The Ice Age Cometh – climate change explained in three graphs (1)
So, geologists 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 (after Jonathon Swift, 1733)
The Big Picture
An Ice Age is a significant period of time (tens of millions of years) when permanent ice exists at both poles. In the Earth’s 4,600,000,000-year-long history (4.6Ga) we have good evidence for at least 7 Ice Ages, but there were probably more. Each lasted around 20-50 million years and together make up around 4% of geological time. Ice Ages are not regularly spaced in time, and they show no apparent correlation with major tectonic or volcanic events or atmospheric CO2 levels.
What caused them? Speculative theories abound but we just don’t know.
The last of the Ice Ages, known to geologists as the Pleistocene Ice Age or Pleistocene Epoch, began around 2.6 million years ago. We live within a subdivision (called a Stage) of the Pleistocene called the Holocene - a brief period of relative warmth. The Holocene is an interglacial and it began rather abruptly around 12-13,000 years ago (12-13Ka).
The Holocene itself is one of a series of at least 45 similar brief partial retreats of ice within the Pleistocene, each of which lasted for 5-15Ka. As you can see from Figure 2 below the Interglacial that immediately preceded the Holocene is called the Eemian. The Eemian lasted for around 15Ka and reached its maximum warmth (much hotter than today) around 120Ka ago. The Interglacial before the Eemian called is called the La-Bouchet. It lasted for 10Ka with peak temperatures around 220Ka. Before La Bouchet there was the Purfleet Interglacial with maximum warmth peaking around 320Ka. Delving back even further into the Pleistocene, none of the other 40 or so preceding interglacials have been given names, at least that I know of.
We live in an Ice Age. It is called the Pleistocene. We do not know how long the Pleistocene will last, but on the evidence of previous Ice Ages, it probably has many tens of millions of years to run. We do not know how long the Holocene Interglacial will last but, on the evidence of previous interglacials in the Pleistocene, the return of the glaciers cannot be long delayed.
A Deep Dive into Deep Time
Earth’s history is divided into four grand and vast periods of time called Eons. These Eons are then subdivided hierarchically into increasingly shorter periods known as Eras, Periods, Epochs and Stages. Using geologist’s shorthand, where Ka is one thousand years: Ma is 1 million years and Ga is 1 billion years, the Eons 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 and even shorter memories we have trouble comprehending these gulfs of time. This can lead us to overestimate the significance of the age in which we live. It is a form of hubris.
It may help to realize that, on comparative geological/human time scales, the Hadean Eon commenced 20 years ago and the Phanerozoic 6 months ago. Eight days ago, towards the end of the Phanerozoic in an Era called the Mesozoic, dinosaurs roamed the land, sea and air and appeared masters of their universe (then the meteor struck). An Ice Age called the Pleistocene Epoch began around 20 hours ago. The evolution of our genus Homo from ape ancestors is a Pleistocene story. Five minutes ago, during a brief Stage of ice retreat and relative warmth within the Pleistocene called the Holocene, an explosion of human innovation enabled our civilisation to take off and thrive.
The lifetime experience of a single human being comes and goes in less than a second.
Such a timetable should teach us humility.
Climate Change Over the Past 540 million Years (the Phanerozoic Eon)
The Phanerozoic began around 540 Ma ago. Its start is defined by an explosion of complex new life forms which as the Eon advanced were able to spread from the oceans to colonise every ecological niche on land, sea and air. If we exclude the ice ages, global temperatures during the Phanerozoic were much higher than today, as were the concentrations of oxygen and CO2 in the atmosphere. Warmth, CO2 and oxygen are essential enablers and drivers of evolution.
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.
The Pleistocene Epoch was preceded by 60 million years of slow cooling from the peak Hothouse conditions of the Late Cretaceous Period. The evolution of our species from arboreal primates to upright, bi-pedal, large-brained, opposable-thumbed, stone-tool-using modern humans began around 2.6 million years ago.
No one knows how long the Pleistocene will last but the geological evidence from previous Ice Ages suggest it still might have tens of millions of years to go.
Climate Change over the Past 543,000 thousand Years (the Latter Part of the Pleistocene)
Figure 2: The graph covers the period from 540 thousand years ago (540Ka) to present – the last 20% of the Pleistocene Ice Age. It shows air temperature (blue line) and atmospheric CO2 concentration (red line) above the Antarctic continent. The data comes from measurement of air bubbles trapped in ancient ice which has been recovered by means of deep drilling. Ice cores provide our best calibrated proxy data and offer 10-year resolution. That data is broadly consistent with results from other types of proxy data from around the globe. Note: CO2 concentrations show good correlation with air temperature, but their highs and lows occur 500-1000 years after the corresponding peaks and troughs of temperature. This suggests that temperature controls CO2 (with a multi-century lag), not the other way around.
During the Pleistocene, glaciers repeatedly advanced south and north from the poles, covering land and sea in mid-latitude areas with kilometer-thick ice, sculpting the earth beneath. Sea levels rose and fell by more than 100 meters. As a result, some species of plants and animals became extinct and new species (including ours) evolved. The habitats of almost all were widely redistributed. Again, and again and again.
If we bore down into the Pleistocene, we see a pattern of at least 45 advances and retreats of the ice. These cycles were initially spaced at 40Ka intervals but from around 1Ma ago the cycles slowed to 100-150Ka of ice advance followed by 10–15Ka intervals of ice retreat and relatively-warm weather – although interglacial temperatures, even at their peak, never reached what been normal for most of the Phanerozoic. We call these brief intervals Interglacials. There were dozens of these glacial/interglacial cycles. The longer 100Ka cycles that began 1Ma ago are called Dansgaard-Oeschger Cycles after the two Danish scientists who first defined them. Figure 2 shows the last five of these D-O cycles. It is widely accepted that D-O events were caused by minor variations in the amount of solar radiation reaching the earth as a result of variations in the earth’s orbit around the sun – a regular astronomical control knob called a Milankovic Cycle, after the Serbian mathematician/astronomer Milutin Milankovic who first defined these cycles in 1914.
To summarise: we live in the Holocene, a Stage of the Pleistocene Ice Age. The Holocene is one of dozens of brief Pleistocene interglacials. The previous Interglacial is called the Eemian. The coldest point between the Eemian and the Holocene is known as the Last Glacial Maximum and occurred around 21 thousand years ago (21Ka). Although the Holocene has overarching significance for the development of human civilisation, it has no particular geological significance. Any other point of view is just human conceit (footnote 2).
An Aside:
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.
Climate Change over the Past 12 thousand Years (the Holocene Stage of the Pleistocene)
Figure 3: A compilation of global average surface temperature over the past 10,000 years. It is based on a compilation different types of proxy data (ice cores, tree rings, deep-sea sediments) from around the globe. Note the steep initial rise in Holocene temperature to a maximum around 8-9000 years before present (BP) followed by a slow and gradual decline to the deep trough and harsh conditions of the Little Ice Age (LIA) at around 175-600 years BP. The upward temperature spike at the extreme right of the graph is based on direct thermometer measurement. It has been called the Modern Warm Period (MWP). Some believe that the MWP is the result of human CO2 production: others that it is merely a recovery from the anomalous cold of the LIA. If a similar brief temperature spike had occurred in the pre-thermometer era it would have been smoothed out in the low-resolution proxy-generated part of the curve. Source: Nature Sci Data 7, 201 (2020). https://doi.org/10.1038/s41597-020-0530-7
As you can see from figure 2 above, all the interglacials of the Pleistocene for which we have good data followed a similar 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 from the Holocene Climate Optimum of 8-4000 years ago (figure 3 above) matches this pattern and is an ominous pointer towards the next cyclical advance in the ice.
Will we survive the next ice advance?
The Modern warming trend (which began around 175 years ago – a geological microsecond)- gives faint hope of a delay in the inexorable return of the glaciers. But when that happens, be it in 100-, 500- or 1000-years’ time, as a species we will surely survive. After all, with lesser technology, we survived the last advance of the ice. But will our civilisation?
Our few descendants (as they shiver before a fire in their caves or trudge through the snow in their furs) will look back with puzzlement on the Global Warming concerns of their 21st Century ancestors.
(1) The average global temperature of the lower atmosphere – known as the Troposphere – where all weather is generated, is the most fundamental single measure of global climate. The three graphs I present seek to show the change in lower troposphere temperature over increasingly longer periods of geological time as we look back from the present day. The graphs summarize the best and most up to date that I can locate, but their detail will inevitably be modified as knowledge advances. The further back in time, the greater the uncertainty and the coarser the resolution. It should also be noted that global trends may be amplified or smoothed by regional factors.
(2) It is a conceit that has unfortunately infected the International Commission on Stratigraphy. The academics who comprise the Commission seem to feel that the times in which they happen to live must be important enough to merit a geological Epoch all of its own. Hence, in current official stratigraphic charts, the Pleistocene Epoch (the Pleistocene Ice Age) is held to have finished around 12,000 years ago to be replaced by a new Epoch, the Holocene. This is the equivalent to declaring that the setting up of the United Nations in 1946 led to a new epoch of global peace (Pax Americana?) that will last 1000 years. At least the ICS did not ratify the absurd neologism “Anthropocene”. In this conceit the men and women of the Stratigraphic Commission were undoubtedly influenced by the current fashionable science meme which holds that human production of CO2 is the cause of the Modern Warm Period and this will be so extreme and long-lived as to prevent any return of Pleistocene Ice Age. But there is no evidence to support this. Knowledgeable readers will already have noticed that in my description of the status of the Pleistocene and the Holocene I do not follow the ICS. I make no apology for this.
I explore the topic of fashionable science memes in more detail in a later post of mine HERE.
Click for higher resolution images.