by Emma Bertran
figures by Rebecca Clements

Over the past few decades, our Earth has undergone global changes, a gradual reshaping that can be witnessed in real time. Since the Industrial Revolution, the global annual mean concentration of atmospheric carbon dioxide (CO₂), a powerful greenhouse gas, has increased by more than 40%, rising from an average of 280 ppm (parts per million; 280 ppm can be visualized by imagining a huge bag of a million marbles where only 280 of them are red)— to over 400 ppm in May 2013, leading to a global warming by 1.7° C. As a result, climate conditions changed: plants and animals in search of better conditions suddenly migrate, glaciers and ice sheets melt, and the oceans acidify. Recent shifts in the United States political landscape, particularly regarding environmental policies, will scale back our country’s commitment to slow down emissions. Many of these decisions are based on the assumption that climate predictions are imperfect and that climate fluctuations have always happened, implying the planet will bounce back. While imperfect, climate predictions are based on evidence and solid science. If one wants to know what the Earth might look like in the future, and whether it can bounce back, one need look no further than the Earth’s past.

The ghost of climate past

Our story takes us to a period of time called the Paleocene-Eocene Thermal Maximum (PETM). 56 million years ago, global temperatures rose as a result of a massive injection of CO₂ into the atmosphere due to the burning of fossil fuels. Hence, the PETM bears marked resemblances to the current global warming, although unlike today, the causes of this burning were far from anthropogenic. Understanding how the PETM unfolded helps foresee what lies ahead.

Reconstructing Earth’s past climate relies on chemical signatures in sediments. As sediments accumulate, they trap minerals and shells containing clues on the composition of the oceans and atmosphere at the time those minerals and shells first fell on the ocean floor. When analyzing 56 million-year-old deep-sea sediments, paleoclimatologists found evidence of significant and rapid global changes.

They saw large shifts in time in the composition of oxygen isotopes in preserved shells. Before I explain why this shift is important, let me briefly explain what an isotope is. An element is defined by the number of protons it has. Isotopes are different versions of a particular chemical element that differ only by mass, as they have the same number of protons but different numbers of neutrons. Isotopes with more neutrons will be heavier than isotopes with fewer neutrons. This, of course, applies to oxygen, which has a number of isotopes, each defined by its mass. Thanks to the development of highly specialized instruments, researchers can distinguish and measure the relative abundance of the different isotopes of a given element in a sample. Their distribution in different materials reflects the processes in which they were involved. For instance, water (H₂O) will bear different oxygen isotopes, some heavier, some lighter. In the following, the water with the heavier oxygen isotopes will be called heavy water, while water with the lighter oxygen isotope will be called light water. Heavy water will evaporate more slowly than light water. Conversely, heavy water will accumulate in snow faster than light water, which will accumulate in seawater. The offset between the relative amount of heavy to light water in seawater and snow is temperature dependent, and hence climate dependent. With these relationships in mind, paleoclimatologists can built time records of the relative amount of heavy to light water in seawater preserved in shells, and infer the changes in global temperatures. The shells found in 56 million-year-old sediments show a large decrease in the abundance of heavy water, and hence increased abundance of light water, suggesting a global rise in temperatures by 5 to 8° C that lasted 20,000 years.

Looking for a cause

With evidence for changing global temperatures, the next step is to determine its cause. A plausible hypothesis is that, during the PETM, the atmosphere accumulated significant amounts of CO₂. To test this, scientists used the record of carbon isotope abundances in carbon-rich materials in sediments. Changes in their relative abundance of heavy and light carbon isotopes reflect changes in the main processes of the global carbon cycle. It turns out the record of carbon isotopes indicates a major perturbation in the cycle due to a massive inflow of CO₂ to the atmosphere. The amount of CO₂ produced 56 million years ago has been estimated to range between 3,000 and 10,000 petagrams (one petagram is 2.2 trillion pounds) over 20,000 years, which is 0.5 petagrams per year during the PETM. Today, fossil fuel combustion represents an inflow to the atmosphere of 6-8 petagrams per year. Thus, the carbon isotope record suggests a substantial buildup of CO2 in the atmosphere over 20,000 years.

A number of natural processes can cause a massive rise in atmospheric CO₂, including volcanic activity. Prior to the PETM, most of Earth’s continents formed a single landmass, Pangaea. But 56 million years ago, Pangaea was breaking up, a process accompanied by increased volcanism. However, to produce the volume of CO₂ implicated by carbon isotope studies, rates of volcanic activity would have had to be unreasonably high, implying CO₂ alone could not have caused the global temperature increase. Scientists now believe this initial increase in volcanism caused huge volumes of molten rock to rise and intrude into the landmass, baking carbon-rich sediments, including coal and oil. The burning of these fossil fuels released two strong greenhouse gases: carbon dioxide and methane. This was enough to initially increase global temperatures, which in turn led to the release of methane previously frozen in the cold seabed. The methane that bubbled its way to the atmosphere amplified the greenhouse effect, raising global temperatures even further. A cascade of responses followed (Figure 1). Droughts exposed forests and peat lands to desiccation, spreading wildfires, which released more CO₂. In Polar Regions, permanently frozen ground containing dead plants thawed. This plant material became food for methane-producing microbes. Combined, these effects pushed the PETM to its hottest point.

What happens when the planet is hotter?

The oceans’ inherent capacity to store carbon helped offset the warming at first, but the continuous flow of CO₂ into the atmosphere proceeded too rapidly for the oceans to keep up. Increasing seawater temperatures lowered the oxygen content of the water itself, as this gas is less soluble in warm waters than in cold ones. This, combined with ocean acidification due to CO₂ dissolution, caused a huge stress on sea life, particularly foraminifera. These small organisms produce a protective shell made of calcium carbonate. They had less oxygen to breathe, and their shells were much harder to maintain in acidic waters. Consequently, 30-50% of those species went extinct. The warming Earth also saw its climate zones shift, both on land and at sea. Indeed, fossils of crocodiles, ferns, and palm trees 56 million years old can be found above the Arctic Circle (Figure 2).

The direction and causes of the changes during the PETM are strikingly similar to those driven by human activity. Of course, 56 million years ago, the climate did bounce back, so one might question why we should worry about the current global warming. There are a few critical differences: the rate of temperature change, and the length of time of the PETM and the current global warming. The PETM lasted 20,000 years, and according to recent computer models built to understand the PETM, it took nearly 200,000 years – ten times as long – for the Earth’s natural buffers to return it to its initial state. Life responds more favorably to slow changes than to abrupt ones. During the PETM, life had time to adapt to its new conditions. The current global warming is occurring at a faster pace: CO₂ productions are ten times higher than during the PETM (Figure 3). This might not give life enough time to adapt. Animals and plants may go extinct, and our world will be changed for the next hundreds to thousands of years. While the idea of palm trees in Greenland may sound idyllic, this image comes at a very high price. Policymakers must consider lessons from the past if we are to at least give life a fighting chance.

Emma Bertran is a graduate student in the Department of Earth and Planetary Sciences at Harvard University. She is an earth historian, as she studies the close interaction between microbial metabolisms and their environment, and how those are reflected in specific chemical signatures in sediments and can be used to explore changes in Earth’s past environments.

For more information:

On the current global warming trends: https://www.epa.gov/climate-change-science/causes-climate-change
On the use of carbon and oxygen isotope records to understand past climate conditions: http://www.ldeo.columbia.edu/~peter/Resources/Seminar/readings/Ravelo%20&%20Hillaire-Marcel.2007.pdf
On the PETM: https://www.scientificamerican.com/article/the-last-great-global-warming/