A Greener Planetary Greenhouse
In recent years Earth-orbiting satellites have seen plants growing more vigorously
than usual over northern parts of our planet.
September 7, 2001: For more than two decades, northern hemisphere
vegetation has become gradually more lush, according to new research based on NASA
satellite data. Researchers confirm that plant life seen above 40 degrees north latitude,
which represents a line stretching from New York to Madrid to Beijing, has been growing
more vigorously since 1981. One possible cause is rising temperatures, linked perhaps to
the buildup of greenhouse gases in our atmosphere.
Above: Along with other northern vegetation,
high-latitude forests have been growing more vigorously since 1981. Image Credit:USDA
Forest Service.
The area of northern vegetation has not actually expanded, but it has increased in
density. The growing season has also increased by several days. Furthermore, Eurasia
appears to be
greening more than North America, with more lush vegetation for longer periods of time.
"When we looked at temperature and satellite vegetation data, we saw that
year-to-year changes in growth and the duration of the growing season were tightly linked
to year-to-year changes
in temperature," said Liming Zhou of Boston University. Zhou and colleagues also
examined the differences in vegetation growth between North America and Eurasia, because
the patterns
and magnitudes of warming on the two continents are different. The greenness data from
satellites were strongly correlated with temperature data from thousands of meteorological
stations on both sides of the world. The Eurasian greening was especially persistent over
a broad area from central Europe through Siberia to far-east Russia, where most of the
vegetation is forests and woodlands.
North America, in comparison, shows a fragmented pattern of change notable only in the
forests of the East and grasslands of the upper Midwest.
Below: Temperature anomalies (red bars) averaged during the months April
through October are correlated with the greenness of vegetation.
Dramatic changes in the timing of both the appearance and fall of leaves are recorded
in these two decades of satellite data. The researchers reported a growing season in
Eurasia that is now nearly 18 days longer than it was before. Spring arrives a week early
and autumn is delayed by 10 days. In North America, the growing season appears to be as
much as 12 days longer. The researchers used a temperature data set developed from the
Global Historical Climate Network. Dr. James Hansen, of NASA's Goddard Institute for Space
Studies in New York, developed this data set and said, "The data were compiled from
several thousand meteorological stations in the United States and around the world. The
stations also include many rural sites where the data are collected by cooperative private
observers."
Above: Researchers used the Normalized Difference
Vegetation Index (NDVI) to measure the "greening" of plant life. The algorithm
relies on spectral data collected by orbiting weather satellites, such as the vegetation
data shown on this map.
Scientists believe the results indicate a greener planetary greenhouse. "This is an
important finding because of possible implications to the global carbon cycle," said
Ranga Myneni of Boston University. "However, more research is needed to determine how
much carbon is being absorbed, and how much longer it will continue." Carbon dioxide
is a main greenhouse gas, and scientists suspect it plays an important role in rising
global temperatures. If the northern forests are greening, they may already be absorbing
carbon -- a process that can impact global temperature changes.
The greening trend revealed by this research provides an important piece of the puzzle of
global climate change, and will help scientists produce more accurate predictions of how
greenhouse gases will affect our climate in the decades to come.
Measuring Earth Vegetation (NDVI and EVI)
by John Weier and David Herring
Though we often take the plants and trees around us for granted,almost every aspect of our
lives depends upon them. They feed us, cloth us, absorb carbon dioxide, provide us with
oxygen, and give us building materials and medications. When drastic changes occur to the
vegetation around us, our health, economy, and environment are all affected. Twenty-five
years ago, for instance, thousands of people starved when the vegetation in the Sahel
region of Africa dried up during an extended drought. Over the past five decades
deforestation in South America has left thousands of acres fallow and has destroyed many
species including possible valuable medications.
Satellite maps of vegetation show the density of plant
growth over the entire globe. The most common measurement is called the Normalized
Difference Vegetation Index (NDVI). Very low values of NDVI (0.1 and below) correspond to
barren areas of rock, sand, or snow. Moderate values represent shrub and grassland (0.2 to
0.3), while high values indicate temperate and tropical rainforests (0.6 to 0.8).
In an effort to monitor major fluctuations in vegetation and understand how they affect
the environment, 20 years ago Earth scientists began using satellite remote sensors to
measure and map the density of green vegetation over the Earth. Using NOAA’s Advanced
Very High Resolution Radiometer (AVHRR), scientists have been collecting images of our
planet’s surface. By carefully measuring the wavelengths and intensity of visible and
near-infrared light reflected by the land surface back up into space, scientists use an
algorithm called a "Vegetation Index" to quantify the concentrations of green
leaf vegetation around the globe. Then by combining the daily Vegetation Indices into 8-,
16-, or 30-day composites, scientists create detailed maps of the Earth’s green
vegetation density that identify where plants are thriving and where they are under stress
(i.e., due to lack of water).
Normalized Difference Vegetation Index (NDVI)
To determine the density of green on a patch of land, researchers must observe the
distinct colors (wavelengths) of visible and near-infrared sunlight reflected by the
plants. As can be seen through a prism, many different wavelengths make up the spectrum of
sunlight. When sunlight strikes objects, certain wavelengths of this spectrum are absorbed
and other wavelengths are reflected. The pigment in plant leaves, chlorophyll, strongly
absorbs visible light (from 0.4 to 0.7 µm)for use in photosynthesis. The cell structure
of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1
µm). The more leaves a plant has, the more these wavelengths of light are affected,
respectively.
Vegetation appears very different at visible and
near-infrared wavelengths. In visible light (top), vegetated areas are very dark, almost
black, while desert regions (like the Sahara) are light. At near-infrared wavelengths, the
vegetation is brighter and deserts are about the same. By comparing visible and infrared
light, scientists measure the relative amount of vegetation. (The variation in shade is
more apparent in the detail of the U.S. West Coast).
The NOAA AHVRR instrument has five detectors, two of which are sensitive to the
wavelengths of light ranging from 0.55–0.70 and0.73–1.0 micrometers. With
AHVRR’s detectors, researchers can measure the intensity of light coming off the
Earth in visible and near-infrared wavelengths and quantify the photosynthetic capacity of
the vegetation in a given pixel (an AVHRR pixel is 1 square km) of land surface. In
general, if there is much more reflected radiation in near-infrared wavelengths than in
visible wavelengths, then the vegetation in that pixel is likely to be dense and may
contain some type of forest. If there is very little difference in the intensity of
visible and near-infrared wavelengths reflected, then the vegetation is probably sparse
and may consist of grassland, tundra, or desert.
NDVI is calculated from the visible and near-infrared light
reflected by vegetation. Healthy vegetation (left) absorbs most of the visible light that
hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse
vegetation (right) reflects more visible light and less near-infrared light. The numbers
on the figure above are representative of actual values, but real vegetation is much more
varied. (Illustration by Robert Simmon).
Nearly all satellite Vegetation Indices employ this difference formula to
quantify the density of plant growth on the Earth —near-infrared radiation minus
visible radiation divided by near-infrared radiation plus visible radiation. The result of
this formula is called the Normalized Difference Vegetation Index (NDVI). Written
mathematically, the formula is
NDVI = (NIR — VIS)/(NIR + VIS)
Calculations of NDVI for a given pixel always result in a number that
ranges from minus one (-1) to plus one (+1); however, no green leaves gives a value close
to zero. A zero means no vegetation and close to +1(0.8 - 0.9) indicates the highest
possible density of green leaves.
NDVI as an Indicator of Drought
Satellite remote sensors can quantify what fraction of the photosynthetically active
radiation is absorbed by vegetation. In the late 1970s, scientists found that net
photosynthesis is directly related to the amount of photosynthetically active radiation
that plants absorb. In short, the more a plant is absorbing visible sunlight (during the
growing season), the more it is photosynthesizing and the more it is being productive.
Conversely, the less sunlight the plant absorbs, the less it is photosynthesizing, and the
less it is being productive. Either scenario results in an NDVI value that, over time, can
be averaged to establish the "normal" growing conditions for the vegetation in a
given region for a given time of the year. In short, a region’s absorption and
reflection of photosynthetically active radiation over a given period of time can be used
to characterize the health of the vegetation there, relative to the norm.
The difference between the average NDVI for a particular
month of a given year (such as August 1993, above) and the average NDVI for the same month
over the last 20 years is called NDVI anomaly. (Compare the August 1993 NDVI anomaly to
August 1993 NDVI and Average August NDVI in North America.) In most climates, vegetation
growth is limited by water so the relative density of vegetation is a good indicator of
agricultural drought.
The above image shows the NDVI anomaly in the U.S. for August 1993. In that year, heavy
rain in the Northern Great Plains (North and South Dakota, Alberta, and Saskatchewan) led
to flooding in the Missouri River. The resulting exceptionally lush vegetation appears as
a positive anomaly (green). Concurrently, in the Eastern U.S., rainfall was very low, and
the region exhibited a strong negative anomaly (dark red).
Today, researchers at NASA and NOAA have two decades of NDVI data over the entire globe.
Comparing this month’s or this year’s NDVI data with the 20-year average reveals
whether the productivity in a given region is typical, or whether the plant growth is
significantly more or less productive. So for the purposes of this Web site, a given
region in which there is a period of reduced plant growth (due to a lack of precipitation)
is labeled as "in drought." Other possible causes for lower than normal NDVI are
exceptionally cold temperatures (which can delay or cut short the growing season) and
clouds
Enhanced Vegetation Index (EVI)
In December 1999, NASA launched the Terra spacecraft, the flagship in the agency’s
Earth Observing System (EOS) program. Aboard Terra flies a sensor called the
Moderate-resolution Imaging Spectroradiometer, or MODIS, that greatly improves
scientists’ability to measure plant growth on a global scale. Briefly, MODIS provides
much higher spatial resolution (up to 250-meter resolution),while also matching
AVHRR’s almost-daily global cover and exceeding its spectral resolution. In other
words, MODIS will provide images over a given pixel of land just as often as AVHRR, but in
much finer detail and with measurements in a greater number of wavelengths using detectors
that were specifically designed for measurements of land surface dynamics.
Consequently, the MODIS Science Team is preparing a new data product–called the
Enhanced Vegetation Index (EVI)–that will improve upon the quality of the NDVI
product. The EVI will take full advantage of MODIS’ new, state-of-the-art measurement
capabilities.While the EVI is calculated similarly to NDVI, it corrects for some
distortions in the reflected light caused by the particles in the air as well as the
groundcover below the vegetation. The EVI data product also does not become saturated as
easily as the NDVI when viewing rainforests and other areas of the Earth with large
amounts of chlorophyll.
Frequency of Coverage
Neither the NDVI nor the EVI product will eliminate all obstacles. Clouds and aerosols can
often block the satellites’view of the surface entirely, glare from the sun can
saturate certain pixels, and temporary malfunctions in the satellite instruments
themselves can distort an image. Consequently, many of the pixels in a day’s worth of
images are indecipherable, and maps made from the daily Vegetation Indices are patchy at
best.
Longer term averages of vegetation data help remove errors caused by clouds, but removes
detail, especially at high spatial resolutions. The above pair of images shows the island
of Borneo during September 1999. On the left is a 10-day average from September
21–30. Heavy cloud cover caused some areas to appear as if they had little or no
vegetation (light brown). The 30-day average for the whole month of September, however,
shows that the entire island is heavily forested.
With the imaging data the MODIS and AVHRR instruments provide,scientists should be able to
use these indices to get daily measurements of vegetation density over most of the
Earth’s surface. The maps are helpful in monitoring and understanding environmental
and climate changes such as deforestation and desertification as well as drought. The maps
also play a major role in other types of satellite measurements. For example, they are
crucial in helping scientists classify different types of vegetation over the world’s
landscapes as well as detecting changes in land surface cover overtime.
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