I started off working on a commentary to the WSJ editorial. In doing so, I looked back at IPCC 1990, which is not necessarily handy for most readers, and found myself starting to quote from it so extensively that I’ve simply typed up an extended excerpt, which I’ll use as a source. I’ll make a couple of comments here, but mostly I’ll re-visit it later.
The Executive Summary to chapter 7 stated (p. 200) stated:
We conclude that despite great limitations in the quantity and quality of the available historical temperature data, the evidence points consistently to a real but irregular warming over the last century. A global warming of larger size has almost certainly occurred at least once since the end of the last glaciation without any appreciable increase in greenhouse gases. Because we do not understand the reasons for these past warming events, it is not yet possible to attribute a specific proportion of the recent, smaller warming to an increase of greenhouse gases. [my emphasis]
Later, as you will quickly see, they discuss the Younger Dryas as an example of such a cooling and warming event. This last sentence conveys very precisely one of my main concerns: attribution of causation for past events. Looking ahead, it is extremely interesting to see what happens to the Younger Dryas and attribution of past changes in IPCC SAR and IPCC TAR. Obviously SAR and TAR spend lots of time talking about the issue of attribution in the 20th century, but how do they go about superceding this very specific 1990 finding. I’m not talking about whether there is or isn’t an explanation in the literature for the Younger Dryas (for example, whether a pulse of meltwater is or isn’t a good explanation), but how or even whether the later IPCC reports turned their collective attention towards attribution of the Younger Dryas and like past events in order to overrule the finding of IPCC 1990.
Similarly, it will be interesting to look in detail as to whether for the later reports is whether changed paleoclimate views, especially in TAR, are due to substantive new information (and, if so, what) or simply spinning differently than 1990. Anyway, back to the running text:
Virtually all our information about modern climate has been derived from measurements which were designed to monitor weather rather than climate change. Even greater difficulties arise with the proxy data…which must be used to deduce the characteristics of climate before the modern instrumental period began. So special attention is given to a critical discussion of the quality of the data on climate change and variability and our confidence in making deductions from these data.
Note that we have not made much use of several kinds of proxy data, for example tree ring data, that can provide information on climate change over the last millennium. [my bold] We recognise that these data have an increasing potential; however their indications are not yet sufficiently easy to assess nor sufficiently integrated with indications from other data to be used in this report. Climate varies naturally on all time scales from hundreds of millions of years to a few years. Prominent in recent Earth’s history have been the 100,000 year Pleistocene glacial-interglacial cycles when climate was mostly cooler than at present (Imbrie and Imbrie, 1979). This period began about 2,000,000 years before the present time (BP) and was preceded by a warmer epoch having only limited glaciations, mainly over Antarctica, called the Pliocene.
Global surface temperatures have typically varied by 5-7 degrees C through the Pleistocene ice age cycles, with large changes in ice volume and sea level and temperature variations as great as 10-15 deg C in some middle and high latitude regions of the Northern Hemisphere. Since the beginning of the current interglacial epoch about 10,000 years BP, global temperatures have fluctuated within a much smaller range, Some fluctuations have nevertheless lasted several centuries, including the Little Ice Age which ended in the nineteenth century and which was global in extent. …
There is evidence that rapid changes in climate have occurred on time scales of about a century which cannot be directly related to orbital forcing or to changes in atmospheric composition. The most dramatic of these events was the Younger Dryas cold episode which involved an abrupt reversal of the general warming trend in progress about 10,500 BP as the last episode of continental glaciation came to a close. The Younger Dryas is an event of global significance; it was clearly observed in New Zealand (Salinger, 1989) though its influence may not have extended to all parts of the globe (Rind et al, 1986). There is, as yet, no consensus on the reasons for this climatic reversal, which lasted about 500 years and ended very suddenly. However because the signal was strongest around the North Atlantic Ocean, suggestions have been made that the climatic reversal had its physical origins in large changes in the sea surface temperature (SST) of the North Atlantic Ocean.
One possibility is that the cooling may have resulted from reduced deep water production in the North Atlantic following large-scale melting of the Laurentide ice sheet and the resulting influx of huge amounts of low density freshwater into the northern North Atlantic ocean (Broeker et al., 1985). Consequential changes in the global oceanic circulation may have occurred (Street-Perrott and Perrott, 1990) which may have involved variations in the strength of the thermohaline circulation in the Atlantic. This closed oceanic circulation involves northward flow of water near the ocean surface, sinking in the sub-Arctic and a return flow at depth. The relevance of the Younger Dryas to today’s conditions is that it is possible that changes in the thermohaline circulation of a qualitatively similar character might occur quickly during a warming of the climate induced by greenhouse gases. A possible trigger might be an increase of precipitation over the extratropical North Atlantic (Broeker 1987) thought the changes in ocean circulation are likely to be considerably smaller than in the Younger Dryas. Section 6 gives further details.
The period since the end of the last glaciation has been characterized by small changes in global average temperature with a range of probably less than 2 deg C (Figure 7.1) though it is still not clear whether all the fluctuations indicated were truly global. However large regional changes in hydrological conditions have occurred, particularly in the tropics. Wetter conditions in the Sahara from 12,000 to 4,000 years BP enabled cultural groups to survive by hunting and fishing in what are today almost the most arid regions on Earth. During this time Lake Chad expanded to become as large as the Caspian Sea is today (several hundred thousand sq km, Grove and Warren, 1968). Drier conditions became established after 4,000 BP and many former lake basins became completely dry (Street-Perrott and Harrison, 1985). Pollen sequences from lake beds of northwest India suggest that period with subdued monsoon activity existed during the recent glacial maximum (Singh et al, 1974) by the epoch 8,000 to 2,500 BP experienced a humid climate with frequent floods.
Original Caption: Figure 7.1. Schematic diagrams of global temperature variations since the Pleistocene on three time-scales: (a) the last million years; (b) the last ten thousand years, and (c) the last thousand years. The dotted line nominally represents conditions near the beginning of the twentieth century.
There is growing evidence that worldwide temperatures were higher than at present during the mid-Holocene (especially 5,000-6,000 BP) at least in summer, though carbon dioxide levels appear to have been quite similar to those of the pre-industrial era at this time (Section 1). Thus parts of western Europe, China, Japan, the eastern U.S.A. were a few degrees warmer in July during the mid-Holocene than in recent decades (Yoshino and Urushibaru, 1978; Webb et al 1987; Huntley and Prentice, 1988; Zhang and Wang, 1990). Parts of Australasia and Chile were also warmer. The late tenth to early thirteenth centuries (about AD950-1250) appear to have been exceptionally warm in western Europe, Iceland and Greenland (Alexandre 1987; Lamb, 1988). This period is known as the Medieval Climatic Optimum. China was, however, cold at this time (mainly in winter) but south Japan was warm (Yoshino 1978). This period of widespread warmth is notable in that there is no evidence that it was accompanied by an increase of greenhouse gases. Cooler episodes have been associated with glacial advances in alpine regions of the world such neo-glacial episodes have been increasingly common in the past few thousand years.
Of particular interest is the most recent cold event, the Little Ice Age which resulted in extensive glacial advances in almost all alpine regions of the world between 150 and 450 years ago (Grove 1988) so that glaciers were more extensive 100-200 years ago than now nearly everywhere (Figure 7.2) Although not a period of continuously cold climate, the Little Ice Age was probably the coolest and most globally extensive cool period since the Younger Dryas. In a few regions, alpine glaciers advanced down-valley even further than during the last glaciation (Miller, 1976). Some have argued that an increase in explosive volcanism was responsible for the coolness (for example Hammer 1977; Porter 1986); other claim a connection between glacier advances and reductions in solar activity (Wigley and Kelly, 1989) such as the Maunder and Sporer solar activity minima (Eddy, 1976) but see also Pittock (1983).
At present there is no agreed explanation for these recurrent cooler episodes. The Little Ice age came to an end only in the nineteenth century. Thus some of the global warming since 1850 could be a recovery from the Little Ice Age rather than a direct result of human activities. So it is important to recognize that natural variations of climate are appreciate and will modulate any future changes induced by man.
They go on to discuss the Pliocene (3,300,000 to 4,300,000 BP), the Eemian interglacial optimum (125,000-130,000 BP) and the Holocene Optimum (5,00-6,000 BP); then a lengthy discussion of the instrumental record, noting factors such as changes in observation time, thermometer exposure, thermometer heights, relocations, urbanisation, bucket bias. The warming from late 19th to the 1980s is estimated at 0.3 — 0.6 deg C. Lots of sensible material here.