Climate change felt in deep of Antarctic Bottom Water

polyna

In 1974, just a couple years after the launch of the first Landsat satellite, scientists noticed something odd in the Weddell Sea near Antarctica. There was a large ice-free area, called a polynya, in the middle of the ice pack. The polynya, which covered an area as large as New Zealand, reappeared in the winters of 1975 and 1976 but has not been seen since.

Scientists interpreted the polynya’s disappearance as a sign that its formation was a naturally rare event. But researchers reporting in Nature Climate Change disagree, saying that the polynya’s appearance used to be far more common and that climate change is now suppressing its formation.

What’s more, the polynya’s absence could have implications for the vast conveyor belt of ocean currents that move heat around the globe.

Surface seawater around the poles tends to be relatively fresh due to precipitation and the fact that sea ice melts into it, which makes it very cold. As a result, below the surface is a layer of slightly warmer and more saline water not infiltrated by melting ice and precipitation. This higher salinity makes it denser than water at the surface.

Scientists think that the Weddell polynya can form when ocean currents push these denser subsurface waters against an underwater mountain chain known as the Maud Rise. This forces the water up to the surface, where it mixes with and warms colder surface waters. While it doesn’t warm the top layer of water enough for a person to comfortably bathe in, it’s enough to prevent ice from forming. But at a cost—the heat from the upwelling subsurface water dissipates into the atmosphere soon after it reaches the surface This loss of heat forces the now-cool but still dense water to sink some 3,000 meters to feed a huge, super-cold underwater ocean current known as Antarctic Bottom Water.

Antarctic Bottom Water spreads across the global oceans at depths of 3,000 meters and more, delivering oxygen into these deep places. It’s also one of the drivers of global thermohaline circulation, the great ocean conveyor belt that moves heat from the equator towards the poles.

But for the mixing to occur in the Weddell Sea, the top layer of ocean water must become denser than the layer below it so that the waters can sink.

To find out what has been going on in the Weddell Sea, Casimir de Lavergne of McGill University in Montreal and colleagues began by analyzing temperature and salinity measurements collected by ships and robotic floats in this region since 1956—tens of thousands of data points. The researchers could see that the surface layer of water at the site of the Weddell polynya has been getting less salty since the 1950s. Freshwater is less dense than saltwater, and it acts as a lid on the Weddell system, trapping the subsurface warm waters and preventing them from reaching the surface. That in turn, stops the mixing that produces Antarctic Bottom Water at that site.

That increase in freshwater is coming from two sources: Climate change has amplified the global water cycle, increasing both evaporation and precipitation. And Antarctic glaciers have been calving and melting at a greater rate. Both of these sources end up contributing more freshwater to the Weddell Sea than what the area experienced in the past, the researchers note…

A weakening of the mixing of water in the Weddell Sea could explain, at least in part, a shrinking in Antarctic Bottom Waters reported in 2012. “Reduced convection would reduce the rate of Antarctic Bottom Water formation,” says de Lavergne. That “could cause a weakening in the lower branch of the thermohaline circulation…”

More immediately, though, a weakening of the mixing in the Weddell Sea could be contributing to some of climate trends observed in Antarctica and the Southern Ocean. By keep warmer ocean waters trapped, the weakening may explain a slowdown in surface warming and expansion in the sea ice, the researchers note.

The weakening of the Weddell Sea mixing has also kept trapped all the heat and carbon stored in those deeper layers of ocean water. If another giant polynya were to form, which is unlikely but possible, the researchers warn, it could release a pulse of warming on the planet.

Paleoclimatologists are the most conservative of their species. They hedge the rare conclusions drawn – appropriately I might add – with language reflecting an absence of immediacy befitting their science. Dimwits of the denialist sect of the flat-earth religion perceive this as contradicting climate change. Complexity continues to confound small minds.

Caution is deserved – for the range of short-term phenomena possibly resulting from the weakening of the Weddell Sea mixing ranges from an outburst of warm moisture for several years or more or conversely drastic realignment of global thermohaline circulation, the great ocean conveyor belt that moves heat from the equator towards the poles, pushing northern Europe into the kind of deep freeze illustrated in the film “THE DAY AFTER TOMORROW”.

Either road, it wouldn’t be fun and games.

Thanks, Mike

12 thoughts on “Climate change felt in deep of Antarctic Bottom Water

  1. Joseph says:

    All very interesting……and the science seems to be moving forward on both sides along with reasonable theories…….and that’ s all we have now…….theories. I don’t understand why we have to constantly attack and ridicule the other side because their conclusions do not conform to ours. For systems as complex as weather and climate, it may take millions of data points and several more decades to get a reasonable picture of what is really going on…….
    Can we stop with the bs and try to work together…..

    • keaneo says:

      Thanks for the lesson in sophistry. Working together and meeting somewhere in the middle may get votes in school board elections. It has nothing to do with the reality. 95% of scientific research agrees on the clarity of climate change, the problems confronting the world. Another tiny percentage of non-scientists reject research, deny conclusions and the methods used to acquire those conclusions.

      There is no reason to “work together” with ignorant people who deny climate change anymore than we should work together to make religious parents feel good about denying their children vaccination or support a small number of businesses in their self-righteous quest to deny service to members of the public they hate because of gender identity. There is no reason to “work together” with folks who think we don’t need speed limits or seat belts. Cripes, even insurance companies know better.

    • morey says:

      Do you think climate change just popped up in time for the 2012 election? There are scientists who have been dedicated to research on climate change for decades. The reason for the growth in numbers agreeing with their conclusions is that – as symptoms increased, more research was dedicated to the questions, and overwhelmingly corroborative conclusions were reached.

      Ain’t nothing new about research in small numbers expanding into overwhelming conclusions. It worked that way with Edison, Jenner, Galileo. Almost all good science started with individual small pursuits.

    • Grace says:

      Joseph, do you ever read any science-based sources? Or just people who have opinions about science? I find the link to RealClimate over in this site’s “blogroll” to be a terrific place to go to on anything to do with legitimate, scholarly research. Articles written by fulltime researchers. Reports representative of broad segments of study – not opinion.

  2. eideard says:

    Unfortunately, hypocrisy is still the rule for climate change denial. I blogged about a study funded by denialists, led by one of the few scientists who was confident computational analysis would prove climate change wasn’t happening.

    By the time he completed his analysis at Berkeley, he had to admit he was wrong. He published his study – and you might expect reasonable people to follow his lead and stop trying to impede political and social action to correct the problem.

    Nada, nuttin’ honey. Charles Koch – one of the primo contributors to the study – still spends just as much or more trying to halt any activity that would aid the repair of our climate.

  3. Cassandra says:

    “Hell hath no fury like a vested interest disguised as a moral principle” and if cornered the Brothers Koch and their ilk persist in maintaining that the brutal logic of the market is that creating value for one’s shareholders trumps all other consequences – even as we rapidly approach a tipping point in which the global ecosystem and resource supply, on which our economies (and species) depend, will face a catastrophic, system wide failure. Of course I could be wrong, perhaps all those canaries that are piling up died of old age.

      • Quinn says:

        “In 2005, British researchers noticed that the net flow of the northern Gulf Stream had decreased by about 30% since 1957. Coincidentally, scientists at Woods Hole had been measuring the freshening of the North Atlantic as Earth becomes warmer. Their findings suggested that precipitation increases in the high northern latitudes, and polar ice melts as a consequence. By flooding the northern seas with lots of extra fresh water, global warming could, in theory, divert the Gulf Stream waters that usually flow northward past England and Norway, and cause them to instead circulate toward the equator. If this were to happen, Europe’s climate would be seriously impacted.” http://en.wikipedia.org/wiki/Thermohaline_circulation

  4. Chump says:

    “Institutionalizing delay: foundation funding and the creation of U.S. climate change counter-movement organizations” by Robert J. Brulle (http://link.springer.com/article/10.1007%2Fs10584-013-1018-7)

    Brulle’s work, which is focused on the United States, shows how a network of 91 think tanks and industry groups are primarily responsible for conservative opposition to climate policy. Almost 80 percent of these groups are registered as charitable organizations for tax purposes and collectively received more than seven billion dollars between 2003 and 2010.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.