Paleoclimate Puzzles: Reconstructing the Earth’s past climate
There are many fundamental mysteries that surround Earth system processes on our planet. It is the existence of these mysteries that motivate Earth scientists to make new discoveries to piece together the puzzle of how our planet formed and has evolved through time. Paleoclimatologists are a type of Earth scientists who attempt to reconstruct the climatic conditions of the past. We use an array of fossil and chemical tools to inform us about climate variables like temperature, humidity, precipitation, pH, ice volume (and many more!) and these tools are referred to as proxies. For example, the oxygen isotopic composition of fossilized plankton is a proxy for ocean temperature. When this value decreases ocean temperature is higher, and when this value increases ocean temperature is lower. In an ideal world, proxies would be perfect recorders of the variables of climatic interest to us. But there are always complications. The oxygen isotopic composition of fossilized plankton, for example, is also a function of ice volume on land in addition to seawater temperature. Therefore, proxies are carefully calibrated, and when the proxy estimate is a function of multiple variables all of these effects are taken into account. These data can then be used to constrain numerical models as we try to suss out why particular climate conditions prevailed. The important question here is why do we do this? We’re of course curious about the history of our planet, but what do we have to gain from studying the Earth’s past climate?
The earth is 4.56 billion years old and in that time has experienced a wide variety of climate states. These range from a completely ice-covered planet known as Snowball Earth, to hot house climates with tropical North and South poles. Currently we live in an ice-house climate. We have permanent ice sheets at the North and South poles. But since the industrial revolution, humans have been modifying the climate by burning fossil fuels and releasing CO2 into the atmosphere. The million-dollar question is what will be the ultimate consequences of this process? How will the rapid addition of CO2 into the atmosphere affect the climate of our planet? This is one of the questions that paleoclimatologists are equipped to answer. Because the Earth has seen everything from ice houses to hot houses, odds are that we can find a past analogue of our current climate situation. Using various proxies, we can identify periods of Earth’s past in which the rate of CO2 input to the atmosphere most closely resembles our current rate of increase. We can then examine climate records preserved in marine sediment, rocks and fossils to determine how that past scenario played out and then we will have expectations of how our climate will change into the future and how the human population will be affected by these changes.
But here is the tricky part; an exact analogue of CO2 release as rapid as it is today does not exist in Earth’s history. The closest event in Earth’s past that resembles the modern increase of CO2 in the atmosphere is an event known as the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago. This event was originally identified in fossilized marine plankton called foraminifera. The foraminfera recorded large excursions in both temperature and carbon isotopes. The carbon isotopic composition of the foraminifera indicate dramatic changes in the carbon cycle; in this case, there was a massive input of isotopically light carbon to the atmosphere and this light carbon signature was transmitted to the surface ocean. Now identified in many marine sediment cores, the question is whether this input of carbon to the atmosphere during the PETM is analogous to the anthropogenic emission of carbon. We have a fairly well-constrained estimate of anthrogopenic carbon emission to the atmosphere: 515 petagrams of carbon (the unit is PgC) since preindustrial times and the estimated total fossil fuel reservoir is 5000 PgC. The PETM estimates are more difficult to constrain. Based on all the available data, modellers and climatologists estimate anywhere from 5400 PgC to 112 000 PgC released over several thousand years during the PETM! Can this event help us to constrain future climate changes due to anthropogenic CO2 emission? The short answer is, maybe. Depending on the amount of carbon released and over what period of time, this can certainly act as an endmember estimate. If 5400 PgC was released over 10 000 years and resulted in 5 degrees worth of warming during the PETM, then 5000 PgC released due to the burning of fossil fuels over 300 years will likely have more dramatic effects.
The PETM is still surrounded by many mysteries. How was the carbon emitted to the atmosphere? What was the source of the carbon? Why is the carbon so isotopically light? There are several different hypotheses put forward that satisfy the criteria of this event that the proxies have laid out for us and more work will be done to determine the extent of PETM as an analogue of modern anthropogenic CO2 emission. In the meantime, there are other paleoclimate puzzles to study and many new discoveries to be made about Earth system processes on our planet.
IPCC Report AR5
Pagani M., Caldiera K., Archer D. and Zachos JC. 2006. An Ancient Carbon Mystery. Science 314:1556-57
Genna Patton is a PhD student in Oceanography at the University of British Columbia in Vancouver. She uses chemical tracers preserved in marine sediment to reconstruct ocean circulation in the past. Her work area is in the Atlantic Ocean with a specific interest in the Labrador Sea and further South in the Ceara Rise.
Views expressed in blog posts reflect those of the author, and not necessarily those of the CFES.