By Alexander de Sherbinin and Robert S. Chen,
Center for International Earth Science Information Network, The Earth
Institute, Columbia University (www.ciesin.columbia.edu),
Palisades, N.Y.
Remote sensing instruments have played a vital role in detecting
environmental changes, including the impacts of human activities on the
landscape, atmosphere and oceans. Without Earth-observation systems, in
a real sense, we would be “flying blind,” missing the information needed
to make informed decisions and adjust our activities to avert future
crises.
Consider the ozone hole. Depletion of the stratospheric ozone layer was
identified by a ground-based research team in 1984, but it wasn’t until
the seasonal hole in the ozone layer was confirmed by NASA’s Total Ozone
Mapping Spectrometer (TOMS) instrument—and depicted visually—that the
public and policy makers became aware of the magnitude of the problem
(Figures 1 and 2). These events paved the way for the Montreal Protocol
in September 1987, widely considered one of the most effective
environmental treaties because of its strict targets and enforcement
mechanisms. TOMS data and images provided scientific and “visceral”
support to efforts to implement and expand the protocol. By
international agreement, ozone-damaging chlorofluorocarbons were banned
in 1995, and their levels in the atmosphere are decreasing.
Changes on the Land
Perhaps remote sensing’s most notable contribution has been its
ability to visualize land use and land coverpatterns, and to assess
changes over time. Since 1972, when Landsat 1 was launched, a
growing number of optical and radar instruments have helped
scientists track tropical deforestation, the expansion of cultivated
land, the growth of urban areas and changes in inland water bodies.
Most of us are aware of changes in our surrounding landscape—the
housing developments on prime farmland or the disappearance of
forests and wetlands—but less easy to fathom is the magnitude of
these incremental local changes on our region or the planet as a
whole. This is where remote sensing plays an important role. For
example, using Landsat imagery, researchers at Rutgers University
found that, from 1986-1995, the state of New Jersey lost the
equivalent of 52 football fields per day to development, roughly
half of the land coming from forests and wetlands and half from
farmland (Figure 3).
Globally, the Forest Resource Assessment (FRA)
of the Food and Agriculture Organization of the United Nations (FAO)
has, since 1990, tracked deforestation by taking a 10 percent
sample-based remote sensing survey of forest lands. In its most
recent assessment, the FRA found a net change in forest area in the
period 2000-2005 of approximately 7.3 million hectares per year (an
area about the size of Panama), down from 8.9 million hectares per
year in the period 1990-2000. This implies a total net loss since
1990 of 125.5 million hectares, or an area roughly the size of Peru.
Of all forest lands, the biodiversity-rich tropical forests of
Africa and Latin America are disappearing at the fastest rates.
Tropical deforestation accelerates biodiversity loss—tied as it is
to habitat loss and fragmentation—as well as climate change because
tropical forests are major carbon sinks that become carbon sources
when cut or burned. Remote sensing technology, such as the
Measurements of Pollution in the Troposphere (MOPITT) instrument
aboard NASA’s Terra spacecraft, shows the dramatic increase of
carbon monoxide emissions during the dry season over the Brazilian
Amazon, when farmers are preparing their fields and ranchers are
clearing forest for cattle (Figure 4).
Satellite monitoring of such burning has been greatly aided by
Moderate Resolution Imaging Spectroradiometer (MODIS) imagery on
NASA’s Terra satellite, which can track forest fires on a daily
basis in near-real time. MODIS imagery is being used by IBAMA, the
Brazilian environmental protection agency, to identify fires and to
direct fire prevention crews to put them out (Figure 5).
Other land cover changes are less spatially widespread, yet are
notable for the intensity of their impact. Urban areas make up only
3 percent of all land cover types globally, yet the foot print of
urban areas on the land is far greater owing to their demands for
water, resources and land for waste disposal (Figure 6).
Controversial large dams, such as the Three
Gorges Dam in China (Figure 7), generate electricity for urban-based
consumers, in the process swallowing thousands of hectares of
productive lands. Also in China, clouds of smog and dust, largely
from urban-based industries, show up in satellite images traveling
as far as the west coast of the United States. Regional air
pollution is a major problem in East Asia because of China’s
dependence on soft-coal deposits for energy and its rapidly growing
vehicle fleet (Figure 8).
Climate Change
Land cover change studies have been a mainstay of the remote sensing
research community for years, but during the last decade satellite
data have been used more frequently to track the effects of
anthropogenic climate forcing on Earth systems. For example, a
variety of sensors have been used to monitor sea surface
temperatures (SSTs), which have a strong influence on global and
regional climatic conditions. SSTs also are used with air
temperature measurements to help assess global mean surface
temperature trends.
Rainfall can also be monitored by satellite, using data from the
Tropical Rainfall Monitoring Mission (TRMM). Rising temperatures and
changing rainfall patterns are predicted as a result of global
warming.
Satellite capture of key climate variables such as SSTs, winds,
albedo and rainfall offer important evidence of global environmental
variability and change that complements the ground-based record. But
it is images of the impacts of climate change that have captured
public attention. Satellite images of the Ross Ice Shelf calving
county-sized icebergs off of Antarctica made front page news in
January 2003 (Figures 9 and 10), and images of glaciers retreating
up mountainsides have become a mainstay of reports on global
warming. Less visible changes, such as the thickness of the floating
ice sheets covering the Arctic Ocean, also have been detected by
satellite instruments, as has the shrinkage of the polar ice cap.
Recent reports, based in part on satellite observations, suggest
that melt water from Greenland’s ice sheet has more than doubled
from 90 cubic kilometers to 220 cubic kilometers a year since 1995,
raising the specter of more rapid sea-level rise than originally
thought. If the entire Greenland ice sheet disappears, sea levels
could rise by about 7 meters (22 feet).
Climate change results not only in changing
temperature and precipitation regimes, but also in increases in the
frequency and intensity of climate-related hazards. Scientists
predict that tropical storms will intensify with the increased
heating of the ocean surface layer induced by global warming,
resulting in more severe hurricanes and cyclones. Florida was
bombarded by hurricanes in 2004, but it was the devastation wrought
by Hurricane Katrina that served as a wake-up call in the United
States about the degree to which climate change could affect
virtually everyone (Figure 11).
Seasonal droughts also are predicted to increase in duration and
intensity owing to climate change. The normalized difference
vegetation index (NDVI), a ratio of the near infrared and red bands,
is used extensively to assess greenness on the landscape,
which is closely related to rainfall. NDVI-based analyses using
Advanced Very High Resolution Radiometer (AVHRR) data in the 1990s
helped to confirm, and sometimes refute, claims of desertification,
the progressive spreading of the desert margin in regions like the
western United States and Africa’s Sahel. MODIS data also have been
used to show the difference between normal and drought conditions in
the American West, a region particularly hard hit by droughts in
recent years (Figure 12).
Oceans and Coasts
Although they haven’t been around as long as land-based
applications, marine and coastal applications of remote sensing have
made major strides in the last decade, helping us better understand
the impact of human activities on the world’s oceans. Dead zones,
characterized by anoxic conditions in which the areas have low or
completely zero concentrations of dissolved oxygen, are developing
in many of the world’s deltas as a result of agricultural and soil
runoff. Because few organisms can tolerate the lack of oxygen in
these areas, they can destroy the habitat in which numerous
organisms make their home. Such conditions can’t be detected
directly by remote sensing because they occur at substantial depths,
but they are often highly correlated with visible plumes of
sediments, such as those detected by the Sea-viewing Wide
Field-of-view Sensor (SeaWiFS) aboard GeoEye’s OrbView-2 satellite
(Figure 13).
Similarly, harmful algae blooms, often referred to as “red tides,”
have become more prevalent because of increased nutrient loading in
coastal waters. Such blooms can be detected directly by SeaWiFS
(Figure 14). Red tides are caused by tiny algae that grow on the
surface of the ocean, occasionally giving it a reddish-brown tint.
Thus, scientists can use SeaWiFS to map the extent of red tides and
monitor how they spread over time. Satellites detect changes in the
way the sea surface reflects light. These changes can be linked to
concentrations of chlorophyll, showing where algae and other ocean
plants are concentrated in the ocean.
In an innovative use of remote sensing for public education, the
International Coral Reef Action Network’s ReefBase (www.reefbase.org)
provides a combination of MODIS and Landsat images in a global
mosaic to help track areas of coral bleaching and mangrove loss.
Coral bleaching occurs because of increases in ocean temperature
related to global warming. Mangrove loss is a global phenomena
heavily related to expansion of the shrimp aquaculture industry.
Both problems particularly have afflicted tropical coastal areas and
represent well the two faces of global change: problems truly global
in scope that can only be solved through coordinated global action,
and localized problems that manifest themselves everywhere and are
amenable to local solutions.
Future Prospects
This article has described many of the ways in which remote sensing
has helped scientists document the symptoms of global change. Recent
cover stories in National Geographic (“Global Warning”) and Time
(“Global Warming: Be Worried. Be Very Worried.”) demonstrate that
the media and the public have begun to understand the magnitude of
the changes our planet faces. Remote sensing has played a
significant role in raising that awareness.
But unlike the success story of the TOMS images, stratospheric ozone
depletion, and the development of the Montreal Protocol, solutions
to many of the issues identified here may be more difficult to
design and implement. In the case of the chlorofluorocarbons that
damaged the ozone layer, reasonable substitutes were found by the
key industries involved. In the case of land cover change, demands
for housing, agriculture, grazing lands, energy and infrastructure
among the growing number of the world’s affluent, not to mention the
drive to meet the Millennium Development Goal of halving the number
of hungry people, will by necessity put increasing pressure on land
resources around the world. In the case of climate change, easing
society’s thirst for fossil fuels will be difficult, to say the
least. And in the case of oceans, curbing overfishing and limiting
nitrogen discharges into coastal areas will require concerted
international cooperation and investment.
Yet here again remote sensing may play a role. If global agreements
become more stringent, then remote sensing can be used as a
verification tool to ensure that parties to a treaty meet their
commitments. Remote sensing already has been used by Global Forest
Watch to monitor illegal logging. In the future, remote sensing
instruments may be sophisticated enough to monitor biomass volume
across entire forest systems. This will serve not only to monitor
biodiversity-related commitments, but also aforestation,
reforestation and deforestation under the Kyoto Protocol. Radar
remote sensing also has been used to a limited degree to track oil
spills on the high seas under the Bonn Agreement. It is conceivable
that it could be used in conjunction with ship-based Global
Positioning System units to monitor illegal fishing in marine
protected areas. So, as global governance mechanisms become more
sophisticated, we are likely to see remote sensing shifting from an
assessment tool—diagnosing the problems and providing decision
makers with the information they need to shape policies—to a
compliance monitoring tool. This shift, if it occurs, will ensure
remote sensing’s pre-eminent role as a data source for global change
research and policy.
Authors’ Note: To learn more about remote sensing’s role in global
environmental agreements, visit CIESIN’s NASA-funded Socioeconomic
Data and Applications Center (SEDAC) Remote Sensing and
Environmental Treaties Web site at http://sedac.ciesin.columbia.edu/rs-treaties.
