History of Climate Change

We shall start with a relatively non-contentious idea that climate change has been happening on planet Earth for a very long time. We might then describe the extremes of climate change in terms of a series of ice-ages , which extend back in geological time long before humans were around to influence the global climate. Of course, while we will need to better understand why such extremes of climate change occurred, we possibly need to outline the overall scope of successive ice-ages. The table below defines the major ice-ages interspersed by more temperate periods, where all figures are in millions of years calculated from the present time.

Age Start Duration Cold Warm
Huronian 2400 300 300
temperate 2100 1250 1250
Cryogenian 850 214 214
temperate 636 176 176
Andean-Saharan 460 40 40
temperate 420 60 60
Karoo 360 100 100
temperate 260 257.42 257.42
Quaternary 2.58 2.58 2.58
656.58 1743.42

While some of these ice-ages lasted millions of years, the evolution of life on planet Earth has also been supported by longer periods of more temperate climates. As a general definition, an ice age is a period of long-term reduction in the temperature of Earth's climate, which results in an expansion of the northern and southern polar ice sheets, but not necessarily to the same degree. However, an ice-age may also include cycles of colder and warmer periods known as glacial and interglacial epochs, which correspond to glaciers advancing and retreating respectively. However, what might not be obvious from the table above is that we are still living in the ‘quaternary’ ice-age, such that we need to provide a little more detail, where the figures in the next table are now in thousands of years from present time.

Ice-Age Epoch Type Start End Duration
Quaternary Pleistocene Glacial 2580 11.7 2568.3
Holocene Interglacial 11.7 0 11.7

In terms of the geological time of planet Earth, which covers some 5 billion years, the evolution of life is assumed to have started from single-cell organisms, possibly as long ago as 3.8 billion years. Then, somewhere in the temperate period after the Huronian ice-age, life evolved to take on multi-cell forms, which in the Cambrian era exploded in diversity, some 550 million years ago. Again, we might recognise that this was in another temperate period after the Cryogenian ice-age. Of course, the evolution of life has many chapters, such that we need to  jump forward to the start of quaternary ice-age in which some of the earliest hominoids started to appear in Africa, presumably far from the advancing glaciers of the Pleistocene epoch. However, we know that earliest homo-sapiens may have started to migrate out of Africa hundreds of the thousands of years ago long before the interglacial retreat of the Holocene epoch. However, the main exodus towards Europe, which would have still been in the grip of the Pleistocene glacial epoch, is assumed to have taken place around 70,000 years ago. At this time, the harsh climate conditions in the northern hemisphere may have acted as another natural selection criterion to which many other species succumbed, but humanity possibly survived by using its intelligence. In this respect, one of the key differences between life and death was possibly the ability to light fires to stay warm and cook food.

Note: In the context of the present-day debate about climate change, it is possible that those of us lucky enough to be the beneficiaries of an earlier industrial era, in which fossil fuels were increasingly used, may sometimes forget our ongoing dependency on ever more energy. Therefore, even if CO2 was the only driver of present-day climate change, any solution to reduce CO2 cannot be at the expense of increasing energy demands, especially those in developing countries, who may not have access or be able to afford the cost of renewable sources of energy. As such, the CO2 debate is not just a climate change problem for scientists to solve, but also a humanitarian problem with social, economic and political consequences.

We know that humanity continued its expansion into the current Holocene epoch, which began after the Pleistocene epoch ended, some 11,500 years ago. We might also assume that the warmer periods within the Holocene interglacial epoch, which saw the glaciers in the northern hemisphere retreat, also helped support the expansion of humanity in many geographic regions. Of course, the development of successive civilizations would also ultimately lead towards a global urbanisation, which now defines much of the developed world, where 80% of its energy dependency is still based on the use of fossil fuels. However, we have jumped over a lot of climate change history, which may be relevant to the debate, where all figures in the following table are now in years.

Epoch Transition Start End Duration Comments
Pleistocene Glacial
2,580,000 12,800 2,567,200 Last glacial maxima
12,800 11,500 1,300 Interglacial transition halted
Holocene Interglacial
11,500 9,000 2,500 Initial warming transition
9,000 5,000 4,000 Human civilisation flourish
5,000 1,060 3,940 To 950CE:
Increasing climate changes
1,060 760 300 950-1250CE:
Warmer than today
760 660 100 1250-1350CE:
More extreme weather
660 160 500 1350-1850CE

At the end of the last glacial epoch, it is estimated that the average temperature was anywhere between 5-20oC lower than today across various geographies, while the huge volume of water locked up in the polar ice-caps may have lowered sea-levels by up to 120 metres. However, after humanity transitioned into the Holocene epoch, it was to become a beneficiary of some 4000 years of relatively benign climate conditions known as the ‘ Holocene climate optimum’ , which may have seen average temperatures 1-2oC higher than today, although this was not uniform across all geographies and latitudes. We might also realise that the warming conditions started to melt huge volumes of polar ice, which then caused sea-levels to start rising, although this trend was sometimes reversed during a period of cyclic change between 5000-1060 years ago. Again, as we enter the period known as the ‘ Medieval anomaly’ temperatures had stabilised and estimated to possibly have been warmer than today. Finally, we see the start of a period called the ‘Little Ice-Age’ , which lasted some 500 years up until 1850 in the common era (CE). During this period, many geographies in the northern hemisphere experienced much colder climates than today and, in some regions, glaciation started to advance, while in Northern Europe, agriculture became increasingly difficult.

But what else might all these climate cycles be telling us?

In the context of the Holocene epoch of the last 11,500 years, we can see multiple periods of warming and cooling of the Earth’s climate, which predates any significant impact of human industrialisation, i.e. CO2 emissions. However, we now know that CO2 concentration have increased by about 40% since 1750CE, i.e. from 280 to 390ppm, although it is not clear that we know how much the world would have warmed if CO2 levels had not changed. For, in some respect, the ending of the ‘little ice-age’ could simply be representative of one of the many cycles between colder to warmer climates, as seen throughout the Holocene epoch.

Note: While we will not immediately address the issue outlined above, we might note that current IPCC climate models have not necessarily always been accurate in their temperature projections, such that it is not unreasonable to still question the actual causes of climate change since 1750CE. For while a 40% increase in atmospheric CO2 may seem significant, it only represents a 0.011% change to the composition of the Earth’s atmosphere over the last 250 years or so.

If we put the issue of CO2 levels to one side for the moment, it is clear from this outline of the different ice-ages, especially in terms of the more detailed outline of the Holocene epoch, climate change has occurred without any help from humanity. If so, then other mechanisms must be at work that require further understanding. However, before attempting to address this issue, the next discussion will return to some of the wider implications of enforcing CO2 reductions on today’s global population, because as previously suggested, humanity is now very dependent on energy, mainly produced using fossil fuels. For energy has come to underpin all aspects of modernity, such that its cost and availability have social, political and economic implications, where any failure to meet increasing demand, especially in developing economies, may contribute to even more global instability.