Starting around the Pliocene Epoch, 5.3 - 1.8 million years ago, the Earth’s temperature has been getting every more jittery as it cools. Something is making the temperature unstable! And as the graph above suggests, these fluctuations are gradually taking longer, as well as slowing.
These temperature fluctuations are not really periodic, despite the optimistic labels on the above graph saying “41 kiloyear cycle” and “100 kiloyear cycle”. And beware: the data in the above graph was manipulated so it would synchronize with the Milankovitch cycles. For details, see:
O is a climate proxy: something we can measure now, that we believe are correlated to features of the climate long ago. It is the change in the amount of oxygen-18 (a less common, heavier isotope of oxygen) in carbonate deposits dug up from ancient ocean sediments. These deposits were made by foraminifera and other plankton. The more oxygen-18 there is, the colder we think it was.
Here’s a graph that shows more clearly the noisy nature of the Earth’s climate in the last 7 million years:
Note that in the usual style of paleontology, the present on the left instead of on the right.
By the beginning of the Pleistocene Epoch, 1.8 - .01 million years ago, the Earth’s jerky temperature variations became full-fledged glacial cycles. In the last million years there have been about ten glacial cycles, though it’s hard to count them in any precise way—it’s like counting mountains in a mountain range:
Now the present is on the right, but up means cold, or at least more oxygen-18. This graph is copied from:
• Barry Saltzman, Dynamical Paleoclimatology: Generalized Theory of Global Climate Change, Academic Press, New York, 2002, fig. 1-4.
We can get some more detail on the last four glacial periods from the change in the amount of deuterium in Vostok and EPICA ice core samples, and also changes in the amount of oxygen-18 in foraminifera (that’s the graph labelled ‘Ice Volume’):
As you can see here, the third-to-last glacial ended about 380,000 years ago. In the warm period that followed, the first signs of Homo neanderthalensis appear about 350,000 years ago, and the first Homo sapiens about 250,000 years ago.
Then, 200,000 years ago, came the second-to-last glacial period: the Wolstonian. This lasted until about 130,000 years ago. Then came a warm period called the Eemian, which lasted until about 110,000 years ago. During the Eemian, Neanderthalers hunted rhinos in Switzerland! It was a bit warmer then that it is now, and sea levels may have been about 4-6 meters higher.
The last glacial period started around 110,000 years ago. This is called the Winsconsinan or Würm period, depending on location; a more neutral name is simply last glacial period.
A lot happened during the last glacial period. Homo sapiens reached the Middle East 100,000 years ago, and arrived in central Asia 50 thousand years ago. The Neanderthalers died out in Asia around that time. They died out in Europe 35 thousand years ago, about when Homo sapiens got there. The oldest cave paintings are 32 thousand years old, and the oldest known calendars and flutes also date back to about this time. It’s striking how many radical innovations go back to about this time.
The glaciers reached their maximum extent around 26 to 18 thousand years ago. There were ice sheets down to the Great Lakes in America, and covering the British Isles, Scandinavia, and northern Germany. Much of Europe was tundra. And so much water was locked up in ice that the sea level was 120 meters lower than it is today.
Then things started to warm up. About 18 thousand years ago, Homo sapiens arrived in America. In Eurasia, people started cultivating plants and herding of animals around this time.
There was, however, a sudden return to colder weather 12,700 years ago: the Younger Dryas episode, a cold period lasting about 1,300 years. The Younger Dryas ended about 11,500 years ago. The last glacial period, and with it the Pleistocene, officially ended 10,000 years ago.
Carbon dioxide concentrations in ice cores are closely correlated to other climate proxies, though sometimes they lead or lag. The blue curve here shows CO2 concentrations as measured from the Vostok ice core over the last 420,000 years:
The carbon dioxide data from the Vostok ice core can be found here:
The Earth began a long cooling trend in the Oligocene, 34-24 million years ago. There are two main theories. First, this is when India collided with Asia, throwing up the Himalayas and the vast Tibetan plateau. Some argue that this led to a significant change in global weather patterns. For example, these regions became icy and the Earth’s albedo increased. Second, this is when the supercontinent Gondwanaland finally broke up, with Australia and South America separating from Antarctica. Some argue that the formation of an ocean completely surrounding Antarctica led to the cooling weather patterns: water can now go round and round without ever getting pushed into the tropics.
Why did glacial cycles start in the Pleiocene and intensify in the Pleistocene? The so-called Panama Hypothesis is that the closure of this seaway caused a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets.
The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7 Ma.
This quote is from:
P. Molnar, Closing of the Central American Seaway and the Ice Age: A critical review, Paleoceanography 23 (2008), pp. PA2201.
D. N. Schmidt, The closure history of the Central American seaway: evidence from isotopes and fossils to models and molecules, in Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies, 2007, pp. 427-442.
G. Haug and R. Tiedemann, On Atlantic Ocean thermohaline circulation, Nature 393 (1998), 673-676.
D. R. Newkirk and E. E. Martin, Circulation through the Central American Seaway during the Miocene carbonate crash, Geology 37 (2009), 87-90.
D. Lunt et al., Late Pliocene Greenland glaciation controlled by a decline in atmospheric CO2 levels Nature 454 (2008), 1102-1105.
C. H. Lear et al., The closing of a seaway: ocean water masses and global climate change, Earth and Planetary Science Letters 210 (2003), 425-436.
G. Bartoli et al., Final closure of Panama and the onset of northern hemisphere glaciation, Earth and Planetary Science Letters 237 (2005), 33–44.
B. Schneider and A. Schmittner, Simulating the impact of the Panamanian seaway closure on ocean circulation, marine productivity and nutrient cycling, Earth and Planetary Science Letters 246 (2006), 367–380.
An alternative hypothesis:
To understand what drives this important climate change, a number of hypotheses have been proposed, most of which focused on the climatic influence of high latitudes, because high latitudes are the places where ice sheets occur. For example, enhanced North Atlantic Deep Water production driven by tectonic process (“Panama Hypothesis”; Haug and Tiedemann, 1998; Keigwin, 1982) and stratification in the subarctic Pacific (Haug et al., 2005) have been interpreted to increase precipitation, favoring ice sheet formation and accumulation in Northern Hemisphere high latitudes; favorable Milankovitch orbital parameters were also thought to facilitate the continental glacier buildup (Maslin et al., 1998). However, these scenarios are still being debated, and the ultimate driving force of NHG initiation is by no means resolved (Bartoli et al., 2005; Klocker et al., 2005; Lunt et al., 2008a, 2008b; Molnar, 2008).
Changes in low-latitude climate systems such as the Asian monsoon were generally thought unimportant when studying NHG onset. However, recent studies highlighted the key role of low-latitude climate dynamics in regulating global climate changes.
A. C. Ravelo et al., Regional climate shifts caused by gradual global cooling in the Pliocene epoch Nature 429 (2004), 263-267.
G. H. Haug et al., North Pacific seasonality and the glaciation of North America 2.7 million years ago, Nature 433 (2005), 821-825.
S. D. Nikolaev et al., Neogene–Quaternary variations of the ‘Pole–Equator’ temperature gradient of the surface oceanic waters in the North Atlantic and North Pacific, Global and Planetary Change 18 (1997), 85–111.
M. E. Raymo, The initiation of Northern Hemisphere glaciation, Annual Review of Earth and Planetary Sciences, 1994.
A major tipping point of Earth’s history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG) and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001). The severe deterioration of climate occurred in three steps between 3.2Ma (warm MIS K3) and 2.7Ma (glacial MIS G6/4) (Lisiecki and Raymo, 2005). Various models (sensu Driscoll and Haug, 1998) and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2–3ºC) (Bartoli et al., 2005). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007), which probably led to an enhanced precipitation and fluvial runoff, lower sea surface salinity (SSS), and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6ºC and freshening by ∼2 psu from 3.25/3.16–3.00 Ma, right after the first but still reversible attempt of closing the CAS.
It’s widely hypothesized that quasiperiodic changes in the Earth’s orbit called Milankovich cycles trigger glacial cycles under the right conditions. For more see:
These parts of Mathematics of the environment are also relevant: