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A “where were you?” with atmospheric carbon dioxide concentrations

Many thanks to Melannie Hartman for providing me the idea for this post.

From superfamous via Flickr
From superfamous via Flickr

By Nell Campbell

At NREL there are many ways in which the current, past, and future atmospheric concentrations of carbon dioxide (CO2) are important to our research. As an institution we spend a great deal of time working on topics like plant productivity, soil microbial processes, and carbon cycling across scales. All of these topics have direct linkages to the anticipated shifts in precipitation and temperature associated with rising concentrations of CO2 in the atmosphere, as well as to the cycling of carbon that leads to rising atmospheric CO2 directly. In my weekly lab group (Dr. Keith Paustian’s lab) meeting just this past Friday, for example, we debated the importance of considering increasing atmospheric CO2 when using the DAYCENT ecosystem model to simulate a set of long-term ecological research (LTER) experimental sites *. Where the science of soils and plant production are involved, linkages to increasing atmospheric CO2 concentrations are not hard to find.

An interesting observation that came up during my lab group discussion, and the inspiration for this post, is the idea that many people can ‘date’ themselves by their earliest reference parts per million (ppm) atmospheric CO2 concentration. In other words, the answer to the question: “what was the concentration when ‘you’ started thinking about climate change?” can be revealing for a large number of scientists working in this field.

Why would this be revealing? Well a good place to start is with the current atmospheric CO2 concentration: within the last year we hit the 400 ppm mark (a fact highlighted in our 2013 yearly review). This benchmark concentration is higher than our earth has experienced in millions of years; it emphasizes our place in an era with no past analogues for comparison across the development of our society and the ecosystems we depend upon. At the other end is the starting point at the pre-industrial level of 280 ppm; prior to the widespread use of fossil fuels for industry, this was the relatively stable value that humans have experienced for much of our cultural development. Our scientific understanding of biogeochemical cycles tell us that in the long-term (we’re talking millenia here) everything will balance out, eventually. However it’s all the short term change and uncertainty that creates scientific concern, as well as the impetus for the vast array of ecological research projects aiming to understand where our global ecosystems are headed.

Keeling Curve from, updated daily.

Between 280 and 400 ppm we have a net increase of 120 ppm, and the ‘revealing’ part comes from that the fact that much of this change has been experienced by our current generation directly. In our lab meeting the idea of ‘dating’ yourself by your initial ppm value was supported immediately by our model; as a part of our discussion, we spent some time contemplating the calculations used to add the effect of increasing atmospheric CO2 concentrations, and a ‘current’ value of 350 ppm was written into that section of model code. Trace that back in time with the Keeling Curve and that dates the last use of this particular piece of code to the 1990’s!

Of course I then turned this idea back on myself, remembering first the value of 370 ppm with special significance which, in fact, coincides with when I started doing ecology as my undergraduate major in 2001. I also distinctly remember having to ‘update’ my baseline ppm when I taught high school biology to 380 ppm, which lines up perfectly with my one year of teaching prior to graduate school from 2005 – 2006.

The next step was to see if this idea plays out with others at NREL. Naturally I started with fellow current and past editors (specifically Jocelyn, Aaron, and Shinichi) who collectively rang in at the 370-380 range, likewise coinciding with their undergraduate years. Then I broadened out into the NREL research community; Melannie Hartman, the source of this idea, came in at 360ppm which coincided with her start work on this topic at NREL in 1993. Dr’s Bill Parton and Keith Paustian both rang in in the 350’s, lining up with the start of their respective consideration of the topic in the late 80’s and early 90’s.

My conclusion: yes, initial atmospheric CO2 concentrations among scientists working in fields related to climate change can say something about the scientists themselves and how long they have been engaged in this topic. However I have to draw a second conclusion: it is amazing how quickly our climate is changing.

Where were atmospheric CO2 concentrations when ‘you’ started thinking about climate change?


*To explain the Paustian lab discussion: LTER sites are a key data source to study and test models expressing our understanding of the biogeochemical linkages between plants and soils. Soils change slowly through time, and so the comparatively long timespan for LTER data (30 years and beyond) across diverse ecosystems in North America provides an invaluable testing ground for model development and application. At this particular meeting we debated the merits of having the model more accurately reflect the increasing atmospheric CO2 concentrations across these long experimental time periods. In our current model runs we use a single, static value for atmospheric CO2 concentrations. However since atmospheric CO2 concentrations have the potential to increase plant production by directly affecting the process of photosynthesis in plants that use the C3 photosynthetic process, these sites might be more accurately simulated by including a CO2 effect. On the other hand large-scale testing of the impacts of CO2 concentrations on plant growth have shown variable results, so this effect may or may not be necessary to accurately model plant/soil relationships at the LTER sites. The modeling approach used to include this in the DAYCENT model (a version of the CENTURY model) is described here.

Nell Campbell is an EcoPress editor and dates herself at 370ppm.