The Ice Age Cometh – climate change explained in three graphs [1]
So, climatologists observe, a cycle
Hath smaller cycles that on them prey
And these have yet smaller ones to bite ‘em
And so proceed ad infinitem
Thus, every Era in its kind
Repeats all those that come behind
RM (with apologies to Jonathon Swift, 1733)
At the most fundamental level, Earth’s history is divided into four vast periods of time called Eons. Using geologist’s shorthand, where Ma is 1 million years and Ga is 1 billion years, they are:
the PHANEROZOIC (0 to 543Ma); the PROTEROZOIC (543Ma-2.5Ga); the ARCHAEAN (2.5Ga-4Ga) and the HADEAN (4Ga-4.6Ga).
With our short life spans, we have trouble comprehending these gulfs of time. This can lead us to overestimate the significance of the age in which we live. – a form of hubris.
It may help to realize that, on comparative geological/human time scales, the Hadean commenced 20 years ago and the Phanerozoic 6 months ago. Eight days ago, towards the end of the Phanerozoic, dinosaurs roamed the lands, seas and air of the world and likely thought they were masters of their universe (then the meteor struck). An Ice Age called the Quaternary began around 20 hours ago. Five minutes ago, during a brief period of ice retreat in the Quaternary called the Holocene, our turbocharged civilisation took off and thrived. A lifetime of human experience is gone in less than a second.
Such a timetable should teach us humility.
543 million years of Climate Change
There have been (at least) five Icehouse Earths (a.k.a Ice Ages) in the geological record, separated by Hothouse Earths. What caused these occasional periods of intense cold? It may be variations in the output of the sun, it may be changes in the distribution of land and sea caused by Plate Tectonics, but the short answer is, we just don’t know.
Each Ice Age lasted between 20 and 50 million years and, between them, make up around 4% of geological time. Three of them occurred in the Phanerozoic. This is the Eon when general warmth, along with high levels of oxygen and CO2 in the atmosphere, drove the evolution of complex multicellular life forms on land and sea to produce the earth as we know it today.
Figure 1: 543Ma (million years) of Climate Change. The horizontal axis shows millions of years before present. On the vertical axis, temperatures are estimated from rock proxies. Average temperature during these 540Ma was 20°C, but range from 28° to minus 12° (In 2016 it was 14°). PETM=Paleocene-Eocene Thermal Maximum (55.8Ma); EEOC=Early Eocene Climate Optimum (42Ma); MECO=Mid-Eocene Climate Optimum (15-30Ma); EOT=Eocene-Oligocene Transition (40-33Ma); MMCO=Mid-Miocene Climate Optimum (15-13Ma); LGM-Last Glacial Maximum (21Ka); PAW= Post-Anthropogenic Warming (a projected future temperature from IPCC worst-case computer models). White stars indicate rapid cooling events: black stars rapid warming events. Reference: Christopher R Scotese, 2016 (Northwestern University) – Phanerozoic Temperature Curve. Palaeomap Project, Evanston II.
Homo Sapiens evolved during the last of these Ice Ages, a Period known as the Quaternary. It began 2.6 million years ago following 60 million years of slow cooling from the Late Cretaceous hothouse conditions. We do not know how long the Quaternary Ice Age will last, but the evidence suggests it still has many tens of millions of years to go.
450 thousand years of Climate Change
During the Quaternary, glaciers advanced south and north from the poles, covering land and sea in kilometer-thick ice, sculpting the earth beneath. Sea levels fell and species were widely redistributed.
If we bore down into the Quaternary, we see a pattern of repeated advances and retreats of the ice: 100-150 thousand years of advance followed by 10–15-thousand-year intervals of retreat and moderately-warm weather. We call these brief intervals Interglacials. Throughout the Quaternary there were dozens of these glacial/interglacial cycles. They are called Dansgaard-Oeschger Events (after two awkwardly named Danes). It is widely accepted that D-O Events were caused by the near-metronomic beat of wobbles in the tilt of the Earth’s axis and variations of its orbit around the sun (the Milankovitch Cycles).
We live in a Quaternary Interglacial called the Holocene. The previous Interglacial is called the Eemian.
Figure 2: This chart shows temperatures during the last 20% of the Quaternary Ice Age (540 thousand years to present). it is based on deep ice cores from Antarctica. As ice accumulated over time through surface snow fall, the ratios of oxygen isotopes in the entrained air provide a continuous record of ancient atmospheric temperature. Ice cores from Antarctica provide our longest and best calibrated record. Calculated temperatures are consistent with results from other proxies around the globe. Reference www.climatedata.com
During the Eemian Interglacial (which was warmer than today) hippopotami gamboled in the River Thames. 100,000 years later, with the arrival of the Holocene, human civilisation took off, and large mammals gamboled once again on the Thames.
10 thousand years of Climate Change
As you can see from figure 2, all the Interglacial events of the Quaternary followed the same pattern – a rapid 8°-10°C rise in air temperature over a just a few decades, followed by a slow fall over the succeeding millennia. The relentless fall of temperatures through the course of the Holocene Interglacial from the peak of 8-9000 years ago (figure 3) matches this pattern and is an ominous pointer to what is in store for us.
Figure 3: A compilation of global average surface temperature over the past 10,000 years. Based on different types of proxy data from around the globe. Note the gradual decline of temperatures from the Holocene Climate Optimum of 8-4000 years ago, the minor temperature peaks of the Minoan, Roman, Medieval and Modern warm periods, and the deep trough of the Little Ice Age. Nature Sci Data 7, 201 (2020). https://doi.org/10.1038/s41597-020-0530-7
There is nothing surer in science than that the ice will return. In fact, it is overdue.
Will we survive the next ice advance?
The Modern warming trend (which began in 1976 – a microsecond ago) gives faint hope of a delay in the inexorable return of the ice. But when it does return in 100-, 500- or 1000-years’ time, as a species we will most likely survive. After all, with lesser technology, we survived the last advance of the ice. But will our civilisation?
Our few descendants (as they shiver in their caves) will look back with puzzlement on the Global Warming hysteria [2] of their 21st Century ancestors.
Image from the internet
[1] The graphs summarize the best available data and will inevitably be modified as knowledge advances. They show global trends which regional factors may locally amplify or smooth.
[2] Hysteria? You think perhaps I indulge in hyperbole here? I read today (H/T Alan Moran) that the World Bank has just approved a large loan to Guinea Bissau so that said GB can reduce their CO2 emissions by 30% in the next 5 years. Guinea Bissau is a corrupt (they come at number 163 out of 180 countries in the 2023 World Corruption Index), politically unstable (8 military coups since independence, the last in 2023) West African nation of 2 million people where two thirds live below the poverty line and 70% have no access to mains electricity or piped running water. The country’s annual CO2 emissions are about that of a Taylor Swift concert tour. This sad country needs many things, but decarbonization is not one of them. Only in a world where irrational hysteria about Global Warming and rising CO2 levels ignores real world data or even common sense, could such a mad decision be made.