A condensed version of this post originally appeared at Yale e360.
In October 2008 scientists with the Royal Botanical Garden at Kew discovered a host of previously unknown species in a remote highland forest in Mozambique. The find was no accident: three years earlier, conservationist Julian Bayliss identified the site—Mount Mabu—using Google Earth, a tool that’s rapidly becoming a critical part of conservation efforts around the world.
As the discovery in Mozambique suggests, remote sensing is being used for a bewildering array of applications, from monitoring sea ice to detecting deforestation to tracking wildlife. The number of uses grows as the technology matures and becomes more widely available. Google Earth may represent a critical point, bringing the power of remote sensing to the masses and allowing anyone with an Internet connection to attach data to a geographic representation of Earth.
Brief history of remote sensing for environmental applications
A lot of environmental monitoring is possible today only through remote sensing. Detecting changes in sea ice across the sub-freezing Arctic is one example, but remote sensing also allows monitoring of hostile and sometimes war-torn deserts, vast expanses of ocean, the dense Amazon rainforest, and isolated mountain ranges—monitoring which would be cost-prohibitive or impossible without eyes from above.
Landsat image revealing “fishbone” deforestation along roads in the Brazilian Amazon
“Sending people in by foot to survey these places is costly, time-consuming, and potentially dangerous,” Ruth DeFries, a GIS specialist at Columbia University, told mongabay.com. DeFries authored a comprehensive review last year on the use of remote sensing for terrestrial environmental monitoring. “Remote sensing is really the only way to do this work.” she said.
Early earth observation satellites focused on weather, but scientists quickly devised ways to use their data to analyze vegetation cover. In 1972 Landsat became the first non-weather satellite for civilian use, giving scientists the ability to observe any place on Earth every 18 days. The satellite initially was used for crop analysis but today, seven satellite generations later, higher resolution and additional spectral bands have vastly expanded Landsat’s functionality for a wide array of applications.
Since the 1990s, Landsat has become only one example of many sensing technologies. Landsat 7, the most recent Landsat satellite, carries several passive sensors using different wavelengths to decipher Earth’s features from above. A passive optical sensor, for example, is much like a camera, operating off visible light reflected from Earth’s surface. This reliance has its shortcomings—notably it will only take pictures of what it can see. Clouds, smoke, and other factors can interfere with or block its sensing capabilities. Meanwhile, infrared detects the amount of heat emitted from an object at the earth’s surface, making it effective for identifying fires and other sources of heat like cities.
Landsat 7 satellite in the cleanroom prior to launch
Still, Landsat offers immense utility to researchers, especially when used in concert with other sensors like the National Oceanic and Atmospheric Administration (NOAA)’s Advanced Very High Resolution Radiometer (AVHRR) and NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) as well as technologies developed by private companies and national and transnational space programs. Landsat and AVHRR have the advantage of 20-30 years of data, making them ideal for comparisons over time. This is especially important for monitoring changes in forest cover, sea ice, desertification, and urbanization.
But remote sensing isn’t limited to passive sensing. Active sensing—which sends out pulses of energy and reads the radiation that bounces back to the sensor—can provide detailed information about Earth’s surface, including the structure of a forest or the distinction between secondary and primary forest.
These technical advances, which make remote sensing data more relevant and timely, have been accompanied by favorable political and economic trends. Landsat data is now freely available to the public. At the same time a proliferation of commercial satellites offers a range of remote sensing products. Remote sensing data is no longer limited to the military, specialized institutions, or the academic world.
This image shows Arctic sea ice concentration on September 8, 2008, as observed by the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) sensor on NASA’s Aqua satellite. The observations are collected on a pixel by pixel basis over the Arctic. The percentage of a 12.5-square-kilometer pixel covered by ice is shown in shades of dark blue (no ice) to white (100 percent ice). The gray line around the Arctic basin shows the median minimum extent of sea ice from 1979-2000. NASA image created by Jesse Allen, using data obtained courtesy of the National Snow and Ice Data Center (NSIDC). Caption by Rebecca Lindsey.
One of the most prominent uses for earth observation satellites is monitoring sea ice. With records dating back to the 1970s, remote sensing observations have established a baseline for tracking the rapid loss of sea ice in the Arctic. Due to persistent cloud cover, which obscures optical sensors, scientists from the National Snow and Ice Data Center (NSIDC) at the University of Colorado, Boulder, use infrared (IR) sensors to infer the amount of heat emitted from the surface. Because sea ice is much colder than surrounding water, the contrast between the two is stark—at least during the winter. In the summer, melting sea ice approaches the temperature of the surrounding ocean, making it difficult to distinguish between the two. NSIDC says the new generation of multispectral sensors has helped fine-tune monitoring. What researchers have found hasn’t been encouraging, at least for polar bears: summertime sea ice extent in 2007 fell half below average for the past three decades.
Unlike sea ice, fires are relatively easy to detect using thermal infrared bands provided by MODIS sensors. Fire data is acquired at least daily, enabling researchers to monitor, in near real-time, fires burning anywhere in the world. The Fire Alert System—developed by Madagascar’s ministry of Environment, the International Resources Group, and Conservation International using data from the University of Maryland and NASA—has put this information to practical use. The system alerts subscribers via e-mail whenever burning is detected, potentially enabling them to take action on the ground when fires threaten protected areas or human settlements. MODIS data is also regularly used to monitor burning in the world’s tropical forests. In 2007 when commodity prices were peaking, MODIS revealed a surge in the number of hotspots burning across the Brazilian Amazon.
Accurate and timely monitoring of deforestation and forest degradation may present the next great frontier for remote sensing due to the potential emergence of REDD (Reducing Emissions from Deforestation and Degradation), a mechanism for compensating tropical countries for conserving their forests. To date, one of the biggest hurdles for the concept has been establishing credible national baselines for deforestation rates—in order to compensate countries for “avoided deforestation,” it must first be known how much forest the country had been clearing on a historical basis. For the remote sensing community, REDD presents an opportunity to showcase the power of remote sensing and generate a source of funding for improved sensing capabilities.
Presently optical sensing can do a reasonably good job distinguishing between cleared forest and natural forest—assuming cloud cover is minimal, a big assumption in the tropics. It does less well identifying and distinguishing between recovering forests, selectively logged forest, tree plantations, and degraded forests. New active sensing technologies, like cloud-penetrating radar and LIDAR, may change this. Some of these technologies may allow scientists to directly measure biomass in dense forests—currently many sensing technologies are limited by their tendency to “saturate” at a threshold well below the actual biomass in such forests.
Josef Kellndorfer, an associate scientist at the Woods Hole Research Center in Massachusetts, says that a new JAXA (Japan Aerospace Exploration Agency) satellite—known as ALOS for the Advanced Land Observation Satellite—offers great promise for monitoring deforestation and degradation.
Radar image mosaic is a composite of nine individual scenes (45,000 km2) of Bali, Indonesia acquired by the PALSAR sensor carried on board ALOS. The image acquisitions were made between September 9 and October 10, 2007. From New Eyes in the Sky
“ALOS features three sensors: an optical sensor, a typical multi-spectral sensor with visible and infrared bands; a precise stereo mapper for generation of very high resolution elevation models and topographic information, which is also optical-based; and a radar sensor named PALSAR,” he told mongabay.com.
PALSAR will allow scientists to get an annual snapshot at 20-meter resolution of all the world’s biomes, allowing the scientists to establish a baseline for forest cover every year. The sensor also has a 100-meter resolution mode that provides near-global coverage every six weeks and can be used to detect illegal logging activities even under cloud cover.
“These capabilities are exciting because we have the complete pan-tropical forest cover for 2007. We’ll get the same on an annual basis for the life of the ALOS mission. This will enable us to build a data record of forest cover on an annual basis no matter what the cloud conditions are,“ Kellndorfer said.
“Every year, within three months, we will have a full resolution pan-tropical assessment of forest cover,” he continued. “ALOS and future missions with dedicated observation strategies can thus be used as a tool to complement the overall remote sensing and monitoring effort of forests.”
Deforestation data from INPE
The sensing exercise will be particularly important in the Amazon—the world’s largest tropical forest. Brazil’s use of satellite data for environmental monitoring is among the most sophisticated on the planet. The country has two systems for tracking deforestation: PRODES (Program to Calculate Deforestation in the Amazon) and DETER (Real-time Detection of Deforestation). Both presently rely on optical sensing—sometimes thwarted by cloud cover—but where it has a clear view, the country can rapidly identify where deforestation is occurring. PRODES, which has a sensitivity of 6.5 ha, provides Brazil’s annual deforestation estimates (measured each August) while DETER, which has a coarser resolution of 25 ha, is a year-round alert system that updates IBAMA, Brazil’s environmental protection agency, every two weeks. This gives authorities the technical capacity—although not necessarily the political will—to combat deforestation as it occurs. The system will be enhanced when Brazil launches the Amazon-1, its own earth observation satellite with cloud-penetrating LIDAR.