Sea Salt Holds Clues To Climate Change
New research coming out of the United Kingdom (U.K.) suggests that the
amount of salt in seawater is varying in direct response to man-made climate
change. Working with colleagues to sift through data collected over the past
50 years, Peter Stott, head of climate monitoring and attribution at the Met
Office in Exeter, England, studied whether or not human-induced climate
change could be responsible for rises in salinity that have been recorded in
the subtropical regions of the Atlantic Ocean, areas at latitudes
immediately north and south of Earth’s tropics.
By comparing the data to climate models that correct for naturally occurring
salinity variations in the ocean, Stott has found that man-made global
warming -- over and above any possible natural sources of global warming,
such as carbon dioxide given off by volcanoes or increases in the heat
output of the sun -- may be responsible for making parts of the North
Atlantic Ocean more salty.
Salinity levels are important for two reasons. First, along with
temperature, they directly affect seawater density (salty water is denser
than freshwater) and therefore the circulation of ocean currents from the
tropics to the poles. These currents control how heat is carried within the
oceans and ultimately regulate the world’s climate. Second, sea surface
salinity is intimately linked to Earth’s overall water cycle and to how much
freshwater leaves and enters the oceans through evaporation and
precipitation. Measuring salinity is one way to probe the water cycle in
greater detail.
In the last half-century or so, the subtropical Atlantic has been getting
gradually saltier -- a less than 1 percent increase in real terms, but an
effect that is nevertheless significant. “It might sound like quite a small
change,” says Stott, “but the overall salinity of our oceans is naturally
relatively steady, so it’s actually a lot of freshwater being factored out
of the ocean.”
Stott’s analysis suggests that global warming is changing precipitation
patterns over our planet. Higher temperatures increase evaporation in
subtropical zones; the moisture is then carried by the atmosphere towards
higher latitudes (towards the poles), and by trade winds across Central
America to the Pacific, where it provides more precipitation. This process
concentrates the salt in the water left behind in the North Atlantic,
causing salinity to increase.
Water bearer
These are just the sort of effects that Gary Lagerloef and Amit Sen hope to
uncover over the next few years. Lagerloef and Sen are, respectively,
principal investigator and project manager of Aquarius, part of a brand new
satellite mission due to be launched into orbit in May 2010. Aquarius is the
first NASA instrument designed to track sea salinity from space and will be
the primary payload on the SAC-D spacecraft, which has been built by the
Argentinian Space Agency or Comision Nacional de Actividades Espaciales (CONAE).
The three-year mission is named after the “cup-bearer to the gods” in Greek
mythology.
Sea saltiness has been measured for centuries. Most of the data we have
today consist of direct measurements taken at sea (traditionally by ships
and, nowadays, more often by automated buoys and profiling floats). But
there are vast areas of the ocean surface -- a quarter in total -- where
salinity has never been measured. By covering the entire globe once every
seven days, Aquarius will fill in the blanks and provide an unprecedented
global picture of salinity.
Scientists measure salt levels using a practical salinity scale. One
practical salinity unit or psu almost exactly represents the number of grams
of salt in a kilogram of seawater. Salinities in the open ocean, free of ice
or land mass, generally lie between 32 and 37 psu (the Pacific and Atlantic
Oceans have maximum surface salinities around 35 and 37 respectively). “With
our instruments we will be able to measure salinity to an accuracy of 0.2
psu,” explains Sen, who works at NASA’s Jet Propulsion Laboratory (JPL) in
Pasadena, Calif. “If you take half a gallon of water and put a pinch of salt
in it, that’s about 0.2 psu. We will be able to detect that from space,
while flying about 650 kilometers [about 404 miles] above Earth.”
This is no mean feat and is possible because of some impressive radiometer
technology that will fly on board the spacecraft. A radiometer is
essentially a sensitive radio receiver, which, in this instance, detects
microwave radiation given off by the sea surface. The radiated power of the
microwaves that are emitted enables scientists to calculate the saltiness of
the water at the surface.
What’s special about the three radiometers designed for Aquarius is their
calibration stability -- over a seven-day period, their temperature cannot
stray more than 0.1 kelvin (0.18 degrees Fahrenheit). This calls for very
precise thermal control and is the reason Aquarius will be able to measure
salinity with unprecedented precision.
Boom boom
“We measure salinity in the top one to three centimeters of water because
that is the crucial layer that connects the atmosphere and the oceans,”
explains Simon Collins, instrument manager for Aquarius who is also based at
JPL. “As such, one of the largest errors in our measurement comes from
ripples in the surface of the sea.” To correct for this, Aquarius also
carries with it a scatterometer -- a state-of-the-art radar instrument that
senses roughness in the sea surface by booming microwave pulses down to the
ocean and detecting the scattered pulses bounced back to the satellite.
While Aquarius has not yet set off, it has been a long journey for the
project’s scientists and engineers, who are now ready to ship their
instrument from JPL to Argentina. There it will be installed on the SAC-D
spacecraft, before being transported to Brazil for functional and
environmental testing and returned to the United States in April 2010, ready
for its trip to space.
“People don’t realize that there is so much water and so little land,” Sen
remarks. Aquarius, flying high above us, will shed light on El Niño and La
Niña, phases of the world’s most powerful climate phenomena, reveal insights
into how monsoons develop and, most importantly of all, how a pinch of salt
can change our lives.
Journal references:
- R. Curry, B. Dickson & I. Yashayaev. A change in the
freshwater balance of the Atlantic Ocean over the past four decades.
Nature, 2003; 426 (6968): 826 DOI:
10.1038/nature02206
- P. A. Stott, R. T. Sutton & D. M. Smith. Detection and
attribution of Atlantic salinity changes. Geophysical
Research Letters, 2008; 35 (21): L21702 DOI:
10.1029/2008GL035874
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