Over the last four years, I have seen some reports on scientific research into climate mechanisms involving the ocean and the atmosphere (lower and upper) that I regard as both fascinating and (apparently) often misunderstood. So here’s a quick note on what I understand in general to be the import of these mechanisms. As always, there will probably be mistakes in my summary: Caveat lector.
The Fate of GreenlandThat actually was the title of a book by (iirc) MIT researchers that summarized the first, and imho most major, of these mechanisms. It works like this: warm surface water flows in the Gulf Stream current northeast. Off Greenland, it dives down to the deep ocean, and becomes a current flowing south through the Atlantic. When it reaches Antarctica, it flows as a current halfway around the world and starts flowing north again, to surface somewhere around the Bering Strait – a journey of maybe 100-150 years. This current is therefore continually refreshing the warmth of the deep ocean, and is one mechanism by which the heat of the surface water is transmitted to the deep ocean.
What causes the Gulf Stream to sink is that it becomes relatively salt-heavy compared to the water around it. However, periodically, when temperatures get too warm, sea ice in the area melts. Sea ice when it originally forms expels the salt; when it melts, therefore, it releases relatively non-salty water – and the Gulf Stream stops sinking. This, in turn, stops the ocean “conveyor” of deep-water heat and, 100-150 years later, stops the warming of the Bering Strait (and therefore Arctic) water as well – which causes the water around Greenland to cool again and the Gulf Stream to dive again. You may remember that this is the phenomenon that caused many to fear just such a Gulf Stream failure due to global warming – and so you had doomsday predictions of a collapse of temperatures in northern North America and northern Europe, which are today warmed beyond their latitude by the Gulf Stream. Alas, all indicators are that such a stoppage, if it is happening, is happening only slowly, and by the time the Gulf Stream stopped a rise in temperature that would more than compensate may well have happened.
So how has this ocean current played into global warming so far? I believe it has played a minor but significant role in Arctic melt. Bear in mind that Arctic sea ice never extends to the bottom of the Arctic sea, and below it is salt-heavy water that flows (bearing the ice with it) more or less from the Bering Strait across the North Pole to Greenland and Iceland and the North Atlantic. The warming of 1910 is now surfacing in the Bering Strait and going to warm the ice in the Arctic from below and the side – a small warming, according to measurements in the Bering Strait, but I believe a significant factor in the melting of Arctic sea ice we have seen over the last 20 years, along with increased summer heat melting the ice from above. That effect, while still minor, can only increase as, over the next 50 years, we begin to enter the period of more rapid ocean-surface/Gulf-stream warming from 1913-1963. And, of course, the warming of the waters around Greenland will grease the skids of Greenland’s land glaciers, accelerating the raising of ocean levels.
Boys, Girls, and OscillationsBy now, probably, many have heard of the El Nino (“the boy”) effect, and some of the contrary La Nina (“the girl”) effect. Briefly, periodically an unusually warm ocean-surface current (el Nino) arrives in Ecuador from the Pacific. This is the crest of a decade-long or so period when this current is unusually warm. Likewise, periodically an unusually cool current (la Nina) arrives, and is the trough of a decade-long or so period when this current is unusually cool.
The ripple effects of an el Nino are global. We see higher global temperatures, especially in places like the US western seacoast. It may affect a characteristic of the northern latitudes called the North Atlantic Oscillation (basically the location of a place of low [?] pressure in the Atlantic near/in the Arctic), which when “positive” brings hotter weather to northern Europe and cooler weather to North America, and when “negative” brings cooler weather to northern Europe and hotter weather to North America. An el Nino seems to be mostly associated with a “positive” NAO, and a la Nina with a “negative” NAO.
A recent study shows that in the period of today’s global warming, an el Nino period corresponds to 10-20 years of accelerated global warming, and a la Nina period to 10-20 years of “hiatus”, meaning no or slower global warming. Over the last fifteen years, we have seen a hiatus period following an unusually strong el Nino in 1998, plus acceleration in underlying global warming. Thus, although we have not broken the 1998 global temperature record, we are continuing to see global temperatures rise from year to year, from 1999 onwards. What is worse, the la Nina period should be coming to its end – indeed, some expected it to end in 2012. It is possible that global warming’s effects on ocean-surface and deep-ocean warming may have disrupted the timing of el Nino – but, failing that, what we have seen over the last 15 years may have been a relatively halcyon period.
The other interesting finding from this study and a previous one is that el Nino has an effect not only on weather patterns, but also on the ocean – more specifically, on transmission of heat from the ocean’s surface to the deep ocean. The off-Greenland “heat sink”, it turns out, is only one of four such transmission places from surface to deep ocean, one in the Northern Pacific, one in the southern Pacific near Antarctica, and one in the southern Atlantic near Antarctica. In all of these places, the wind patterns essentially form an ellipse around a center. El Nino affects these winds such that transmission to the deep ocean decreases, and there is more surface heat and less deep-ocean heat, and la Nina makes the transmission increase, so that there is more deep-ocean heating and less of the surface heat that contributes to global warmth. Note also that global-warming-driven greater energy in the circum-Antarctic current may have actually increased the “heat sink” there, slowing global warming independently of the el Nino effect, but no one is quite sure of that one.
So what we have so far is a longer-term (100-150 years) ocean global-warming effect that over the next 100 years will become more and more serious, plus a decades-long oscillation that over the long term has zero effect on global warming but can deceive you about the trend – if you let it.
Mongolian WeirdingOK, this one is based a bit on preliminary research, and is the least important in the medium and long terms as regards global warming. But it’s so weird …
As I understand it, it begins when the temperature somewhere above the Tarim desert “basin” near Mongolia warms just enough to allow the heat of that basin to rise towards the troposphere (upper atmosphere). That warming is automagically transmitted northward to the Arctic, where for some reason the transmission of heat to the troposphere removes much of the heat-trapping ozone there (?). As a result, especially in winter, the Arctic actually becomes colder, and this in turn propels a greater differential with temperatures farther south. This, in turn, means that the jet stream, which operates in the troposphere, fluctuates more north and south, and this in turn causes more frequent intrusion of cold Arctic temperatures to the south. Hence, apparently, last winter. And we can blame it all on Mongolia …
Apparently, because of global warming that effect has been happening more often. And yet, compared to the other effects I’ve discussed, in the medium and long terms this is pocket change. So, the jet stream oscillates more often; but the Arctic is also warming much faster than everywhere else, to the tune of 10-20 degrees Fahrenheit in the winter, so the effects of jet-stream-induced cold from the Arctic are steadily being muted. Clearly, as the incredibly warm North American winter of two years ago shows, in the medium term Nino/Nina and the NAO dominate, and in the long term CO2-induced global warming dominates. But the Mongolian effect is just so weird …
Prehistorical MinimumI see a few who in analyzing these effects attempt to apply a simplistic rule: “The last time this happened …” As in, the last time we reached 400 ppm the Arctic was ice-free year-round, and the seas were 100 feet higher. Referring to the past, in the case of these three effects, is likely to understate global warming and its effects over the next 100 years.
For example, beyond perhaps 10 million years ago there was no join between North and South America, and therefore the Pacific el Nino probably flowed directly through the Atlantic until it hit Europe, while there was probably no Gulf Stream. Was there therefore less of a “Greenland heat sink” in those days? Probably, but it was far outweighed by the relative slowness of the CO2 increase in a Milankovitch cycle or a undersea-volcano-driven CO2 increase, and also outweighed by the ability of weatherization and the like to catch up to and counteract CO2-driven increases in ocean-surface temperature.
For another thing, there was an open question a decade ago about the rate of global warming for a certain amount of CO2 – this discussion of “heat sinks” seems to have ended most of the uncertainty. In other words, one could look at the rate of warming and, depending on the rate in which heating went into the ocean surface and into the deep ocean, come up with a wide range of estimates for directly CO2-driven global warming. Well, now we appear to know that we were looking during a period of unusual la-Nina-driven deep-ocean heating, and therefore our estimates of CO2 “forcing” are narrowing in on the high side of the estimate range. Not good news …
So I guess my overall message is that most of these effects have served in the past to allow some to underestimate the seriousness of global warming. Now that we know them, their effects are clearly fascinating but minor in the grand scheme of things. Fun to write about; but let’s not take our eyes off the prize. If we do, that prize will likely be Pandora’s box – without the Hope.