Atlantic's salt balance poses threat, study says
The Toronto Globe and Mail, Dec. 18, 2003
The delicate salt balance of the Atlantic Ocean has altered so dramatically in the last four decades through global warming that it is changing the very heat-conduction mechanism of the ocean and stands to turn Northern Europe into a frigid zone.
The conclusions are from a study in the journal Nature that is to be published today. The study describes planet-scale changes in the regulatory function of the ocean that affect precipitation, evaporation, fresh-water cycles and climate.
"This has the potential to change the circulation of the ocean significantly in our lifetime," said Ruth Curry of the Woods Hole Oceanographic Institution in Massachusetts, the study's lead author.
The study catalogues profound changes in the salinity of the tropical and sub-tropical parts of the Atlantic Ocean. That part of the ocean is becoming saltier because higher average global temperatures are evaporating more ocean water than normal.
Over time, that water vapour travels north and south to the poles, where it eventually replenishes colder oceans.
Because the overall salt content of the world's oceans is constant, this means that as the equatorial ocean becomes saltier, the oceans at the poles are becoming less salty and more fresh.
The implications of this are enormous. Until recently, scientists had no idea how critical salt levels in the oceans are to the hydrological cycle. They now understand that salinity is key to the ocean's ability to move warm water around the planet.
Under normal conditions, the saltier waters of the far north and south sink from the surface of the ocean to the deep abyss. Then they are pulled along the bottom of the ocean back to the equator. As that massive amount of water moves from north to south, warm water from the southern Gulf Stream moves north closer to the ocean's surface to fill the void.
It's akin to a gigantic water conveyor belt.
That warm water pulled up to the north also warms the air. It is the reason England and Ireland are relatively balmy in winter and not frigid like southern Alaska, which is at the same latitude, said Professor Ransom Myers, holder of the Killam Chair in Ocean Studies at Dalhousie University in Halifax and a key researcher with the Census of Marine
But as the waters of the far north become less salty, they lose their ability to sink from the surface of the ocean to the abyss. The conveyor belt is slowing down and may eventually stop.
"Northern Europe will likely experience a very significant cooling," Ms. Curry said, adding that it is expected to be in the order of 5 to 10 C cooler in winter in the course of 10 to 50 years, which is considered to be extremely rapid climate change.
The trend in Europe will run counter to that of most of the planet, which has been warming as greenhouse gases concentrate in the atmosphere.
The last time Northern Europe recorded temperatures that cold was during the so-called little ice age that lasted from 1500 to about 1800, she said.
If the ocean conveyor belt shuts down entirely, Northern Europe will become as cold as Alaska and is unlikely to be able to support as many people as it now does, Prof. Myers said. "We're changing the Earth in ways that are just inconceivably large." he said.
The problem is that this change feeds on itself and scientists don't know where it will stop, Ms. Curry explained.
When water evaporates more quickly in the tropics -- as it is now from the effect of greenhouse gases that humans have emitted into the atmosphere -- that water vapour creates a greenhouse effect that helps to speed global climate change, including warming. In turn, that increases evaporation, which increases the greenhouse effect.
Saltier Atlantic may help decipher global warming
Cape Cod scientist gauges changes in salinity, evaporation
The Boston Globe, Dec. 18, 2003
Researchers announced yesterday that the tropical Atlantic Ocean is much saltier than it was 50 years ago, a discovery that may help shed light on a poorly understood climate system that could have implications for global warming.
For years, scientists assumed that global warming would speed evaporation in parts of the world's oceans but had no direct way of measuring the change. Ocean evaporation is a fundamental part of earth's climate system, a giant hydrological cycle that sends water from tropical seas into the atmosphere to fall in high latitudes as rain or snow.
In the journal Nature, researchers led by Ruth Curry at the Woods Hole Oceanographic Institution compared the salinity of the Atlantic Ocean over several decades. If evaporation was increasing, they reasoned, then more fresh water would be removed from the ocean and salinity would increase. They found that evaporation was increasing and estimated a 10 percent rise in tropical evaporation rates during the last 15 years.
At the same time, they also found a corresponding freshening of water in far northern and southern Atlantic.
''They found a neat way of getting at it,'' said Peter Stone, professor of climate dynamics at the Massachusetts Institute of Technology. ''They found what looks like the kind of acceleration of the hydrological cycle people have been expecting.''
The report is a new piece of evidence in a global-warming puzzle that could help scientists make more accurate predictions about the earth's climate. Global temperatures are increasing, scientists say, in part because of human-related activities, such as the burning of fossil fuels that release greenhouse gases into the atmosphere. These gases trap heat and radiate it back to earth, slowly warming the planet.
Curry's article examined the salinity of the Atlantic from the tip of Greenland to the tip of South America, using data compiled since 1955. Aiding her were two researchers from the Centre for Environment, Fisheries and Aquaculture Science in England and the Bedford Institute of Oceanography in Nova Scotia.
Although global warming may be the reason ocean evaporation is increasing, Curry believes that increased evaporation may itself, in turn, be accelerating global warming. Water vapor is a potent greenhouse gas that can trap more heat in the atmosphere as evaporation rates increase.
''The system appears to be revving up,'' Curry said.
Research in the Mediterranean Sea and the Indian and Pacific oceans has yielded parallel salinity trends, but no one has been able to measure a change over time until this research, she said. ''The climate system is slow to change, but we've definitely kicked it, and we are beginning to see the fingerprint of that change.''
Scientists have long hypothesized that increasing evaporation rates could eventually disrupt a key ocean system, a ''conveyor belt'' that includes the Gulf Stream and transports warm tropical water up to the North Atlantic, where it loses heat and warms the air. That system explains why London is a temperate city, even though it is farther north than Toronto.
Driving the conveyor belt is cold, dense salty water in the North Atlantic that sinks, allowing warmer water to enter the region. Scientists have long recognized that if North Atlantic waters become too fresh, the cold water wouldn't be heavy enough to sink -- and the conveyor belt could stop or slow, causing more severe winters in New England and Europe.