The Ice Age Cometh – climate change explained in three graphs

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 (with acknowledgment to Jonathon Swift, 1733)

The Big Picture

An Ice Age is a significant period of time (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. They are not regularly spaced in time and show no correlation with major tectonic events or atmospheric CO2 levels.

What caused them?  Speculative theories abound but we just don’t know.

The last of these Ice Ages, called by geologists the Quaternary Period, began around 2.5 million years ago. We live in a brief period of relative warmth (known as an interglacial) within the Quaternary. This geological stage is called the Holocene, and it began 11-13,000 years ago. Technically, this post should be named “The Glaciers Cometh”. We are already living in an Ice Age.

The Holocene itself is the last of a series of a series of at least 40 similar brief pauses and partial retreats of ice within the Quaternary. Each lasted 10-15000 years. The interglacial that immediately preceded the Holocene is called the Eemian, but none of the others have been given names. The Eemian reached its maximum warmth 125,000 ago.

We live within the Holocene Stage of the Quaternary Period, the last subdivisions of the Phanerozoic Eon. Obviously, we cannot know for sure how long the Quaternary will last, but on the evidence of previous Ice Ages, it probably has many tens of millions of years to go.

A Reflection on Deep Time

At the most fundamental level, Earth’s history is divided into four vast periods of time called Eons. Using geologist’s shorthand, where Ka is one thousand years: 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 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 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 Quaternary began around 20 hours ago. The evolution of our species is a Quaternary story. Five minutes ago, during a brief Stage of ice retreat and relative warmth in the Quaternary called the Holocene, an explosion of 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 Eon began 543 Ma) ago. General warmth during the Phanerozoic (much higher than today), along with high levels of oxygen and CO2 (also much higher than today) in the atmosphere drove the evolution of complex multicellular life forms enabling them to colonize the land.

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 evolution of our species from arboreal primates to upright, large-brained, stone-tool using modern humans began around 2.5 million years ago. We are a Quaternary Ice Age species. The Quaternary was preceded by 60 million years of slow cooling from the Late Cretaceous hothouse conditions.

We do not know how long the Quaternary will last. The evidence of previous Ice Ages suggest it still has many tens of millions of years to go.

Climate Change over the Past 543,000 thousand Years (the Latter Part of the Quaternary Period)

During the Quaternary, glaciers repeatedly advanced south and north from the poles, covering land and sea in kilometer-thick ice, sculpting the earth beneath. Sea levels rose and fell by more than 100 meters. Some species became extinct, new species (including ours) evolved. The habitats of almost all were widely redistributed. Again, and again.

If we bore down into the Quaternary, we see a pattern of repeated advances and retreats of the ice: 100-150 thousand years of ice 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 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).

The Quaternary Interglacial that we live in is called the Holocene (see figure 2). The previous Interglacial is called the Eemian (figure 2). The deep trough in global temperatures between the Eemian and the Holocene is known as the Last Glacial Maximum (LGM) which reached its nadir 21 thousand years ago (21Ka). The LGM was the coldest period of the Quaternary Ice Age (so far). The Holocene is merely the latest in a cycle of all too brief interglacials and has no particular geological significance. We just happen to be living in it.

 Figure 2: The graph covers the period from 540 thousand years ago (540Ka) to present – the last 20% of the Quaternary Ice Age. It shows air temperature (blue line) and CO2 concentration (red line) above the Antarctic continent. These are measured from 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.

Reference www.climatedata.com

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 10 thousand Years (the Holocene Stage)

As you can see from figure 2 above, all the Interglacials of the Quaternary 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 below) 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. 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 at around 600-175BP. The steep upward spike at the right of the graph is called the Modern Warm Period. It results from this splicing 160 years of thermometer data with 1-day resolution onto the end 10,000 year’s of 100-year resolution proxy data. Such a brief spike in temperature, if it had existed in the pre-thermometer era, could not have been identified in the proxy-generated part of the curve.

Source: Nature Sci Data 7, 201 (2020). https://doi.org/10.1038/s41597-020-0530-7

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 the 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.

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