinson and
revised according to comments received from a wide range of scientists (see the appendix
to this paper).
*The concept of the climate system was used initially by climate modelers in the
early 1970s to represent this linked system. More recently, research on the physical
aspects of the climate system has been organized internationally through the World
Climate Research Program and within the United States through the National Climate
Program Office.
107
OCR for page 108
108
THE CLIMATE SYSTEM
me'
Fib
Atmosphere
Cryosphere
.
FIGURE 1 The climate system. Adapted from the National Climate Program Office's
National Climate Program Five Year Plan 19881992.
longer time scales. Such changes will influence conditions for life
over the earth. The detection and understanding of these changes
are limited by an inadequate understanding of the natural variability
of the system.
Figure 2 (Jaeger, 1988) shows a range of possible scenarios for
global warming. The figure allows for emissions of all the trace gases
affecting climate and for the clelay of global temperature increase
resulting from oceanic heat uptake. A narrowing in the uncertainty
of the global average changes in the climate system is obviously
needed. However, knowledge of such global averages alone is not
very useful without an understancling of the actual change that wiB
be experienced locally and regionally. A more complete picture re-
quires understanding and predicting the regional manifestations of
global climate change over time, with emphasis on changes in the
hydrological system.
The need to improve and test climate models arises from the
need for better descriptions both of present and past states of the cTi-
mate system and of its natural variability. Documenting the changes
within the climate system, using well-validated and well-caTibrated
climatic and hydrological observations, is an early goal for the global
change program. New observational technologies will provide better
OCR for page 109
109
LU
I 4
~ 3
CC
LU
CL
LL
cc 1
m
o
o
j 1 1
Upper Scenario
(rate 0.8 C/decade)
\~/
Middle Scenario /
(rate 0.30C/decade)
Low Scenario /
(rate 0.06 C/decade) >I
.
/
-1 1 1
1860 1900 1940 1980
1 1 1 1 1 1 1 1 1 1
2020 2060 2100
YEAR
FIGURE 2 Scenarios of changes in globally averaged temperature that might develop
in response to continued emissions of greenhouse gases. Values are plotted as differences
from the 1985 value (Jaeger, 1988~.
detailed descriptions and contribute to improved understanding of
the complex global physical systems—the atmospheric, oceanic, ter-
restrial, and hydrologic systems including biota, snow, and ice. The
exchanges of energy and water mass between these systems must
be better described, quantified, understood, and predicted. Obser-
vations from space have provicled, for the first time, the means to
survey the many features of our planet rapidly, efficiently, and gIob-
aDy with high resolution using a single instrument or coordinated
group of instruments. So revealing are these sateHite-based investi-
gations that they are now recognized to be indispensable for future
research on the climate system.
At the same time, many important aspects of the system can
only be studied through extensive observations made in situ. For
example, element cycling between glacial and interglacial stages over
the last 2 million years is particularly well recorded in the geological
record. Isolation of the physical processes that drive these massive
changes is a key ingredient for understanding the climatic system in
OCR for page 110
110
its present state and hence improving our ability to predict future
climatic states.
Conceptual modeling and numerical simulations are and win con-
tinue to be powerful surrogates for and synthesizers of global obser-
vations. Elaborate data and information systems from the national
level to the level of individual institutions are needed to combine
information from various sources and from different federal agencies
to describe the earth system as a related set of interacting processes,
rather than a collection of individual components.
CLIMATE FORCING FUNCTIONS
The climate system changes either in response to alteration of
climate forcing functions (external forcing) or as a result of Tong-
time-scale internal dynamics of the system (internal forcing). With
respect to the dynamics of the total earth system, the only regular
external forcing terms are solar radiation and the energy released by
the decay of radioactive nuclides from the earth's interior. Episodic
astronomical events such as collisions with large asteroids are very
rare in the earth's history, but when they occur they can have drastic
effects. With respect to individual processes, it may be convenient
to regard also as external forcing the radiative effects of atmospheric
constituents as well as those modifications of the land surface that
change so slowly that they can be considered only weakly coupled
to the climate system on the decadal time scale. However, for con-
sistent predictions of the (long-term) evolution of the global system,
biogeochemical forcing will have to be considered as an internal part
of the system.
Solar and Geological Forcing Functions
Solar Output and Orbital Variations
The radiative energy of the sun, centered in the visible and
near-infrared wavelengths, serves through its differential input as the
principal driver of atmospheric circulation. Thus radiative energy
powers the climate machine. The march of the seasons is ample ev-
idence for the acute sensitivity of the environment of the earth to
changes in the distribution of solar radiation. A more sensitive gauge
of the sun's impact on climate, however, is found in the Milankovitch
effect, by which changes in the earth's orbit and axial orientation
OCR for page 111
1'1
provide slow and subtle changes in the latitudinal and seasonal dis-
tribution of insolation. These variations, far smaller than seasonal
variations but of much longer period, serve to pace the recurrence of
ice ages every 21,000, 41,000, and 100,000 years.
The solar irradiance has been measured accurately only since
1979. The small decreasing trend of 0.02 percent/year reversed
in 1987, apparently with the 11-year solar cycle. For the period
of observed decrease, the change in radiative forcing of the earth-
troposphere system was comparable with, but opposite to, that aris-
ing from increasing atmospheric CO2.
Solar ultraviolet radiation, varying irregularly with solar activity,
dictates the basic chemistry of the upper and middle atmosphere,
including the equilibrium and composition of important trace gases
such as ozone. Lightning is more frequent over continents than
oceans, presumably as a result of the terrestrial concentration of
ionization sources linked to soils and airborne dust. The global
electric field is maintained by thunderstorm activity. Solar-induced
variations in cosmic ray flux or changes in the efficiency of coupling
between the solar wind and the high-latitude ionosphere may perturb
it, with a possible but little-studied influence on climate.
Variations in solar radiation and particle fluxes have substantial
effects on the magnetosphere and ionosphere, and solar irradiance
variations and particle precipitation can affect the upper atmosphere.
Variations in the abundance of cosmic rays alter the production rate
for i4C, which is the basis for much of the dating of records of past
climate over the last 50,000 years.
VoIcanic Activity
VoIcanic eruptions may inject into the stratosphere SO2, which
can condense to aerosols and spread gIobaBy, thereby modifying ra-
diative fluxes into the troposphere. Monitoring these aerosol clouds
and documenting the climatic response can help the interpretation
of trends in surface temperatures and contribute to an understand-
ing of atmospheric transport properties. Individual large eruptions
that have occurred since the beginning of the availability of global
temperature records are known to have decreased global mean tem-
perature by as much as 1°C for up to a few years. How great an
effect might be possible from larger and/or more frequent eruptions
is not known. The relatively uncommon eruption of a major caldera
system could potentially have major, even if transient, influences on
OCR for page 112
112
global climate. Volcanism may provide varying amounts of CO2 over
10-m~lion-year time scales.
Tectonic, Geothermal, Isostatic Rebound, Geomorphological,
and Soil Changes
Tectonic modifications of continental positions and shapes on 10-
million-year time scales are implicated in very large climatic changes.
Past variations of tectonic and volcanic factors may help test our un-
derstanding of the climatic response to large forcing, both as a surro-
gate verification of climate model performance and for understanding
sea level changes and their potential climatic connections.
The geothermal flow of heat from the earth's interior, an essen-
tially constant boundary condition, supplements solar heating, albeit
by only a small amount (1 part in 104~. However, this heat source is
significant for the thermal regimes of permafrost and ice sheets.
The response of the earth's lithosphere to loading and unloading
of surface materials, including large ice sheets, sediments in deltas,
and groundwater withdrawal, significantly affects regional sea levels.
The response of bedrock to ice sheets has major implications for the
dynamics of continental ice sheets.
The geomorphological processes that move, remove, or (reposit
soil and other sedimentary materials over the land surface modify
the landscape and hydrological regime in significant ways. Rock
weathering and soil formation processes, likewise, influence the land
surface and are linked to biogeochemical cycles, especially the global
carbon cycle. Wind-blown soil modulates atmospheric radiation and
thus may contribute to regional climate.
Continental Ice Sheets
Continental ice sheets, currently those of Greenland and Antarc-
tica, have large effects on climate, especially in high latitudes. They
also store much of the worId's fresh water, and for at least the last
20 to 40 million years have been modifying sea level. Ice sheets
normally change significantly in size only over time scales of several
centuries or more. Their growth en cl decline, associated with changes
in the earth's orbit and other causes, have produced past ice ages
and interglacial periods.
OCR for page 113
113
Orbital Changes
Changes in the earth's orbit and axial orientation modulate the
latitudinal and seasonal variation of solar radiation and so apparently
force a significant fraction of the glacial-interglacial fluctuation.
Biogeochemical Forcing (Natural and Anthropogenic)
Carbon Dioxide
The concentrations of CO2 in the atmosphere have risen over the
last century from 280 ppm to 350 ppm and are projected to continue
to increase by 1 to 4 ppm/year over the next century. These increases
in CO2 are derived from anthropogenic sources, primarily from the
burning of fossil fuel and to a lesser extent from land use changes,
e.g., the clearing of forests. CO2 concentration also depends on what
fraction of CO2 release is taken up in the oceans (about half) and
what is taken up or given off by soils and vegetation (a considerably
smaller fraction) versus the fraction that remains in the atmosphere.
Increasing CO2 elevates greenhouse heating by about 6 in~n/nO in
W/m2 (where n = CO2 concentration, no = preindustrial values of
CO2 concentration, W = watts, and m = meters). Atmospheric CO2
provi(les the carbon for growth of vegetation; thus changing CO2
modifies this growth.
Changes in the chemical, biological, and physical characteristics
of the oceans, vegetation, and soils in response to changing climate
will provide a climate-CO2 feedback. A warmer climate may re-
duce the capacity of the upper ocean to store CO2; it may also
enhance both photosynthetic uptake and respiratory release of CO2.
At present, we do not know the sign, let alone the magnitude of
this feedback. The variations of atmospheric CO2 with glacial cycles
(inferred from ciata in polar ice caps) suggest the presence of such
feedbacks.
Methane
The concentration of CH4 has risen from preindustrial values
of less than 0.S ppm to current concentrations of about 1.7 ppm,
adding about 0.5 W/m2 of warming to the global climate system.
Since the atmospheric lifetime of CH4 (about 10 years) is short in
comparison with the time scale of changes in its concentration, shifts
in its concentration reflect changes in the balance between sources
OCR for page 114
114
(mostly biological and anthropogenic) and destruction (mostly by
OH radicals in the atmosphere). Past increases over the last cen-
tury are ascribed largely to increases in CH4 sources associated with
human activities in agriculture, forest clearing (biomass burning),
and fuel exploitation, transport, and consumption. Nevertheless, the
role of past and future changes in rates of CH4 destruction in the
atmosphere also warrants careful scrutiny. Changes in temperature,
inundation period, and inundation area wiD change CH4 flux from
natural wetlands and permafrost regions. There may also be sub-
stantial release of CH4 from methanehydrates present in continental
slope sediments as the ocean responds to atmospheric warming.
Dimethylsulfide
Cloud droplets form around cloud condensation nuclei (CCN).
Oceanic clouds primarily condense around sulfate aerosols, which
are supplied by DMS emitted by the oceans (CharIson et al., 1987~.
DMS is generated by certain kinds of marine phytoplankton, and its
generation rate may depend on the temperature of the ocean surface
waters. An increase in sulfate CCNs would increase the numbers of
cloud droplets and hence increase planetary albedo over the oceans.
We need to better quantify how the flux of DMS now influences cloud
cover and albedo and how various changes might modify the flux of
DMS to the atmosphere.
Other Aerosols
Aerosols of both natural and anthropogenic origin, e.g., from
desert dust or conversion of combustion products, may modify radia-
tive fluxes either directly by their radiative properties or indirectly
through their effects on cloud cover or optical properties. Lifetimes
of aerosols depend on wet and dry deposition processes, which may
vary with climate change.
Other Trace Gases
The CFCs Fell and F-12 may in several decades also build up
to large enough concentrations to add significantly to greenhouse
warming. Likewise, N2O, tropospheric and stratospheric 03, and to
a lesser extent many other trace gases are also of radiative impor-
tance and are undergoing changes in flux rates. Changes in climate
and hy~lrology will feed back on production and destruction rates of
OCR for page 115
115
these trace gases. In particular, O3 decrease and CO2 increase can
reduce stratospheric temperatures by large amounts and so affect the
planetary wave and radiative coupling to the troposphere. Strato-
spheric water vapor is strongly dependent on the temperature of the
tropical tropopause and on CH4 concentrations.
Effects of [and Use Changes on Climate
Changes in agricultural activities, deforestation, and desertifi-
cation are increasingly affecting the climate system and in turn are
affected by changing climate. The impacts of vegetation and land
use changes on climate are of two kinds:
1. Biogeochemical: surface modifications affect global atmo-
spheric composition and its radiative balance. Biomass burning, a
major mode of deforestation, is a source of CO2, CH4, N2O, CO,
particulates, and other trace gases. Deforestation is generally ac-
companied by degradation of soils and enhanced fluxes of CO2 and
N2O to the atmosphere. Increases in rice cultivation and ruminant
population increase the amounts of CH4, while an increase in fertilizer
usage contributes to rising N20 concentrations in the atmosphere.
2. Biogeophysical: modifications of vegetation cover alter re-
gional hydrology and regional surface-energy balance. The impor-
tance of land-surface effects on climate has been suggested by model-
ing sensitivity studies. These studies indicate that increases in albedo
from land degradation in semiarid regions could promote drought.
Other studies have shown that model-simulated climates are also
sensitive to land-surface boundary conditions that affect evapotran-
spiration. Interactions between vegetation and snow cover may also
be important.
The issue of realistically modeling how land processes, in partic-
ular vegetation and soil moisture, interact with the climate system
is only now beginning to be addressed. Such modeling should recog-
nize the two-way coupling between these two systems, as vegetation
changes both its form and its function in response to climatic forc-
ing. The modeling efforts must recognize the small spatial scales over
which vegetation cover and soils vary.
OCR for page 116
116
RESPONSE OF THE CLIMATE SYSTEM
TO CHANGES IN FORCING
Nature of Climatic Forcing
Changes in total solar output, stratospheric aerosol, or atmo-
spheric trace gases that are Tong-lived (fairly well mixed gIobally),
such as CO2 and CH4, affect the climate system through the addi-
tional global radiative forcing that they provide. Some differences
occur in the vertical, latitudinal, and seasonal distribution of atmo-
spheric radiative forcing, but the effects of these differences are not
yet established and are apparently relatively small. Thus studies of
the global response of climate to increases in atmospheric CO2 also
largely apply to increases of other trace gases and to increased so-
lar heating. The practical question is thus how the climate system
responds to the sum of various global inputs in the past and future.
The change in climate from changes in global radiative forc-
ing will be in part manifested by a variety of regional responses,
with changes in some regions more significant than those in others.
Changes in other climate forcing factors, in particular those related
to vegetation and ice cover, win have their largest effects on a re-
gional scale and be specific to the given forcing and location. These
effects largely involve changes in energy exchange processes at the
surface, especially over continental interiors.
Key Areas of Uncertainty in Evaluating
Climate System Response
Cloud Effects
Clouds have multiple properties that are important for climate
change. They modulate both incoming solar radiation and outgoing
thermal infrared radiation. Reflection of solar radiation depends not
only on cloud amount but also on the optical thickness of clouds, on
the liquid water content of the clouds, and on the size distribution
of the cloud (lroplets. The upward flux of thermal infrared emission
from clouds depends on cloucI-top temperature and on cloud emis-
sivity. Only high, thin clouds, i.e., cirrus, have emissivities that are
significantly less than 1.0 and hence have thermal emission controlled
by cloud thickness.
Preliminary and incomplete treatments of cloud feedback on cTi-
mate have been included in recent GCM studies of climate response
OCR for page 117
117
to CO2 doubling. These studies have found that cloud changes sig-
nificantly affect the surface temperature response. As temperatures
increase, clouds are expected to hold more liquid water ant! hence
be more reflective, thereby reducing the warming significantly. In
the case of cirrus, however, increased amounts of thermal infrared
radiation are expected to be trapped and so amplify the greenhouse
warming. Unfortunately, these effects have not yet been included
in GCM simulations of climate change. Different GCM simulations
have shown qualitatively similar spatial patterns in the TongitudinaDy
averaged cloudiness change; they indicate, in particular, a decrease
in cloudiness in the moist, connectively active regions such as the
tropical and midcIle latitude rain belt and an increase in the stable
region near the surface from micIdle to high latitudes as wed as in the
Tower stratosphere. Such cloud changes would be expected to have
significant regional effects on radiative balance and hence surface
temperature.
Actual clouds are often thinner than a model layer and generally
have horizontal scales of 1 to 100 km, i.e., scales unresolved by
the grid of global models; hence, their radiative properties may not
be correctly modeled. Thus we need appropriate data bases and a
conceptual and statistical framework for describing the morphology
of realistic cloud fields made up of clouds of various shapes and
sizes. Also required is an understanding of the radiative, dynamical,
and microphysical processes determining this morphology. Diurnal
variations of cloudiness are poorly characterized but are probably
significant for determining changes in cloud radiative balances and
land surface evapotranspiration feedbacks.
The net global feedback is even more difficult to mode} than the
regional effects of cloud changes. Cloud processes in models need to
be related through observations to other meteorological processes.
The primary line of evidence that actual cloud feedbacks are not
extremely far from what is modeled is the reasonable success of
global models in reproducing seasonal and diurnal cycles of the global
climate system.
High-Latitude Response
The largest regional temperature increases from global warming
are expected in high latitudes as a result of albedo and atmospheric
stability changes linked particularly to the extent of sea ice. Modeling
OCR for page 123
123
Some or all of these possibilities appear rather unlikely over the
next century, but their consequences could be much more severe
than those of gradual global warming. Thus there is a clear need for
scrutiny of evidence in the climatic record for abrupt climate change
and its effects on the system from natural causes. Careful observation
of the current system and studies with joint ocean-atmosphere models
of the climate system that may exhibit multiple equilibria are needed
to better understand the prospects of natural and human-induced
abrupt change. The links between planetary radiation, atmospheric
trace gas content, atmospheric water transport, ocean circulation,
and the marine and terrestrial biospheres also need to be examined
with this possibility in mind.
D O CUMENTATION OF PRES ENT CLIMATE AND THE
FACTORS THAT CAUSE IT TO CHANGE
The following key research issues should be addressed:
i
1. The recovery of the past history of environmental change,
evolving studies of ocean and lake sediments, ice cores, tree rings,
and sea level changes for information about past climate, hydrolog-
ical regimes, ocean circulation, biological interaction, atmospheric
chemical composition, and solar inputs.
2. Comprehensive documentation of changes in the current phys-
ical environment, including observation and understanding of the sun
and near-earth space, the atmosphere, snow and ice, oceans, and the
sails and vegetation of the earth.
The former aspect is discussed in the companion paper on "Earth
System History and Modeling," and the latter is discussed below.
Only within the last few decades have the technology, telecom-
munications, and the logistics necessary to handle large amounts of
data been developed to the point where comprehensive global ob-
servations of the climate system have become feasible. Sustained
progress in understanding the global system wiD require a compre-
hensive continuous observing program, ongoing theoretical investi-
gations, and global modeling efforts. Such a program is necessary to
document climate changes, to further studies of important processes,
and to provide the data needed to construct, test, and utilize models
of the system and its components. Some limited instrumental data
extend back over a century or more and provide much of what is now
known about past change. Such data must be analyzed and their
OCR for page 124
124
collection extended into the future, with considerable care for the
continuity and calibration of these records.
Most studies of the current global atmosphere now rely on the
information provided by the worldwide network of surface and upper-
air sounding stations organized by the World Weather Watch and
the system of four to five geostationary and two polar-orbiting me-
teorological satellites, which has been, more or less, in operation
since the Global Weather Experiment of CARP (about 1979~. This
real-time operational observing and data management system is com-
plemented by various sources of delayed data such as temperature
profiles of the upper oceanic layer, sea level data, and a wide variety
of surface and subsurface hydrological measurements. The system
suffers from several deficiencies, which include (1) serious gaps in the
three-dimensional distribution of wind observations over the oceans;
(2) large uncertainties, resulting from insufficient sampling and mea-
surement errors, in the estimation of precipitation over land areas
and total lack of information over the oceans (and over some coun-
tries that do not make hydrological data available for international
exchange); and (3) significant uncertainties in the three-dimensional
global distribution of water vapor and clouds, and the almost total
lack of global information about surface properties, including soil
moisture. Climate studies are concerned with the quality and con-
tinuous availability of the data, whereas the real-time collection is
primarily of importance for operational agencies.
Current prospects for the geostationary platforms of the system
are satisfactory, since the major meteorological satellite operators
are now in the process of finalizing their plans to replace some of
the existing operational satellites with a second generation of space-
craft with nearly identical advanced sensors. However, the replace-
ment of existing operational environmental satellites in polar orbit
must be undertaken to ensure the continuity of essential atmospheric
measurements as well as to provide adequate facilities for testing
experimental remote sensing instruments.
The measurement of the distribution of all phases of water in
the atmosphere should be given high priority if the fundamental role
of fresh water on time scales of decades to centuries in ah subsys-
tems of the earth system is to be understood. Global patterns and
amounts of precipitation on a year-to-year basis must now be es-
timate(1 by the use of rain gauge networks over land coupled with
correlations of precipitation with cloud top temperatures over oceans.
A Tropical Rainfall Measurement Mission would begin to satisfy a
OCR for page 125
125
critically important requirement for direct space measurements of
global precipitation by testing the feasibility of using active and pas-
sive microwave data together with visible and infrared imagery. A
Tow-incTination orbit could resolve the mean diurnal cycle of rainfall
over the tropics and assess the relationship of latent heat released
into the atmosphere to anomalies in atmospheric circulation.
Studies of atmospheric water vapor are also equally important.
Its variations in space and time are inadequately known. Planned
improvements of operational satellite instruments should start soon
to yield significant information for the lower troposphere on a global
scale. Atmospheric radiation balance is especially sensitive to the
concentrations of water vapor in the upper troposphere and Tower
stratosphere.
Radiative balance of the climate system, and the dynamical pro-
cesses that give rise to cloudiness, are highly significant for climate.
Continuous measurements of the total solar output provide a funda-
mental boundary condition for radiative inputs, as do measurements
of the varying contribution to planetary albedo from stratospheric
aerosols. Current analyses of cloud amounts and their trends are
based upon visual estimates by ground-based observers. The Inter-
national Satellite Cloud Climatology Project (ISCCP) now under
way will assemble a 5-year data set of radiance measurements from
information returned by five geostationary and two polar-orbiting
satellites. The data wiD allow compilation of meaningful statistics
for cloucT amount, type, height, and optical thickness. The First
TSCCP Regional Experiment (FIRE) is a U.S. contribution helping
to vaTiciate algorithms for this derivation. Global information on the
altitude of cloud bases is not included in these measurements, but is
also needed.
A complementary approach to determining the role of clouds
in the radiation balance is the measurement of the components of
the radiation balance itself: the fluxes of solar and infrared ra-
diation at the top of the atmosphere and their inferred values in
an equivalent cloud-free environment. The difference is the "cloud
forcing." This quantity is obtained using a combination of a sim-
ple, wide-field-of-view instrument on sun-synchronous polar-orbiting
satellites together with non-sun-synchronous satellite measurements
of the Earth Radiation Budget Experiment (ERBE). Spectral and
directional models are necessary to relate the narrow-spectral-band,
narrow-field-of-view operational instruments to the total fluxes and
OCR for page 126
126
to infer the fluxes at the top of the atmosphere from those at satellite
altitude.
The International Satellite Land Surface Climatology Project
(TSI`SCP) program, through such field programs as the Hydrologi-
cal Atmospheric Pilot Experiment (HAPEX) in 1986 and the First
ISESCP Field Experiment (FIFE) in 1987, focuses on the develop-
ment and improvement of algorithms for remote sensing of physical
properties of the land surface important for climate models. Progress
has been made in the observation and interpretation of a vegetation
index based on the difference in reflected radiances in the visible and
near-infrared channels of the Advanced Very High Resolution Ra-
diometer (AVHRR) imager and, more recently, in another approach
to a vegetation index based on a microwave measurement. Also
promising are procedures to measure surface radiative temperature,
including its diurnal variations, from window infrared and microwave
thermal emission and procedures to obtain incident surface radiation
from reflectance of visible radiation by clouds. Progress in inferring
surface albedo is also being made, with improvement in atmospheric
corrections including clouc] removal, better calibration, and advances
in unclerstanding the angular dependence of bidirectional reflectance
of lancl surfaces.
Observation from space of land surface climate properties re-
quires considerable information about the atmosphere. First, radia-
tive emissions sensed from the surface are modulated by atmospheric
gases, clouds, and aerosols, whose effects must be quantified. These
modulating influences can be as important as the land properties
being measured, both for the dynamics of the land processes and for
the climate system in general. Second, some key observations such as
the indirect measurement of soil moisture, hence evapotranspiration,
require measurements of near-surface atmospheric properties, e.g.,
humidity and winds. A detailed description of surface-energy bal-
ance over land, like that over the ocean, requires both atmospheric
and surface information.
Observational research on land surface climate processes and
hydrology must be closely coordinated with related meteorological
studies, such as those of the U.S. National STORM Program, and
with global change activities in the areas of biological dynamics and
biogeochemical cycles. Besides satellite systems, there will be a need
for continued application and development of aircraft measurements,
in situ sounding systems, and surface instrumental systems, espe-
ciaDy for process studies. These land surface climate processes are
OCR for page 127
127
[Linked to the many practical facets of the impacts of climate change,
e.g., water resources and agricultural productivity.
Changes in glaciers and small ice caps, in both high-latitude and
mountain environments, contribute to global sea level rise and fall.
Seasonal snow is another sensitive indicator of climate change and
provides positive feedback through its effect on global albedo. The
presence of sea ice, already measured from space, completely changes
the magnitude of the heat and moisture fluxes from the ocean as weD
as the surface albedo. High spatial resolution of measurements is
important to determine the fractional ice-covered areas in regions
of broken ice. Large thermal fluxes can be associated with these
marginal ice zones. Additional measurements are needed of sea ice
thickness, the distribution of ice of differing ages and its motion, and
the extent of surface melting and its relation to ice albedo.
A change in the volume of polar ice could result from an increase
or decrease in global temperatures. Yet at present we have no reliable
means to assess changes in the inventory of ice in the Greenland and
antarctic ice sheets. We need to know whether present ice sheets,
averaged over the seasonal cycle, are in steady state, growth, or decay
phases. Variations in polar ice volume should be detectable in sea
level measurements, but these data are noisy and influenced by the
thermal expansion that results from global temperature change, by
local tectonic movements, and by rebound from the melting of the
last major glacial ice sheet. Ground-based surveys are not definitive,
but satellite altimetry could be applied to this problem.
Current observational programs in the ocean sciences support
the goals of the IGBP to the extent that they focus on ocean sur-
face temperatures and how they might vary with climate change,
and on how the oceans store and exchange radiatively important
trace gases. Surface temperatures are linked to vertical mixing and
convection processes and depend on vertical distributions of temper-
ature and salinity, horizontal advection of these quantities, and hence
ultimately on the oceanic general circulation.
Complementing the sparse data coverage by the many ship-
based programs, satellites provide regular global observations over
the ocean of surface temperatures, sea ice cover, and in the future,
wind stress from roughness, winds over the ocean, and sea level
height. It is necessary to measure the geoid more accurately in order
to use sea level heights to derive ocean surface currents. Long-term
monitoring of sea level height is needed both from surface and from
space platforms.
OCR for page 128
128
PRINCIPAL ISSUES AND RESEARCH CHALLENGES
Sustained, Calibrated, [ong-term Measurements
Long-term monitoring of the more important global climate and
hydrological variables is crucial. This objective requires stable, we0-
calibrated measurements over a multidecadal time frame. Monitoring
of both forcing functions and system responses is needed. The list of
Tong-term monitoring needs cited in NASA's Earth System Science
Committee (1988) report (here reproduced as Table I) merits fun
support. The most critical forcing variables are solar irradiance, vol-
canic emissions, and radiatively important trace species, especially
CO2, CH4, and the CFMs. Atmospheric response variables of high-
est priority for long-term measurement are surface air temperature,
tropospheric temperature, precipitation, and surface pressure. Also
very important are winds, especially in the tropics, components of
the earth radiation budget, cloud amount, type, height, and optical
thickness.
Land surface properties for a variety of representative surfaces
must also be measured over a long term, especially those proper-
ties controlling the fluxes of water between surface and atmosphere.
These properties include precipitation, measures of soil moisture, and
vegetation cover, all on a regional scale. Measurements must include
the surface radiative temperature, incident solar flux, seasonal snow
cover, snow water equivalent, changes in the volume of high-altitude
and continental ice sheets, amount of river runoff, distribution of
permafrost, levels and freezing dates of lakes, extent and seasonaTity
of wetlands, and other surface characteristics such as albedo, rough-
ness, and emissivities in the infrared and microwave bands. Key
ocean variables are sea surface temperature; sea level; sea ice extent,
type, and motion; ocean wind stress; subsurface circulation; and
incident solar flux. Adequate attention should he given to sustained,
calibrated, long-term measurement of the types of variables diiscussed
above and analyzed by NASA 's Earth System Science Committee.
Information Systems
Components of information systems inclu(le data transmission,
quality control, directories, catalogs, and inventories; products of
special analyses; status of data observation, collection, archiving,
and distribution; and agreements for international exchange of data.
OCR for page 129
129
TABLE 1 Sustained, Long-term Measurements of Global Variables Important for
the Study of Global Change on Time Scales of Decades to Centuries (NASA's Earth
System Science Committee, 1988)
Analysis
Product
Variable Importance Quality
External Forcing:
Solar ~rrad~ance *** A
Ultraviolet flux ** B
Index of volcanic emissions * D
Radiatively and Chemically
Important Trace Species:
COO *** A
N,O ** A
CH4 ** ~
Chlorofluoromethanes ** A
Tropospheric O3 ** C-
CO ** D-
Stratospher~c O3 *** C
Stratospheric H?O ** C
Stratospheric NO? * C
Stratospheric HNO3 * C
Stratospheric HC~ * C-
Stratospheric aerosols ** B
Atmospheric Response Variables:
Surface air temperature *** B
Tropospheric temperature *** B
Stratospheric temperature ** C
Pressure (surface) *** A-
Tropical winds ** C-
Extratropical winds * B
Tropospheric water vapor ** D
Precipitation *** C-
Components of Earth radiation ** B
budget
Cloud amount. type, height ** D
Tropospheric aerosols * D
Variable
Land-Surface Properties:
Surface characteristics (for albedo.
roughness, infrared and microwave
emittance)
Index of land-use changes (broad
classification of vegetation types)
Index of vegetation cover
Index of surface wetness
Soil moisture
Biome extent, productivity,
and nutrient cycling
Analysis
Product
Importance Quality
* C-
** F
*** D
* F
*** F
*** F
Ocean Variables:
Sea-surface temperature
Seance extent
Sea-ice type
Sea-ice motion
Ocean wind stress
Sea level
Incident solar flux
Subsurface circulation
Ocean chlorophyll
Biogeochemical fluxes
Ocean CO,,
B
D
C-
C
D
D
C-
C-
C
C
Land-Surtace Properties:
Surface radiating temperature * F
Incident solar flux * C-
Snow cover * C
Snow water equivalent * F
River runoff (volume) * B
River runoff (sediment loading) *
River runoff (chemical constituents) * F
KEY
Importance (for documenting and understanding
global change):
*** Essential ** High * Substantial
Analysis Product Quality (presently available multiyear
global analyses):
A = Good quantitative, well calibrated
B = Well discriminated, absolute' accuracy doubtful
C = Useful, poor discrimination
D = Qualitative index, interpretation doubtful
F = No information
- = Not global coverage
Policies for pricing of data and funding for acquisition must encour-
age, rather than hinder, the required research and analysis with
Tong-term multidecadal data bases. Calibration and long-term sta-
bility of operational measurements must be adequate to ensure the
integrity of the long-term data bases. Plans must include adequate
programs for reanalysis of model-assimilated data bases.
Special attention is needed to develop systems to handle the
large data streams from present satellite observations and the even
larger ones from future coordinated packages as planned by NASA
in its EOS program. The analysis of these data into forms readily
OCR for page 130
130
manageable by the scientific community will be a key issue. Large
amounts of information must be made available to the scientific
community in a friendly, supportive, and timely fashion, through
distributed systems using modern computational (supercomputer)
and workstation technologies. An aggressive and supportive new
approach at the national level is needed to develop a national data
and information system for climatic and hydrological data that is
consistent among agencies and thus allows ready access by a wide
variety of researchers.
Hydrological Cycle
Understanding the cycling of water in its three phases is crucial
for studies of global change. Within the atmosphere and at the sur-
face, water absorbs and reflects solar radiation with a wide range of
albedos. Both clouds and water vapor are more important for trans-
fer of thermal infrared radiation than any of the trace greenhouse
gases. Evaporation removes much of the net radiative heating at the
earth's surface; vertical and horizontal transport of water vapor is a
dominant mechanism for redistribution of energy within the atmo-
sphere. Water is a key ingredient of tropospheric and stratospheric
chemical processes. It is also critical for the presence of life. In par-
ticular the dynamics of terrestrial vegetation, through its dependence
on water supply from precipitation, are linked to climate.
Current information on the global and regional budgets of water
is inadequate, and key ingredients i.e., clouds, atmospheric water
vapor, soil moisture, precipitation, and evaporation are now inad-
equately measured. These variables and the processes responsible
for them need to be better represented in global models. The World
Climate Research Program (WCRP) has (leveloped a concept of a
Global Energy and Water Experiment (GEWEX) to greatly improve
the observational and modeling basis for including the physical hy-
drological system in studies of global change. Building upon the
GEWEX concept of WCRP, a program should be developed to better
define the hydrological cycle and! related energy fluxes by means of
global measurements of observable atmospheric and surface proper-
ties, and to study and mode! processes of the gloloal; hydrological cycle
and its connections to land processes and properties of the oceanic
surface layers.
OCR for page 131
131
Effects of Terrestrial Vegetation on Climate
and the Hydrological Cycle
There are close connections between climate and vegetation cover
of both natural and managed ecosystems. The global patterns of
net radiation, ocean temperatures, and precipitation impose strong
constraints on the dynamics of vegetation. However, there are also
significant feedbacks of the vegetation cover on the climate system.
Important factors controlled by vegetation include surface albedo,
evapotranspiration, and surface roughness. These factors are still
poorly represented in global climate models. They depend in part
on the changes of vegetation cover with seasonal temperatures and
associated extreme weather events, and on variations in precipitation
processes, seasonal or otherwise. They also depend on atmospheric
composition either as a stress factor or as a source of nutrients, e.g.,
the dependence of evapotranspiration on atmospheric CO2 either
clirectly through its control of stomata! closure or indirectly through
its control of ecosystem structure. A program of observations and;
modeling should be developed to improve understanding of the effects
of terrestrial vegetation on climate and to build a foundation for
incorporation of these effects in models of global climate change.
Sources of Biogenic Gases and Dependence on Climate
Some biological processes contribute to greenhouse warming by
changing atmospheric composition. Other processes provide con-
densation nuclei for clouds and so may contribute to art increased
cloud albedo, hence a cooler climate. CH4, the next most important
greenhouse gas after CO2, is poorly understood as afactorin global
change. We do not know why it has increased by more than a factor
of 5 since the last ice age, and we cannot predict how either terrestrial
sources or atmospheric Toss might be modified by changes in climate
or atmospheric composition. DMS is apparently the major source of
cloud condensation nuclei over the oceans; yet we have no idea as
to how its sources might change with climate change. A program of
observation and research is needed to improve understanding of the
sources of atmospheric gases from biological processes and to develop
parameterizations for including these sources in coupled models of
global climate change and biogeochemical cycles.
OCR for page 132
132
Atmospheric Dust and Aerosol
Solid particulates in the atmosphere affect atmospheric radiation
balance directly through their modulations of atmospheric radiation
balance and indirectly through their role as cloud condensation nu-
clei. Atmospheric aerosols either increase or decrease the net ab-
sorption of solar radiation, depending on their optical properties
and those of the underlying surface. Carbon particles are generally
the most absorbing, and pure sulfate particles the most reflective.
Wind-suspended material originating in arid regions is widespread
and has been recorded in vastly increased amounts in the paleocTi-
matic record. Its representation in climate models would help focus
on the transport characteristics of the models (also needed for cou-
pled chemistry studies) and would add representation of a significant
ingredient in the climate system. Research is needed to describe the
surface processes responsible for the lifting of soil and desert aerosol
on a global [oasis, the distribution of this aerosol within the atmosphere,
its global transport and locations of deposition, and the inclusion of
all these processes within a global climate model.
BIB [IO GRAPHY
Charlson, K. S., J. E. Lovelock, M. O. Andreae, and S. G. Warren. 1987. Oceanic
phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326:655-661.
Earth System Science Committee. 1988. Earth System Science, A Closer View.
Washington, D.C.: National Aeronautics and Space Administration.
International Council of Scientific Unions. 1986. The International Geosphere-Biosphere
Program: A Study of Global Change. Final report of the Ad Hoc Planing Group.
ICSU Twenty-first General Assembly, Berne, Switzerland, September 14-19, 1986.
Jaeger, J., 1988. Developing policies for responding to climatic change. World Climate
Program Impact Studies. WMO/TD No. 225. Geneva: World Meteorological
Organization.
National Climate Program Office. 1988. National Climate Program Five Year Plan
1988-1992. Washington, D.C.
National Research Council. 1986. Global Change in the Geosphere-Biosphere: Initial
Priorities for an IGBP. Washington, D.C.: National Academy Press.
Office for Interdisciplinary Earth Studies. 1986. Climate-Vegetation Interactions, C.
Rosenzweig and R. Dickinson, eds. Boulder, Colo.
Office for Interdisciplinary Earth Studies. 1987. Arctic Interactions Recommendations
for a Multidisciplinary Arctic Component with IGBP. Boulder, Colo.
Risser, P. (ed.~. 1985. Spatial and Temporal Variability of Biospheric and Geospheric
Processes. Paris: ICSU Press.
World Climate Research Program. 1987. Report of the Workshop on Space Systems
Possibilities for a Global Energy and Water Cycle Experiment. World Climate
Program-137.
OCR for page 133
133
World Climate Research Progrmn. 1988. Global Energy and Water Cycle Experiment:
Concept and Rationale. Draft report of the Joint Scientific Committee Study
Group on Global Energy and Water Cycle Experiment. WCRP-1572.
APPENDIX: INDIVIDUALS WHO PROVIDED COMMENTS
ON BACKGROUND PAPER ON CLIMATIC AND
HYDROLOGIC SYSTEMS
Richard A. Anthes, National Center for Atmospheric Research
D. James Baker, Jr., Joint Oceanographic Institutions, Inc.
Eric ]. Barron, Pennsylvania State University
Roger Barry, University of Colorado
Larry Benson, U.S. Geological Survey
Ralph Cicerone, National Center for Atmospheric Research
William C. Clark, Harvard University
John A. Dutton, Pennsylvania State University
Tnez Fung, Goddard Institute for Space Science
Peter H. Gleick, University of California, Berkeley
Robert Gurney, National Aeronautics and Space Administration
J. Michael Hah, National Oceanic ant! Atmospheric Administration
Frederick Koomanoff, U.S. Department of Energy Research
Arthur H. Lachenbruch, U.S. Geological Survey
Louis Lanzerotti, AT&T Bell Laboratories
Conway Leovy, University of Washington
Jerry MahIman, Geophysics Fluid Dynamics Laboratory
G. A. McBean, Institute of Ocean Sciences
Mark F. Meier, University of Colorado, Boulder
PhiDip Merilees, National Center for Atmospheric Research
Ellen S. MosTey-Thompson, Ohio State University
Marshall E. Moss, U.S. Geological Survey
K. T. Paw U. University of California, Davis
Veerabhadran Ramanathan, University of Chicago
CIaes Rooth, University of Miami
Jorge Sarmiento, Geophysics Fluid Dynamics Laboratory
Stephen Schneider, National Center for Atmospheric Research
Piers SeDers, NASA Goddard Space Flight Center
Kevin Trenberth, National Center for Atmospheric Research
Tom Van der Haar, Colorado State University
J. M. Wallace, University of Washington
Warren Washington, National Center for Atmospheric Research
Representative terms from entire chapter:
land surface
