B. Historical Normal Temperature Fluctuations
Thermometers did not exist millions of years ago, but temperature can be estimated by various proxies. A proxy is not a direct measurement, but an estimate based on a relationship. For example, a canary in a mine can be used to estimate the amount of oxygen in the mine shaft. If the canary dies, the miner must get out. It is not a direct measurement of oxygen, but it is a proxy for predicting a minimum oxygen level.
In estimating temperatures millions of years ago, scientists use Oxygen isotope ratios taken from ice cores and fossils. The detailed description of this proxy method is available on the internet. Temperature proxies are not as accurate as direct thermometer or thermocouple measurements, but they do provide general trends as well as changes in trends.
Based on these ice cores and fossil sediments, numerous temperature plots have been made. One graph goes back 500 millions of years. [Veizer, J. et al. (1999) 87Sr/86Sr, d13C and d18O Evolution of Phanerozoic Seawater. Chem. Geol. 161, 59-88; and Veizer J, Godderis Y, and Francois L, (2000) Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon, Nature, Vol 408 pgs. 698-701. http://dx.doi.org/10.1038/
The above figure shows that between 500 and 75 million years ago there were large temperature swings (as much as 18°C) with an overall average fluctuation of about ±4°C. Between 75 and 1 million years ago, there was a gradual decline in temperature changes (anomaly) from +14 °C to a -2 °C, exhibiting an overall average fluctuation of about ±1°C. Between 1 million and 20,000 years ago, there were variations of about 9 °C with an average fluctuation of ±2°C. Between 20,000 and 10,000 years ago, there was a gradual warming from around -4 to about 0°C with a fluctuation of about ±1°C. Between 14,000 years ago until today, the temperature change has been remarkably stable.
Another plot going back 5 million years provides more details in the fluctuation record. [Lisiecki, L, Raymo, M. (January 2005), “A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records”, Paleoceanography. 20 (1): PA1003; updated Lisiecki, & L; Raymo, M. (May 2005), correction to “A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records“, Paleoceanography. 20 (2): PA200; data: doi:10.1594/PANGAEA.704257; .Petit, J. ; et. al (1999). “Climate and Atmospheric History of the Past 420,000 years from the Vostok Ice Core, Antarctica”, Nature. 399 (6735): 429–436.]
This graph shows that between 5.4 and 3.5 million years ago, the average temperature fluctuation was ±1°C. Between 3 million years ago and 1 million years ago, the temperature anomaly fluctuations were about was ±2 °C. Between 1 million years ago until today, the overall temperature anomaly has been fluctuating around ±4°C.
Another graph goes back 800,000 years. [Jouzel, J. et al. (2007) Orbital and Millennial Antarctic Climate Variability over the Past 800,000 years; Science 317 pgs. 793-796.] In Figure 2, of that publication, there are a plot of temperatures. It shows that there were several peaks and valleys that varied from 9 to 10°C. The remainder of the time the average fluctuation was about ±2°C.
Yet another graph goes back 2000 years. [Christiansen, B. & Ljungqvist, F. (2012) The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability; Climate of the Past, Vol 8, pgs. 765-786.] This plot shows average temperature fluctuations of about ±1°C with higher peaks and valleys of about ±2°C. Over the last 1 thousand years, the average fluctuation is about ±1.5°C. The temperature 1000 years ago was higher than it is today.
The temperature scales between the various figures are not the same. The plot involving 500 million years has a scale of 20°C. The plot involving 5 million years has a scale of 10°C. Figure 2 (of Jouzel, J. et al.), going back 800,000 years, has a scale of 15°C. Figure 8, show above, has a scale of 4°C. Be careful not be influenced by the size of the fluctuations without taking into account the scale manipulation.
These graphs illustrate that the earth is currently in a relatively stable period with average fluctuations over the last 500 million years between about ±2°C.