Albedo is the fraction of solar energy that
is diffusely reflected back from Earth to
space. Measurements of albedo are essential
for climate research studies and
investigations of Earth’s energy budget.
Different parts of Earth have different
albedos. For example, ocean surfaces and
rain forests have low albedos, which means
that they reflect only a small portion of
the Sun’s energy. Deserts, ice and clouds,
however, have high albedos; they reflect a
large portion of the incoming solar energy.
The high albedo of ice helps to insulate the
polar oceans from solar radiation. Over the
whole surface of Earth, about 30% of
incoming solar energy is reflected back to
space. Because a cloud usually has a higher
albedo than the surface beneath it, clouds
reflect more shortwave radiation back to
space than the surface would in the absence
of the cloud, thus leaving less solar energy
available to heat the surface and
atmosphere. Hence, this ‘cloud albedo
forcing’, taken by itself, tends to cause a
cooling or ‘negative forcing’ of Earth’s
climate.
Surface albedo can be estimated from
shortwave, broadband or multi-spectral
radiometer measurements with good horizontal
resolution. Current measurements of albedo
and reflectance are obtained primarily using
multi-spectral imagers such as AVHRR and
MODIS, Vegetation and instruments on some
geostationary satellites (such as MSG).
Clouds, aerosols and atmospheric gases
affect the achievable accuracy, which is
currently marginal to acceptable, but should
improve as progress is made in interpreting
data from high-resolution, multi-spectral
instruments. Surface conditions (moisture,
surface vegetation, snow cover etc.)
strongly affect albedo and high-quality
ground truth data are necessary in support
of satellite measurements. Better
understanding of the directional reflectance
properties of different surfaces and more
accurate aerosol data (to correct
atmospheric effects) are needed to improve
surface reflectance measurements.
As aerosol concentration increases within a
cloud, more cloud droplets form. Since the
total amount of condensed water in a cloud
does not change much, the average droplet
becomes smaller. This has two consequences:
clouds with smaller droplets reflect more
sunlight and such clouds last longer. Both
effects increase the amount of sunlight that
is reflected to space without reaching the
surface.
The Terra satellite has yielded greater
knowledge of such cloud/aerosol effects, with
MODIS and MISR providing data on cloud
features, and ASTER providing complementary,
high spatial resolution measurements. Terra’s
data provide new insights into how clouds
modulate the atmosphere and surface
temperature. Further multi-directional and
polarimetric instruments (e.g. POLDER) also
provided measurements leading to better
estimates of albedo.
Other sensors, such as GERB and SEVIRI on the
MSG missions (starting with Meteosat-8) have
provided improved capabilities for measuring
surface albedo. Improved sounder performance
will yield more information on the infrared
surface emissivity spectrum. Multi-spectral
imaging sensors such as AVHRR/3 on NOAA and
Eumetsat polar-orbiting satellites, IVISSR on
the Chinese FY-2 series and AWIFS on the
Indian Resourcesat provide global visible,
near-infrared and infrared imagery of clouds,
ocean and land surfaces.
CEOS has undertaken to improve the continuity
of terrestrial climate monitoring through
enhancements to the moderate-resolution
historical record. AVHRR data reprocessing
will be undertaken to ensure a consistent
dataset to contribute to historical albedo.
CEOS will also work to enhance the quality of
the Fundamental Climate Data Records generated
from the AVHRR record.
Essential Climate Variables: Glaciers and
Ice Caps, Ice Sheets, Lake Areas and
Levels
Many modelling activities in Earth and
environmental sciences, telecommunications
and civil engineering increasingly require
accurate, high-resolution and comprehensive
topographical databases with indication of
changes over time, where relevant. The
information is also used by, amongst others,
land use planners for civil planning and
development, and by hydrologists to predict
the drainage of water and likelihood of
floods, especially in coastal areas. In its
Fourth Assessment Report in 2007, the IPCC
predicted that (by conservative estimation
techniques underestimating the melting of
ice sheets) global mean sea level may rise
as much as 28–43 cm by the end of the 21st
century. Potentially, sea-level rise will
cause severe flooding, with disastrous
impacts on large, densely populated,
low-lying coastal cities and deltaic areas,
such as Bangladesh.
Satellite techniques offer a unique,
cost-effective and comprehensive source of
landscape topography data. At present, most
information is obtained primarily from
multi-band optical imagers and synthetic
aperture radar instruments with stereo image
capabilities. The pointing capability of
some optical instruments allows the
production of stereo images from data
gathered on a single orbit (e.g. by ASTER on
Terra or HRS on SPOT-5) or multiple orbits
(e.g. by SPOT and Pleiades series). These
are then used to create digital elevation
maps, which give a more accurate depiction
of terrain.
Since SARs can also be used in
interferometric mode to detect very small
changes in topography, they have important
applications in monitoring of volcanoes,
landslides, earthquake displacements and
urban subsidence. Current missions include
Envisat (operations concluded in April
2012), RADARSAT-2, the Cosmo-SkyMed
constellation and TerraSAR-X. From 2014,
ESA’s Sentinel-1 mission, and JAXA's ALOS-2
will also contribute to such information.
Radar altimeters can also provide coarse
topographic mapping over land. They have
been supplemented by a new generation of
laser altimeters, such as GLAS (on ICESat,
ended 2010) which can provide landscape
topography products with height accuracies
of order 50–100 cm, depending on slope.
The role of these satellites and their
importance in mitigating geo-hazards, such
as earthquakes, landslides and volcanic
eruptions, is the focus of the GEO
Supersites initiative.
GCOS notes that measurements of lake area
and lake level give an indication of the
volume of the lake, an integrator variable
that reflects both atmospheric
(precipitation, evaporation-energy) and
hydrological (surface water recharge,
discharge and ground water tables)
conditions. GCOS threshold requirements for
these variables are currently met by
existing missions.
Soil moisture plays a key role in the
hydrological cycle. Evaporation rates,
surface runoff, infiltration and percolation
are all affected by the level of moisture in
the soil. Changes in soil moisture have a
serious impact on agricultural productivity,
forestry and ecosystem health. Monitoring
soil moisture is critical for managing these
resources and understanding long-term
changes, such as desertification, and should
be developed in proper coordination with
other land surface variables. There is a
pressing need for measurements of soil
moisture for applications such as crop yield
predictions, identification of potential
famine areas, irrigation management and
monitoring of areas subject to erosion and
desertification, as well as for the
initialisation of NWP models.
Direct measurement of soil moisture from
space is difficult. Most of the active and
passive microwave instruments provide some
soil moisture information for regions of
limited vegetation cover. However, under
many conditions remote sensing data are
inadequate and information regarding
moisture depth remains elusive. While recent
studies have successfully demonstrated the
use of infrared, passive microwave and
non-SAR sensors to obtain soil moisture
information, the potential of active
microwave remote sensing based on SAR
instruments remains largely unrealised. The
main advantage of radar is that it provides
observations at a high spatial resolution of
tens of metres compared to tens of
kilometres for passive satellite
instruments, such as radiometers, or non-SAR
active instruments, such as scatterometers
(e.g. QuikSCAT, ERS, Envisat and MetOp). The
main difficulty with SAR imagery is that
soil moisture, surface roughness and
vegetation cover all have an important and
nearly equal effect on radar backscatter.
These interactions make retrieval of soil
moisture possible only under particular
conditions, such as bare soil or surfaces
with low vegetation, or through complex
modelling to ‘subtract’ the
contributions/effects of vegetation.
An appropriate instrument for measurements
of soil moisture would appear to be the
passive microwave radiometer, although some
success has been achieved by radar, despite
the complications of analysing the signals
reflected from the ground. Microwave
radiation emitted at the ground can be
monitored to infer estimates of soil
moisture. Passive microwave sensors can be
used to do this, based on detection of
surface microwave emissions, although the
signal is very small and frequently polluted
by radio-frequency interference from illegal
sources. Reliable data (high signal-to-noise
ratio) need to be taken over a large area,
which introduces the problem of
understanding how to interpret the satellite
signal, since it consists of radiation from
many different soil types.
SAR data currently provide the main source
of information on near-surface (10–15 cm)
soil moisture. ASCAT (an improvement of the
ERS-1/2 scatterometer) on EUMETSAT’s MetOp
series also provides data from which soil
moisture information can be inferred.
AMSR-E on Aqua (ended late 2011) and now
AMSR-2 on GCOM-W (since mid 2012) provide a
variety of information on water content by
measuring weak radiation from Earth’s
surface..
Launched in late 2009, the first mission to
satisfy requirements for observing soil
moisture from space for the primary
applications of hydrologic and
meteorological modelling is ESA’s Soil
Moisture and Ocean Salinity mission,
carrying the MIRAS (Microwave Imaging
Radiometer using Aperture Synthesis) passive
L-band 2D interferometer. The new
capabilities provided by SMOS will help to
reduce process uncertainties and improve
climate models. NASA's SMAP mission, planned
for launch in late 2014, is also aimed at
providing soil moisture monitoring
capabilities.
Essential Climate Variables: Land Cover,
Fire Disturbance (Burnt Area), Leaf Area
Index (LAI), Fraction of Absorbed
Photosynthetically Active Radiation (fAPAR),
Above-ground Biomass
Changes in land cover are important aspects
of global environmental change, with
implications for ecosystems, biogeochemical
fluxes and global climate. Land cover change
affects climate through a range of factors
from albedo to emissions of greenhouse gases
from the burning of biomass.
Deforestation inter alia increases the
amount of carbon dioxide (CO2)
and other trace gases in the atmosphere.
When a forest is cut and burned to establish
cropland and pastures, the stored carbon
joins with oxygen and is released into the
atmosphere as CO2. The IPCC Third
Assessment Report (2001) noted that about
three-quarters of the anthropogenic
emissions of CO2 to the
atmosphere during the past 20 years was due
to fossil fuel burning. The rest was
predominantly due to land use change,
especially deforestation. The IPCC Fourth
Assessment Report (2007) confirmed this
statement with improved confidence
levels.
In 2005, a number of developing countries
proposed to incorporate deforestation
prevention into the Kyoto Protocol, in part
through an emissions trading system. The
initiative, known as REDD, (Reducing
Emissions from Deforestation in Developing
countries) would allow developing countries
to sell emissions savings from forest
conservation. Developed countries would buy
the savings to credit against their own
emissions targets. In 2009 CEOS agencies
began actively supporting efforts within GEO
for the development of a global forest
carbon tracking framework, providing
satellite data acquisitions and related
expertise.
IGOS set up an Integrated Global Carbon
Observation Theme to develop a flexible,
robust strategy for international global
carbon observations over the next decade. A
key component of IGCO is terrestrial carbon
observations aimed at the determination of
terrestrial carbon sources and sinks with
increasing accuracy and spatial resolution.
The IPCC has highlighted an improved
understanding of carbon dynamics as vital in
tackling one of the biggest environmental
problems facing humanity. The IGCO report
(2004), further developed as a GEO Carbon
Strategy (2010), is providing an essential
input to the implementation of the United
Nations Framework Convention on Climate
Change, particularly on the role of natural
sinks in meeting targets under the UNFCCC
Kyoto Protocol.
Satellite observations allow scientists to
map land cover and the dynamics of fire
disturbance, and track two key elements of
Earth’s vegetation – the ‘Leaf Area Index’
(LAI) and the ‘Fraction of absorbed
Photo-synthetically Active Radiation’
(fAPAR). LAI is defined as the one-sided
green leaf area per unit ground area in
broadleaf canopies, or as the projected
needle leaf area per ground unit in needle
canopies. fAPAR is the fraction of
photosynthetically active radiation absorbed
by vegetation canopies. Both LAI and fAPAR
data are necessary for understanding how
sunlight interacts with Earth’s vegetated
surfaces.
Multiple types of satellite observations are
used in agricultural applications. Space
imagery provides information which can be
used to monitor quotas and to examine and
assess crop characteristics and planting
practice. Information on crop condition, for
example, may also be used for irrigation
management. In addition, data may be used to
generate yield forecasts, which in turn may
be used to optimise the planning of storage,
transport and processing facilities.
Classification and seasonal monitoring of
vegetation types on a global basis allow the
modelling of primary production – the growth
of vegetation that is the base of the food
chain – which is of great value in
monitoring global food security.
A number of radiometers provide measurements
of vegetation cover, including the ATSR
series, AVHRR/3, MODIS, MERIS (ended 2012),
SEVIRI and Vegetation. These instruments are
helping production of global maps of surface
vegetation for modelling of the exchange of
trace gases, water and energy between
vegetation and the atmosphere.
Multi-directional and polarimetric
instruments (such as MISR and POLDER)
provide more insights into corrections of
land surface images for atmospheric
scattering and absorption, as well as
Sun-sensor geometry, allowing better
calculation of vegetation properties.
Synthetic aperture radars are used
extensively to monitor deforestation and
surface hydrological states and processes.
The ability of SARs to penetrate cloud cover
and dense plant canopies makes them
particularly valuable in rainforest and
high-latitude boreal forest studies.
Instruments such as ASAR (Envisat - ended
2012), SAR (RADARSAT-2) and, in the near
future, PALSAR (ALOS-2, launch early 2014)
provide data for applications such as
agriculture, forestry, land cover
classification, hydrology and
cartography.
CEOS and GCOS have concluded that many of
the Essential Climate Variables related to
vegetation and supported from space will
require reprocessing of the moderate
resolution historical record
(in particular AVHRR) to be of greater value
for climate purposes, and appropriate
actions have been defined.
Essential Climate Variables: Fire
Disturbance (Active Fires), Land Surface
Temperature
Land surface temperature varies widely with
solar radiation. It is of help in
interpreting vegetation and its water stress
when the ranges of temperatures between day
and night and from clear sky to cloud cover
are compared.
Estimates of greenhouse gas emissions due to
fire are essential for realistic modelling
of climate and its critical component, the
global carbon cycle. Fires caused
deliberately for land clearance (agriculture
and ranching) or accidentally (lightning
strikes, human error) are a major factor in
land cover changes, affecting fluxes of
energy and water to the atmosphere.
On a local scale, surface temperature
imagery may be used to refine techniques for
predicting ground frost and to determine the
warming effect of urban areas (urban heat
islands) on night-time temperatures. In
agriculture, temperature information may be
used, together with models, to optimise
planting times and provide timely warnings
of frost. Measurements of surface
temperature patterns may also be used in
studies of volcanic and geothermal areas and
resource exploration.
Land surface temperature measurements are
made using the thermal infrared channel of
medium/high-resolution multi-spectral
imagers in low-Earth orbit. In addition,
visible/infrared imagers on geostationary
satellites also provide useful information,
with the advantage of very high temporal
resolution.
However, difficulties remain in converting
the apparent temperatures as measured by
these instruments into actual surface
temperatures – variations due to atmospheric
effects and vegetation cover, for example,
require compensation using additional
imagery/information.
A number of capable sensors designed to
provide land surface temperature data are
currently operating or planned. These
include advanced sounders (IASI, HIRS/4) on
operational meteorological platforms. On the
Suomi NPP satellite (and future JPSS
missions), VIIRS combines the radiometric
accuracy of AVHRR with the high spatial
resolution of the DMSP’s OLS instrument.
The Hot Spot Recognition Sensor (HSRS) on
BIRD (launched 2001) demonstrated its value
as a purpose-built fire detection instrument
until its partial failure in 2004, while
MODIS provides regular sampling of active
fires, SEVIRI observes the diurnal cycle of
fire occurrence in Africa and the (A)ATSR
series, despite not being designed for
active fire observations, has produced the
longest record of hot spot detection (at
night). ESA offered a monthly world fire
atlas product available online at
dup.esrin.esa.it/ionia/wfa until Envisat
concluded operations in 2012.
Essential Climate Variables: Land Cover
(Including Vegetation Type)
The spatial information that can be derived
from satellite imagery is of value in a wide
range of applications, particularly when
combined with spectral information from
multiple wavebands of a sensor. Satellite
Earth observation is of particular value
where conventional data collection
techniques are difficult, such as in areas
of inaccessible terrain, providing cost and
time savings in data acquisition –
particularly over large areas.
At regional and global scales,
low-resolution instruments with wide
coverage capability and imaging sensors on
geostationary satellites are routinely
exploited for their ability to provide
global data on land cover and vegetation.
Land cover change detection is important for
understanding global environmental change
and has profound implications for
ecosystems, biochemical fluxes and climate.
Instruments on satellites with wide and
frequent coverage provide data useful for
spin-off applications. AVHRR on NOAA’s polar
orbiting satellite series was originally
intended only as a meteorological satellite
system, but it has subsequently been used in
a multitude of diverse applications, while
the Envisat MERIS instrument has been used
to generate global land cover imagery at 300
m resolution.
On national and local scales, the spatial
resolution requirements for information mean
that moderate resolution imaging sensors,
such as those on the SPOT, Landsat and IRS
series, and imaging radars (such as those on
Envisat and RADARSAT) have been most useful.
Such sensors have been routinely used as
practical sources of information for:
— Agriculture monitoring, farming and
production forecasting;
— Resource exploration and management, e.g.
forestry;
— Geological surveying for mineral
exploration and identification;
— Hydrological applications such as flood
monitoring;
— Civil mapping and planning, involving
cartography, infrastructure and urban
management;
— Coastal zone monitoring, including oil
spill detection and monitoring;
— Topographic mapping, generation of
DEMs.
The Landsat 5 satellite operation was
suspended in November 2011 after a 27-year
long mission. Landsat 7 (launched 1999)
continues to collect imagery worldwide with
partially compromised quality for some
applications. Landsat 8 – also known as
Landsat Data Continuity Mission or LDCM –
was launched in February 2013. The evolving
SPOT series has been discontinued, with only
SPOT 5 still in operation in 2013. The ESA
Sentinel-2 series (first launch late 2014)
is expected to extend Landsat-type and
SPOT-type moderate resolution imagery
acquisition for a minimum period of 15
years.
SAR data are particularly useful in
monitoring and mapping floods because they
are available even in the presence of thick
cloud cover. Instruments on RADARSAT-2 and
TerraSAR-X continue to provide improved
capabilities in this field. Such
multi-incidence, high-resolution SAR systems
will also be useful for landslide inventory
maps and earthquake prediction. Moreover,
InSAR techniques can be used to document
deformation and topographic changes
preceding, and caused by, volcanic
eruptions. Volcanic features also have
distinctive thermal characteristics which
can be detected by thermal imagery, such as
that provided by the ASTER radiometer flying
on Terra. The IGOS Geo-hazards Theme report
provides a comprehensive guide as to the
value of satellite Earth observations for
such applications. Future SAR instruments
will continue to be important for land
imagery because of their all-weather, day
and night observing capability and high
spatial resolution (1–3 m), as provided by
RADARSAT-2 and SAR-2000 on the
Cosmo-SkyMed satellites.
Innovative instruments, such as AVNIR-2 and
PRISM on ALOS (completed April 2011),
provided enhanced land observing technology
and improved data products. In general,
future sensors will benefit from a greater
number of sampling channels. NOAA’s VIIRS
instrument, for instance, has multi-channel
imaging capabilities and combines the
radiometric accuracy of AVHRR with the high
spatial resolution of the OLS flown on DMSP
missions.
CEOS has initiated a virtual constellation
study team for land surface imaging to
provide the coordination framework necessary
to secure continuity of moderate resolution
imagery used for many land surface
applications, including their relation to
climate.
