Climate change explained in three graphs

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 (2) - a period of relative warmth. The Holocene is an interglacial and it began, rather abruptly, around 12,000 years ago (12Ka).

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, 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 Interglacial, 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 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 within the Pleistocene, the return of  ice sheets to mid- and high-latitude land areas 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.

Three Ice Ages (Scotese labels them Icehouses) are apparent on the graph. The most recent of these, on the extreme right, is called The Pleistocene Ice Age and it began 2.6 Ma ago. The Pleistocene 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 in the Pleistocene.

No one knows how long the Pleistocene will last but a glance at the duration of previous Ice Ages suggests it still has 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 around 20-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-and high-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, were much lower than what been normal for most of the Phanerozoic. The longer 100Ka cycles 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 both the 40Ka and  100Ka cycles 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 and wobbles (known as precession) of earth’s rotational axis. These regular astronomical cycles are called Milankovic Cycles after the Serbian mathematician/astronomer Milutin Milankovic who first defined them 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 21Ka ago. Although the Holocene has overarching significance for the development of human civilisation, it has no particular geological significance. Any other point of view is human conceit (see footnote 2).

An Aside:

During the Eemian Interglacial (warmer than today) hippopotami gamboled in the River Thames. With the Last Glacial advance, they retreated south. 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 Epoch (2))

Figure 3: Air and sea temperatures over the past 12,000 years measured from proxy data in Greenland and the Eastern Pacific Ocean. Greenland ice core data from Vinther et al 2009: (https://www.nature.com/articles/nature08355). Macassar Strait seafloor-sediment data from Rosenthal et al 2013: (http://science.sciencemag.org/content/342/6158/617). Compilation of the two data sets by Andy May. The correlation between two regions 9,000 km apart, using two different proxy methods, is compelling.

 Note the very steep initial rise in Holocene temperatures to a maximum around 8-9000 years before present (known as The Holocene Climate Optimum) followed by a slow and gradual decline to the deep trough in temperatures from around 200 to 800 years before present (known as The Little Ice Age). The upward temperature spike at the extreme right of the graph is based on daily thermometer measurements that began, when there was sufficient global coverage, around 1850. Such a spike, had it occurred during the earlier period of coarser-resolution proxy data, would not have been apparent. 

As you can see from figures 2 and 3, 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 global temperatures from the Holocene Climate Optimum 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 (3).

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