You sometimes hear people say that the temperature on a particular day was the “hottest ever”. But what they mean is the hottest ever recorded by humans. Because the truth is that, throughout almost all of earth’s 4,600 million year history, its average global temperature was about 8°C to 15°C warmer than today. This was so hot that, during most of earth’s history, there was no ice anywhere on earth.

Occasionally, however, these warm conditions were interrupted by very cold periods (called ice ages) in which the poles and mountains were covered in ice. We are living in an ice age now. The current ice age has lasted about 2 million years so far.

There are times during an ice age when the earth gets slightly warmer. Some but not all of the ice melts. Warm periods like this are called interglacials. We are living in an interglacial now. It began about 15 thousand years ago. An ice age has many of these interglacials. After a few tens of thousands of years the temperature drops and the ice gets thicker and covers huge areas near the poles. This is called a glacial period.

The current interglacial was warmest about 5 thousand years ago. Since then it has been steadily getting colder. The earth was heading towards another glacial period. From about 1300 to 1850 there were times when rivers froze completely and crops failed. This happened in the 1780s and it was a factor in causing the French Revolution. The average global temperature was then around 13°C, about 2°C colder than today.

There is reason to believe that, had people not started burning fossil fuels, we would now be entering glacial conditions. But around 1750 people started burning more and more fossil fuels.

By 1850 there was enough CO2 to make the temperature reverse this downward trend and start to increase to its present level. Carbon dioxide in the atmosphere rose from 280 parts per million (ppm) to the present level of about 400 ppm and the average global temperature rose to its current value of about 15°C.

If we keep burning fossil fuels and the global temperature keeps rising, we will almost certainly bring the current ice age to an end. We can expect average global temperature to reach the sort of level it has been throughout earth’s history, somewhere between 23°C and 30°C, with local and seasonal variations taking it sometimes much higher.

This will completely transform agriculture and have a devastating effect upon all life everywhere on earth. Migrations and extinction of animals and plants, which have already started because of the warming we have already caused, will affect far more species. Humanity will face mass flooding of many cities, desertification of many fertile lands and widespread shortage of fresh water. It seems highly likely that wars will occur over who controls the little remaining inhabitable land.

It is not too late to prevent this disaster, but it will require that every one of us change our life styles. Indeed the whole basis of human activity will have to change to put environmental protection at the heart of what we do. The choice is yours.

But even that will not solve the problem. Civilization has emerged during an interglacial, which is a very unusual time in earth’s history, neither too cold (a glacial) nor too hot (the normal situation on earth). Is it really going to be possible to maintain the climate at this delicate balancing point for thousands or millions of years?

Coping with the current global warming is only the beginning of humanity’s struggle to make its home planet habitable in the long term. Will our descendants look back at us and say their ancestors were living in a fool’s paradise?

The history of earth’s climate is called paleoclimate. More information on Wikipedia and on my website History of the Universe.