Imbrie and Imbrie: Ice ages: solving the mystery: 1979

Firstly, let me say that this is an excellent book, and I recommend it to anyone interested in the question of ice ages. Now on...

Almost incredibly, this book was cited in support of the notion that an-immenent-ice-age-was-preditcted-in-the-1970s. Did the citer misremember the book? Fail to read it at all? Or deliberately misrepresent it? We may never know...

Too impatient to trouble yourself with reading even carefully chosen extracts? Then read my conclusion.

So: lets look at...

Chapter 16: the coming ice age

Perhaps some people never got past reading the chapter title? ;-)

Lets quote some of the chapter:

Unless there is some fundamental and unforeseen change in the climate system, most scientists who have examined the evidence agree that the world will experience another age of ice. But when? On this question, geologists disagree. Some predict that the present interglacial age will last another 50,000 years. Others, finding that the earth has been cooling for some time, believe that an ice age is already on its way - due within the next few centuries, according to one extreme view.

So: note that there is disagreement, and that the view that it would arrive within a few centuries is characterised as "one extreme view".

After discussing exactly what an ice age is, I+I put in:

By common agreement, therefore, a Pleistocene interglacial age is defined as any interval of time during which oak and other deciduous trees are widespread in Europe.

Now, how should we attempt to predict future ice ages?

...One approach treats the geological record of cliamte statistically, and uses the known duration of previous interglacials as a basis for estimating the remaining lifespan of the present interglacial... But only insurance companies are satisfied by statistical predictions of this kind.

So: this approach is not recommended by I+I. Continuing...

A more satisfactory way of predicting... is to project current cliamtic trends into the future

I would disagree: this is not a good method of prediction. Ho hum. On...

One such trend is the long-term cooling that began 7000 years ago... If this trend were to continue, global temperatures would reach ice-age levels (6 oC cooler than they are today) some 18,000 years from now... A cooling trend of much shorter duration than 7000 years was first identified by J. Murray Mitchell, Jr in 1963... global climate has been cooling since 1940. Over a 20-year period, he found, the average temperature of the northern hemisphere has fallen about 0.3 oC

Note the uncharacteristically careless use of global (my emphasis) above: they mean NH.

As Mitchell pointed out, however, there is only one thing certain about climatic trends: they can and do reverse themselves. And by the mid 1970s, it was by no means clear that the cooling trend that began in 1940 was continuing... And since no one has yet been able to produce a convincing explanation of Mitchell's trend, those prophets of doom who use such data to predict an early end to the present interglacial age are in danger of repeating the error [of assuming that cycles are trends]

So... without saying anything much about the 7000-y trend, I+I are clearly not convinced by the since-1940s trend. Note that recent (2001) climate model results can explain the cooling trend of those times, mostly from anthropogenic sulphate aerosols from relatively "dirty" fossil fuel consumption (ref: Hadley Centre).

Now, I+I come on to more plausible (physically based techniques):

The astronomical theory of the ice ages provides a basis for forecasting the future climate that avoids the uncertainties inherent in prediction based on trends...

And the result is:

...the cooling trend that began 7000 years ago will continue into the future, and lead to a maximum advance of the glaciers 23,000 years from now

So, there we have it? Not quite...

However, the long-term cooling trend predicted by the astronomical theory will undoubtedly be modified by climatic oscillations of much shorter duration... If Denton and Karlen are correct, the warming efect of the present cycle (which began in 1700) will soon override the cooling effect of the astronomical cycle and cause temperatures to rise for the next 1000 years

So... is that now it? No...

Such is the forecast suggested by present knowledge of natural climatic cycles: 1000 years of warming followed by 22,000 years of cooling. But this forecast does not take into consideration the impact of an "unnatural" agent... Viewed in a geological perspective, it is likely that consumption of the bulk of the worlds known fossil fuel reserves would plunge the planet into a "super-interglacial age", unlike anything experienced in the last million years. Moreover, the effect of carbon dioxide would endure for a thousand yearrs or more after the use of fossil fuels ceased..."

And then goes on to say that once CO2 levels return to "normal", the astronomical cycles would reassert themselves, leading to cooling.


It is clear from the above that I+I regard that the short-term (1000-y) prediction from astonomical cycles is warming; that they expect astronomical cycles to be overwhelmed by CO2 forcing in the near-term; and that CO2 forcing would lead to warming.

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