Will the Ice Age Cycle stop Global Warming?

I promised  in the previous post to complete my review of long term influences on climate by taking a look at the Milankovitch cycles on our climate - the cyclical effect of changes in Earth's relationship to the Sun.

First a really long look back. This figure looks back 542 million years. Click on the images if you need to make them bigger.

Fig 1

http://commons.wikimedia.org/wiki/File:All_palaeotemps.png

Note that the sections have different time scales, with the more modern periods, towards the right, measured in thousands, and the earlier section measured in millions.  The Earth formed 4.54 billion years ago, so palaeoclimatologists have quite a bit more work to do. On the left we have earlier temperatures,  when it was about 8C hotter than present. Dinosaurs began during the Triassic (230 million years ago, in the yellow-red section), and lasted until 65 million years ago - at the beginning of the second panel from the left.  Homo sapiens emerged around 200,000 years ago, so we have been around only for one and a half Ice Age cycles.

Figure 2 shows the latter half of Figure 1. Note that the writing is backwards because I have flipped it so that modern times are on the right, as in Fig 1. I have numbered the ice ages 1-5 for convenience. We live on number 5, at a peak of the upswing which has lasted about 10,000 years.

Our civilisations began about 12,000 years ago. It took about 6,500y for temperatures to climb from the last Ice Age to the present warmth, so civilisations began in the warming period, and also in warmer parts of the world. Global temperatures were perhaps about 4*C cooler than today.

Fig 2

The graph runs back 400,000 years. Temperatures are blue, CO2 is green, and the red is dust. Glacial (low temperature) periods are cooler and drier, hence the dust.

Notice that the normal CO2 at the top of an upswing ("Interglacial") is 280 ppmv, as it was in 1750 before we began to burn fossil carbon which had been taken out of the atmosphere millions of years ago. It is now approaching 400 ppmv, higher than it has been for the last four Ice Ages. See Fig 3.

Looking more closely at our warm period, the Holocene, in Fig 1 (remember that the last panel in Fig 1 represents the rightmost blue band in Fig 2) we can see that the temperatures have been declining slowly since the Climatic Optimum. The Holocene temperatures have been warm for longer than the other four peaks, so we are due to begin a downswing any time soon. "Soon" in geological terms is measured in tens of thousands of years. 

Looking at the length of time for each of the maxima at the modern level of temperature, their duration varies from 4000 y to 18000 y. Our interglacial, the Holocene, stands in the middle of the range - about 11,000 years. So we could begin to move to the next glacial period any time between now and 7000 AD. Or it may not come until 19,000AD.

Is there some kind of natural limit on the temperature of the planet represented by the peaks reached? 

Ice Ages 2 and 4 (on Fig 2) are nearly 2*C higher than modern levels, so it looks as if we have not hit any natural limit yet.

Also, CO2 is significantly higher than in previous Ice Ages:

Fig 3

Therefore, there is no reason that our CO2 should not take us into uncharted planetary heating territory.

Will the next Ice Age descent, due to takes place sometime in the next 19,000 years, be sufficient to cool us down? 

We can work out how much impact our enhanced greenhouse effect will have on that descent.

Descent to a glacial period from an interglacial peak takes somewhere between 5,000 and 20,000 years.  If we take an average figure of 15,000 years for global temperatures to fall 8.5*C, this gives a 1C drop every 1,765 yrs, or 0.006C per decade.

A rapid fall lasting only 5,000 yrs would be at a rate of 0.017C per decade.

Now assuming that CO2 will double in three hundred years, (1750-2050) and that that doubling will raise planetary temperatures by 1.2C, we calculate 1.2/300*10 = 0.04.

That is, anthropogenic warming gives 0.04C warming per decade. Which is 6.7 times greater than average Ice Age cooling.

My estimate of warming is conservative. Spencer and Christy, well-known climate contrarians and satellite temperature scientists, give rates of warming as 0.14C per decade over the three decades to 2010. This warming is 8 times faster than the fastest cooling expected in the next Ice Age.

So the Milankovitch cycle will just slow the rate at which we heat the planet, it will not cool us down again. Even the mighty Ice Age cycle will not save us from the effects of our fossil fuel addiction.

I am surprised at this, and I cheerfully accept that I may be wrong. I may have messed up on my calculator, or on my measurements (paper, pencil and ruler). The sheer crudity of my bucket climatology may have missed something. Though on the other hand, it is so simple, I cannot see what I may have missed.

If anyone can do better - particularly, if anyone can show that there is some reason that the next Ice Age will save us - then please let me know. Unfortunately, there are proper scientists who believe that our CO2 will delay the next Ice Age.

If I am in the right ball park, then there is no more point looking to Milankovitch Cycles to save us than there is in praying to the Climate Fairy.

We have to save ourselves by decarbonisation of our energy systems and sequestration of atmospheric CO2.

See also