All of Human civilization fits neatly into the current interglacial period. The development of agriculture, settlements and societies were all enabled by a beneficial climate for the last 10,000 years. Interglacials usually average ~10,000 years, so is our luck about to run out? It turns out that the answer is no, because we are very fortunate that human society has developed during an interglacial when the earth’s orbit has very low eccentricity. Eccentricity is important because it regulates the strength of polar maximum summer insolation caused by precession of the equinoxes every 21,000 years. Precession determines the distance from the sun during a Polar summer. If summer coincides with the earth’s perihelion then summer insolation can be up to 20% higher than average. However if the earth’s orbit is nearly circular, as it is today, then precession has little effect at all. That is why we have about 12000 years left before cooling begins.
The peak of the warmer Eemian interglacial 120,000 years ago lasted less than 10,000 years, because the much larger eccentricity at that time enabled the first minimum precession summer to increase the spread of northern ice sheets.
One needs to go back 420,000 years to find a similar glacial cycle to the current one at low eccentricity. This is known as the Anglian glaciation because the ice sheets spread as far south as Anglia and diverted the Thames southwards from the Wash to its current basin. The Anglian had very similar orbital parameters to those we experience today.
This result implies that the the current interglacial would naturally last another 20000 years. However, the alignment of precession is not perfect, and the north-south precession cycle is inverted. Despite this, at very low eccentricity, it is only obliquity that really counts. We conclude that within 12000 years the earth would naturally be returning to a new ice age lasting 100,000 years. The earth then enters another long period of high eccentricity lasting a further 400,000 years. Future Interglacials will last only ~10,000 years, before the cycle repeats. One only needs to look at how transient interglacials were 600,000 years ago when eccentricity was high.
Now we look at an even more remarkable correlation. Eccentricity has an even longer cycle with a time base lasting 2.8 million years. The following plot is the result of calculations by Laskar and his group covering 50 million years.
The mid-Pliocene was much warmer than today with CO2 levels similar to those caused by man today (400ppm or above). This is about as warm as most climate models predict the earth will be by 2100 – i.e. about +2C above current temperatures. The full 5 million year record of Benthic Fora data gives clear evidence of Milankovitch cycles throughout the period, including a 420K eccentricity cycle in earlier times. However by 3 million years ago glacial cycles had begun to follow a regular 41K obliquity cycle. It was only much later (< 1 million years) that 100k deep glacial cycles began.
So can we learn anything by looking at the data 2.8 million years ago in the super-cycle? The plot below shows the Benthic Fora data (Ice volume proxy for temperature) compared to the Milankovitch cycles
Even 2.8 million years ago it seems that the decrease in polar insolation due to decreasing obliquity needed a helping hand to enter another glaciation period.
Even without human induced increases in CO2 the current interglacial was set to last nearly as long as the Anglian. The minimum obliquity is due to be reached in 11,000 years time and a minimum Arctic summer in 13,000 years time. A spread in ice sheets and cooling would normally be expected to have started before then. Has global warming delayed this process and if so then by how much ?
If we assume emissions continue to the end of this century and then reduce as we develop other energy sources, then the temperature response might look something like this.
We can argue about how warm the peak temperature will get and CMIP5 models vary about this roughly between 2 and 5 degrees. However this manmade climate disturbance should last for not much more than 3000 years so long as our emissions are reduced before 2100. The real question is what level we should then try to keep CO2 to avoid another devastating glaciation in 13,000 years time? If we want to survive long term then probably we should never let CO2 fall below 300ppm ever again!