Then and Now: CO2 Levels


It’s no secret that the Earth can endure extreme environments and climates that change over time. At over 4.5 billion years old, the Earth has undergone numerous fluctuations of global temperatures and naturally occurring global warmings and coolings. These fluctuations range from the ice age that caused “snowball Earth,” when temperatures were estimated to be minus 14 degrees Centigrade [1], to the Pre-Archean period, when temperatures were over 1000 degrees Centigrade [2]. Today, we see a climate change projecting to continue over this century and beyond, which arouse important questions: how did organisms survive such great changes before? Will humans be able to adapt to the changing environments and climates as well?

The current global warming is coined as the greatest environmental threat of our time [3]. According to the Natural Resources Defense Council (NRDC), the current situation is responsible for the rise in sea levels and the fluctuations of weather causing storms, fires, drought, and floods. Although the Earth has gone through many periods of natural global warming, the current global warming is different in that it is evidenced to be caused by human activities. The burning of fossil fuels and oil, clearing of land for human uses, and engaging in other human activities within the last century have significantly increased the concentration of carbon dioxide (CO2) in our atmosphere [4].

Throughout the Earth’s history, there has never been a time where global warming has occurred so rapidly, and scientists foresee that we are headed for a tripling or more of CO2 concentrations in a shorter period of time than those seen under natural causes. In addition, the NRDC found that the current maximum rates of CO2 release due to fossil fuel burning are much higher than that from that of the Paleocene-Eocene Thermal Maximum (PETM). The PETM, which occurred between the Paleocene and Eocene epochs over 55 million years ago, has been characterized by a massive carbon injection, resulting in global temperatures increasing by 5-8 degrees Centigrade [5]. As seen by these discrepancies between the current global warming to those from the past, it is very challenging to conclude the course of the next century and the impacts it will have on our society.

However, we can still observe and compare the Earth’s history to our current situation, which may shed some light of where we might be headed. The most recent event of global warming was the PETM and luckily, we can observe some of the effects through sediment and isotope data. Though we still don’t know exactly how PETM came to be, scientists have stated that the PETM and associated carbon pulse “are often touted as the best geologic analog for the current” manmade rise in CO2 levels [6].

According to geologists, there was a doubling in carbon dioxide levels during the PETM. Correspondingly, the surface of the ocean turned acidic due to extreme ocean acidification, which may have been a factor of global temperatures rising [7]. It is hypothesized that as a result of a decrease in dimethylsulfide production by phytoplankton due to the ocean acidification, which is also our biggest source of biologically created sulfur, affects the cloud production [8]. Cloud production affects the amount of sunlight reflected back to space and in turn, affect climate. The climate change resulted in an equatorial heat-death, as noted by a 2009 Nature article [9], causing survival and evolution of some species and death of many others.

Because the climate change during the PETM was so gradual, many ecosystems were able to adapt to the changes. However, there were some devastating consequences for some species, as evidenced by the composition of the calcareous nannofossils. The fossils included coccoliths and coccospheres (phytoplankton) of haptophyte algae found in marine deposits. Their survival was short-lived following the course of the PETM and many scientists suggest this was due to the warm surface waters and low pH conditions [10]. As the current global warming is occurring at a rate so quickly, it may have many more detrimental effects than during the PETM.

Dr. Andrew Somerville, a biological anthropologist at UCSD, agrees with the idea that for the species that lived on, size evolution resulted due to the rising temperatures [11]. Fossils indicate that turtles were as big as breakfast tables and snakes were as long as buses during this era [12]. Interestingly, Paleocene mammals, such as ancestors of modern-day horses, were as small as common day housecats. Due to the rising heat, ectotherms, also known as cold-blooded animals, needed a bigger surface area to regulate their heat, so  the bigger ectotherms were the winners and survived. On the other hand, it was more challenging for mammals to regulate body heat and, due to the limited nutrition available due to heat killing off many food sources, smaller mammals were able to survive better [12]. As evidenced by these changes, current global warming may result in evolution of humans and other warm-blooded mammals to much smaller beings, coexisting with much larger reptiles.

In addition, Dr. Somerville notes that the environment notably changed into trees and dense vegetation due to the hotter climates. As environment and evolutionary adaptation, a taxonomic and morphological diversification, is highly correlated with climate, some species were able to thrive more than others. Subsequent to the global warming, more grasslands were observed due to the cooling and drying temperatures. Interestingly, our human ancestors have coevolved with the grasslands, which may explain the rise of bipedalism [13]. Our diets also have been greatly affected by grasslands. Dr. Andrew Somerville states that since the US consumes about a third of the staple crops derived from the grasslands, including cereals (corn, grasses, rice, wheat, etc) and dairy products, our main food sources would ultimately be depleted alongside the degradation of grasslands. As the temperatures and CO2 levels rise, we may be going back to dense vegetative environments with many trees and the growth of angiosperms, as plants do well in hotter and wetter temperatures.

Although there isn’t direct evidence, other scientists predict that during the Paleocene era, there was great variation in weather events due to the volatility and the increase in heat energy in the environment. Results from helium isotopes and clay mineralogy suggests that a hydrologic change in western Europe was triggered by the PETM. This increased freshwater flux, which greatly affected the weather in the eastern North Atlantic [14]. As seen current day, sea levels are at a rise, which is predicted to affect the current conveyer belt of the North Atlantic. People predict that the current that brings warmth to UK and England will be affected by the current global warming, causing the area to be inhospitable and too cold for life.

Lessons from the previous global warming event clearly shows the correlation between global warming, environment, and its inhabitants. Although current global warming is caused by unnatural, anthropogenic causes and is occurring at a speed faster than ever seen before, we have the scientific background to predict how the world would change if we stay anywhere near our current emissions path.





Hey I'm Michelle S. Lee and this is my fourth year at UCSD. I'm double majoring in biochemistry/cell biology and psychology and I'm a part of Saltman Quarterly, Flying Samaritans, and Phi Delta Epsilon at UCSD. In my spare time, I love to go jogging, snowboarding, hiking, and shopping.