Conclusions about the current state of the climate are based on a relatively short time span of 100 to 150 years of data from instrumented global observing sites. Some nations in the Northern Hemisphere (especially in Europe and North America for example) have local records that span a longer period from which we can infer important global climate events. The year 1816 for example, was known as the “year without a summer,” resulting in vast food shortages and thousands of deaths. This cold period was believed to be caused by the eruption of Mt. Tambora in Indonesia and other volcanoes in that region, leading to volcanic ash layers that reflected much of the sun’s incoming rays. There is also evidence for a “Little Ice Age” between 1500 and the 1700’s marked by much cooler than normal temperatures (although there was never really any glaciation during that time).
Climate information for a much longer period (hundreds of thousands of years) can be determined by analysis of “proxies,” or data sets from which temperature and other variables can be inferred using isotope analysis or other techniques. Examples of proxy data include: tree rings, ice cores from glaciers, lake or ocean bed soil cores, and even stalagmites from caves.
Analysis of ice cores from the deep Antarctic ice sheets at the Russian Vostok research station has yielded climate information back 400,000 years or more.  The ice cores at this location were around 3300 meters (10,000 feet) deep. Each layer of ice can show the amount of snowfall accumulation (and melting) during a year’s time, but more than that, it can tell us something about the CO2 and CH4 concentration of the atmosphere (parts per million – ppm) by analyzing air bubbles trapped in the ice. The accuracy is believed to be within a few ppm. Also, the average atmospheric temperature around the time each layer of ice was created can be estimated from the concentration of deuterium, (also known as heavy water) an isotope of hydrogen. Since the air bubbles are several thousand years younger than the ice surrounding them, a correction must be made to compare the timelines accurately. 
The results of their analysis reveal an astonishing cycle of four distinct cold, glacial episodes, separated by shorter interglacial periods about 100,000 years apart (Figure 1). The swings in global mean temperature during these episodes have been as much as 10 degrees C. During each inter-glacial period, the earth’s atmospheric temperature warms, and CO2 and CH4 concentrations rise. And of course, the glaciers recede in area over a period of 20,000 years or more. You can see from the graph that we are now in an inter-glacial period that began about 12,000 year ago, known as the Holocene epoch. This accounts for a natural occurrence of many of the conditions we observe today. Similar results were obtained for even deeper ice cores in another location in Antarctica (Dome C) that extends back to 800,000 years, and from dozens of deep sea bed cores .
Figure 1. Change in CO2 concentration (blue), surface temperature (red), methane concentration (green), and solar insolation with deuterium concentration (orange) over the past 450,000 years. (From Barnola, et al. 1987)
Another important finding from the ice core studies was that the increases in global temperatures seem to precede slightly the rises in atmospheric CO2 concentrations, not vice versa. This suggests that increases in CO2 prior to each interglacial period did not cause the warming, but occurred as a result of the warming. How can this happen? When the ice and permafrost melted and the oceans warmed through natural causes, CO2 in vast quantities was released into the atmosphere.
Recent research has studied the ratio of oxygen isotopes from rainwater that deposits as minerals in stalagmites in a Chinese cave and found a relationship with precipitation amounts that extends back for a period of 640,000 years.  The intensity of the Asian monsoon can thus be estimated for a much longer period (nearly 300,000 years) than previously found in earlier studies. They found that pronounced dry, weak monsoons were observed with the termination of the last seven glacial periods, coincident with a rise in sea level as glaciers began to melt rapidly.
What caused these pronounced, periodic, natural changes in the Earth’s temperature, CO2 and CH4 compositions, ice coverage and even precipitation? They are believed to be due to what are known as Milankovitch cycles, named after a Serbian astronomer who developed his theory in the 1920’s.  These are slow changes in the Earth’s orbit that bring our planet slightly closer to the Sun (thus bringing more insolation to the atmosphere) during interglacial periods and farther away during glacial episodes (Figure 2). The earth’s orbit is not circular but slightly elliptical, more like the shape of a fat egg. Changes in the orbital shape are due to the slight gravitational pull by the largest planets, Saturn and Jupiter that cause the orbit to rotate. The tilt of the Earth’s axis relative to the orbital plane (which gives us our seasons) also varies with time and rotates, somewhat like a spinning top (see Figure 2). The interaction of all of these effects is complicated, leading to major observed peaks every 100,000 years, and weaker ones every 20,000 and 40,000 years.
Figure 2. Depiction of the Milankovitch cycles in the Earth’s orbit and tilt. The eccentricity of the Earth’s orbit is exaggerated for illustration. (Source: University Corporation for Atmospheric Research)
What effect does the current man-made increase in greenhouse gases such as CO2 have on the natural glacial and inter-glacial cycles? Recent research  suggests that anthropogenic effects could overwhelm orbital effects, and lengthen the current inter-glacial period 20,000 to as much as 50,000 years (depending on the GHG emissions scenario). Certainly, this is a good thing for our descendants!
In summary, looking at the Earth’s climate in the distant past helps to put our current situation in better perspective. The melting of the glaciers and sea ice, warming temperatures, and CO2 increases are to be expected from natural processes during this current inter-glacial epoch. Human activity adds to these conditions, resulting in record levels of CO2 in recent years, but it does not initiate them.
(Updated Oct. 30, 2020)
- The Year without a Summer. Wikipedia, source: https://en.wikipedia.org/wiki/Year_Without_a_Summer
- Little Ice Age. Wikipedia, source: https://en.wikipedia.org/wiki/Little_Ice_Age
- Barnola, J-M., Raynaud, D., Korotkovich, Y. and Lorius, C., 1987: Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329, pages 408-414.
- Barnola, J-M., P. Pimienta, D. Raynaud, and Y. Korotkevich, 1991: CO2-climate relationship as deduced from the Vostok ice core: a re-examination based on new measurements and on a re-evaluation of the air dating. Tellus, 43B, pages 83-90.
- Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, et al. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present.Nature453: 379-382.
- Cheng, H. et al., 2016: The Asian monsoon over the past 640,000 years and ice age terminations. Nature, Volume 534, 640-646.
- Lee, J., 2012: Milankovitch cycles. Encyclopedia of Earth. Source: http://www.eoearth.org/view/article/154612/
- Ganopolski, A., R. Winkelmann, and H. Schellnhuber, 2016: Critical insolation-CO2 relation for diagnosing past and future glacial inception. Nature, 529, 200.
Next: Short Period Natural Climate Variations