Carbon Dioxide Through Time

Carbon Dioxide Through Time

In the late 1950s, Roger Revelle, an American oceanographer based at the Scripps Institution of Oceanography in La Jolla, California began to ring the alarm bells over the amount of CO₂ being emitted into the atmosphere. Revelle was very concerned about the greenhouse effect from this emission and was cautious because the carbon cycle was not then well understood. So, he decided that it would be wise to begin monitoring atmospheric concentrations of CO₂. In the late 1950s, Revelle and a colleague, Charles Keeling, began monitoring atmospheric CO₂ at an observatory on Mauna Loa, on the big island of Hawaii. Mauna Loa was chosen because its elevation and location away from industrial centers made it as close to a global signal as any other location. The record from Mauna Loa, one of the most classic plots in all of science, shown in the figure below, is a dramatic sign of global change that captured the attention of the whole world because it shows that this "experiment" we are conducting is apparently having a significant effect on the global carbon cycle. The climatological consequences of this change are potentially of great importance to the future of the global population. The CO₂ concentration recently crossed the 400 ppm mark for the first time in millions of years!  In 2022, the yearly average was 417 ppm (check that number with the curve below!).

The record of CO₂ measured at Mauna Loa, Hawaii shows seasonal cycles superimposed on a longer-term rise in the yearly average (black line). The seasonal cycles are related to seasonal variations in photosynthesis and soil respiration in the Northern Hemisphere, where most of the land mass is located at present. The long-term trend is related to the addition of CO₂ to the atmosphere through the combustion of fossil fuels.

Credit: NOAA

As the Mauna Loa record and others like it from around the world accumulated, a diverse group of scientists began to appreciate Revelle's concern that we really did not know too much about the global carbon cycle that ultimately regulates how much of our CO₂ emissions stay in the atmosphere.

The importance of present-day changes in the carbon cycle, and the potential implications for climate change became much more apparent when scientists began to get results from studies of gas bubbles trapped in glacial ice. As we learned in Module 1, the bubbles are effectively samples of ancient atmospheres, and we can measure the concentration of CO₂ and other trace gases like methane in these bubbles, and then by counting the annual layers preserved in glacial ice, we can date these atmospheric samples, providing a record of how CO₂ changed over time in the past. The figure below shows the results of some of the ice core studies relevant for the recent past -- back to the year 900 A.D.

The recent history of atmospheric CO₂, derived from the Mauna Loa observations back to 1958, and ice core data back to 900, shows a dramatic increase beginning in the late 1800s, at the onset of the Industrial Revolution. At the same time, the carbon isotope composition (δ13C is the ratio of 13C to 12C in atmospheric CO₂) of the atmosphere declines, as would be expected from the combustion of fossil fuels, which have low values of δ13C. The inset shows a more detailed look at the last 150 years, where we can see that the rise in CO₂ coincides with the rise in the burning of fossil fuels.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

The striking feature of these data is that there is an exponential rise in atmospheric CO₂ (and methane, another greenhouse gas) that connects with the more recent Mauna Loa record to produce a rather frightening trend. Also shown in the above figure is the record of fossil fuel emissions from around the world, which show a very similar exponential trend. Notice that these two data sets show an exponential rise that seems to begin at about the same time. What does this mean? Does it mean that there is a cause-and-effect relationship between emissions of CO₂ and atmospheric CO₂ levels? Although we should remember that science cannot prove things to be true beyond all doubt, it is highly likely that there is a cause-and-effect relationship -- it would be an extremely bizarre coincidence if the observed rise in atmospheric CO₂ and the emissions of CO₂ were unrelated.

How serious is our modification of the natural carbon cycle? Here, we need a slightly longer perspective from which to view our recent changes, so we return to the records from ice cores and look deeper and further back in time than we did in the figure we have been examining.

The record of atmospheric CO2 over the last 400,000 years shows that the recent rise in CO₂ is unlike anything we’ve seen in the past 400 kyr both in terms of the rate of increase and the levels to which it is rising. Before this recent rise, CO₂ fluctuated by about 80 ppm in connection with the ice ages (which as you can see have a regularity to their timing); this pattern has clearly been interrupted by the recent trend. The data shown here come from a variety of ice cores (blue, green, red, and cyan) and the Mauna Loa observatory (black).

Credit: Robert A. Rohde (original PNG), User: Jklamo (SVG conversion) [CC BY-SA 3.0]

In addition to providing a record of the past concentration of CO₂ in the atmosphere, as we learned in Module 1, the ice cores also give us a temperature record. By studying the ratios of stable isotopes of oxygen that make up the glacial ice, we can estimate the temperature (in the region of the ice) at the time the snow fell (glacial ice is formed by the compression of snow as it gets buried to greater and greater depths). From these data, shown in the figure below, we can see the natural variations in atmospheric CO₂ and temperature that have occurred over the past 160,000 years (160 kyr).

Data from the Vostok (Antarctica) ice core for the past 160 kyr show the relationship between variations in heat-trapping gases CO₂ (carbon dioxide) and CH₄ (methane) concentrations in parts per million (ppm) and parts per billion (ppb) and the temperature at Vostok. Note that each curve has its own scale for the vertical axis, but they all share the same time scale. The dashed blue line at the end shows the very recent rise in CO₂ to the present day value of about 410 ppm, indicated by the arrow. The gas concentrations come from tiny bubbles trapped in the ice as it forms near the surface, while the temperature variations come from studying isotopes of oxygen and hydrogen in the ice itself. The ice cores thus provide us with an exceptional picture of atmospheric gas concentrations in the past and their relationship with temperature.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

In fact, looking at this much longer span of time enables us to clearly see that the present CO₂ concentration of the atmosphere is unprecedented in the last several hundreds of thousands of years. As geoscientists, we are interested in more than just the last few hundred kiloyears, and so we look back into the past using sediment cores retrieved from the deep sea. Geochemists studying these sediments have been able to reconstruct the approximate concentration of CO₂ in the atmosphere and the sea surface temperature (SST).

The longer history of atmospheric CO₂ as reconstructed from studies of deep-sea sediments. In the upper right, the blue region represents the upper and lower estimates back through time — you can see that it is difficult to be too precise going back this far in time — and you can see that the last time the midpoint of these estimates rose above the current level was around 2.5 Myr ago. This was a time when there was far less ice on Earth; the Arctic was apparently 15 to 20°C warmer than it is today, and sea level was about 20 meters higher than the present. As we go further back in time, we see that the atmospheric CO₂ concentration rises to very high levels. The Earth was a very different place before about 30 Myr ago — sea level was perhaps 100 m higher and there was practically no ice on Earth.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

To find atmospheric CO₂ levels equivalent to the present, we have to go back 2.5 million years. This means that, to the extent that the state of the carbon cycle is closely linked to the condition of the global climate, we are pushing the system toward a climate that has not occurred any time within the last several million years -- not something to be taken lightly.

The farther back in time we go, the more difficult it is to figure out how CO₂ concentrations have changed, but that has not stopped some from attempting:

The history of atmospheric CO₂ over the last 550 Ma, based on modeling, shows extremely high levels about 100 Ma (million years ago) and before 350 Ma. Note that there are huge uncertainties associated with these estimates, but the mid-range of the estimates suggests that CO₂ levels were very high during this time period. Interestingly, these periods of high CO₂ more or less coincide with periods of high sea level, as can be seen in the lower panel.

Credit: David Bice © Penn State University is licensed under CC BY-NC-SA 4.0

One thing that is clear is that further back in time, CO₂ levels have been much, much higher, and the average global temperatures have also been much higher. Why does the CO₂ concentration change so much? This is a big question whose answer involves many factors, but consider two that are relevant to what we'll learn about in this module. Photosynthesis only started in the Silurian (S on the timescale in the figure above), and photosynthesis is a major sink or absorber of atmospheric CO₂. Sea level was much higher during the two big peaks in CO₂ — this leaves less room for photosynthesis and it also decreases the planet's albedo, making it warmer. A warmer ocean cannot absorb atmospheric CO₂ and instead, it releases it to the atmosphere.

In conclusion, from this brief look at the record of fossil fuel emissions and atmospheric CO₂ concentrations, it is clear that we have cause for concern about the effects of the global CO₂ "experiment". Because of this concern, there is a tremendous effort underway to better understand the global carbon cycle. In the remainder of this module, we will explore the global carbon cycle by first examining the components and processes involved and then by constructing and experimenting with a variety of models. The models will be relevant to the dynamics of the carbon cycle over a period of several hundred years -- these will enable us to explore a variety of questions about how the system will behave in our lifetimes and a bit beyond.