Tag Archives: AMOC

AMOC Amok: Global Warming Bad News

January 6, 2017UncategorizedAMOC, Climate Change, Climate Change, Global Warming, Global Warming, North Atlantic, Ocean CurrentsGreg Laden

You already know abut the North American Conveyor current. Briefly: The major ocean currents happen because the equatorial ocean is warmer, and since water (unlike land) can move (though not as fast as air) the dissipation of this heat across the surface of the Earth results in warm water moving, at the surface, north or south away from the Equator, where it loses its heat and finds it way back to the equatorial regions, usually as deeper, cooler water.

Conveniently, this process also involves increasing the salinity of the water far from the equator, as evaporating water becomes saltier. This saltier water is therefore both cold and dense, so it sinks, drawing the warm surface water into the evaporation regions. Something like this is happening at a small scale around all the oceans, but the density driven conveyor is the biggest driver of ocean currents, most significant with respect to weather, and most famous, in the North Atlantic.

With global warming, the fresh water budget and distribution in the northern latitudes, in the Atlantic, changes, with more fresh water coming out of the Arctic and off of Greenland. This freshens up the hypersaline engine of the Atlantic Conveyor, also known as the Atlantic Meridional Overturning Circulation (AMOC). When that engine slows or shuts down, the currents in the entire North Atlantic, and beyond, change.

Here is the number one reason this is important (though number two may be more important, I’ll get to that in a moment). You know how England is warm and Maine is cold, though they are both really far north? London, Saskatoon, and Adak are all at about the same latitude. Paris, Quebec City, and Thunder Bay. Northern Europe is warmish, and habitable, even in Scandinavia, because of the heat that the AMOC transfers from ocean to land.

If the AMOC shuts down or moves really far south, Scandinavia, which is at the same latitude as Hudson Bay, will act more like Central Canada, which it does not do today.

Visiting London from Minneapolis in the Winter now means going to a warmer (if dreary and foggy) place. Without the AMOC, it will be more like going to central Canada.

The above strip maps make it look like there is an equivalence across different longitudes at a given latitude. This is not true. The ocean, even without the AMOC, will still warm Western Europe. But now, there is a gradient of warmth from eastern North America over to Europe, where a mostly non freezing winter shifts north to a degree that is nothing short of spectacular. Without the AMOC, that shift will be modest. And, interior areas in Eurasia, such as Moscow, will also cool down (though relatively not as much).

Newly published research tells us something new and troubling about AMOC deterioration. Current climate models suggest that this may happen, but it is unclear to what degree and when. Physical evidence shows the actual real life weakening of AMOC in recent years. So, reality seems to be outpacing the models. Some have suggested that this means that AMOC varies a lot, and will likely swing partly out and back in. Others are not so sure.

The recent research identifies a bias in the generally used climate models that causes AMOC to be more stable and long lasting, under global warming, than it might in real life. When the model is run with and without the bias corrected, you get very different results (see graph above).

This is a preliminary finding. The model has not been run enough times, and a few other things that are usually done have not yet been done. But the results are interesting enough that it is getting some serious attention.

One of the world’s experts on this topic, Stefan Rahmstorf, has written this up on RealClimate: The underestimated danger of a breakdown of the Gulf Stream System. The original research is here, but you may need a subscription.

I should mention that the collapse of the AMOC that happens when this model is run occurs in the somewhat distant future. That makes it worse, of course, because even more people will be living in, and depending on, the affected region than today. But it also allows us to ignore the problem because, hey, who cares about what happens to our children anyway, right?

Oh, and on that other thing that could happen if AMOC shuts down. This is speculative, but we do know that in the past large areas of ancient versions of the Atlantic Ocean and other seas have essentially died, become anoxic over large areas, so they become sources of dead matter rather than edible fish and stuff. This is how many of the major oil supplies we exploit today formed. I would imagine that shutting off the relatively restricted North Atlantic basin from much of the global circulation would be a first step in killing the ocean. So, there goes that food supply, and possibly, that source of oxygen. You know, for eating and breathing and stuff.

How Did Climate Change Cause The Great More’Easter of 2016?

January 25, 2016UncategorizedAMOC, Blizzard of 2016, Climate Change, Climate Change, Global Warming, Global Warming, Severe weather, what caused the blizzard?Greg Laden

Storms like last weekend’s blizzard and widespread snowfall can happen, in theory, any winter, but large snowfall storms in the US Northeast have been significantly more common in recent years than in previous recorded history. Over the last few years we’ve seen these large snowfalls happen farther south than usual, as was the case with the 2016 Blizzard. Climate scientists are pretty sure that this blizzard was either outright caused or significantly enhanced (you really can’t tell the difference) by human caused global warming. How can a blizzard, a big cold thing, be caused by warming? Because climate is not a simple thing.

Just trust me, this was an effect of global warming. Or, if you like, read on, and I’ll give you the gory details.

There are two factors that needed to come together to make a storm into a large southern-offset blizzardy mess like this one. First, there needed to be cold air tracking farther south than usual, and this happened as a result of trade wind and jet stream meanderings which have become more common with climate change, and made more likely this year, probably, because of El Niño. Second, there needed to be more moisture in the air coming off the Atlantic Ocean. This happened last weekend, and during other recent storms over the last few years, because the Atlantic is much much warmer than it usually is in the immediate region of the coast. Warmer water provides more moisture to the atmosphere via evaporation, and that relationship is not linear. More sea surface warmth equals more more moisture.

The Atlantic hasn’t been just a bit warmer. This region of the Atlantic has been anomalously very warm for several years and has been getting more warmer annually.

There are two reasons for this extra warmth. One is pretty straight forward. Sea surface temperatures globally are warmer because of human caused greenhouse gas warming of the surface of the planet. This has been enhanced over recent months because of El Niño, but it is a larger and longer term phenomenon with El Niño warming riding on top of that overall increase. Any randomly chosen patch of the world’s ocean is likely to be warmer today than it was ten or twenty years ago.

The second reason is a little more complex. Weather (and it’s big brother, climate) happen because of the uneven distribution of the Sun’s energy on the surface of the earth. Extra heat accumulates near the equator (which is pointing, relatively, more directly at the Sun), and this heat is redistributed through the movement of air and sea currents towards the poles. However, since the oceans and continents are not evenly or symmetrically distributed, or otherwise laid out to make this redistribution of heat efficient, this gets pretty complex. For example, the Pacific is huge while the Atlantic is narrower and restricted as one goes north. Notice also that the Indian Ocean is not connected directly to northern regions, only to the south, so extra heat builds up there and has to make its way towards both poles via long and convoluted currents.

One result of this complexity is what we call the Atlantic Meridional Overturning Circulation (AMOC). This is sometimes referred to as the Atlantic Conveyer and people will sometimes use the term “Gulf Stream” to refer to part of that, but really, it is all more complex than that and not so easily labeled.

Warm water that started near the Equator (including both in the Atlantic and the Indian Ocean, via South Africa) moves north in the Atlantic, on the surface. Up in the North Atlantic, this warm water becomes relatively even warmer (since the air is cooler in the north) and passes as well into areas where the air may be relatively dry. This causes heat to leave the water carried by the current, and evaporation to take place. Evaporation not only cools the water, but makes it extra salty. Saltier water is denser, so the cooling, hyper-saline waters at the northern reaches of the currents sink to the bottom of the ocean, pulling even more of the north-flowing surface current with it. This is like the electric motor that turns a conveyor belt. The lower part of the “belt” is the saltier, colder water now flowing back south, in the opposite direction, towards equatorial regions where it can later re-emerge and warm up again.

That is the simple version. If you just put water in a big place it will rotate because energy supplied by winds (or other currents) will be deflected by the Earth’s rotation, so you get, in the simple case, a counter-clockwise rotation (in the Norther Hemisphere). To the side of such a rotating masses of water, one tends to get counter-gyres (running clockwise). Trade winds push surface waters along, contributing to currents. Between the movement of the currents themselves, differential heat across the sea surface and at some depth, and air the currents, the surface of the ocean tends to not be very flat, though it looks rather flat from any given normal human vantage point. At present, the North Atlantic is mounded up in such as way that the sea surface is lower along the North American east coast than it would be were none of these things were happening.

All this results in a big blob shaped area in the North Atlantic where the surface waters are relatively cold, into which warmer currents mostly from the south (including the Gulf Stream) flow, cooling, sinking, being part of the conveyor.

What happens if you turn this conveyor off? For one thing, heat that is normally contributed to the atmosphere at northern latitudes as part of the process is no longer available to the various trade winds that pass over them. So, downwind regions (i.e., northern Europe) may experience cooling. Under certain conditions, this could cause a shift in climate in the direction of an Ice Age. We are currently experiencing such warming planet wide that this is not a possibility, though there is a famous movie in which this (rather unrealistically) happens.

Another effect can be a change in the mounding of water around the North Atlantic, with an effective regional sea level rise (measurable in inches, probably) along the Northern Hemisphere east coast.

Another effect is, of course, that the hot water moving north into the North Atlantic where it might otherwise cool gets stuck, almost like it is backed up, and becomes warmer and warmer.

All of these effects can happen with a mere slowdown in the AMOC, not only if it stops completely, and we seem to have seen these effects.

Stefan Rahmstorf, a scientist who studies these things, has an excellent writeup about a slowing AMOC and its effects, here at RealClimate.

The graphic at the top of this post is from his post. This shows sea surface temperatures across the world’s ocean as relative change caused by doubling the planet’s normal CO2 level. This is a model indicating that in the North Atlantic, there would be cooling in the far north, and extreme heating along the Northern Hemisphere’s east coast. So that is what the physics says is likely to happen in a warming world.

Here is a portion of the Climate Reanalyzer daily summary showing today’s actual sea surface temperature anomalies (how far above or below a long term average the actual sea surface temperature is measured to be).

Find the purple spots in the North Atlantic. That is the head of the AMOC, more or less, and here we have record low relative sea surface temperatures. Along the east coast are several blobs of red, showing near record or record high sea surface temperatures. There are stripes and blobs of very warm water all along the coast, made relatively warmer first by the simple fact that the sea surface is warmed by global warming, then made even more extra warm because of the recent slowing down of the AMOC. (Click through to see the whole globe, the scale, and to play with the data.)

Why is the AMOC slowing down?

First, note, that this is not a short term oddity of weather. Rahmstorf asserts that this is a long term condition.

(1) The warm sea surface temperatures are not just some short-term anomaly but are part of a long-term observed warming trend, in which ocean temperatures off the US east coast are warming faster than global average temperatures.

(2) Climate models show a “cold blob” in the subpolar Atlantic as well as enhanced warming off the US east coast as a characteristic response pattern to a slowdown of the AMOC.

Stefan and other scientists have effectively argued that this slowdown is caused in large part by the addition of fresh water from melting glaciers in Greenland. The fresh water interferes with the process by which waters at the head of the AMOC becoming hyper-saline, and thus slows down the conveyor belt. There are probably also increases in freshwater flow from major rivers into the North Atlantic, also resulting from climate change, that contribute to this.

Let me clarify something here in case there is some confusion. The cooling of the regions of the North Atlantic having to do with AMOC did not provide wintery conditions to cause this blizzard. That is something happening much father away. We may be seeing cooling effects in part of Europe because of this (I’m not discussing that here) but the Blizzard of 2016 (which we hopefully don’t bother to call “2016A” assuming there will not be another) was not hyped up because of that cooling, but rather, from the backed up surface warmth much nearer New England and the rest of the US East coast.

The slowing down of the AMOC has been going on for decades, and seems likely to continue. It is not that clear what would happen if the AMOC simply shut down, or even if it could. Will the action simply move to a new latitude, or will some sort of conveyor system continue but with a very different configuration? Will additional slowdown of the AMOC cause important sea level rise in the US East? One thing that seems very likely is this. With increased surface warmth, and no reasonable expectation that warming will slow or reverse in the near future, Greenland will continue to contribute abundant fresh water to the region, and quite possibly, increased rainfall in major river basins will add even more freshening. The AMOC is not likely to stop slowing down, or to regain its strength.

The slowing and other changes in the AMOC may be a qualitative and long term outcome of anthropogenic global warming. It seems likely that enhanced sea surface warmth off the US East Coast will be with us for the long term. A blizzard like the one we had over the weekend is much more manageable in regions that normally have frequent heavy snow storms, like Massachusetts and Upstate New York. If they happen now and then father to the south, that is a bit of a disaster, but if it is only now and then, it is not likely that we could or would do much about it.

But if annual or nearly annual middle-Atlantic blizzards are now part of the “new normal” of our disrupted climate, then infrastructural changes may be required. Roads and parking lots, and even sidewalks, are constructed with the prospect of frequent snowfalls in mind in northern states. Maybe that is what we should be doing in the formerly less snowy regions along the Atlantic. Snow plows … lots of them … will be needed. Complex and annoying (and costly) parking rules to make room for snow clearing are common in snowy states. Should “snow emergency” procedures and parking rules be set up for the mid-Atlantic?

People will have to learn, either the easy way or the hard way, that during a blizzard warning, one does not simply venture out onto the highways. Minnesotans and northern New Englanders and everyone in between keep blizzard kits in their cars. These are life saving items for when you do get stuck for 30 hours on a highway in the middle of nowhere. People who commute to Washington DC may consider this inexpensive investment. And so on.

Finally, will there be another Snopocalypse this winter, somewhere in the US? I think not. With El Nino, things are warming up, and even in the usually blizzardly places, like New England or around the Great Lakes, I suspect we’ll have more slush and rain than deep snow. But you never know. On the other hand, global warming and El Niño enhanced storminess and raininess could cause more flooding, both inland and in coastal regions. But climate science denying Senator Jim Inhofe may have to wait until next winter to get a new snowball.

New Research Shows Exceptional Slowdown In Major Atlantic Ocean Currents (UPDATED)

March 23, 2015UncategorizedAMOC, Climate Change, Climate Change, Global Warming, Global Warming, Gulf StreamGreg Laden

Climate scientists have noticed a disturbing pattern in the North Atlantic. This is the relative cooling of surface waters in the area fed by the Gulf Stream. This pattern has emerged over recent decades, and may portend very rapid and potentially disruptive climate change in the upcoming decades. The cooling is not subtle at all, and looks like this:

Map based on NASA GISS data of warming 1901-2013

So what does this mean? A paper out just today describes, explains, and discusses this odd anomaly and its potential consequences. First, a bit of context.

The Earth’s climate follows certain patterns. Most obviously it is warmer at the equator, colder at the poles. Less obvious if you’ve not looked into this is the presence of a very wet band around the middle of the earth, flanked to the north and south by irregular dry bands (that’s where most of the deserts are), with these flanked by the temperate zone, where you have more moisture and highly seasonal temperatures, and so on.

This pattern emerges as a complex response to two major inputs. First the Earth is spinning, and second, the Earth is heated more at the equator than the poles, so heat must move through air and water currents towards the north and south.

One of the major systems that moves heat away from the equator is known sometimes as the Atlantic Conveyor, which is really part of a lager system of sea currents that includes the Gulf Stream. Notice that the Indian Ocean is sequestered mostly in the Southern Hemisphere, bordered along the west by Africa and the north by Asia. Extra warm water in the Indian ocean tends to make its way around the southern tip of Africa, and up the Atlantic, which is a round about route. This water eventually makes its way to the North Atlantic, where it cools, and owing to evaporation, becomes extra salty. This drives the formerly warm surface water into the depth of the ocean, where it flows along the bottom of the Atlantic south, eventually returning (I oversimplify a bit) to the Indian Ocean and elsewhere.

This system is also known as the AMOC (Atlantic Meridional Overturning Circulation) and is part of the global “Thermohaline Circulation” system.

Meanwhile, a smaller but similar aspect of this system starts with the Gulf of Mexico. This water becomes quite warm from the Sun, but is blocked from moving directly north by the presence of North America, with Florida adding to the captive nature of those waters. But the water does make its way around Florida and flows north along the East coast of the US, and eventually also reaches the North Atlantic, and similarly, contributes to the saline deep currents.

Because salinity partly, even largely, drives this system, adding fresh water to the North Atlantic may interfere with this system of currents. How do you get enough fresh water to do this? In the past, huge volumes of fresh water probably entered the North Atlantic every now and then as large outflows of giant inland lakes, formed by melting glaciers, broke through barriers of ice or sediment. There is some evidence that in the past this sort of thing may have partly, or even completely, shut down the Atlantic Conveyor system, which would have had huge impacts on climate.

Today there seems to be two main sources of extra fresh water in the area. One is during years (or decades) when there is a larger than usual number of ice bergs floating into the North Atlantic from the Arctic. The other, potentially, is from melting of Greenland’s fast glaciers, a process that has recently speeded up because of human caused greenhouse gas pollution warming the Earth.

By now you may recognize this scenario as the basis for the Hollywood disaster movie “The Day After Tomorrow.” In that movie the thermohaline circulation system shut down and an ice age instantly gripped the planet. Giant frozen tornadoes came plummeting down from the Stratosphere. One of them hit the helicopter the British Royal Family was escaping in. Everybody in the US ended up in Mexico.

Every one who survived, that is.

The thing is, now, this can’t happen. Well, that particular scenario can’t ever really happen. But yes, the shutting down of this system can theoretically cause the onset of an ice age, or at least a mini-ice age, and has done so in the past. But no, it can’t now because our planet has warmed too much from human greenhouse gas pollution to allow that to happen. That may be the one good thing about global warming.

The new research does suggest, though, that this major pattern of circulation appears to be slowing down. This will have a number of effects. It will likely change the weather in Europe a bit. It will likely cause an increase in sea level along the US East Coast, because the current (and former) system piles up water towards the east and lowers it in the west, within the North Atlantic. That could be worth a few inches.

According to lead author Stefan Rahmstorf, “It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up. Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” says Rahmstorf. If the slowdown of the Atlantic overturning continues, the impacts might be substantial. Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”

The researchers used a combination of sea surface, atmospheric, and proxy (mainly coral) indicators of temperature to indirectly measure changes in ocean currents over time.

According to climate scientist Jason Box, “Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation. So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further.” Michael Mann, another author of the paper, adds, “Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both. That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”

What happens if the system actually turns off completely? It was formerly thought that the chances of this happening were small, but this research, conforming to a growing body of expert opinion, suggest that the chances of that may be higher than previously thought. Were this to happen the main characteristic of any effects would be rapidity. Whatever happens would happen fast, and rapidly changing climate is generally regarded as bad no matter what the change itself really is.

UPDATE ADDED:

A criticism of this work has emerged, suggesting that another study indicates that there is no a long-term slowdown of the Atlantic Meridional Overturning Circulation (as suggested by the research covered here). That criticism is incorrect. Michael Mann, one of the AMOC study’s author has written a clarification on his facebook page. He begins:

Some critics have tried to make hay over a previous article from last year by URI Graduate School of Oceanography scientist Tom Rossby (see: http://www.gso.uri.edu/b…/rossby-gulf-stream-is-not-slowing/) they claim contradicts our recent Nature Climate Change study finding evidence for a long-term slowdown of the Atlantic Meridional Overturning Circulation (“AMOC”). …

Rossby employs direct measurement of Gulf Stream transport using a ship-board acoustic Doppler current profiler (ADCP) over the interval 1993-2012. I have no reason at all to doubt Rossby’s findings. And they do *not* conflict with our own findings (though some have misleadingly sought to assert they do) for two fundamental reasons:

Mann’s entire post is HERE and you should go read it.

Additional Resources:

The article:

Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): Evidence for an exceptional 20th-Century slowdown in Atlantic Ocean overturning. Nature Climate Change (online) [DOI: 10.1038/nclimate2554 ]

Stefan Rahmstorf, lead author, has this blog post at RealClimate: What’s going on in the North Atlantic?

Figure caption from the original article, goes with the graphic at the top of the post:

Figure 3. Surface temperature time series for different regions. Data from the proxy reconstructions of Mann et al.12,13, including estimated 2-? uncertainty bands, and from the HadCRUT4 instrumental data49. The latter are shown in darker colours and from 1922 onwards, as from this time on data from more than half of all subpolar-gyre grid cells exist in every month (except for a few months during World War II). The orange/red curves are averaged over the subpolar gyre, as indicated on Fig. 1. The grey/black curves are averaged over the Northern Hemisphere, offset by 3 K to avoid overlap. The blue curves in the bottom panel show our AMOC index, namely the difference between subpolar gyre and Northern Hemisphere temperature anomalies (that is, orange/red curves minus grey/black curves). Proxy and instrumental data are decadally smoothed.

A neat video of the thermohaline circulation system.

A movie produced by Peter Sinclair, that goes along with THIS blog post.

Coverage by Chris Mooney at the Washington Post: Global warming is now slowing down the circulation of the oceans — with potentially dire consequences

Greg Laden's Blog

Tag Archives: AMOC

AMOC Amok: Global Warming Bad News

How Did Climate Change Cause The Great More’Easter of 2016?

New Research Shows Exceptional Slowdown In Major Atlantic Ocean Currents (UPDATED)

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