Icehouse
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Up to the early 1800s, nearly all scientists and experts generally agreed that the Earth had been progressively cooling since its birth.
In 1837, the Swiss biologist and geologist Louis Agassiz turned this notion on its head when he presented a theory of ice ages at the annual Swiss Society for Natural Research (1). Agassiz argued that the Earth's climate history actually consisted of a series of alternating ice ages, also known as "icehouse periods," and warmer ages, or "greenhouse periods." An icehouse period is denoted by the existence of extensive ice sheets and glaciers in the polar and continental regions, year-round. By contrast, the intervening greenhouse periods are marked by the absence of any ice sheets or glaciers at all (2). It may be surprising to learn that the Earth has largely existed in a greenhouse state, with no polar ice sheets or glaciers to speak of, for 85% of its entire history.
The most recent (and ongoing) "true" ice age is the Quaternary Glaciation, which started around 2.6 million years ago (this is also when the first humans appeared) (3). The Quaternary Glaciation has been characterized by enormous ice sheets in the past, including the Antarctic, Greenland, Laurentide, and Weichselian ice sheets (the first two still exist, but the latter two have since receded). At smaller levels of scale, the Quaternary Glaciation has consisted of alternating glacial and interglacial periods. During a glacial period, each of which lasts thousands of years, the temperatures cool down, and the ice sheets and glaciers advance (4). Conversely, in an interglacial period, the temperatures warm up, and the ice sheets and glaciers recede (5). |
Temperature changes over the last 500,000 years of the Quaternary Glaciation, which has consisted of intermittent glacial and interglacial periods (6). |
The Last Glacial Period occurred 115,000 to 12,000 years ago (7). The Last Glacial Period is often referred to as the "Last Ice Age," although this is not correct, given that the Antarctic and Greenland ice sheets existed, year-round, before the Last Glacial Period even began. Thus, it may be better to think of the Last Glacial Period as the "most recent cold spell" of the Quaternary Glaciation, which is the last true (and current) ice age. During the Last Glacial Period, the aforementioned ice sheets blanketed large regions of the Earth, including the Greenland and Antarctic Ice Sheets. In the northern hemisphere, the top half of North America was also covered by the Laurentide Ice Sheet, and in northern Eurasia, the Weichselian Ice Sheet rolled over much of Britain, Germany, Poland, and Russia. Moreover, many large glaciers existed in central Asia and Africa, and in the southern hemisphere, ice sheets and glaciers blanketed the southern regions of Australia, New Zealand, and Africa. |
The Laurentide Ice Sheet, with the smaller Cordilleran and Greenland Ice Sheets flanking it (8). |
The Last Glacial Period ended 12,000 years ago and has since given way to the current interglacial period, which is commonly known as the Holocene (9). During the Holocene, human civilization as we know it has proliferated across the globe, and with it the emergence of agriculture, religion, and technology. However, despite being relatively stable in temperature, the Holocene has also been marked by occasional fluctuations in temperature. The most recent of these dips in temperature lasted for several centuries and was well-documented throughout the world.
The Little Ice Age
The Little Ice Age was a period of cooling that largely affected the northern hemisphere and lasted from 1300 to 1850 or so (10). Most hypotheses attempting to explain the underlying reasons for the Little Ice Age try to relate it to inherent fluctuations in solar radiation, volcanic activity, ocean circulation, and global climate (in other words, nobody knows why it occurred).
The Little Ice Age had many regional impacts, mostly in North America and Europe. In North America, intermittent crop failures were common and winters became exceptionally cold. In Europe, agricultural crops had to be modified, and people in Iceland and Scandinavia were forced to switch to a non-agricultural diet. Record snowfalls were recorded all over Europe. The Baltic Sea froze over twice, and both Iceland and Greenland were entirely surrounded by ice. Beyond North America and Europe, excessive cold spells were also documented in Mexico, Central America, South America, Asia, Africa, and the Middle East. |
Winter, by Adriaen van de Venne (1614). |
The Little Ice Age also had a fairly profound influence on human behaviour. In North America, the colder winters discouraged many settlers, which led to the collapse of numerous settlements. In Europe, drought, disease, and unemployment led to rising rates of robbery and murder. Marginalized groups were often blamed for the cold, which led many to believe "magic" was responsible, such that many witchcraft trials were held. Despite these negative effects, the Little Ice Age did produce some positive outcomes in Europe, particularly a notable rise in winter landscape paintings. Elsewhere, the Little Ice Age also made its mark - the Ottoman Empire, one of the most powerful in the world, experienced droughts and agricultural challenges that led to multiple uprisings from which it did not recover for 100 years. |
The Reverend Robert Walker Skating on Duddingston Loch, by Henry Raeburn (1790s). |
The Post-Little Ice Age
The Little Ice Age ceased to exist well over 150 years ago. In the greater scheme of things, it was really just a minor event, a relatively insignificant dip in temperatures that lasted about 500 years, which is only 4% of the 12,000 years of the Holocene interglacial period, and is not even 1% of the roughly 100,000 years of the Last Glacial Period, which itself lasted for only 4% of the entire Quaternary Glaciation. Yet at the time, most humans perceived the Little Ice Age to be a major event, one that shaped human behaviour and the course of human history in general.
Many of the historical responses to the Little Ice Age may seem silly now, particularly the idea that any particular group of humans was responsible for it. Yet, this may have seemed sensible at the time. Now we are in the midst of a post-Little Ice Age, with the Earth's surface temperature having risen by 0.8 degrees Celsius since 1880 (11). Again, this can be perceived as a major event, or it can, perhaps more objectively, be seen as a relatively insignificant bump in the greater scheme of things. |
All temperature fluctuations since the Last Glacial Period, with the Little Ice Age just a small dip at the end. |
Earth's natural climate alternates through "icehouse" and "greenhouse" periods. The former give way to the latter, and the latter to the former. Importantly, this occurs over multiple levels of scale. So before we get too caught up in our predictions about future scenarios (11), this broader cyclical change needs to be acknowledged, and hopefully understood at a deeper level. So that we may discover and comprehend explanations that are closer to the truth and avoid falling for reactive and superficial (or superstitious) beliefs.
And, even more importantly, to engage in objective, critical thinking - particularly in the face of popular opinion.
Solace.
References:
(1) https://en.wikipedia.org/wiki/Ice_age.
(2) https://en.wikipedia.org/wiki/Greenhouse_and_icehouse_Earth#Greenhouse_Earth.
(3) https://en.wikipedia.org/wiki/Quaternary_glaciation.
(4) https://en.wikipedia.org/wiki/Glacial_period.
(5) https://en.wikipedia.org/wiki/Interglacial.
(6) https://courses.lumenlearning.com/suny-sustainability-a-comprehensive-foundation/chapter/milankovitch-cycles-and-the-climate-of-the-quaternary/.
(7) https://en.wikipedia.org/wiki/Last_Glacial_Period.
(8) https://digitalatlas.cose.isu.edu/geog/native/text/history.htm.
(89 https://en.wikipedia.org/wiki/Holocene.
(10) https://en.wikipedia.org/wiki/Little_Ice_Age.
(11) https://science.nasa.gov/earth/climate-change/secrets-from-the-past-point-to-rapid-climate-change-in-the-future/.
(12) https://en.wikipedia.org/wiki/Climate_change.
