In Part 1 of this series on climate reconstruction and dating, we briefly discussed the evidence that leads scientists to believe our current period of warming is caused by human activity. We then launched into a popular technique for reconstructing past climate conditions – radiocarbon dating. In Part 2, we’ll focus more on a series of simpler methods, starting with dendrochronology.
Okay, we understand if radiocarbon dating was rather complicated, but we promise it gets easier from here on out. Dendrochronology is the study of using tree rings to determine the age of a tree and the climate conditions the tree experienced. As you probably know, you can figure out the age of a tree by counting the number of rings in its trunk. Each year, a tree adds a new layer of growth consisting of xylem cells, which are responsible for the tree’s water uptake. These layers are the rings we see in a tree stump or core sample. Tree rings can tell quite a bit about various climate settings the tree lived through. For example, thick rings indicate a period of significant growth, implying ample water (precipitation, flooding), space, or sunlight. Thinner rings suggest growth-stifling conditions, like droughts. Researchers can age the rings using radiocarbon dating and surmise how long ago these events occurred, and make inferences about the regional climate at the time.
Long-term Climate Dating – Stable Isotopes, Oceans, and Ice Cores (Oh my!)
The previous climate reconstruction methods we discussed are only useful for recent events (within the past 50,000 years or so). But how do we determine ice ages or carbon levels from hundreds of thousands or even millions of years ago? For starters, let’s take a look at stable isotopes. As the name implies, these are isotopes of an element that do not undergo radioactive decay. A common stable isotope is Oxygen-18 (18-O).
18-O located in layers of sediment at the bottom of the oceans can help unlock secrets about past ice ages. How?
- 18-O, along with the more common Oxygen-16 (16-O), can be found in water molecules. The proportion of 16-O to 18-O is constant.
- Water with 16-O is lighter than water with 18-O, and is more likely to evaporate. Normally, 16-O will precipitate back into the ocean, keeping the ratio of 16-O:18-O constant.
- During an ice age, ice sheets tie up ocean water. The 16-O that evaporates ends up in the ice sheets. This results in an enrichment 18-O during an ice age.
Ocean basins have tens of millions of years worth of fossils and sediment built up in annual layers. The deeper you go, the older the layers get. Each year’s sediment layer contains fossils, dust, and oxygen isotopes from that time. Analyzing layers with significantly higher levels of 18-O indicate the layers were deposited during an ice age. This gives us a record of when ice ages happened over the past several million years, which explicitly gives us information about Earth’s temperature changes as well.
So stable isotopes in ocean sediments at least partially help us reconstruct temperature records for the past several million years. But how do we determine what the atmosphere was like eons ago? One way to do so is to examine cores taken from ancient ice sheets in Antarctica. Much like annual layers of sediment, snow and ice build up in defined layers over the years. Winter ice deposits and summer ice deposits are darker and lighter in color, respectively, making it fairly easy to measure annual layers. Now, air bubbles can get trapped in the ice at the time of deposition. The air in these bubbles represents the composition of gases in the atmosphere when their ice layer was formed. Researchers can take an ice core as a sample to analyze the gas content in the air bubbles in order to reconstruct ancient atmospheric compositions. Below is an example of a 19 cm ice core section from Antarctica, taken from a depth of over 1800 meters. The arrows point out the lighter summer layers.
Source: National Oceanic and Atmospheric Administration
Antarctic ice core data has provided us with an 800,000 year record of atmospheric carbon dioxide. Matching this data with corresponding temperature charts of the same time period displays a strong correlation between the two variables.
Source: Rhode Island Public Radio. Temperature Data from Zachos et. al 2001 Transformed as in Hansen & Sato, 2012; Co2 Data: Luthi et al., 2008.
“But wait!” one may cry out. “Correlation does not necessarily equal causation!” A valid argument in many cases, but unfortunately not in this one. Combining what we know about the Greenhouse Effect, the molecular relationship between carbon dioxide and light/heat, and our temperature and atmospheric records for the past 800,000 years indicates a strong causal relationship between global temperatures and carbon dioxide. And this is scientific fact.
800,000-year Ice-Core Records of Atmospheric Carbon Dioxide (CO2). (n.d.). Retrieved October 24, 2017, from http://cdiac.ess-dive.lbl.gov/trends/co2/ice_core_co2.html
Espinoza, A. (n.d.). Battle With The Sea: What We Know And How We Know It. Retrieved October 24, 2017, from http://ripr.org/post/battle-sea-what-we-know-and-how-we-know-it#stream/0