An atmospheric perspective on land and ocean ecosystems in a time of global change

Abstract:

We are now century or more into the "great geophysical experiment" of rising levels of greenhouse gases in the atmosphere. As this "experiment" slowly plays out, a major need is to track changes in land and ocean ecosystems which are impacted both changes in physical climate and chemical climate (e.g. changing CO₂ levels.). Measurements of CO₂ concentration and related variables (such as O₂ and the isotopes of CO₂) in background air continue to provide key insights into how these systems are changing.

One surprise coming from these observations is evidence that land ecosystems are serving as a sink for a significant fraction the excess carbon dioxide from fossil-fuel burning. Ecosystems remote from direct human influences have evidently been thrown out of steady state by a range of processes and are accumulating carbon unusually. Evidence for this sink is compelling found from a variety of methods, including from measurements of atmospheric O₂ abundance. Additional compelling evidence for large changes in land ecosystem comes from measurements of the seasonal cycle in atmospheric CO₂.

In contrast, there is little evidence that marine ecosystems - at least those remote from coastal influences - have undergone such large changes. While the oceans are absorbing excess CO₂, this is mostly a physical, not a biological, process. But just as the absence of evidence is not evidence for absence, it could just be that we lack strong long-term atmospheric constraints on marine ecosystems. Whereas atmospheric CO₂ data provides a wealth of information on land processes, they do a poor job of resolving ocean biological processes because the buffering chemistry of CO₂ in seawater slows the response of atmospheric CO₂ to processes originating from within the ocean.

This talk will highlight evidence for changes in land ecosystems from measurements and CO₂ and its isotopes, and will illustrate a path forward to resolving changes in marine ecosystems and related impacts on the ocean carbon sink via measurements in atmospheric O₂ concentration, particularly via the tracer "atmospheric potential oxygen" APO, which is effectively the sum of the O₂ and CO₂ concentrations, which shows rich variability of marine origin.