The Pleistocene: Much warmer than today, but with low CO2 levels

Changes in ocean circulation patterns have a significantly larger effect on atmospheric temperature than the concentration of CO2, study finds

By Dr. Matthew Wielicki, Principia Scientific International, June 6, 2023

The Pleistocene epoch is a significant division of geologic time that occurred within the Cenozoic era, following the Pliocene epoch and preceding the Holocene epoch, which is the current epoch. The Pleistocene epoch spanned from approximately 2.6 million years ago (mya) to about 11,700 years ago.

During the Pleistocene, Earth experienced a series of repeated glaciations and interglacial periods. Vast ice sheets covered large portions of the Northern Hemisphere, including parts of North America, Europe, and Asia.

The cycles of advancing and retreating ice sheets had a profound impact on landscapes, carving out valleys, creating moraines, and reshaping the Earth’s surface.

The Pleistocene epoch was also marked by the presence of diverse megafauna, including large mammals such as mammoths, mastodons, saber-toothed cats, and giant ground sloths. These animals adapted to the harsh climate and were well-suited for survival in tundra-like environments.

The appearance of early humans, including Homo erectus and Homo neanderthalensis, also occurred during this epoch.

Towards the end of the Pleistocene epoch, approximately 11,700 years ago, the Earth transitioned into the Holocene epoch. The retreat of the large ice sheets and the onset of warmer climatic conditions allowed for the expansion of forests and the emergence of modern plant and animal species.

The Pleistocene epoch holds significant importance for the understanding of Earth’s climate dynamics, the evolution of life, and the interactions between early humans and their environment.

Temperatures of the Pleistocene Epoch

Recently, multiple studies have shown that the Pleistocene Epoch was significantly warmer than today. In fact, on the order of 9–19 °C above contemporary values, at least in Northern Greenland.

That’s on the order of 10 times the current warming of 1.5°C since pre-industrial times, as reported in the journal Nature (found here). A 2022 study published in Nature, “A 2-million-year-old ecosystem in Greenland uncovered by environmental DNA”, states:

Palaeoclimatic records show strong polar amplification with mean annual temperatures of 11–19 °C above contemporary values. The biological communities inhabiting the Arctic during this time remain poorly known because fossils are rare.

Here we report an ancient environmental DNA (eDNA) record describing the rich plant and animal assemblages of the Kap København Formation in North Greenland, dated to around two million years ago.

The record shows an open boreal forest ecosystem with mixed vegetation of poplar, birch and thuja trees, as well as a variety of Arctic and boreal shrubs and herbs, many of which had not previously been detected at the site from macrofossil and pollen records.

The DNA record confirms the presence of hare and mitochondrial DNA from animals including mastodons, reindeer, rodents and geese, all ancestral to their present-day and late Pleistocene relatives.

The presence of marine species including horseshoe crab and green algae support a warmer climate than today.

Climate alarmists will warn that Northern Greenland does not represent global temperatures. However, it’s not reasonable to assume that regions that were 10 times warmer than pre-industrial levels in the Pleistocene were somehow isolated from the rest of the planet.

What caused the incredible warmth during the early Pleistocene Epoch?

One thing we can rule out is elevated atmospheric CO2 concentration as highlighted in this study published in 2019 in Nature titled “Low CO2 levels of the entire Pleistocene epoch”:

Quantifying ancient atmospheric CO2 provides valuable insights into the interplay between greenhouse gases and global climate.

We reconstructed CO2 during 2.6–0.9 million years ago, which documents overall low CO2 levels ( less than 300 ppm) comparable with ice-core records, indicating that the Earth system has operated under late Pleistocene CO2 levels for an extended period.

The CO2 levels do not show statistically significant differences across the mid-Pleistocene Transition (ca. 1.2–0.8 Mya), suggesting that CO2 is probably not the driver of this important climatic event.

So, if it’s not elevated atmospheric CO2 concentration, what could have produced these incredibly warm temperatures?

A 2014 study in the journal Geophysical Research Letters, “Dynamical changes in the tropical Pacific warm pool and zonal SST gradient during the Pleistocene,” states:

If the regional radiative effects of CO2 were the only agent of change, tropical Sea Surface Temperature (SST) gradients should have remained similar as CO2 varied with time.

Instead, a new record of SST from the west Pacific shows that tropical SST gradients were different, even reversed, in the past, suggesting an important role for dynamical circulation changes.

Specifically, changes in the temperature of upwelled source water, in addition to local CO2 forcing, influenced tropical Pacific SST. These dynamical changes, rather than CO2, may have shifted the background state of the tropics and even helped set the stage for the mid-Pleistocene transition.

This evidence suggests … dynamic circulation changes, rather than simply a response to “top-down” CO2 local forcing.

So it appears that changes in ocean circulation patterns …have a significantly larger forcing on the atmospheric temperature than the concentration of CO2.

Recent studies have shown that significant amounts of heat are trapped in the oceans and this has always been attributed to “top-down” heating of the oceans due to anthropogenic greenhouse gas (GHG) emissions.

However, the oceans have approximately 1000 times the energy of the atmosphere. So, this is analogous to having a large campfire and then lighting a match and claiming the match is heating the campfire.

The oceans are heated by multiple sources (see this earlier post), and it appears that the changes in ocean circulation could account for 10 times the amount of warming we have thus far experienced as well as act as a huge source of increased atmospheric CO2.

Ocean circulation, not CO2, drives warming

In summary, the early Pleistocene was on the order of 9–19 °C warmer than the present-day, in multiple locations, and at pre-industrial levels of CO2.

This incredible warmth, 10 times greater than the observed warming since 1850, was likely due to changes in ocean circulation patterns.

This data highlights the relative insensitivity of surface temperature to atmospheric CO2 concentration and provides a great example of the complex nature of the climate system.

Distilling the climate system to only GHG concentrations is reductionist and ignores this amazing complexity.

With this in mind, we should have no expectation of a measurable response in surface temperatures to a reduction of atmospheric GHG concentration.

This article has been edited and condensed. To read the original article click here.

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