It’s finally here, the first Climate Aware post, and I’m wondering what kind of science-y stuff I should talk about. I’m thinking Oxygen; it’s kind of important right? I mean, we need it to survive, but it also tries to slowly kill us, which is why our bodies need antioxidants (think blueberries). But what else makes oxygen so important? If you said “because we use it as a proxy for historical climatic variability,” you’re 100% correct…. or you’re standing directly behind me as I type this. Either way, you’re here now and you should keep reading.
Many scientists have seen this before (Fig. 1), or maybe something similar, but there are many of you who haven’t, so what is it? Figure 1. is a stacked graph showing a measure in ppm (parts per million) of oxygen 18 (on the y-axis) versus time in thousands of years (on the x-axis). To create this beautifully constructed graph, data was collected from 57 sediment cores from around the world and time correlated (a discussion I’ll save for another time), going back roughly 5.3 million years. Thanks to Lorraine Lisiecki and Maureen Raymo (2005), two amazing women in science, all of the hard work has been done. All right, good, but what is 18O, and what does it have to do with the CLIMATE?
Well it turns out that oxygen has a few different “forms”, called isotopes. Three different isotopes occur naturally 16O (most abundant), 17O (trace amounts), and 18O (next most abundant). The superscript refers to the atomic mass of oxygen, thus 18O is heavier than 16O because it has two more neutrons than 16O. Now you may be thinking: “this is way too much chemistry for me, just get to your point!” I’m getting there, but like all good stories, you need a dramatic setting first.
It turns out that because 16O is lighter than it’s counterpart 18O, it preferentially evaporates. Conversely, because 18O is heavier than 16O, it preferentially condenses within the atmosphere. This is extremely important for climatologists because they can measure the amount of 18O in seawater, glacial ice, and sea floor sediment. The measured amount is then compared to a standard ratio of oxygen isotopes in seawater, at a depth of 200 to 500 meters. (Riebeek, 2005)
Figure 2. displays the difference in 18O concentration in annual precipitation compared to average global annual temperatures. What we find is that colder temperature locations have a lower 18O concentration and, consequentially, higher 16O concentration of isotopes. Knowing that 18O preferentially condenses, and therefore precipitates out of the atmosphere first, it makes sense that warmer climates would contain a higher concentration of 18O in the seawater measurements, and that is exactly what we find. (Riebeek, 2005) When air rises it cools, both due to the adiabatic temperature lapse rate and as it moves towards higher latitudes. Beginning at low latitudes, moisture within an airmass condenses and eventually falls as precipitation containing the heavier 18O isotope. Remaining moisture, which contains mostly 16O, continues to move poleward where it will eventually precipitate out as rain, snow, or ice.
The implications for this are HUGE! Since there is a correlation between temperature and a ratio of 16O to 18O we are able to use oxygen as a proxy for historical global climate change. For example (Fig. 3) Zachos et al., (2001) reconstructed major geologic, climatic, and biotic events for the past ~65 million years, with the help of oxygen 18.
Climate proxies such as oxygen 18 and oxygen 16 (oxygen is just one of many) enable us to resolve global climate on a much larger scale. Understanding past climate variability then allows us to better deduce the results of more finely tuned research. This is extremely important when discerning our current climate, and helps in comparing the similarities and differences that exist between what we are observing now and what has occurred in the past.
And that, my friends, is why we care about oxygen.
- Lisiecki, L. E., and Raymo, M. E., 2005, A Pliocene-Pleistocen stack of 57 globally distributed benthic 18O records: Paleoceanography, v. 20, p. 1-17
- Riebeek, H., 2005, Paleoclimatology: the Oxygen Balance: http://earthobservatory.nasa.gov/Features/Paleoclimatology_OxygenBalance/
- Zachos et al., 2001, Trends, Rhythms, and Aberrations in Global Climate 65 Ma to the Present: Science, v. 292, p. 686-693