Guest post by William McClenney
We will initiate this discussion by looking at some of the early, and then later, recognitions of some of climate change’s “best lap times” in terms of sheer speed. Perhaps one of the more poignant moments in all of climate science occurred in 1992, documented by John D. Cox, writing in “Climate Crash: Abrupt Climate Change and What it Means for our Future (John Henry Press, an imprint of the National Academies Press, ISBN: 0-309-54565-X, 224 pages, 2005), which describes the initial discovery of Abrupt Climate Change (ACC) and also introduces a main character, Dr. Richard B. Alley:
“They knew they had the critical layer of ice in their snow cave [where the ice cores were first processed-wm]. Wanda Kapsner, a Penn State graduate student, had been taking thin sections about every 20 meters along the lengths of core laid out in the cave. She told Alley, ‘This section is in Holocene ice and the next section 20 meters down is in Ice Age ice, and so between these two is where you’re going to find it.’”
“This team of scientists was about to complete their six-week stint at Summit, and a new team was about to take its place. So it was up to project leader and paleoclimatologist Paul Mayewski to decide which team was going to handle this important ice. On hearing the news from Kapsner, Mayewski told Alley, ‘Fine, before we get out of here, we’re going to do that ice.’”
“The ice that had formed from falling snow during the transition from the last of the cold, dry, windy ice ages to the first of the warm, wet calms of the modern 10,000-year-long Holocene climate is 1,678 meters, just over a mile, down the GISP2 core. Rendered in ice, what exactly would it look like, this boundary of epochs? The young American scientists had read the literature from Chet Langway, Willi Dansgaard, Hans Oeschger, Wally Broecker, and others, and they had heard from the Europeans, who were about a year ahead of them in drilling at Summit. Yet still they were not entirely prepared for what they saw that day in the ice, for the suddenness of it.”
“’You did not need to be a trained ice core observer to see this,’ recalled Alley. ‘Ken Taylor is sitting there with the ECM and he’s running along and his green line is going wee, wee, wee, wee—Boing! Weep! Woop! And then it stays down.’ Dust in the windy ice age atmosphere lowered the acidity of the core to a completely new state. ‘We’re just standing there and he just draws a picture of it,”’Alley said.”
“Spontaneous celebration was followed by a sudden and unexpected quiet. ‘I think we cheered,’ recalled Alley, ‘and then we were all a little sobered. Because it was just so spectacular. It was what we’d been looking for, and there it was, and then we’re sitting there. Holy crap.’”
“The instant of recognition that summer of 1992 had a raw feel to it, although eventually the disquiet would find concrete expression in numerous articles and presentations as the scientists became accustomed to the large truth of abrupt climate change and immersed themselves in its fine details. Alley recalled later: ‘Those of us who were down there in that trench at that time knew right then that our picture of the world had changed. There’s a whole bunch of us who came out of that ice core project who have since dedicated ourselves to understanding abrupt climate change.’”
“In the GISP2 science trench, the tray holding the section of core rolled down the assembly line and then it was Alley’s turn at the ice. “It slides across in front of me and I’m trying to identify years: ‘That’s a year, that’s a year and that’s a year, and—woops, that one’s only half as thick.’ And it’s sitting there just looking at you. And there’s a huge change in the appearance of the ice, it goes from being clear to being not clear, having a lot of dust.”
Paper after paper began to roll off the scientific presses from 1992 on, and just like the unfolding recognition of plate tectonics which preceded it by a few decades, it was literally riveting for all of us geologists fascinated by the Quaternary. So we get our first trap-speed: climate can switch abruptly from its cold to its warm state in just one year. Our first peg on the lower-end of natural noise.
Stocker and Marchal (2000) begin their discussion with this:
“Among the archives recording past climate and environmental changes, ice cores, marine and lacustrine sediments in anoxic environments, and tree rings have seasonal to annual resolution. Changes in dust level (1), snow accumulation (2), summer temperature (3), and indicators of the productivity of marine life (4) suggest that some of the climate changes have evolved on time scales as short as a few years to decades.” (“Abrupt Climate Change in the Computer: Is it real?” (PNAS, vol. 97, no. 4. February 15, 2000, pp. 1362–1365)
By 2002, we have this from the National Research Council:
“Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed. For example, roughly half the north Atlantic warming since the last ice age was achieved in only a decade, and it was accompanied by significant climatic changes across most of the globe. Similar events, including local warmings as large as 16°C, occurred repeatedly during the slide into and climb out of the last ice age. Human civilizations arose after those extreme, global ice-age climate jumps.”
“The new paradigm of an abruptly changing climatic system has been well established by research over the last decade, but this new thinking is little known and scarcely appreciated in the wider community of natural and social scientists and policy-makers.” (“Abrupt Climate Change – Inevitable Surprises”, Committee on Abrupt Climate Change, National Research Council of the National Academy of Sciences, 2002, ISBN: 0-309-51284-0, 244 pages, Richard B. Alley, chair).
We will step a little closer in time and take a look at a rather fascinating ACC event.
The Younger Dryas:
Proxy and isotopes become more difficult to work with over time, carbon 14 gets us back some 50kyrs, but no further. The longest and presumably best ice cores come from Greenland and so far extend no further than 135kyrs or so. So we will begin to examine some of the better resolved climate change trap-speeds from the Younger Dryas.
From two National Academies Press publications:
“The ice was always melting back. Rivers were getting wider and flowing over new lands. Warm times were lasting longer. Forests were growing larger and richer, as were the big grasslands and their wild grains. Some 12,000 years ago, hunters were in the embrace of a stable, congenial climate stretching back farther than memory, perhaps farther than the stories told by the elders. For hundreds of years, the ice had been shrinking and a large swath of landscape had been bathing in steadily increasing warmth. In the fertile lands of West Asia, clans of an ancient people that would become known as the Natufians had long been accustomed to the good hunting of game and the easy gathering of fruits and wild grains. This was the way of the world for people 12,000 years ago. Nothing in the sky or the sea or on the land and nothing in memory warned them of what was to come. The change in climate was a surprise of life-bending power, as if the whole world turned against the clans. Cold, dry winds began sweeping the countryside, withering every living thing in their paths. First the fruits were lost and then the grasslands, and eventually the forests were driven back. The rivers shrank and some became choked with advancing ice. Without warning or recognition, in just a few years, a colder, harder, shorter, and more difficult way of life set in. From beginning to end, as far as the Natufians were concerned, the time of wind and cold lasted forever. Like the old time of warmth and plenty, the hard new climate stretched out beyond memory, holding humanity in its grip for something like 470,000 days.
What in the world happened? Why did the climate over much of Earth so suddenly reverse itself? What is it about the way of the world that makes such a thing possible? Why did this crippling cold, windy, dry regime hold sway over the sparsely populated lands of the Northern Hemisphere for 13 centuries?”
Asks John D. Cox in “Climate Crash”. Three years before, the National Research Council, of which Richard Alley was chair, stated:
“Briefly, the data indicate that cooling into the Younger Dryas occurred in a few prominent decade(s)-long steps, whereas warming at the end of it occurred primarily in one especially large step (Figure 1.2) of about 8°C in about 10 years and was accompanied by a doubling of snow accumulation in 3 years; most of the accumulation-rate change occurred in 1 year. (This matches well the change in wind-driven upwelling in the Cariaco Basin, offshore Venezuela, which occurred in 10 years or less [Hughen et al., 1996].)”
“Ice core evidence also shows that wind-blown materials were more abundant in the atmosphere over Greenland by a factor of 3 (sea-salt, submicrometer dust) to 7 (dust measuring several micrometers) in the Younger Dryas atmosphere than after the event (Alley et al., 1995b; Mayewski et al., 1997) (Figure 2.1). Taylor et al. (1997) found that most of the change in most indicators occurred in one step over about 5 years at the end of the Younger Dryas, although additional steps of similar length but much smaller magnitude preceded and followed the main step, spanning a total of about 50 years.”
The above quotes from page 27 of “Abrupt Climate Change – Inevitable Surprises” (referenced above).
Richard B. Alley, NRC chair of the Committee on Abrupt Climate Change, states in Quaternary Science Reviews (2004):
“In the case of the termination of the Younger Dryas cold event, for example 1/3 to 1/2 of the entire 10,000-year deglacial warming in Greenland was achieved in order of 10 years (Cuffey et al., 1995; Severinghaus et al., 1998), with most of the accumulation-rate change apparently in a single year (Alley et al., 1993). The scaling of 1/3 to 1/2 the deglacial change in about a decade probably applies in many places (Alley, 2000). Any event for which rate of change is anomalous by two to three orders of magnitude merits attention. Older events were even larger, with a rapid shift of about 16 C in Greenland in one case (Lang et al., 1999).”
Steffensen et al (Science, vol. 321, 1Aug2008) abstract it thusly:
“The last two abrupt warmings at the onset of our present warm interglacial period, interrupted by the Younger Dryas cooling event, were investigated at high temporal resolution from the North Greenland Ice Core Project ice core. The deuterium excess, a proxy of Greenland precipitation moisture source, switched mode within 1 to 3 years over these transitions and initiated a more gradual change (over 50 years) of the Greenland air temperature, as recorded by stable water isotopes. The onsets of both abrupt Greenland warmings were slightly preceded by decreasing Greenland dust deposition, reflecting the wetting of Asian deserts. A northern shift of the Intertropical Convergence Zone could be the trigger of these abrupt shifts of Northern Hemisphere atmospheric circulation, resulting in changes of 2 to 4 kelvin in Greenland moisture source temperature from one year to the next.”
More recently the Akkadian Empire under Sargon (2,300-2,200 BC), mankind’s first empire ever, succumbed to climate change that happened rather suddenly. A 300 year long period of drought struck this nascent civilization and toppled what turned out to be only a 100 year empire. The Old Kingdom of Egypt and the Harappans of the Indus Valley suffered a similar fate 4,200 years ago, succumbing to an abrupt drought that ended those civilizations, with Egyptians “forced to commit unheard of atrocities such as eating their own children and violating the sacred sanctity of their own dead (Fekri Hassan, 2001)”. The Mayans had pretty much the same luck with three periods of extreme drought at 810, 860 and 910 AD. Sadly just two years after the last drought, which saw 95% of the Mayan population gone, wet years returned to the Yucatan.
A reconstruction from fossil algae in sediments from Drought Lake in North Dakota of the past 2000 years found that dry conditions were far and away the rule in the High Plains, with the Dust Bowl conditions of the 1930’s one of the lesser dry spikes found in the record.
The Wisconsin glacial, which preceded the Holocene, the interglacial in which all of human civilization has occurred, is littered with ACC. D-O oscillations average 1,500 years, and have the same characteristic sawtooth temperature shape that the major ice-age/interglacials do, a sudden, dramatic, reliable, and seemingly unavoidable rise of between 8-10C on average, taking from only a few years to mere decades, then a shaky period of warmth (less than interglacial warmth), followed by a steep descent back into ice age conditions. Each D-O oscillation is slightly colder than the previous one through about seven oscillations; then there is an especially long, cold interval, followed by an especially large, abrupt warming up to 16C (a Bond cycle). During the latter parts of the especially cold intervals, armadas of icebergs are rafted across the North Atlantic (Heinrich events), their passage recorded reliably by the deep ocean sediment cores which capture the telltale signature of these events in dropstones and detritus melted out of them.
“The ice age ended in one year” according to Dorthe Dahl-Jensen, professor at the Center for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen:
“We have analysed the transition from the last ice age to our current warm interglacial period and there is such an abrupt change in climate that it is as if someone just pushed a button”
What should be fairly apparent is that in terms of sheer speed, major natural climate change can occur in the 1 year bracket, the bottom end of the natural noise range, and not just at the glacial terminations. We see evidence of many cycles, small and large, that occur from 3-7 years (El Nino range), on the decadal (AMDO/PDO) to millennial (D-O) scale. Not to mention the 100 million year major ice age cycles. Therefore, in order to qualify as a signal, and subsequently recognizable as anomalous, the signal must somehow exist outside of a range spanning 1-100,000,000 years.
“Signal-to-noise ratio (often abbreviated SNR or S/N) is a measure used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. While SNR is commonly quoted for electrical signals, it can be applied to any form of signal (such as isotope levels in an ice core or biochemical signaling between cells).”
Think of SNR as possibly the simplest statistical test.
This time we will work backwards from recent time (present to 70 million years ago [mya]). Mainly in terms of the two primary components of climate change, temperature and sea level. This part will be primarily visual, with the intent being to exercise your perhaps newfound skills in discerning signal relative to background.
[In the interest of full disclosure, not all of these graphics I thought at the time to reference (when jpeg’ing them from the pdf’s), or after downloading them from wherever.]
So we will start with the projected anthropogenic signal from the IPCC’s Assessment Report 4 (AR4) (2007). This is Figure 10.33 from page 821 of Chapter 10:
I got this from the C3 website some time ago, but I do remember going to the source and digging up the data. Assuming the 75 year smoothed Law Dome 1,000 year record evinces the LIA and MWP, then the recent thousand-year noise is nominally say 1.5C, but also potentially inclusive of 3.1C. High SNR of about 0.4:1 (0.6/1.5), or 40% of the noise level, to low SNR of 0.2:1 (0.6/3.1), or 20% of the noise level. Not anomalous (need >1:1).
Post “8.2k event” sea levels from Rio Grande do Norte, Brazil. Late Holocene sea level noise range of maybe 6 meters, -2 to +4 AMSL. A SNR of 0.1:1, or signal at 10% of the noise level. Not anomalous.
Here we see the range perhaps -32.4 to -28.7, or a temperature noise envelope of 3.7C through the majority of the Holocene interglacial. The “8.2k event” and the Holocene Climate Optimum paired and evident. As is the rocky ride down since the Minoan Warming. SNR similar but lower still than the 1kyr comparison (0.16:1, 16% of the noise). Not anomalous.
In the majority of the Holocene, the noise level is simply too high to accommodate recognition of the AR4 worst case AGW “signal” scenario. If we take what is widely recognized as the dual-thermal sea level maxima at MIS-5e (6 meters) then at the end-Holocene, the SNR degrades to just 10% of the latest post-MPT end-interglacial noise. Not anomalous. Beyond this point, SNR itself becomes subject to the larger post-MPT glacial/interglacial switch between the warm and cold states which have dominated climate for the past ~800kyrs or so.
As we exit the Holocene (stage left), we have this interpretation of our present interglacial’s sea level from “Holocene sea-level fluctuations inferred from the evolution of depositional environments of the southern Langebaan Lagoon salt marsh, South Africa, John S. Compton, The Holocene 11,4 (2001) pp. 395–405 (sorry, best reference preserved in the downloaded paper I found online August, 2009).
Again, I do not remember from what paper I extracted this. Note the gradual, post-PETM evolution of climate. Noise, at even more alarming levels is apparent not only in the primary decline, but in the range about the late mean. This takes us out to 70mya.
Now that you have taken the “wild climate ride”, which just predates the extinction of the dinosaurs to present, the Holocene trapping with a top SNR of just 0.4:1, ACC which traps in the one year bracket, and with respect to the definition of an anomaly, what would your present estimation be of the AR4 AGW worst-case “signal” prognostications?