Satellite observations and measurements*
Earth-orbiting satellites and other technological advances have also enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.
Since 1978, with the launch of Nimbus 7 from Vandenburg Air Force base in California, a plethora of satellites has also assisted in monitoring the changing climate of our planet and its atmosphere. In particular focus are:
- the Earth’s radiation budget (the balance between incoming solar radiation and outgoing longwave radiation),
- changes at the surface (sea/land ice; changes to forests and other ecosystems,
- land use changes , sea level rise);
- changes in greenhouse and other gas concentrations, and
- changes in aerosol loadings and properties. Aerosols[1] scatter light but so do molecules and the land surface and separating thee components can be very challenging.
Satellites can measure both scattered/reflected sunlight and thermal emission both from the atmosphere and the land/ocean surface. They are generally placed in sun-synchronous orbits which precess[2] to provide near global coverage, so that any environmental changes can be reliably detected. The object is to achieve a balance between high resolution and surface coverage.
Today we have a whole range of satellites which measure radiation in various wavelengths to determine the attenuation caused by various gases and particles. (Here, aspects of the narrative here are highlighted to reflect what is being looked for). One of them, SAGE[3] has studied aerosols and gases in the atmosphere very successfully for some 40 years. The most recent version (launched 19 February 2017) will be mounted to the International Space Station where it will use the unique vantage point of the ISS to make long-term measurements of ozone, aerosols, water vapour, and other gases in Earth’s atmosphere.[4]
In fact, NASA now has a suite of spacecraft in sun-synchronous orbits around the Earth making outstanding contributions to our knowledge of the atmosphere, oceans and land surface. The flagship is Terra (EOS AM-1)[5], which lies in a near polar 705 km sun-synchronous orbit with a 16 day repeat cycle. It began collecting data on February 24, 2000, and carries five remote sensors designed to monitor the state of Earth's environment and ongoing changes in its climate system.
One of these, known as MODIS[6], has 36 spectral channels measuring both reflected sunlight and emitted terrestrial radiation, and sees every point on our world every 1-2 days in 36 discrete spectral bands[7]. It is designed to provide information on “the climatology and dynamics of atmospheric properties, It provides data on land/cloud/aerosol properties; clouds being one of the key areas of focus, ocean colour and productivity, chemistry, temperature, water vapour, “the impact of human activity on the regional and global environmental, and the interaction and subsequent impact of man on terrestrial and marine biota”.
In June and early July 2019, a heat wave in Alaska broke temperature records, as seen in this July 8 air temperature map (left). The corresponding image from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on Aqua on the right shows smoke from lightening-triggered wildfires.
Another instrument, MISR[8], measures reflected sunlight in various bands, and has 9 cameras, which view the Earth with cameras pointed at nine different angles. One camera points downwards, and the others provide forward and aft view angles, at the Earth’s surface, of 26.1°, 45.6°, 60.0°, and 70.5°. As the instrument flies overhead, each region of the Earth’s surface is successively imaged by all nine cameras in each of four wavelengths (blue, green, red, and near-infrared). “Most satellite instruments look only straight down, or toward the edge of the planet. To fully understand Earth’s climate, and to determine how it may be changing, we need to know the amount of sunlight that is scattered in different directions under natural conditions. MISR is a new type of instrument designed to address this need”[9].
It can distinguish different types of clouds, aerosol particles, and surfaces and will monitor the monthly, seasonal, and long-term trends in:
- the amount and type of atmospheric aerosol particles, including those formed by natural sources and by human activities;
- the amount, types, and heights of clouds; and
- the distribution of land surface cover, including vegetation canopy structure”.[10]
CERES[11] measures the Earth’s total radiation budget and provides cloud property estimates that enable scientists to assess clouds’ roles in radiative fluxes from the surface to the top of the atmosphere. One CERES instrument operates in a cross-track scan mode and the other in a biaxial scan mode[12].
MOPITT[13] is a Canadian instrument designed to measure methane and carbon monoxide in order to determine their sources and sinks, with particular emphasis on the distribution, transport, sources, and sinks of carbon monoxide in the troposphere. Carbon monoxide, which is expelled from factories, cars, and forest fires, hinders the atmosphere’s natural ability to rid itself of harmful pollutants. MOPITT has a sensor which measures emitted and reflected radiance from the Earth in three spectral bands. It ‘sees’ the Earth in swaths 640 km wide, and can measure the concentrations of carbon monoxide in 5-km layers down a vertical column of atmosphere, to help scientists track the gas back to its sources.[14]
ASTER[15] is a Japanese instrument which provides high resolution images of the land surface, water, ice and clouds in 14 different wavelengths of the electromagnetic spectrum, ranging from visible to thermal infrared light[16].
* Foundation source for this material: Associate Professor Michael Box's WEA course: "Our atmospheric environment", 2.5.
[1] Recall that aerosols (also known as particulate matter) encompass particles, liquid droplets and mixtures in the atmosphere, which may be natural or anthropogenic. Examples of primary or natural aerosols include desert dust, sea salt fog, and geyser steam. Examples of artificial aerosols are haze, dust, particulate air pollutants and smoke
[2] A Sun-synchronous orbit is a geocentric orbit that combines altitude and inclination in such a way that the satellite passes over any given point of the planet's surface at the same local solar time. Source: https://en.wikipedia.org/wiki/Sun-synchronous_orbit
[3] The Stratospheric Aerosol and Gas Experiment, now in its third stage.
[4] https://en.wikipedia.org/wiki/SAGE_III_on_ISS
[5] https://terra.nasa.gov/; https://terra.nasa.gov/about/terra-instruments
[6] Moderate-resolution Imaging Spectroradiometer at https://terra.nasa.gov/about/terra-instruments/modis
[7] Ibid.
[8] Multi-angle Imaging SpectroRadiometer
[9] Citation from Associate Professor Michael Box, WEA course “Our Atmospheric Environment”.
[10] https://terra.nasa.gov/about/terra-instruments/misr
[11] Clouds and the Earth's Radiant Energy System
[12] https://terra.nasa.gov/about/terra-instruments/ceres
[13] (Measurement of Pollution in the Troposphere)
[14] https://terra.nasa.gov/about/terra-instruments/mopitt
[15] The Advanced Spaceborne Thermal Emission and Reflection Radiometer
[16] https://terra.nasa.gov/about/terra-instruments/aster
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