Plainly stated, science which does not take into account facts that are relevant to the issue under study is bad science. Period.
The usually overlooked fact that is relevant to the issue of AGW comes straight from the temperature record for the last 450 ky as measured from the ice cores drilled at the EPICA Kohnen Station in the Dronning Maud Land (EDML) and at the Vostok Antarctic sites. More recently, the EPICA Dome C (EDC) ice core has provided a record for the last 800 ky. (Unless explicitely stated otherwise, time in the charts goes from right to left.)
Figure 1. Antarctic temperature for the last 450 ky from EPICA and Vostok
The temperatures may be seen in a somewhat greater detail in the following chart for just the Vostok measurements.
Figure 2. Antarctic temperature and CO2 levels for the last 450 ky from Vostok
The following is the nomenclature for [interglacial periods (from present to oldest); warming events respectively leading to them]
Present: [Holocene; Termination 1 (TI)]
1st previous, 120 ky ago: [Eemian or Marine Isotope Stage 5 (MIS 5); Termination 2 (TII)]
2st previous, 240 ky ago: [Marine Isotope Stage 7 (MIS 7); Termination 3 (TIII)]
3st previous, 330 ky ago: [Marine Isotope Stage 9 (MIS 9); Termination 4 (TIV)]
4st previous, 410 ky ago: [Marine Isotope Stage 11 (MIS 11); Termination 5 (TV)]
From this record it is clearly seen that, for the last 450 ky, the Earth has been in a cycle where benign interglacial periods like the Holocene (last 11.5 ky) have been more or less brief spikes occurring every 80-120 ky between much longer cold periods with temperatures around 6 ºC, and as much as 9 ºC, lower than today's.
It is also clear that coolings from tops, although not as steep as warmings from bottoms, are nevertheless quite fast.
Therefore, to evaluate the prospects for the global temperature as far as the natural forcings are concerned, we can start by comparing the Holocene with appropriate previous tops. The first choice would seem to be the Eemian: it is the closest in time and its duration is comparable to that of the Holocene so far. MIS 7 and MIS 9 on the other hand should be discarded because of their extreme sharpness, while MIS 11 looks like another good candidate. We'll later see that, in fact, MIS 11 could indeed be the Holocene's appropriate match.
To compare the Holocene with the Eemian, I made the following graph based on the first in Stuart Staniford's post "Living in the Eemian". I took the Holocene's last 12 ky and superimposed it onto the corresponding period of the Eemian, 121 ky ago. This is exactly what Stuart did in the second and third graphs of his post, only that here I show it within the bigger picture.
Figure 3. Antarctic temperature for the last 150 ky from Vostok, with the Holocene shifted 121 Ky and superimposed onto the Eemian
From that chart it is clear that, as far as the natural forcings driving climate change are concerned, and IF those forcings had now the same values as they had in the Eemian, the Earth's temperature would start to drop very soon (in about 3 ky) and be 6 ºC lower than today's in about 15 ky.
It should be noted at this point that the cited Stuart's post of 2006 was about the risk of the global temperature (and sea level) reaching the Eemian's peak values, which for the sea level was 5 (five) meters higher than today's. Here we are focusing on the risk of the global temperature (and sea level) plunging as they did after the end of the Eemian, which for the sea level meant a drop of 50 (fifty) meters in 10 Ky, as from the following chart:
Figure 4. Sea levels for the last 140 ky
This possibility in principle could be viewed as actually good: a sea level drop could mean more arable land to compensate for that lost to advancing ice and colder temperatures. However, a new ice age could bring about yet another problem: drought. Quoting from http://en.wikipedia.org/wiki/Last_Glacial_Maximum
"In warmer regions of the world, climates at the Last Glacial Maximum were cooler and almost everywhere drier. In extreme cases, such as South Australia and the Sahel, rainfall could be diminished by up to ninety percent from present, with floras diminished to almost the same degree as in glaciated areas of Europe and North America. Even in less affected regions, rainforest cover was greatly diminished, especially in West Africa where a few refugia were surrounded by tropical grassland.
Most of the world's deserts expanded. ... In Australia, shifting sand dunes covered half the continent, whilst the Chaco and Pampas in South America became similarly dry. Present-day subtropical regions also lost most of their forest cover, notably in eastern Australia, the Atlantic Forest of Brazil, and southern China, where open woodland became dominant due to drier conditions. In northern China - unglaciated despite its cold climate - a mixture of grassland and tundra prevailed, and even here, the northern limit of tree growth was at least twenty degrees further south than today."
The quoted Wiki page cites as reference the studies listed at the Paleoclimate Modelling Intercomparison Project page http://pmip.lsce.ipsl.fr/publications/pub21k.shtml . However, there is the possibility that the Wiki page authors' work strayed from the findings of the referenced studies. And since most of them require a journal subscription for access, I could only have a look at The Paleoclimate Modeling Intercomparison Project, S. Joussaume and K. E. Taylor, proceedings of the third PMIP workshop, Canada, 4-8 october 1999, in WCRP-111, WMO/TD-1007, edited by P. Braconnot, 9-24, 2000.
"According to both sets of PMIP simulations the LGM (Last Glacial Maximum) climate is also more arid over most of the northern continents and in the tropics (Figure 7). Larger precipitation however occurs over tropical oceans, especially over the warmer pools of CLIMAP SSTs. Much smaller changes are found over tropical oceans when computing SSTs since the models simulate colder ocean temperatures than CLIMAP. At the regional scale, the simulations are characterised by a number of common features, including a reduction in the strength of the Afro-Asian monsoon (Braconnot, et al., subm.) and increased intertropical aridity, corroborated by various paleoindicators."
To note, this description of global climate and vegetation is about the Last Glacial Maximum, i.e. the coldest period immediately preceding TI. Now, how can we be sure that the "First Glacial Maximum" after an interglacial period does not share the same features?
And finally, it could be argued that, should the Holocene show signs of ending, the time for mankind to plan and prepare for it would be orders or magnitude greater than that allowed by AGW. Not so, according to this 2006 study: http://www.pnas.org/cgi/content/abstract/104/2/450
Evidence for last interglacial chronology and environmental change from Southern Europe
"Establishing phase relationships between earth-system components during periods of rapid global change is vital to understanding the underlying processes. It requires records of each component with independent and accurate chronologies. Until now, no continental record extending from the present to the penultimate glacial had such a chronology to our knowledge. Here, we present such a record from the annually laminated sediments of Lago Grande di Monticchio, southern Italy. Using this record we determine the duration (17.70 ± 0.20 ka) and age of onset (127.20 ± 1.60 ka B.P.) of the last interglacial, as reflected by terrestrial ecosystems. This record also reveals that the transitions at the beginning and end of the interglacial spanned only 100 and 150 years, respectively."
Therefore, if there were not anthropogenic warming forcings in play, the possibility that natural cooling forcings would bring the Earth into a new ice age within a few millenia cannot be discarded a priori. And since that outcome could set in fast and be most disruptive to civilization, it must be explicitely considered and discarded (or accepted as lesser evil) as part of any scientific study of AGW from which policy recommendations are to be derived.
Plainly stated, any scientific approach to the issue of AGW, particularly with the aim of offering policy recommendations based on it, which does not take into account the temperature record for at least the last 450 ky is bad science. Period.
The recent position statement from the American Geophysical Union is a fine example thereof:
"During recent millennia of relatively stable climate, civilization became established and populations have grown rapidly."
Sure folks, but the record for the last 450 millennia shows that periods like this of benign, stable climate have a length of just a few millenia, and that they are always followed by plunges into much longer ice ages. So prolonging the climate of the recent millenia for much longer may not be an option in the system design specifications, in which case either we do our best to break out of the cycle into a warmer period or we plunge into a new ice age soon.
"In the next 50 years, even the lower limit of impending climate change—an additional global mean warming of 1°C above the last decade—is far beyond the range of climate variability experienced during the past thousand years and poses global problems in planning for and adapting to it."
So, should we do our best to stay within the range of the past thousand years while hoping that the natural forcings will not push us down the slope this time? And if they do, would planning for and adapting to a new ice age pose no global problems?
"Warming greater than 2°C above 19th century levels is projected to be disruptive, reducing global agricultural productivity, causing widespread loss of biodiversity, and—if sustained over centuries—melting much of the Greenland ice sheet with ensuing rise in sea level of several meters."
So, should we opt for the risk of cooling greater than 6ºC instead? Because that path may be way more disruptive and ruinous for global agricultural productivity. Unless you are betting on the ensuing sea level drop of more than 50 meters to solve all those problems.
In contrast, the following position statements (if real, of course) would be examples of good science and sound recommendations:
A. "After having achieved a complete understanding of the natural forcings driving climate change, we have unequivocally determined that, purely out of them, the current interglacial period will have a total duration of around HH Ky - i.e. it will span a further FF Ky into the future - and that, as a result, mankind can safely embark on an aggressive reduction of CO2 emissions without fearing that the onset of a new ice age could take place in the next millenia."
B. "After having achieved a complete understanding of the natural and anthropogenic forcings driving climate change, and an accurate quantitative assessment of their relative weight, we have unequivocally determined that, if CO2 emissions are reduced according to the KKK protocol, the anthropogenic warming forcings will exactly compensate for the natural cooling forcings and the Earth will remain in the Holocene for ever after."
C. "After having achieved a complete understanding of the potential consequences of global warming versus those of the onset of a new ice age, we see unequivocally that the latter would be much more bearable for mankind. Therefore, we recommend an aggressive reduction of CO2 emissions while stocking up on sweaters."
I will next look at where current scientific knowledge stands relative to the possibility of issuing statements A. or B. above. (I don't think they will dare to state C.!) To that purpose, I will use as reference the reading materials used by Dr Ian Lawson in his 2007-2008 Palaeoclimate course at the University of Leeds, together with his comments on them.
Starting with hypothetical statement B., it is common knowledge that the natural forcings driving climate change are related to the periodical changes in the Earth's orbit around the Sun, collectively known as Milankovitch cycles, of which there are three with their respective periods:
Eccentricity (orbital shape): 413 ky for major component (eccent. variation ±0.012) (*)
Obliquity (axial tilt): 41 ky
Precession (wobble): 23 ky (actually 21-26)
(*) A number of other terms vary between 95 ky and 136 ky, and loosely combine into a 100 ky cycle (variation of −0.03 to +0.02).
Dr Lawson's comments on this subject should be taken seriously...
"Milankovitch is a very complex subject area – Huybers and Wunsch (2005) (*) estimate that there are more than 30 competing hypotheses to explain the relationship between orbital variations and climate! Use the reading materials below to consolidate and broaden your understanding of the topic, but bear in mind that you will only be examined on the subjects discussed in the lecture – the trick here will be to avoid getting bogged down in the complexities of the arguments."
(*) "Obliquity pacing of the late Pleistocene glacial terminations"
... and should lead us to think that scientific knowledge is still far from the point of being able to make an accurate quantitative assessment of the combined effect of natural and anthropogenic forcings that would enable the issuance of statement B.
That leaves us with statement A. Therefore, either science can determine that, purely out of the natural forcings, the Holocene will be much longer than the Eemian (the "Long Holocene" hypothesis), in which case it makes sense to embark on a policy of aggressive reduction of CO2 emissions, or we are left with the possibility that the Holocene, out of the natural forcings, could be just as wide as the Eemian (the "Short Holocene" hypothesis), in which case the outcome of such a policy might turn out to be a new ice age with a substantially lower sea level, an ice-covered North America, and drier climate everywhere. And at a time when there will be no fossil fuels to keep people warm.
The foremost proponents of the "Long Holocene" hypothesis are A. Berger and M. F. Loutre, researchers at the Institut d'astronomie et de géophysique Georges Lemaître, Université catholique de Louvain. They suggested that an orbital analogy could be made between the Holocene and MIS 11. This analogy results from a similar low level of eccentricity (the distance between MIS 11 and today corresponds to the 413 ky period in eccentricity). Notably, their first paper on the subject "Marine Isotope Stage 11 as an analogue for the present interglacial" was received by the Global and Planetary Change journal in 1999 but accepted only in 2002. The comparison was further strengthened by simulations conducted with a two dimensional climate model (the "Louvain-la-Neuve" or LLN model). Such simulations suggested that the orbital forcing would not be triggering a glacial inception until 50 ka after present. Their results were published in the 2003 paper "Clues from MIS 11 to predict the future climate – a modelling point of view" whose abstract says:
"Simulations performed with the LLN two-dimensional Northern Hemisphere climate model have confirmed that climate is largely triggered by changes in insolation forcing although atmospheric CO2 concentration also plays an important role, in particular in the amplitude of the simulated variations. Marine isotope stage 11 (MIS 11) some 400 kyr ago and the future share a common feature related to climate forcing, i.e. the insolation at these times displays small similar variations. MIS 11 can be considered an analogue for future natural climate changes. Different simulations were performed to identify the conditions constraining the length of the MIS 11 simulated interglacial. Clearly its length strongly depends on the phase relationship between insolation and CO2 variations. It is only when insolation and CO2 act together towards a cooling, i.e. they both decrease together, that the climate enters quickly into glaciation and that the interglacial may be short. Otherwise each forcing alone is not able to drive the system into glaciation and the climate remains in an interglacial state. The same situation applies for the future. However, we already know that CO2 and insolation do not play together. Indeed, insolation has been decreasing since 11 kyr BP and CO2 concentration remains above 260 ppmv, with a general increasing trend over the last 8000 yr. Therefore we conclude that the long interglacial simulated for the future is a robust feature and the Earth will not enter naturally into glaciation before 50 kyr AP."
On the other hand, the foremost proponent of the "Short Holocene" hypothesis is William F. Ruddiman. His hypothesis is actually more radical than mine. Let's recall that, from the chart in Figure 3, I had raised the possibility that as far as the natural forcings driving climate change are concerned, and IF those forcings had now the same values as they had in the Eemian, the Earth's temperature would start to drop very soon (in about 3 ky) and be 6 ºC lower than today's in about 15 ky. In contrast, what Ruddiman postulated in his 2003 paper "The Anthropogenic Greenhouse Era Began Thousands of Years Ago" was:
The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere. A different hypothesis is posed here: anthropogenic emissions of these gases first altered atmospheric concentrations thousands of years ago. This hypothesis is based on three arguments. (1) Cyclic variations in CO2 and CH4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO2 trend began an anomalous increase 8000 years ago, and the CH4 trend did so 5000 years ago. (2) Published explanations for these mid- to late-Holocene gas increases based on natural forcing can be rejected based on paleoclimatic evidence. (3) A wide array of archeological, cultural, historical and geologic evidence points to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia, including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago. In recent millennia, the estimated warming caused by these early gas emissions reached a global-mean value of 10 ppm in the last 1000 years are too large to be explained by external (solar-volcanic) forcing, but they can be explained by outbreaks of bubonic plague that caused historically documented farm abandonment in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for the observed CO2 decreases. Plague-driven CO2 changes were also a significant causal factor in temperature changes during the Little Ice Age (1300–1900 AD)."
A year later, using simulations with the "GENESIS 2" model, in a co-authored paper "A Test of the Overdue-Glaciation Hypothesis" (2MB PDF) Ruddiman et al reached the conclusion that, were it not for pre-industrial anthropogenic GHG emissions, global temperature today would already be 2ºC cooler. In their words:
"The ~2ºC mean-annual cooling simulated by removing anthropogenic greeenhouse gases is equivalent to roughly one third of the global-mean difference between full-interglacial and full-glacial climates. It also represents 80% of the warming simulated by the GENESIS 2 model in response to a doubling of modern CO2 (Thompson and Pollard, 1997). Without any anthropogenic warming, Earth’s climate would no longer be in a full-interglacial state but well on its way toward the colder temperatures typical of glaciations."
Interestingly, Ruddiman addressed the "Long Holocene like MIS 11" hypothesis in 2004 in "Cold climate during the closest Stage 11 analog to recent Millennia". Even more interestingly, Crucifix, Loutre and Berger published also in 2004 a Commentary on “The Anthropogenic Greenhouse Era Began Thousands of Years Ago” stating:
"We think that the only way to resolve this conflict is to properly assimilate the palæoclimate information in numerical climate models. As a general rule, models are insufficiently tested with respect to the wide range of climate situations that succeeded during the Pleistocene. In this comment, we present three definitions of palæoclimate information assimilation with relevant examples. We also present original results with the Louvain-la-Neuve climate-ice sheet model suggesting that if, indeed, the Holocene atmospheric CO2 increase is anthropogenic, a late Holocene glacial inception is plausible, but not certain, depending on the exact time evolution of the atmospheric CO2 concentration during this period."
Keeping in mind that these are the foremost proponents of the "Long Holocene" hypothesis, let's pay attention to the emphasized sentence: a delayed onset of the new ice age "is plausible, but not certain, depending on the exact time evolution of the atmospheric CO2 concentration"! (But fear not, for all the wise people currently advocating for drastic cuts in CO2 emissions are certainly aware of the full contents of this and subsequents studies from these researchers, and they have made sure that their recommended policies still allow for ample margin to avoid an early Holocene glacial inception.)
BTW, this is in line with Ruddiman's comment in a Dec 2005 archived discussion: "the fact that Berger’s model produced an extremely long interglaciation if he only used insolation forcing, but a much shorter one if he added CO2 (from Vostok) to the forcing;"
Moreover, the three Belgian researchers published in 2006 "The Climate Response to the Astronomical Forcing" stating:
"The Louvain-la-Neuve climate-ice sheet model has been an important instrument for confirming the relevance of Milankovitch’s theory, but it also evidences the critical role played by greenhouse gases during periods of low eccentricity." (And the Holocene is such a period.)
Therefore, two years ago the struggle between the two hipotheses was still unsettled. Looking now to other researchers for guidance, in Jul 2006 Peter Huybers published the potentially breakthrough paper "Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing" in which he states:
"Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, matching the period of changes in Earth's obliquity. The resolution of this 40,000-year problem is that glaciers are sensitive to insolation integrated over the duration of the summer. The integrated summer insolation is primarily controlled by obliquity and not precession because, by Kepler's second law, the duration of the summer is inversely proportional to Earth's distance from the Sun."
After which a numerous research team, using the EPICA Dome C measurements and the "ECBILT-CLIO" intermediate complexity climate model, in their Oct 2006 paper "Past temperature reconstructions from deep ice cores: relevance for future climate change" reviewed both Ruddiman's and Berger's hypotheses, and building on the just cited Huybers papers, concluded:
We propose that, when the eccentricity is small and therefore the precession minima and maxima are weak, the role of obliquity cannot be neglected in triggering deglaciations and glaciations. In this respect, it is important to see that there is no perfect orbital analog for the presentday and future orbital context (Berger et al., 1998). Today, the obliquity is at an intermediate value, whereas the end of MIS11 occurred when both the summer insolation and the obliquity were minimal. If obliquity is playing a larger than invoked role on ice ages, then it cannot be expected that an ice age would be to occur over the next 50 ka, until minima of obliquity and NH summer insolation coincide.
This analysis should be extended by taking into account the obliquity imprint on climate feedbacks such as the atmospheric dust and greenhouse gas content,"
While the conclusions of this study heavily lean towards the "Long Holocene out of just natural forcings" hypothesis, a 2005 study on a completely different area yielded results supporting Ruddiman's case: "Similarity of vegetation dynamics during interglacial periods" Analysing polen records, the authors found that the pattern of relative variation of different bioclimatic affinity groups (BAGs) in the modern records corresponded to that seen at the end of previous interglacials, and they ask:
"Does the pattern we observe in the uppermost part of the Velay pollen record indicate the end of the Holocene interglacial as it was seen in the previous interglacials? If not, then the Holocene would be a unique interglacial period during which either a reexpansion of BAGs 12 and/or 10 would take place or the coniferous forests would persist at the temperate latitude for several thousand years until the onset of the next ‘‘natural’’ interglacial period. Such a situation has not been observed during the prior four interglacials."
To sum up, advancing scientific knowledge seems, by the end of 2006 to have shifted the balance towards the "Long Holocene" hypothesis, although it is still far from reaching a firm consensus. Which leaves the treatment and policy recommendations on the subject of AGW issued before that date looking rather reckless, to say the least.
As for how far science still is from a firm consensus, a newly published paper by Raymo and Huybers is most eloquent:
"It is widely accepted that variations in Earth's orbit affect glaciation, but a better and more detailed understanding of this process is needed. How can the 41,000-year glacial cycles of the early Pleistocene be explained, let alone the 100,000-year glacial cycles of the late Pleistocene? How do the subtle changes in insolation relate to the massive changes in climate known as glacial cycles? And what are proxy climate records actually measuring? The field now faces these important questions, which are made all the more pressing as the fate of Earth's climate is inexorably tied to the vestige of Northern Hemisphere glaciation that sits atop Greenland, and to its uncertain counterpart to the south."