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NPOESS
and Climate:
Part 1: The Challenges Ahead
George Ohring
Mitch Goldberg
Dave Jones
This is the seventh in a series of articles on the
National Polar-orbiting Operational Environmental
Satellite System (NPOESS). One of the challenges for
NPOESS is to collect critical information for operational
use, especially for numerical weather prediction, while
collecting high quality observations for longer-term
monitoring and research on the Earth’s changing climate.
This special two-part article will focus first on
identifying the differences between weather and climate
and the importance of performing measurements that satisfy
both. In this piece the differences between weather and
climate, and the distinction between climate variations
and climate change are explained. The second part in next
month’s issue will focus on describing how NPOESS will
measure climate.
Introduction: The Importance of
Climate
The National Polar-orbiting Operational
Environmental Satellite System (NPOESS), once operational,
will meet the nation’s needs for environmental
measurements from satellites far into the future. Among
these needs, none is more critical than climate; for
climate strongly influences the nation’s and the
world’s economy, sustainability, as well as the safety
of life and property. Consider the year-to-year variations
of climate and their impacts, from the effects of El Niño,
droughts, and hurricanes to the long-term trends in
climate, such as increasing surface temperatures, ozone
depletion, and sea level rise. Climate is a global
phenomenon; as such, having the ability to map the Earth
globally on a daily basis is essential to observing and
understanding our climate and its dynamic nature.
Polar-orbiting satellites are the ideal platforms for
climate monitoring and study. NPOESS presents a unique
opportunity to make sustained long-term measurements and
monitor the Earth’s global climate.
Differentiating Weather,
Climate,
Climate Variation, and Climate Change
The distinction between the common terms
“weather” and “climate” is not always clear.
Weather represents the day-to-day changes of the
atmosphere, and more broadly of the ocean and land. We
experience it as wet or dry, warm or cold, windy or calm.
Climate is the long-term average of weather conditions,
where the average, or “normal,” is usually computed
from a recent 30-year period of weather observations. For
example, the July “normal” climate of New York City is
characterized by average rainfall of 4 inches, an average
high temperature of 84° F, and an average low temperature
of 68° F. In more technical terms, climate is a
statistical description of weather, including its
variability and extremes as well as averages.
Climate conditions are not steady; they vary or
fluctuate. Scientists generally talk about two major kinds
of climate variations—short-term and longer-term.
Short-term climate fluctuations are departures from
average weather conditions over a period of a month or
more; they are generally referred to as climate
variations. El Niños and droughts are examples of climate
variations.
Variations in the weather and climate over the
longer-term (decades, centuries, etc.) are generally
referred to as climate changes; the ice ages are an
example. Another distinction between weather and climate
involves the controlling processes. In general, the
weather is governed mainly by the atmosphere, its
circulation and the processes within it, such as the
formation of clouds and rain. Climate generally depends on
additional atmospheric processes including chemical
reactions that determine the concentrations of important
constituents, such as ozone and methane, and processes
that alter the radiation budget, such as the interactions
between clouds and radiation. Climate also depends on the
important interactions between the atmosphere and the
other components of the Earth’s climate system—the
oceans, the land, the cryosphere (the snow and ice cover
of the Earth), the biosphere—and the sun. The Sun’s
radiation striking the Earth is the driving force for
climate. Its diurnal (daily), latitudinal, and annual
variations cause the day to night, equator to pole, and
summer to winter climate extremes.
The oceans play an important role in climate,
acting as a reservoir for heat. Surface and subsurface
ocean currents transfer large amounts of heat around the
globe, modulating climate and climate change. Heat,
momentum, and water are constantly being exchanged between
the ocean and the atmosphere. Productivity within the
ocean, a function of the immense populations of small
drifting plants, also acts to absorb or release carbon,
thus impacting its global cycle on the planet.
The land surface, including its vegetation and
seasonal snow cover, also influences climate. The land
affects the flow of air over it, the absorption of solar
energy, and the water and carbon cycles. In addition,
explosive volcanic eruptions may release large quantities
of dust and chemicals, such as sulfur dioxide that is
converted to sulfate aerosols in the stratosphere; these
sulfate aerosols reflect solar radiation causing
significant global cooling at the Earth’s surface and
lower atmosphere for a year or more. Cryospheric snow and
ice cover, including sea-ice in the Arctic and southern
oceans and the land-based ice-sheets of Greenland and
Antarctica are excellent reflectors of sunlight. Any
process that significantly alters snow and ice-cover can
affect polar climates as well as that of the entire
planet.
While natural forces are the major determinant of
climate, the activities of humankind may be altering its
path. Burning of fossil fuels releases carbon dioxide and
smoke aerosols into the atmosphere, and these pollutants
perturb the Earth’s radiation balance, which leads to
climate change. Further research is ongoing to measure and
calculate the amount of influence humankind is having on
climate change.
The potential socio-economic effects of long-term
climate change can only be roughly estimated. However,
studies suggest that global climate change will have
profound impacts on society, as shown in Figure 4.
Measuring Climate
Both weather and climate measurements must be made
on a global scale. For climatic variables with large
diurnal variations (e.g., solar radiation, temperature,
clouds, and precipitation), it is also desirable to sample
the complete daily cycle. Because climate is an average of
weather conditions over time, observations of all the
weather elements, such as temperature, humidity, wind,
rain, etc., are needed.
Additional observations of the sun and other
components within the climate system are required.
Long-term variations in solar radiation do occur and can
affect the climate; satellites are excellent platforms for
observing such variations because they fly above the
perturbing effects of the atmosphere. The radiation budget
represents the balance between incoming energy from the
Sun and outgoing thermal (longwave) and reflected
(shortwave) energy from the Earth. Changes in the
radiative energy balance of the Earth-Atmosphere system
(caused, for example, by increasing amounts of carbon
dioxide and aerosols) can cause long-term changes in
climate. Satellites orbiting above the atmosphere are
ideal for measuring the radiative energy streams into and
out of the Earth-Atmosphere system.
Measurements of additional atmospheric variables of
critical importance to climate, including cloud
properties, ozone, aerosols, and greenhouse gases, such as
carbon dioxide are needed. Aerosols are defined as
suspensions of liquid droplets or solid particles in the
atmosphere, e.g., smoke, dust, sand, volcanic ash, sea
spray, polar stratospheric clouds, and smog. Ocean
variables that are important for climate include sea
surface temperature, sea level, salinity, and ocean color
(for information on ocean productivity and the oceanic
component of the Earth’s carbon cycle). Even very small
annual variations in sea level can have devastating
consequences for small islands and low-lying coastal
areas. Needed land observations include vegetation (for
the terrestrial component of the Earth’s carbon cycle),
soil temperature, and soil moisture. Important cryospheric
observations include snow cover and depth, sea-ice cover
and thickness, and ice-sheet cover and thickness.
Challenges in Climate Observing
from Space
As detailed in previous articles in this series, a
sun-synchronous, polar-orbiting satellite can observe each
location on the Earth every 12 hours. With the NPOESS
constellation of three orbiting platforms, each spot on
Earth will be observed approximately every four hours near
the Equator and more frequently at higher latitudes. Thus,
NPOESS will provide complete global coverage daily and
resolve the diurnal cycle for some observations.
Geostationary satellites, with their continuous
observations of the same areas of the Earth, can help to
fill in the diurnal cycle of some of the climate
variables.
There are several important distinctions between
observations needed to predict the weather and
measurements required to monitor and understand the
Earth’s climate and climate change. For one, predicting
weather requires a good set of observations at a single
time; monitoring climate fluctuations calls for a longer
time series of observations.
The anticipated signals associated with global
climate change (e.g., temperature increase or decrease of
tenths of a degree centigrade per decade; global mean sea
level rise of 2-3 centimeters per decade) represent
difficult measurement challenges. Observations must be
able to resolve very small variations and they must be
made continuously over a sufficiently long time period.
The term Climate Data Record (CDR) has been introduced to
denote a time series that has sufficient stability,
length, and continuity to define climate variations and
change. Observing long-term climate change also requires
instruments whose measurement characteristics do not
change appreciably with time.
The generation of satellite-based CDRs requires
many inter-related activities and steps. These include:
inter-calibration of identical instruments carried on
different NPOESS spacecraft as well as intercomparison
with similar instruments carried on other spacecraft
(e.g., National Aeronautics and Space Administration
[NASA] research satellites); development of
processing algorithms; detection
and elimination of systematic errors in data; generation
of stable time series; validation of data products;
reprocessing of data as improvements are made to
processing algorithms; and quality control and analysis of
data.
In addition, satellite systems require that certain
factors be minimized or accounted for in the creation of
stable CDRs. These include the biases inherent in the
observing instruments; changes in instrumentation;
satellite orbital drift; system calibration; sensor
degradation; and system malfunctions.
Measuring the required climate variables and doing
so with the accuracy and long-term stability needed to
detect climate variations and change are major challenges
for NPOESS. Part 2 of this article will detail how NPOESS
expects to meet these observational challenges.
The contents of this paper are solely the opinions
of the authors and do not constitute a statement of
policy, decision, or position on behalf of the NOAA, NASA,
or the U.S. Government.
About the Authors:
George Ohring is a consultant to the National
Oceanic and Atmospheric Administration’s (NOAA) National
Environmental Satellite, Data and Information Service (NESDIS).
He is the former Chief of the Climate Research and
Applications Division in the NESDIS Office of Research and
Applications and can be reached at [email protected].
Mitch Goldberg is the Chief of the Satellite
Meteorology and Climatology Division in the NESDIS Office
of Research and Applications. He has been instrumental in
preparing for NPOESS by insuring that the advanced
research instruments on NASA’s Earth Observing System
satellites are rapidly exploited for operational weather
and climate prediction. Mitch can be reached at
[email protected].
Dave Jones is Founder, President and CEO of
StormCenter Communications, Inc. (stormcenter.com).
He is also President of the Foundation for Earth Science
and sits on the Executive Committee of the ESIP Federation
(esipfed.org).
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