The slow cooling of our interglacial
Here's my latest Mind and Matter column in the Wall Street Journal, with added links and charts. On interglacials.
The entire 10,000-year history of civilization has happened in an unusually warm interlude in the Earth's recent history. Over the past million years, it has been as warm as this or warmer for less than 10% of the time, during 11 brief episodes known as interglacial periods. One theory holds that agriculture and dense settlement were impossible in the volatile, generally dry and carbon-dioxide-starved climates of the ice age, when crop plants would have grown more slowly and unpredictably even in warmer regions.
This warm spell is already 11,600 years old, and it must surely, in the normal course of things, come to an end. In the early 1970s, after two decades of slight cooling, many scientists were convinced that the moment was at hand. They were "increasingly apprehensive, for the weather aberrations they are studying may be the harbinger of another ice age," said Time in 1974. The "almost unanimous" view of meteorologists was that the cooling trend would "reduce agricultural productivity for the rest of the century," and "the resulting famines could be catastrophic," said Newsweek in 1975.
Since then, of course, warmth has returned, probably driven at least partly by man-made carbon-dioxide emissions. A new paper, from universities in Cambridge, London and Florida, drew headlines last week for arguing that these emissions may avert the return of the ice age. Less noticed was the fact that the authors, by analogy with a previous warm spell 780,000 years ago that's a "dead ringer" for our own, expect the next ice age to start "within about 1,500 years." Hardly the day after tomorrow.
Still, it's striking that most interglacials begin with an abrupt warming, peak sharply, then begin a gradual descent into cooler conditions before plunging rather more rapidly toward the freezer. The last interglacial—which occurred 135,000 to 115,000 years ago (named the Eemian period after a Dutch river near which the fossils of warmth-loving shell creatures of that age were found)—saw temperatures slide erratically downward by about two degrees Celsius between 127,000 and 120,000 years ago, before a sharper fall began. See charts here and here.
Cyclical changes in the earth's orbit probably weakened sunlight in the northern hemisphere summer and thus caused this slow cooling. Since the northern hemisphere is mostly land, this change in the sun's strength meant gradually increased snow and ice cover, which in turn reflected light back into space. This would have further cooled the air and, gradually, the ocean too. Carbon-dioxide levels did not begin to fall much until about 112,000 years ago, as the cooling sea absorbed more of the gas.
Our current interglacial shows a similar pattern. Greenland ice cores and other proxy records show that temperatures peaked around 7,000 years ago, when the Arctic Ocean was several degrees warmer than today, trees grew farther north in Siberia and the Sahara was wet enough for hippos (Africa generally gets wetter in warm times). Data from the southern hemisphere reveal that this "Holocene Optimum" was global in extent. Here's an example:
source: Willis Eschenbach, wattsupwitthat
An erratic decline in temperature followed, with Minoan, Roman and Medieval warm periods peaking at successively lower temperatures, culminating in the exceptionally cool centuries of the "Little Ice Age" between 1550 and 1850, when glaciers advanced all over the world. In the Greenland ice cores, these centuries stand out as the longest and most consistent cold spell of the current interglacial.
In other words, our own interglacial period has followed previous ones in having an abrupt beginning and a sharp peak, followed by slow cooling. The question is whether recent warming is a temporary blip before the expected drift into glacial conditions, or whether humankind's impact on the atmosphere has now reversed the cooling trend.
Comments (12)
So much for wasting trillion of dollars combatting global warming.
"..many scientists were convinced that the moment was at hand". Groan. This was (now famously) not true and its disappointing to see this falsehood spread here. Time Magazine and Newsweek are not scientific journals and on this matter were talking out of their asses.
Matt Ridley, its quite easy to research this and find the truth. Just google "ice age myth", or watch this video http://www.youtube.com/watch?v=EU_AtHkB4Ms which debunks the endlessly regurgitated 70s myth.
There have been glacial and interglacial periods. Some last longer and are cooler or warmer than others. Variations within the different cycles are quite variable.
Do intelligent people really make climate predictions based on such data?
And did Ehrlich expect an imminent ice age to validate his forecast of Malthusian collapse in the 1970s just as now he uses runaway AGM to predict exactly the same thing?
The word “planet” means wanderer. The ancients, with their lives lived largely outdoors and without artificial lighting, were much more intimately acquainted with the heavens than are we moderns, unless we specialize in astronomy. They noticed that although there was a fixed pattern of stars for the most part, some of them wandered around in complicated patterns.
It was some time before concepts shifted enough for it to be realized that the Earth, too, was a wanderer. The comfortable, solid notion of a fixed Earth that was the center of the universe was grimly defended by the medieval Church but ultimately fell to the advance of science.
There seems to be an unstated assumption in the neo-religious Gaian fundamentalism that is based on this kind of geocentrism — “it’s so big, how could it move?” — in terms of the Earth’s biosphere and climate. There’s also an element of original sin ...
http://www.foresight.org/nanodot/?p=3209
(by yours truly in '09)
This brief column and the two selected graphs reveal in a nutshell the entirely normal oscillations of natural climate variability in the Holocene interglacial - the time frame of modern man.
This inconvenient and revealing perspective is precisely the one that so much policy-driven effort is devoted towards concealing from general public awareness, in attempts to foster support and meek acceptance for the imposition of an ever more draconian green political agenda.
Are you sure about that Arthur? By my recollections, the CRU at UEA got it's start under Hubert "The Iceman" Lamb.
http://books.google.ca/books?id=50HjSi5o8J0C&pg=PA285&lpg=PA285&dq=histo...
Arthur, you can debunk it as much as you like, but some of us were highly suggestible teenagers at the time and can remember that some serious scientists were suggesting a new ice age.
I gave a sixth form seminar on this topic in the mid eighties.
Be carefull of that GISP2 temeprature graph (the last graph) - it seems that the 'Present' is not 2000 AD but 1855 :
http://sciblogs.co.nz/hot-topic/2011/01/04/easterbrook%E2%80%99s-wrong-a...
That GISP2 ice core record does look a little ominous, especially after the Minoan Warming peak, when it starts to resemble a sort of jagged staircase, heading downward... The great trough that appears just before the Medieval Warming peak is an interesting one, as the lowest point would appear to coincide with the early to mid 9th century - about the time of Charlemagne, when Europe was experiencing relatively cold conditions (or so I've read). However, just as GISP2 is descending into that trough, at very roughly the 1400 mark, would surely correspond to the 6th century, when some sort of very severe cooling and extreme weather events are said to have taken place (author David Keys theorises that a massive volcanic eruption happened around then.) The (Cold and) Dark Ages?
The problem with the second graph is that it is fabricated by Don Easterbrook and the actual temperatures on the right hand side are about 2 C higher.....http://tinyurl.com/247awua
Arthur,
since you consider the fact that Time and Newsweek are magazines means they are not to be believed, maybe you should also ask the IPCC to remove the 30% of references in the fourth assessment report which have been found to come from non-peer reviewed papers, green activist PR and, yes, magazines
Arthur...
Somewhere in the CBC's archives (good luck finding it) is a mini-documentary that ran circa 1978 on their National News broadcast. Put together by none other than Adrian Clarkson. It was all about the cooling trend and whether it forebode a coming ice-age. Reported that growing seasons in N. America had shortened by as much as two weeks, and that crops were taking a beating. Very...*ahem*...alarmist.
The only thing that's "mythical" is AGW. There was never a time in the multi-billion year history of this planet when the climate wasn't changing. That we...civilization...exists at all because of what amounts to a temporary break in a multimillion year ice age is the reality.
Watched an interesting documentary recently. Explained that dinosaurs were able to get as big as they did during the Cretaceous/Jurassic periods was due to exceedingly abundant vegetation to feed them, which was the result of CO2 levels multiple times greater than currently. Notably, the age of gigantic dinosaurs lasted over 100,000,000 years. Human civilization will be damn lucky if we make it much past 10,000 years before most of it gets snuffed by another round of mile thick glaciers.