Climate History

by Harrison H. Schmitt

[Senator Schmitt wrote four articles that include climate history. To make it easier for you to follow this climate history, I summarize the climate history of his four articles here. The numbers in the text are references he cited in his articles. – Ed]

The Last 600 Million Years

Extraordinarily complex natural processes underlie changes in the Earth’s climate. They represent decadal to millennial to epochal variations in weather patterns as nature continuously attempts to compensate for solar heating imbalances in and between the atmosphere, oceans, and landmasses.

Nature’s attempts to restore heat balance at and near the Earth’s surface take place under many complicating influences. These include the rotating Earth’s seasonally variable orientation relative to the sun; periodic differences in Earth’s orbital positioning; movement and release of heat stored in the oceans; atmospheric circulation; the sun’s variable irradiance and magnetic fields; frequent and unpredictable volcanic eruptions; and geologically slow but exorable redistribution and reconfiguration of land, ocean, and ice masses.

Geologic investigations indicate that over the last 600 million years average global surface temperature appears to have been buffered roughly at a maximum of about 72 degrees Fahrenheit (22 degrees Celsius)4. The last 53 million years being significantly colder than the previous average5, as indicated by oxygen isotopes of shells in sea floor cores,6 but comparable to other cold periods in the geologic past. During this 600 million year period, major cold perturbations to about 54 degrees Fahrenheit (12 degrees C) occurred about every 150 million years.7

Over that period, carbon dioxide decreased from an estimated maximum of about 7000ppm 550 million years ago to minimum of about 300ppm around 300 million years ago8 (current level is 385ppm) without changing the long-term average temperature at the Earth’s surface. Around 43 million years ago, declining carbon dioxide concentration reached about 1400ppm, followed by three oscillations during the next 10 million years with amplitudes of about 1000ppm.9

Temperature apparently remained relatively constant during these ancient carbon dioxide oscillations except during the most recent when oxygen isotope ratios indicate a sharp drop in temperature10 33.5 million years ago, that is, about the time ice sheets began to accumulate on Antarctica.11 Relative to today’s values, declining atmospheric carbon dioxide levels remained relatively high (740-1400ppm) as Antarctica cooled.

About 22 million years ago, with Antarctica’s continued slow migration away from Africa, Australia, and South America, the ocean distribution and ocean currents around Antarctica began to resemble modern configurations,12 with partial deglaciation of that continent beginning about 14-15 million years ago.13 A particularly warm two million years for the tropical Earth latitudes developed about 4 million years ago even as sea surface temperatures slowly declined toward present levels.14

This seemingly contradictory situation apparently related to a long-term north-south expansion of the warm tropical ocean waters resulting in a factor of four reduction in the sea surface temperature gradient from the equator to at least 34ºN (~2ºC gradient versus ~8ºC, today) that lasted until about 1.5 million years ago.

The Last 1 Million Years

Along with disruptions of the El Niño Southern Oscillation, convective tropical Hadley circulation apparently slowed during this long period with both effects probably leading to significant global climate impacts. About 2.75 million years ago,15 major ice ages began to oscillate with interglacial periods of warmth. This occurred in spite of the concurrent anomaly in the tropical sea surface temperature gradient.

Ten specific high latitude ice ages took place in the last million years, apparently correlated with a change in the dominant solar influence on cooling from the Earth’s 41,000-year orbital obliquity cycle to its 100,000-year eccentricity cycle.16

*A significant decrease in the overall concentration of atmospheric carbon dioxide occurred at about the same time as this change in orbital influence with even greater, temporary reductions associated with each ice age; however, the reported data do not support a causal association of this decrease in carbon dioxide with the overall cooling during this million-year period.17

The rise or fall in average ocean temperature would be expected proceed any effect on stored carbon dioxide due to the oceans’ relatively high mass and slow circulation.

The lesson in these variations in atmospheric carbon dioxide and global temperature through geologic time, at least at a million-year or so time-resolution, appears to be that increases and decreases in carbon dioxide have not triggered global temperature changes as derived from fossil oxygen isotope ratios.

As carbon dioxide release from the oceans due to warming lags warming by hundreds to thousands of years, no support exists for a conclusion that a specific natural carbon dioxide change forced a specific temperature change.

Other long-term geological and solar-related phenomena may have overwhelmed any broad greenhouse effects related to carbon dioxide; or, alternatively, the proxies used for estimating ancient atmospheric carbon dioxide concentrations may be invalid.22

Compilations of temperature changes in the oceans and seas, as preserved by oxygen isotope variations in shells from cores of bottom sediments, provide a record of natural cycles of major climate change back for 1.8 million years.18

For example, geological analysis of sea level changes over the last 500,000 years show a remarkable correlation with major natural climate change.19 These data further indicate that the Earth probably is approaching the peak of the warming portion of a normal climate cycle that began with the end of the last Ice Age.20 Terminations of past ice ages appear to be associated with increased solar heating (insolation), as orbital influences changed, and not with triggering increases in carbon dioxide levels; although such increases certainly accompanied the terminations.

About 22,000 years ago, during the last Ice Age, low sea levels created a land bridge between Asia and North America allowing migrations of modern humans from Asia into the Americas. 29

Studies of Antarctic ice cores indicate that Earth-surface temperatures several degrees warmer than present existed during the four preceding interglacial periods of the last 420,000 years.23

The Last 10,000 Years

Since the last Ice Age ended about 10,000 years ago11 (the glacial maximum lasting between 33,000 and 19,000 years ago12), geological and tree-ring records for that interval document prolonged periods of warmth and cold, ranging from 3000 years to a few hundred years in duration.13

Since the last vestiges of the most recent major Ice Age about -10,000 years ago (end of the Younger Dryas cold period 25), decades-long periods of warming and cooling have been superposed on even longer cycles. The longest of these cycles repeats about every 1500 years and the shortest about every 55-60 years.26 These latter, short, multi-decade intervals of rapid warming and cooling27 have occurred during the current, 350-year long 4 general warming trend.

The most recent short-term variations have been cooling between 1935 and 1975, warming between 1975 and 1995, and now cooling again since 2000. In short, nothing out of the ordinary natural climate variations have occurred since fossil fuel use accelerated in the last Century.

The Medieval Warm Period

A particularly prolonged warm period in the current interglacial between 9000 and 6000 years ago has been documented, most recently in oxygen isotopic analyses of Greenland ice sheet cores.26 That warm period resulted in significant thinning of Greenland’s ice sheet to thicknesses within a 100m of those of today.

Several other warm periods have occurred since, the most pronounced of which has been termed the Medieval Warm Period of 500-1300. 27

The Little Ice Age

The Little Ice Age of 1400-1900,14 following a century-long transition from the Medieval Warm Period, recorded the last significant period of global cooling during the last 10,000 years, although decades-long cooling has occurred several times since.

By 1400, Arctic ice pack had enclosed Iceland and Greenland and driven Viking settlers away from their farms on those islands.15

By the end of the 1600s, in response to the earlier climate cooling, glaciers had advanced over valley farmlands cultivated as those same glaciers receded during the Medieval Warm Period.16

Indeed, essentially all of the consequences of warming prior to 1300 reversed during the next several hundred years of the Little Ice Age.

Since about 1660, the middle of the last 70-year phase of the Little Ice Age, global surface and near surface temperatures have risen an average of about 0.9 degree Fahrenheit (half a degree Celsius) each 100 years, although decades-long cooling events have interrupted this trend..17

In response, a general retreat of world glaciers has taken place over the last century or more, not just in the last decades of the 20th Century,18 repeating the documented pattern of the Medieval Warm Period.

Arctic Ocean Ice

The Arctic Ocean ice pack has retreated northward since about 180019  and, as during the Medieval Warm Period, may have largely disappeared during some summers, depending on the high latitude atmospheric circulation.20

Similarly, though only on a decadal rather than a century scale, satellite observations since 1979 show that the decrease in the area of the Arctic ice pack since 1996 appears to have reversed from its 2007 summer minimum. 21

Antarctic Ice

Antarctic sea ice also retreated from the extent reported by many explorers and whalers early in the 20th Century22.

Antarctic sea ice, however, has been expanding northward for about two decades 34 after indications in the Law Dome ice core of an additional gradual decline between about 1960 and 1990. 35


Natural variations in climate have been very complex, often extreme, and all before human industrial activity existed.


Harrison H. Schmitt is a former United States Senator from New Mexico as well as a geologist and former Apollo Astronaut. He currently is an aerospace and private enterprise consultant and a member of the new Committee of Correspondence.

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