RADIATIVE ENERGY FLUXES
Total Solar Irradiance
Changes in the amount of electromagnetic energy reaching the Earth from
the Sun, known as solar irradiance, may significantly affect climate change.
As this figure shows, changes in the solar activity which are indicated
by changes in sunspot number correspond to changes in the total solar irradiance.
Variations in solar irradiance of a few tenths of a percent could cause
a global surface air temperature change of 0.1 degrees C. Therefore, it
is necessary to monitor solar irradiance.
The figure shows measurements by NASA´s Nimbus 7/Earth Radiation Budget
(ERB) instrument, Solar Maximum Mission (SMM)/Active Cavity Radiometer Irradiance
Monitor (ACRIM) and the Earth Radiation Budget Satellite (ERBS) and NOAA-9
satellite/Earth Radiation Budget Experiment (ERBE) instruments.
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UV Spectral Irradiance
It is important to measure the spectral composition of ultraviolet radiation
emitted by the sun to improve our understanding of the roles played by different
wavelengths of radiation in both the formation and destruction of atmospheric
ozone and other atmospheric gases.
Solar ultraviolet (UV) light is primarily responsible for both creation
and destruction of ozone in the Earth's stratosphere. Ozone is the molecular
form of oxygen, made up of three 3 atoms, that shields the Earth's surface
from solar UV radiation by absorbing much of this radiation. The same process
also causes the temperature in the stratosphere to be higher than in the
upper troposphere.
Stratospheric ozone densities are known to vary with the 11-year solar cycle.
Solar variability over the solar cycle causes expansion and contraction
of the outward extension of the Earth's atmosphere into space. Scientists
use UV spectral observation of solar irradiance along with measurements
of atmospheric gases and winds to better model the processes occurring in
the Earth's upper atmosphere, particularly involving the creation and destruction
of ozone.
The chart provides data from the Solar Ultraviolet Spectral Irradiance Monitor
(SUSIM). Irradiance is measured in Watts per meter cubed, and wavelength,
which is the distance between corresponding points on a wave along the direction
of propagation measured in nanometers. An AU is an astronomical unit, or
the mean distance of the Earth from the Sun, which is approximately 93 million
miles.
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Earth Radiation Balance
Weather, described by temperature, humidity, winds, and precipitation, is
ultimately determined by location, topography, and the exchange of radiant
energy between the Sun, Earth, and space. Climate is defined as the average
weather over various time periods ranging from a few weeks to decades to
geological timescales. For the Earth as a whole, this energy transfer must
be constant when averaged over annual timescales. Otherwise, the mean temperature
of the Earth-atmosphere system must change to establish the equilibrium
dictated by thermodynamics.
The Earth's climate system constantly tries to maintain a balance between
the energy from the Sun that is absorbed by the Earth, and the energy that
goes from Earth back out to space. Scientists refer to this process as Earth's
"radiation budget." The components of the Earth system that are
important to the radiation budget are the planet's surface, atmosphere,
and clouds.
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Radiation Budget at the Top of the Atmosphere
The Earth's radiation budget accounts for the balance of incoming shortwave
(SW) radiation from the Sun and longwave (LW) radiation exiting from the
Earth-atmosphere system at various time and space scales.
When incoming shortwave solar radiation, known as insolation, enters the
Earth's climate system, a portion of it is absorbed at the Earth's surface,
causing the surface to heat up and a portion is reflected back to space
as outgoing shortwave radiation. Some of the absorbed energy is then radiated
outward in the form of longwave infrared radiation. Cloud layers trap some
of the radiation from the Earth's surface, and then emit longwave radiation,
both out to space and back to the surface.
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Net Radiation
Annual average net radiation determined from 1985 to 1986. Net radiation
is the difference between incoming solar radiation that is absorbed by Earth
and outgoing infrared radiation from Earth that is lost to space. The net
radiation is generally positive at low latitudes (greater heating represented
by oranges, reds, and pinks) and negative at high latitudes (greater cooling
represented by greens and blues).
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Outgoing Radiation
Both the longwave and shortwave radiation images show smaller amounts of
outgoing radiation in colors ranging from blue to green, with blue being
lowest; higher amounts are shown in colors ranging from green to pink, with
pink being highest. Energy emitted by the Earth's climate system tends to
maintain a balance with solar energy coming into the system. This balance,
known as the radiation budget, allows the Earth to maintain the moderate
temperature range essential for life as we know it.
The images represent the average amount and distribution of reflected shortwave
radiation and longwave radiation that was radiated from the Earth to space
by the Earth's climate system during July 1985, as measured by the NASA
Earth Radiation Budget Experiment (ERBE) instruments on the Earth Radiation
Budget Satellite (ERBS) and the NOAA-9 satellite.
Continuing research wil use data from the Clouds and the Earth's Radiant
Energy System (CERES) instrument scheduled for flight on the Tropical Rainfall
Measuring Mission (TRMM) and on the EOS Terra and Aqua satellites.
The color scales are measured in Watts per meter squared. Higher numbers
represent greater amounts of radiation.
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Cloud Radiative Forcing
The overall effect of all clouds together is that the Earth's surface is
cooler than it would be if the atmosphere had no clouds.
The shortwave rays from the Sun are scattered in a cloud; many of the rays
return to space. The resulting "cloud albedo forcing," taken by
itself, tends to cause a cooling of the Earth.
When a cloud absorbs longwave radiation emitted by the Earth's surface,
the cloud re-emits a portion of the energy to space and a portion back toward
the surface. The intensity of the emission from a cloud varies with its
temperature and also depends upon several other factors, such as the cloud's
thickness and the makeup of the particles that form the cloud.
This process is called "cloud greenhouse forcing" and, taken by
itself, tends to cause a heating or "positive forcing" of the
Earth's climate. Usually, the higher a cloud is in the atmosphere, the colder
is its upper surface and the greater is its cloud greenhouse forcing.
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Cirrus Cloud Warming
The high, thin cirrus clouds in the Earth's atmosphere act in a way similar
to clear air because they are highly transparent to shortwave radiation,
yet they readily absorb the outgoing longwave radiation. Like clear air,
cirrus clouds absorb the Earth's radiation and then emit longwave, infrared
radiation both out to space and back to the Earth's surface.
The overall effect of the high thin cirrus clouds is to enhance atmospheric
greenhouse warming.
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Stratocumulus Cloud Cooling
Stratocumulus clouds reflect much of the incoming shortwave radiation but
also re-emit large amounts of longwave radiation. Their cloud albedo forcing
is larger than their cloud greenhouse forcing, resulting in a net cooling
of the Earth.
The longwave radiation emitted downward from the base of a stratocumulus
cloud tends to warm the surface and the thin layer of air in between, but
the preponderant cloud albedo forcing shields the surface from enough solar
radiation that the net effect of these clouds is to cool the surface.
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Deep Convective Cloud Near Balance
Deep convective or cumulonimbus clouds emit little longwave radiation at
the top but much at the bottom. They also reflect much of the incoming shortwave
radiation. Their cloud greenhouse and albedo forcings are both large, but
nearly in balance, resulting in neither warming nor cooling.
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Aerosol Radiative Forcing
Atmospheric aerosols physically affect the heat balance of the Earth, both
DIRECTLY by reflecting and absorbing incoming solar radiation and INDIRECTLY
by influencing the properties and processes of clouds and possibly by changing
the atmospheric chemistry associated with greenhouse gases. These climatically
important processes are perturbed by aerosols from natural sources such
as volcanoes, forest fires, and dust storms, and by aerosols from such human
activities as the input of sulfur dioxide gases into the atmosphere during
fossil fuel burning.
Direct Effect
Perturbation of the Earth's radiative budget due to scattering and absorption
of solar and terrestrial radiation by aerosols is called direct aerosol
radiative forcing. In major industrialized regions, the radiative forcing
of climate by anthropogenic sulfate aerosols has been calculated to exceed
in magnitude the forcing by anthropogenic carbon dioxide, although the signs
of the two forcings are opposite.
Consequently, much attention has been given in recent years to improving
the model calculations of the direct aerosol radiative forcing, and to quantifying
the uncertainties of the estimates. At present, there is a great need for
observational data on aerosols relevant to aerosol forcing of climate.
These are images from the book titled Aerosol Forcing of Climate, edited
by R. J. Charlson and J. Heintzenberg, John Wiley & Sons, 1995.
Aerosol cooling: July mean geographic distribution of direct radiative forcing
by anthropogenic sulfate aerosols; highest cooling is shown in blue.
Greenhouse gas warming: July mean geographic distribution of radiative forcing
due to anthropogenic greenhouse gases; greatest warming shown in red and
yellow.
Net cooling/warming: July net radiative forcing due to greenhouse gases
and sulfate aerosols; warming shown in red and yellow, cooling shown in
white and blue.
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Indirect Effect
Clouds polluted by aerosols (from smoke, for example) have more numerous
and smaller drops. These clouds may have a higher albedo and reflect more
of the sun's radiative energy, thereby leading to cooling of the climate.
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Ship Tracking
Although the impact on climate remains controversial, the recent rediscovery
of such phenomena as ship tracks has stimulated research into the effects
of cloud-aerosol interaction on climate. These two satellite images show
a stratocumulus cloud system off the West Coast of the United States. The
large image was constructed from NOAA-9 AVHRR data on June 27, 1987, at
a shortwave infrared wavelength. The smaller inset (top left) was constructed
from reflected solar radiation measurements at a visible wavelength. The
MODIS instrument on the EOS AM and PM satellite platforms will provide a
substantially improved set of measurements for identifying and studying
cloud-climate processes, including the ship track phenomena.
The streaks revealed in the images are due to a reduced droplet size in
clouds contaminated by the exhaust from ships. Particles emitted by ship
increase concentration of cloud condensation nuclei (CCN) in the air. Increased
CCN, in turn, increase the concentration of cloud droplets and reduce the
average size of the droplets.
Increased concentration and smaller particles reduces production of drizzle
(100 µm radius) droplets in clouds. Liquid water content increases
because loss of drizzle particles is suppressed. Therefore, along ship tracks,
clouds are OPTICALLY both thicker and brighter, and cloud single scattering
albedo increases.
In the words of John Aitken (1880), "...when water vapor condenses
in the atmosphere, it always does so on some solid nucleus. The dust particles
in the air form the nuclei on which it condenses. If there were no dust
in the air, there would be no fog, no clouds, no mist, and probably no rain."
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Natural Greenhouse Effect
On average, the Earth is in radiative balance with outgoing infrared radiation
equaling absorbed solar radiation. Because there are greenhouse gases in
the cooler atmosphere above, the Earth's surface is warmer than it would
be without those gases.
The natural greenhouse effect causes the mean temperature of the Earth's
surface to be about 33 degrees Celsius warmer than it would be if natural
greenhouse gases were not present. This is fortunate, for the natural greenhouse
effect creates a climate in which life can thrive and humans can live under
relatively benign conditions.
You can see how the greenhouse gases absorb radiation in this simplified
diagram illustrating the greenhouse effect. Shortwave solar radiation can
pass through the clear atmosphere relatively unimpeded, but longwave infrared
radiation emitted by the warm surface of the Earth is absorbed partially
and then re-emitted by a number of trace gasesparticularly water vapor and
carbon dioxidein the cooler atmosphere above.
Some solar radiation is reflected by the Earth and the atmosphere whereas
some of the infrared radiation is absorbed and re-emitted by the greenhouse
gases. The effect of this is to warm the surface and the lower atmosphere.
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Enhanced Greenhouse Effect
The enhanced greenhouse effect refers to the possible raising of the mean
temperature of the Earth's surface above that occurring due to the natural
greenhouse effect as a result of an increase in the concentrations of greenhouse
gases due to human activities. Such a global warming would probably bring
other, sometimes deleterious, changes in climate; for example, changes in
precipitation, storm patterns, and the level of the oceans.
Global Warming
This figure shows that the global air temperature at the Earth's surface
has increased about one-half degree Celsius during the past century. These
data were gathered from about 2000 locations around the world. Points on
the blue line indicate the differences between the annual temperature averages
and the 1951-1980 30-year average. The large jumps between points on this
line show that average temperatures vary widely from year to year. Overall,
however, average air temperatures at the Earth's surface have increased.
The red line was plotted as a 5-year "running average" to smooth
out annual jumps and highlight the overall trend. The running average is
obtained by averaging the temperatures for a 5-year period.
The warming trend indicated in the graph is important because it provides
physical evidence in support of the predictions of climate models, which
indicate that an increase in atmospheric carbon dioxide would cause a rise
in global temperatures.
Further scientific investigations and satellite-based observations planned
as part of the ESE/EOS Program will advance the understanding of global
warming processes and help to provide the predictive capability needed to
assess future changes in surface air temperature and the global climate.
One very important aspect of greenhouse investigations has been the development
of models for global climate change studies as well as predictions. So far,
these models indicate a warming trend that is mostly a by-product of human
intervention. There is an important need to further develop and verify the
climate models through acquisition, assembly, and analysis of reliable climate
data in order to monitor and predict our effect on the planet. The Earth
Observing System allows the collection of highly accurate and long-term
data sets toward that end.
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