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Last Time The World Was Warm

A Return to Pliocene Conditions

Contemplating history is necessarily relative to your frame of reference. Atmospheric carbon dioxide (CO2) levels and global average temperatures are the highest in recorded history — a time frame covering the last 5,000 years. But 5,000 years represents a mere 0.0001% of Earth’s history. All of human history is a drop in the bucket of our planet’s history. Fortunately, we only have to gaze back in time about three million years to find a period when Earth was warmer and atmospheric CO2 levels were about the same as today. That time was the middle Pliocene.

The Pliocene age spans a period ranging from 5.3 to 2.6 million years ago. The International Commission on Stratigraphy divides the Pliocene into two stages, the Zanclean, followed by the Piacenzian. The second stage (Piacenzian) was when the world last saw greenhouse gas levels as high as we are experiencing today.

But it wouldn’t be fair to picture the Pliocene as a greenhouse world because the overall trend throughout the Pliocene was global cooling. Also, other changes were occurring throughout the Pliocene. The Central American land bridge between North and South America formed, allowing animals to migrate between the two continents. In Africa, apes came down from the trees and onto the emerging savannas, starting their evolution into human beings.

The last time Earth experienced CO2 levels equivalent to today was during the second stage of the Pliocene (Piacenzian) from about 3.3 to 3.0 million years ago. This period was not a major thermal spike but rather a warming bump during the middle phase of a cooling trend. In other words, temperatures were dropping, and after the Middle Pliocene, Earth continued to cool, only returning to higher temperatures in modern times.

It is legitimate to ask what kind of world existed in the Middle Pliocene because the answer to that question may provide us a look at our own future. We think of sea levels as relatively stable, even though they are now rising, but when viewed over millions of years, the oceans are in constant flux, rising and falling in response to larger climatic changes. The Pliocene was no exception, and sea levels fluctuated up and down by 20 to 30 meters (66 to 98 feet). So, this is our first clue. In a world with today’s CO2 levels, sea levels could be some 60 to 90 feet higher.

Researchers estimate that during the mid-Piacenzian Warm Period global temperatures were about two to three degrees Celsius above modern pre-industrial temperatures. Of course, today, we have global temperatures over one degree Celsius above pre-industrial levels, so we are well on our way to mid-Pliocene conditions. The Pliocene oceans’ estimated response to higher temperatures and CO2 levels was a rise of about 56 feet above today’s mean sea level. The Pliocene sea-level fluctuations were mainly a response to changes in polar ice storage. Still, they also include about three to four feet of sea-level rise from thermal expansion of the warmer ocean waters.

Carbon Dioxide

Atmospheric CO2 levels today hover just below 420 ppm. We can compare this to when America’s founding fathers set foot on a relatively pristine North America, and CO2 levels were between 270 and 280 ppm. The change between then and today represents a 50% rise in CO2 concentrations, but most of this increase occurred over the past 50 years. During the 200 years between 1750 and 1950, CO2 levels rose by only 13%. Over the past seventy years, CO2 concentrations in the atmosphere increased by about 35%.

The change of 140 ppm from the pre-industrial era doesn’t seem like much, but a little CO2 goes a long way. Without greenhouse gases — CO2 being the primary component — the Earth’s average temperature would be about -18 degrees Celsius. As CO2 concentrations rise, so does the temperature. Mid-Pliocene CO2 levels were about the same as today, but the planet was several degrees warmer, and sea levels were significantly higher.

So, if Pliocene atmospheric CO2 concentrations were the same as today, why are global temperatures today lower than in the Pliocene? It is true that because Earth is a complex system, there are many variables affecting climate. In particular, ocean circulation systems can significantly affect how heat is distributed around the planet. But the most probable answer to our question is that we are currently experiencing a temperature lag effect.

The oceans are vast, and stupendous amounts of heat are required to even slightly raise the average global ocean temperature. More than 90 percent of the extra heat absorbed by Earth over the past 60 years has ended up in the oceans. The amount of energy absorbed by the oceans over the past 60 years is equivalent to the energy released by 2.8 billion Hiroshima-sized atomic bombs, or about three bombs exploding every 2 seconds for 60 years

The oceans create a buffering system where initial increases in heat are absorbed into the seas. This process dampens the response time of the planet’s temperature to increases in CO2 and greenhouse warming. There is reason to suspect that our current levels of atmospheric CO2 will eventually lead to mid-Pliocene temperatures. This, of course, works against our efforts to mitigate global warming. Even if we were to freeze greenhouse gas emissions at current levels, the Pliocene analog tells us we would probably see another two degrees Celsius of warming because of the lag in climate response time.

Rate of Change

The rate at which human activity has changed atmospheric CO2 levels is also important to consider. From the perspective of geological time, the 70 years from 1950 to the present is an unnoticeable moment. The Earth usually changes slowly, allowing millions of years for life to adjust. The few periods of extremely rapid change in the geological past are also known as mass extinction events, where life failed because rapid climate change overwhelmed life’s ability to adapt and evolve. But even the conditions around mass extinctions usually evolve over much longer periods than Anthropocene climate change. The late Permian mass extinction, also known as the “Great Dying,” is believed to have been heavily influenced by rapid increases in CO2 levels from the formation of the Siberian Traps. Rapid, in this case, means the basalt flows from this event erupted for two million years.

With the exception of an asteroid strike, Anthropocene warming is the most rapid global climate change event in geological history. The rapidity of change exacerbates the problem of response lag time. History tells us that we have only lit the spark, and the fire is yet to come. I think we can be assured that the changes wrought so far will provide momentum for future warming regardless of what measures we take.

The middle Pliocene is our closest analog to present-day greenhouse gas levels. It provides a probable peek into our future — several more degrees of global temperature rise and many meters of sea-level rise. In all likelihood, this outcome is a bullet we can’t dodge. Our best option is to prepare and adapt at a faster rate than the environment around us changes, lest we become part of the Anthropocene mass extinction.

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Mid-Pliocene sea level and continental ice volume based on coupled benthic Mg/Ca palaeotemperatures and oxygen isotopes (Gary S Dwyer and Mark A Chandler; The Royal Society) 

Constraints on global mean sea level during Pliocene warmth (Oana A. Dumitru, Jacqueline Austermann, Victor J. Polyak, Joan J. Fornós, Yemane Asmerom, Joaquín Ginés, Angel Ginés, & Bogdan P. Onac, Nature) 

Atmospheric CO2 Mirroring Levels Seen 15 Million Years Ago (by Carol Konyn; 

Feature Image: Savanna Lands (Modified by ArcheanWeb) — Original Credit: By Vincent Mugaba — Own work, CC BY-SA 4.0, Wikimedia Commons 

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.