What Geology Has to Say About Global Warming

by |July 11, 2014
Cobscook Bay State Park, Maine. Photo: W. Menke

Cobscook Bay State Park, Maine. Photo: W. Menke

Last month I gave a public lecture entitled, “When Maine was California,” to an audience in a small town in Maine. It drew parallels between California, today, and Maine, 400 million years ago, when similar geologic processes were occurring. Afterward, a member of the audience asked me what geology had to say about global warming. The following is an expanded version of my answer. Note that I use the word geology to mean any element of the earth sciences that is focused on earth history, and do not distinguish the many sub-disciplines about which a specialist would be familiar.

Geologists think of the last 50 million years as the recent past, both because they represents only about one percent of the age of the earth, and because plate tectonics, the geologic process that controls conditions within the solid part of the earth, has operated without major change during that time period. This is the time period that is most relevant to gaining insights about earth’s climate that can be applied to the present-day global warming debate.

The geological record of ancient climate is excellent. Ancient temperatures can be determined very precisely, because the composition of the shells of corals and other marine organisms varies measurably with it.  Furthermore, the plants and animals that lived during a given time and are now preserved as fossils indicate whether the climate was wet or dry. The overall climatic trend has been cooling, from an unusually warm period, called the Eocene Optimum, 55-45 million years ago, to an unusually cool period, colloquially called the Ice Age, which ended just 20,000 years ago. The overall range in temperature was enormous, about 35°F. The earth was so warm during the Eocene Optimum that Antarctica was ice-free; ice caps did not start to form there until about 35 million years ago. Palm trees grew at high latitudes and cold-blooded animals, such as crocodiles, lived in the Arctic.

Lesson 1. The earth’s climate (including its average temperature) is highly variable.

Notwithstanding very divergent conditions, life flourished both during the Eocene Optimum and the Ice Age, though in both cases life was more abundant in some parts of the world than in others. The fossil record indicates that forests were common during the Eocence Optimum, yet some areas were sparsely vegetated steppes and deserts.  While the great glaciers of the Ice Age were lifeless, extremely large mammals such as Woolly Mammoth and Giant Ground Sloth inhabited lower latitudes. The changing climate produced both winners and losers. Some species adapted; others went extinct.

Lesson 2. Life flourished during both warm and cold periods; changes in climate produced both winners and losers.

Roque Bluffs State Park, Maine. Photo: W. Menke

Roque Bluffs State Park, Maine. Photo: W. Menke

An important issue is whether climate variability is due to processes occurring on the earth, or to changes in the intensity of sunlight – for it’s the sun that keeps our planet warm. The geological evidence, though subtle, strongly supports earthly, and not solar, causes. This evidence is drawn from the study of the many shorter period climate fluctuations, some which last millions of years and other just thousands, which are superimposed on the long-term cooling trend.

Climate during the Ice Age (the last 4 million years) has been particularly unstable, with many swings of more than 10°F.  These fluctuations are recorded in the annual layers of snow preserved in glaciers and in marine sediments, whose properties track the temperature at which they were formed. The timing of these swings closely follows regular fluctuations in the tilt of the earth’s axis and the shape of its orbit around the sun. Called Milankovitch cycles, they are due to the gravitational influence of the moon and planets. Their magnitude can be reliably calculated, since they are due to fluctuations of the position and orientation of the earth relative to the sun, and not to any change in the sun’s brightness. Surprisingly, they are too small to account for the large swings in temperature, unless the earth’s climate system is acting to amplify them. Here’s the subtle part of the argument: This mismatch between the feeble amplitude of the Milankovitch cycles and the large swings in climate is strong evidence that internal processes can cause strong climate variability.

Lesson 3. Variations in climate are mainly due to processes occurring on the earth, as contrasted to in the sun.

Ice Age carbon dioxide levels are well known, because bubbles of Ice Age air are preserved within the Antarctic and Greenland glaciers. More ancient carbon dioxide levels are difficult to measure, since no samples of older air have been preserved. Several indirect methods are in use, one based on the effect of ocean carbon dioxide levels on the composition of marine sediments, and another on its effect on now-fossil plant leaves. These measurements show fairly convincingly that the long-term cooling trend over the last 50 million years is associated with a gradual decrease in carbon dioxide levels, from 2000-3000 parts per million during the Eocene Optimum to 200 p.p.m. during the Ice Age. The cause of this decrease is not fully understood, but seems to indicate that the total amount of carbon that can influence climate (carbon in the atmosphere, biosphere and ocean) is slowly decreasing, possibly because an increasing amount of carbon is being tied up in sedimentary rocks such as limestone.

Lesson 4. Atmospheric carbon dioxide levels are highly variable, with the highest levels being associated with warm periods and the lowest levels associated with cold periods.

The correlation of atmospheric temperature with carbon dioxide reflects the latter’s role as a greenhouse gas. By absorbing heat radiated from the earth’s surface and re-radiating it back downward, it causes the earth’s surface to be warmer than it otherwise would be. The earth would be uninhabitable without the greenhouse effect, as can be seen by comparing the earth’s average temperature of about 60°F to the minus 100°F average temperature of the moon, which receives exactly the same amount of sunlight. An important question is whether the high carbon dioxide level at the time of the Eocene Optimum was the cause of the high temperatures that occurred during that time period.

Ascribing causes to fluctuation in climate is a tricky business, because atmospheric carbon dioxide level is only one factor among several that determine earth’s climate. Other important factors include: the amount of water vapor (another greenhouse gas) in the atmosphere; the percentage of the sky covered by clouds, which reflect sunlight back into space; the percent of land covered with ice and snow, which are also very reflective; and the percentage covered by oceans and and forests, which are very absorbing. All factors act together to maintain a given temperature; yet they feed back upon one another in complicated ways. Thus, for instance, had the Antarctic been glaciated during the Eocene Optimum (and the geological evidence is that it was ice-free), the world would have been somewhat cooler due to the high reflectivity of the ice. On the other hand, glaciers were absent precisely because the world was so warm. Geologic evidence alone cannot prove that the high levels of atmospheric carbon dioxide during the Eocene Optimum caused the high temperatures then, since the contribution of other factors, such as clouds and water vapor are unknown. Nevertheless, global climate models seem to indicate that such a high temperature only can be maintained in a world with high carbon dioxide; no other combination of factors can explain it.

Changing global temperatures induce changes in patterns of rainfall, winds and ocean currents, all of which can have a profound effect on the ecosystem of a given region. A large decrease in rainfall will, of course, turn rainforest into a desert. However, geology has few specifics to offer on the subject of how any particular region will be affected. The factors that cause climate change at a given geographical location are too varied to allow convincing geological analogues. However, geology shows that variability is the norm. Some of today’s deserts were forested a few million years ago, and some of today’s forests were formerly deserts. From the human perspective, climate change has the potential of causing some areas to become less agriculturally productive (and therefore less inhabitable), and other to become more so.

Lesson 5. Local climates are very variable, changing dramatically over periods of thousands to millions of years.

Wolfe Neck Woods State Park, Maine. Photo: W. Menke

Wolfe Neck Woods State Park, Maine. Photo: W. Menke

Changing global temperature can cause a rise or fall in sea level due to the accumulation or melting of glacial ice.  This effect is global in extent and one that can have an extremely deleterious effect on us human beings, since so many of us live near the coast. The geological evidence is very strong that sea level was higher by about 200 feet at times, such as during the Eocene Optimum, when Antarctica was ice-free, and was about 400 feet lower during the height of the Ice Age. The range is enormous; the world’s coastlines are radically altered by such changes. The continental shelves were substantially exposed during the low stands, and many low-lying coastal areas were underwater during the high stands. Woolly Mammoths roamed hundreds of miles offshore of Virginia during the Ice Age. Beach sand deposits in inland North Carolina indicate that the shoreline was far inland during the Eocene Optimum.

Lesson 6. Sea level has fluctuated as the world’s glaciers grow or recede, and was about 200 feet higher at times when Antarctica was ice-free.

Carbon dioxide levels have risen since the end of the Ice Age, first to a natural level of about 280 p.p.m. just before the start of the Industrial Era, and then to 400 p.p.m. as people burned coal and petroleum in large quantities. Carbon dioxide is currently increasing at a rate of about 2.6 p.p.m. per year.

A critical question is the level of atmospheric carbon dioxide 35 million years ago, when glaciers began to form in Antarctica, for it serves as a rough estimate of the concentration needed to melt present-day Antarctica. It’s a rough estimate only, for geological conditions were not exactly the same now and then. In particular, strong ocean currents that today keep warmer waters away from Antarctica were not present 35 million years ago, owing to the somewhat different configuration of tectonic plates. Unfortunately, the best currently-available estimates of atmospheric carbon dioxide during this critical time period have large uncertainties. Carbon dioxide decreased from 600-1400 p.p.m. at the start of the glaciations to 400-700 p.p.m. several million years later. These measurements are consistent with modeling results, which give a threshold of about 780 p.p.m. for the formation of a continental-scale ice cap on Antarctica. This value will be reached by the year 2150 at the present growth rate of atmospheric carbon dioxide – or sooner if emission rates continue to soar – suggesting that Antarctica will be at risk of melting at that time.

Antarctic ice will not melt overnight even should the threshold be reached. The deglaciation at the end of the Ice Age provides a useful example. The rate of sea level rise was initially low, just one-tenth of an inch per year. It then gradually increased, peaking at about 3 inches per year about 14,000 years ago, which was about 5,000 years after the start of the deglaciation. This rate persisted for 1,600 years, during which time sea level rose a total of 60 feet. The average rate of sea level rise was slower, about a half-inch per year.

Lesson 7. Sea level rise as fast as a few inches per year can persist over thousands of years.

The most extreme scenario for future carbon dioxide levels considered by the Intergovernmental Panel on Climate Change (IPCC) predicts about 0.4 inches per year of sea level rise over the next century. This rate is less than, but similar in magnitude, to the average rate during the Ice Age deglaciation, but considerably smaller than its peak.  Because of its focus on the current century, a reader of the IPCC report might be left with the sense that sea level rise will be over by 2100. Precisely the opposite is true! Geology demonstrates that melting accelerates with time and can last for several thousand years.

The most important lessons drawn from geology are that the earth’s climate can change radically and that the pace of change can be rapid. Geology also supports the theory that past periods of especially warm temperature were caused by high atmospheric carbon dioxide level. Of the many effects of global warming, geology is currently most relevant to sea level rise caused by melting glaciers. The precision of the measurement is currently too poor to give an exact answer to a critical question, At what carbon dioxide level are we in danger of melting Antarctica? However, while crude, these estimates suggest that this threshold will be reached in 150-300 years, if carbon dioxide levels continue to rise at the current rate.


Further reading:
“When Maine was California” (pdf) — talk to Downeast Lakes Land Trust
Intergovernmental Panel on Climate Change report (pdf)

William Menke of the Lamont-Doherty Earth Observatory is a professor of earth and environmental sciences.


Fieldwork in Maine, June 2014. Photo: W. Menke

Fieldwork in Maine, June 2014. Photo: W. Menke

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15 thoughts on “What Geology Has to Say About Global Warming

  1. Mark Wagner says:

    Dr. Menke, while I appreciate your considerable qualifications as a seismologist, I strongly suggest that you leave lecturing about atmospheric conditions alone unless you get another doctorate in climatology. Potentially flawed logic connecting atmospheric CO2 concentrations to global temperatures in the way you have propagates the notion that CO2 caused warming. Could the converse be true? Quoting the IPCC does not impress me with your clear and independent thinking skills; of which I hope you have in abundance!

    As a Petroleum Engineer, I do not have the deep climatology skills necessary to opine about anthropogenic impacts on the global temperature, but I can recognize someone extending their qualifications and “reaching” for a pulpit to preach that man is going to kill mankind.

    Your use of the word subtle is suggestive, inferring that you do not want to be bold in your position on “earthly” causes compared to those produced via the Milankovitch cycles but subtle. Given that your audience is not acedemic, they assume an implied expertise in the subject matter (qualifying yourself as a geologist and connecting that with climatology. How do you do that? How are you qualified as an Environmental Scientist? Plate techtonics?). I think you spend too much time with those at your school who derive a meaningful percentage of their income from their support of man-caused global temperature increase.

    As you know, data correlations are a tricky art. Even when it appears that action A creates effect B, another “artist” can manufacture a plausible effect C as a derivative. Are you an artist or a scientist?

    As an educated man, you of course recognize the vast amount of unknowns in the way our Earth regulates itself. Do you remember the Macondo accident, and the dire predictions about the oil “blob” creeping around Florida to destroy the east coast? I have looked extensively and cannot find that it actually happened. Wonder why? I did, so I investigated the event and as a hobby earth scientist was humbled to learn that oil consuming bacteria, largely ignored (except in my industry) procreated at an incredible rate to devour the oil, just as they have done for millions of years in the ocean depths! Remember, it is estimated that thousands of barrels of oil leak from the earth each day. The variation in that estimate is wide, and could be hundreds of thousands of barrels. But no matter; the earth heals and will continue in ways we have yet to understand. Long live the hungry bacteria!!

    In conclusion, please do not take this as an ad hominem attack, but as a criticism of your taking a position of trust too far. I would never question your work on seismology, but unless presented with better qualifications, continue to do so on your theories about global temperatures.

    So, thoughts?

    Mark Wagner

  2. Dennis N Horne says:

    “the notion that CO2 caused warming”


    The author is doing no more than explaining science “settled” by the Royal Society, National Academy of Sciences, American Association for the Advancement of Science, American Physical Society, American Chemical Society …

    Anyone wanting to learn basic climate science can go to the pages of any of those organisations.

  3. Tim Black says:

    So what I’m reading here is that assuming u are right that gas levels will rise with in 150 200 years to a level that melts the ice whether we change anything or not but faster if we don’t and that these events are cyclical so do we make changes and entirely change our way of life or do we look at technology to offer protection from the consequences or to potentially reduce gas levels I have no qualifications in science at any level however as I see it u can either sit and wait counting down or just maybe we could look at science a clearly unscientific idea that comes to mind is what other gas could have a potentially cooling effect when added to green house gas at any rate take the money invested in to climate change and look at solutions rather than an extension

  4. Roger Young says:

    As a Registered Professional Geologist, I think this is a great article that factually summarizes rather straightforward historical geology, geomorphology, and climatology.

  5. Natasha says:

    Mark Wagner describes himself as a “Petroleum Engineer” who I does “not have the deep climatology skills necessary to opine about anthropogenic impacts on the global temperature”…

    Yet Mr Wagner is happy to knock the author of this post, William Menke, on those same grounds – that ONLY an individual with the correct professional qualifications can give an opinion worth respecting – and that the opinions of those with different professional qualifications are somehow axiomatically less credible. Is that so Mr Wagner?

    Its very simple to step over Mark Wagner’s blatant professional (“Petroleum Engineer”) bias here: weigh the evidence for yourself, including the messenger(s) beginning with the observation that the professionals who do have the correct expertise on AGW do tell us that anthropomorphic CO2 emissions ARE already affecting the climate deleteriously. Check out the ‘death’ of the Great Barrier Reef for example.

    This issue does not need “Petroleum Engineer(s)” muddying the discussion with veiled ‘critical thinking’ 101 errors, when its obvious that long term humanity will be forced to stop using fossil fuel or die out.

    So lets talk about solutions like fast breeder nuclear instead?

  6. Dr. Bradley Wolff, D.C. says:

    Dr. Menke’s explanation of coastal changes associated with Earth’s “recent” timeline (55 million, to “just” 20,000 years ago) laid it out clearly:

    Fifty-five MILLION years ago, the Earth was once ice free at the poles – crocodiles roamed the Arctic, and sea levels were HUNDREDS of feet higher than they are today.

    Time Passes

    As the last Ice Age wraps up twenty THOUSAND years ago, the Earth was cold, and sea levels were 400 HUNDRED feet lower than they are today.

    A spread of eight HUNDRED feet in coastline sea level height change…all taking place before planes, trains, or automobiles scurried about the surface.

    To presume that humans can actuate any sort of meaningful, Global-wide, and long-term Geo-climatic change on a human (rather than geologic) timeline is both arrogant and ludicrous — we (like the dinosaurs before us) are less than fleas on the back of this planet.

  7. Ron Chin says:

    What has not mentioned here, and something Natasha should be considering is that our earth’s condition is also impacted by solar activity. The sun itself goes through changes and phases that impact our climate. A current snap shot shows the nutrient supplies in the great barrier reef diminishing and a world phenomena known as bleaching occurring, but the scholar or student of the earth knows this has occurred many many times before man even existed, and will many many times after man has run its cycle on earth. Dr. Wolff is correct, we are mere fleas on the back of our earth. Current solar data actually shows we are entering a cooling solar phase which will be noticeable over the next 15 years. The last time we had a cooling trend like this was in the 1700’s which was the mini ice age. If that data can be relied upon, the barrier reef’s death will reverse itself, and the northern ice cap will again rebuild. Its a mistake to look at the earth’s climate as a current snapshot when it has been changing dramatically changing for millions of years. But what do I know, I have to geology degree or climatology degree or astro-physics degree, I’m just a farmer who can read.

  8. JL Eaton says:

    Thanks for your exceptional article. Don’t give up on educating people, despite those who are singing to themselves, so as not to hear the information. The Chiropractor above actually addresses a compelling issue without understanding it: “time passes” is key. Tens of millions of years pass as the sea levels change with natural Earth cycles; but the mere 150 or so recent years’ rate of change is absolutely essential to comprehending the problem. A log graph showing the comparative rates-of-change might do wonders for educating those rare individuals still amenable to education. And thank you!

  9. Bill Smith says:

    To be brief, I’ll go with Natasha’s comment. Otherwise, I’m not reading much about US. My interest in global warming is strictly layman. I have read from legitimate sources about the ups and downs of temperature over the millions of years and the various causes, but I hear very little about US. This time, whatever length of time that is, we will be the cause of the increase in CO2 which is happening at an increased rate, due to the arrogance, ignorance and greed of the petroleum industry.

  10. charlie says:

    It’s nice to read that may be the climate modelers are interested in the history of climate too.
    May be the climate change theory and concerns is why we should all be hoping that quantum computing is feasible after all.

  11. Alex Paye says:

    Thanks you guys
    “The present is the key to the past”

  12. michael anderson says:

    280 ppm is not a natural co2 level. The last time co2 levels were as low as they have been recently, was in the last half of the carboniferous. Then it rose drastically during the triassic, to around 1500 ppm. The corellaton between co2 and temperature is almost nonexistent, when looking at earth’s history. On a finer scale, in the last few hundred years, co2 follows temperatre, it does not lead it. We are still in a cooler period than during the medival warming period. But how warm is it really? many of the data stations are in the middle of cities or other areas being artificially warmed. I talked to a guy, once, who dropped off research students from UAF, who were collecting soil temperature data right off the road embankment. That’s bad data! And it’s part of your models. But, we should not want to cool, because that is what caused huge famines and plauges, in the past. There are rediculous claims of co2 rising to 5000 ppm by the 24th century and other such nonsence, all based on a linear extrapolation of the most recent fluctuation of a chaotic, onlinear system. It ‘s like expecting a highly variable penny stock to hit 20000 in a month, based on it’s last peak. Would you bet your money on that? In adition, humans output 6 billion tons of CO2 per year. Sounds like a lot, right? Not compared to the 186 billion tons released anually through all sources. So, even if CO2 caused climate change, how are humans responsible for it when they are only responsible for 3.2% of total co2 output? Just face it, anthroprogenic climate change is a farce used by states to scare people into taxation and control and unfortunately, science has been corrupted, and their accademies have been turned into the modern day church, with billions of dollars in grants from those states to push it. I was getting emails, almost 10 years ago, to put my money into buying “carbon credits”, one of the best devised finacial scams in history, even better than pet rocks or expensive collector beanie babbies. The state pays for preconcieved solutions, then scares and guilts people into paying money for nothing so they, in the state, can fly around on private jets, averaging 0.5 to 2 mpg and pay for multiple vacation homes and mansions. This is objective science? I think not. Quit being suckers and Quit landing yourself in the middleground just to look “rational” to politicians (professional mafia men) with a high degree of psychopathy.

  13. michael anderson says:

    I meant 6 billion and 186 billion tons of CARBON, not CO2. percentage is the same, though. But it has increased to 9.5 billion tons of human produced carbon, since that number was published. That makes human contribution of carbon, or co2, a total of 5% of total carbon release. Couple that with the fact that water vapor accounts for 95% of the greenhouse effect, human produced carbon is only responsible for 0.27% of the greenhouse effect. Be glad you have a greenhouse effect, otherwise, earth’s average temp would be about -18C and we wouldn’t be here.

  14. Buddy says:

    Is it not possible that, rather than our use of fossil fuels, the increase in CO2 is resulting from mans quest to clear cut more and more forested areas, pave over more and more fields? Isn’t it likely that CO2 levels are rising because we are reducing the earths ability to process CO2 and create Oxygen?

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