Climate Change and CO2 Uptake

by |May 18, 2009

The Keeling Curve, a record of rising atmospheric carbon dioxide since 1958, is one of the most recognizable images in modern science. Despite its ubiquity, this iconic graph tells only half – or, more precisely, 57% – of the story. That’s because the other 43% of cumulative anthropogenic emissions haven’t made it to the atmosphere yet. Rather, they’ve been absorbed by oceans and the terrestrial biosphere.

The existence of such large carbon sinks is, well, a happy turn of events for humans who have only recently recognized the downside of increasing atmospheric carbon. But the ability of these sinks to continue to absorb carbon in a warmer, more carbon-saturated world is not particularly well understood. Indeed, according to Lisa Welp – who stopped by Lamont on Friday to give a talk entitled “Is Recent Climate Change Enhancing CO2 Uptake in the High Northern Latitudes?” – the relationship between warming and CO2 uptake is just beginning to be worked out.

Welp is a post doc at Ralph Keeling’s CO2 Lab at Scripps. She also worked with Xuhui Lee at the Yale School of Forestry and Environmental Science and got her PhD in environmental science and engineering at Caltech. Her work focuses on the impact of climate change on CO2 uptake in high northern latitudes where temperatures are below freezing for long parts of the year.

Initially, Welp explained, scientists assumed that warming in such naturally cold areas would create a negative feedback cycle – that is, warmer temperatures lead to more vegetation, which leads to more CO2 uptake. In recent years, however, the evidence seems to point the other way. Specifically, CO2 uptake is stagnating and may even be decreasing. It has been posited that this may be because of an increased number of summer droughts and the “browning” of the boreal forest, but the exact relationship is not well understood.

Welp has made a contribution to our understanding of the issues by examining the relative effect of the carbon balance on different ecosystems. Boreal forests cycle naturally through regimes of aspen, black spruce, and recent burn; Welp’s research indicates that the deciduous aspens are much more sensitive to spring warming and summer drought than black spruce. Because of this, Welp concludes that these deciduous ecosystems contribute disproportionately to observed variability in atmospheric CO2 concentration. She hypothesizes that forest disturbances will increase as a result of climate change and other factors – leading to more deciduous forests and, ultimately, to greater variability in carbon uptake in the high northern latitudes.

Welp also used global scale inversion modeling to determine whether climate change has affected the high latitude carbon balance over the past two decades. She found a slight increase in CO2 uptake in the 50 N to 60 N band, but no significant effect above that. She also found an increase in the seasonal amplitude of northern CO2 fluxes. Interestingly, this increase occurred despite a relative decrease in vegetation. Welp doesn’t have a ready explanation for the decoupling of CO2 uptake from vegetation, but she’s hoping further tests will better illuminate the relationship. Given the amount of CO2 currently stored in the high northern latitudes, I hope so too.

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Would it be possible to record these talks and post them for downloading (along with the accompanying slide shows, where applicable)?