Once glacial conditions have been established (and they will), they will be unmistakable. Ice sheets will have covered large parts of the northern hemisphere making large swathes uninhabitable. Sea levels would have dropped by about 100 m. Global mean temperature would be around 10-12 ºC rather than the 15 ºC in an interglacial.
Habitable and fertile land would have increased around the equator and in the tropics – but not as much as would be rendered uninhabitable by the ice sheets. Modern technology and recourse to energy would still allow some exploitation of resources under the ice sheets. Precipitation levels would reduce however (with so much of the water cycle being bound up in the ice sheets). Some of the equatorial regions would see a desertification. New resources would be available due to the 100 m drop in sea level. Population would probably be significantly lower than during an interglacial but what population could be sustained comfortably will be strongly dependent upon the availability of energy and the ease of energy conversion. River flows and hydropower will dry up. Fossil fuels and nuclear energy is what will make the difference.
But whenever it comes, it will not happen overnight. It would take not less than a few hundred years for the transition from interglacial to glacial conditions but it might take 1000 years or more.
But how will we know if the transition has started? What are the signs to look for? For example a few years of reduction of global precipitation may mean nothing if at the same time an increase of water locked up as ice is not also evident.
Probably the most potent feedback loops (forcings) for the transition to glacial conditions is the ice cover on the earth’s surface and the cloud cover in the upper reaches of our atmosphere. Both of these act directly on the sun’s energy being reflected away from the earth and will shift the earth to a different paradigm of solar energy input. There may be other parameters which cause incoming solar insolation to vary but how much the earth reflects away of whatever is coming in is controlled by the ice and the high clouds. We can consider the interglacial and glacial conditions to be semi-stable equilibrium conditions, each representative of a particular level of solar energy input to the earth system.
So the first real indicators will be the growth of ice cover and an increase in high clouds. All other prior indicators must finally show up as ice cover or high cloud. Even global temperature (which is merely an averaged, composite, weighted artefact) is not of great relevance except when it shows up as ice or cloud. Note that ice cover at a lower latitude is of greater significance since it causes a greater reduction of received solar insolation than at the poles. For ice cover to be on an increasing path we will first see a reduction in the melt of the previous season’s ice – regularly. We should see this not only at both poles but also at lower latitudes on high ground. We will see warming factors being neutralised. We will see a decrease in precipitation but this will probably lag the reduction of ice cover and the reduction of sea level by many years. It might begin to show up first as a reduction of low rain clouds and increase of high clouds. We should see the sea level increase characteristic of an interglacial, level off and then begin to fall – slowly at first and then accelerating.
An impulse or trigger is needed to shift from one semi-stable equilibrium state to another. What that trigger – or those impulses – might be is unknown. But I note that
- A cooling cycle of 30 + years may have begun and may well be a trigger for a transition.
- Expected global temperature increase due to the undoubted (but small) effect of carbon dioxide concentration in the atmosphere is not happening and will not any time soon.
- Expected increase of sea level (even if based on fallacious CO2 based climate models) is not happening and sea level rise seems to be decelerating.
- Antarctic ice cover is at its highest level ever and has been increasing over the last few years.
- Arctic ice has recovered from its decrease of a few years ago and is at the same level as about 2 decades ago.
- Some unusual sign of fresh glacier formation have been observed on Ben Nevis.
- Some Himalayan glaciers. and even Alpen glaciers have shown signs of growth or reduced rates of decrease.
Just indicators of course – and another 50 or so years should tell.
But one thing is clear. Our future depends upon the availability of energy – and that will be primarily fossil and nuclear (and fusion if that has been developed by then). The pointless (and futile) attempts to curtail exploration for and the use of fossil fuels will have to cease – and better they be abandoned sooner rather than later.