Climate Change Daily Earth Science Repost

An ancient analog to global warming

Fifty-five million years ago, the average surface temperature of the earth spiked to about 27 degrees Celsius (80.6 degrees Fahrenheit); 13 degrees Celsius higher than our average temperature today. Rapid global climate warming was enabled by a temperature-related tipping point. This event is known as the Paleocene-Eocene Thermal Maximum (PETM).

The PETM is the closest historical global warming analog that we have to today. The ocean pH fell by 0.4 units during the PETM event. It doesn’t seem like much, but it represents a 100% increase in acidity from the pre-PETM levels. A pH decrease of greater than 0.1 units in human blood can kill a person. PETM changes occurred over 20,000 years at a rate of about 0.1 unit decrease in pH every 5,000 years. At the same time, atmospheric temperatures rose about 1 degree Celsius every 5000 years.

Present-day climate warming has also produced ocean acidification. Ocean pH has decreased by 0.1 units in about 200 years, and approximately 70% of that drop occurred in the last 100 years. Earth’s average temperature has increased by about one degree Celsius over the same time frame. These rates of change are about 25 times greater than during the PETM event. So, even though the PETM is a reasonable analog to help understand some aspects of rapid climate warming, it cannot provide an exact match to the hyper-rate of temperature change that the earth’s climate is currently undergoing.

Agents of Warming

Many studies of the PETM event identify rapid increases in atmospheric carbon dioxide (CO2) levels as the initial agent of change. The effects of atmospheric CO2 increases are twofold. The atmosphere received a huge injection of greenhouse gases to spike warming, and 25% – 30% of the CO2 released into the atmosphere was absorbed into ocean waters. Then the CO2 and water reacted to produce carbonic acid causing the oceans to become more acidic.

There is no certainty as to the causes of the atmospheric CO2 increase leading up to the PETM. But speculative reasons include increased volcanic activity, the eruption of a massive kimberlite field, or even a comet impact. All of these potential CO2 sources have pros and cons regarding their significance, but the fact remains that large amounts of CO2 from any source will cause climate warming.

The rapidity, strength, and short life span of the PETM temperature spike are unusual. Therefore, the unusual nature of this event is what makes it a useful comparison to today as earth experiences rapid warming and acidification of the oceans. One explanation for the rapid PETM temperature rise is methane release through a positive feedback loop.

Methane Clathrates

In the depths of the oceans where temperatures are low, and pressures are high, conditions are ripe for the formation of methane hydrates, or methane clathrates as they are also known. Large amounts of methane get trapped within water crystals to form a substance similar to ice. Large deposits of these clathrates exist today on the ocean floors, and the oceans 55 million years ago probably had the same types of deposits.

Ocean water temperatures rose 6 to 8 degrees Celsius during the PETM, and temperatures before the PETM were already significantly higher than today. One theory is that at some threshold temperature, large methane clathrate deposits destabilized and released significant amounts of methane into the atmosphere. Methane is a more potent greenhouse gas than CO2, and it traps heat 20 times more effectively than CO2.

The released methane amplified the warming trend causing even more clathrates to decompose. At the right tipping-point temperature, the warming and methane release cycle became self-sustaining and continue until all of the ocean’s methane-clathrates were gone. This massive release of methane created conditions that amplified climate warming to produce an abnormal temperature spike.

If there is one lesson to take away from the PETM, it is that tipping points matter. Once they are passed, there is no going back.


Arctic Warming (Source: ArcheanWeb) – Also:

Embrace a Hot Earth (Source: ArcheanWeb) – Also:


Feature Image: Eocene in Wyoming – “Life-history of our planet … Illustrated by M. Gunning” Author: GUNNING, William D. Contributor: GUNNING, Mary. Shelfmark: “British Library HMNTS” Page: 10 Place of Publishing: Chicago Date of Publishing: 1876 Issuance: monographic Identifier: 001544493

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Ocean Acidification

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William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.