Tuesday, March 24, 2015

Is Cold Spot In North Atlantic Dangerous? And Is It Climate Change?

Graphic indicates how much almost all of
the world has warmed up since 1901. But note
that spot in the North Atlantic that
has actually cooled in that time frame.  
Climate and weather watchers have been focused this winter on the patch of cold weather in the eastern United States in Canada in a world that has otherwise been really, really warm over the past few months.  

But there's another patch of chilliness that keeps appearing over the North Atlantic between Greenland and Ireland that might ultimately be more important.

It could mean that the Gulf Stream, that warm, huge current of air that flows north and northeast through the Atlantic toward Europe might be slowing down.

This has been predicted since at least the 1990s in computer models that depict the future of global warming.

It was also the plot device in the movie "The Day After Tomorrow," in which global warming shuts down the Gulf Stream, setting off a meteorological chain reaction that ultimately flooded, then sent New York City and many other parts of the world into a deep, deep fatal ice age.

Before you start panicking, we're not about to plunge into a scary ice age. The very cold and snowy winter and early spring in New England and southeastern Canada this year appears to have nothing to do with the developing cold pool in the North Atlantic.

Still, it's cause for some worry.

Eventually, if this slowing trend in the ocean currents in the North Atlantic continues, it could muck up fisheries, accelerate sea level rise along the United States and Canada east coasts, and really screw up the weather in northwestern Europe, says the online publication Vox. 

Scientists refer to the whole pattern of warm water flowing north, and colder water flowing south in the eastern Atlantic as the Atlantic overturning circulation, or if you like jumbles of letters, the AMOC.  And this circulation seems to be getting less vigorous.

Says Vox in a discussion about newly released research on the matter:

"'Nature Climate Change' argues the Atlantic overturning circulation already appears to be weakening. The researchers, led by Stefan Rahmstorf of the Potsdam Institute for Climate Research created an index of regional climate conditions going back centuries.

They find that the weakening of the AMOC appears unprecedented in the past 1,100 years, possibly due to an influx of freshwater from Greenland's melting ice caps."

As the planet warms, more and more ice melts from Greenland. That meltwater isn't salty, like the ocean. That fresh water coming into the Atlantic from Greenland reduces the density of the water in the North Atlantic, and that in turn would weaken the circulation patterns.

Like I said, climate scientist have thought this was a possibility for quite a few years now. What's surprising about the study is that the circulation pattern appears to be slowing down faster than anyboy expected.

"The slowdown we see in the data is not what you see in the climate models," Rahmstorf said.

Of course, it's always been hard to directly observe the ocean current in the Atlantic, and even harder to reconstruct how they behaved in the past.

There's always been a lot of variability to it. Sometimes it's stronger, sometimes it's weaker. The new data shows this variability, but also an overall downward trend amid the little peaks and lulls in the pace of the water's movement.

Obviously, more study is going to have to go on to figure out exactly what's happening out there.

Plus, scientists have even less of a clue about what a slowing Atlantic circulation would have on climate and the ecosystem.

There are glimmers of knowledge. Scientists do know that fisheries might collapse or change threatening the livelihood of some coastal communities in North America and Europe.

Worse, as I mentioned, slowing currents could mess with sea levels on the U.S. East Coast. Sea levels are slowly rising worldwide, but the water's going up faster in some areas than others. That has to do with ocean currents, whether the land itself is rising and sinking due to geological factors, or other issues going on.

Since the circulation as it has been working pulls warm water north, cooler water comes in to the immediate coastal areas of the East Coast to replace the warm water that flowed away. Cool water is more dense than warm.

If the circulation slows more warm water could linger near the East Coast. Since warm water is less dense, it expands a bit, which would contribute to a faster pace of sea level rise over the decades in places like New York and Boston.

Then there are uncertainties as to what effect the slowdown in circulation would have on climate. The slowdown would probably affect something called the North Atlantic Oscillation.

When the oscillation is in its positive phase, an area of low pressure that's normally near Iceland gets stronger. When that low is strong it tends to pull winds eastward, faster in eastern North America during the winter. That prevents Arctic air from plunging south very often, so you get milder winters there.

If the oscillation is negative the Iceland low is weak, there's not as much wind moving west to east, so Arctic air masses can plunge south into places like southeastern Canada and the eastern half of the United States.

The big question is, will the slowdown in the Atlantic circulations weaken or strengthen the Icelandic low in the winter? As you can see, that would have a big effect on what kind of weather we get during future winters.

Though the Atlantic Ocean circulation is expected to continue weakening, at least in general, as the climate warms, it is highly unlikely to completely shut down this century.

Which is a good thing.

The makers of that movie, "The Day After Tomorrow," got their idea from the last time the Atlantic circulation shut down, about 13,000 years ago.

The last glacial period was ending, and scientists think a big amount of freshwater melted off Greenland. Plus, something holding back a big freshwater lake that was expanding from meltwater let go, causing an immense surge of water into the North Atlantic.

All this shut down the current (remember the fresh water mucks it up), says NOAA.

That caused the northern hemisphere to plunge back into an ice age for a little while.  (In geological terms, "little while" often means thousands of years.)

The flow of freshwater abated, and then the northern hemisphere emerged from the last glacial period, and warmed up to the point we're at today.

"The Day After Tomorrow" exaggerated the speed and strength of all this, but you get the idea.

It was below zero in Vermont this morning. As I noted, this early spring cold has pretty much nothing to do with the North Atlantic and we're certainly not plummeting into another, temporary ice age.

Yet, nobody wants to inflict on distant, future generations springtimes that makes this frigid early season feel like a balmy walk along the beach in Barbados.

The North Atlantic research also demonstrates how in the long term, climate change can really screw things up in all parts of the Globe. And we don't know exactly how, or what extent this might happen, if it happens at all.

The science that climate change is certainly settled. But the eventual consequences of this change is very, very far from certain.

There will be lots and lots of work from scientists trying to sort this out in the coming years.

No comments:

Post a Comment