Enhanced Seasonal Exchange of CO2 by Northern Ecosystems – Observations and Models

Thursday, 18 December 2014: 4:45 PM
Heather D Graven1, Ralph F Keeling2, Stephen C Piper2, Prabir Kumar Patra3, Britton B Stephens4, Steven C Wofsy5, Lisa R Welp2,6, Colm Sweeney7, Pieter P Tans7, John J Kelley8, Bruce C Daube5, Eric A Kort9, Gregory Santoni5, Jonathan D Bent2, Rebecca Thomas10 and Iain Colin Prentice10,11, (1)Imperial College London, London, SW7, United Kingdom, (2)Scripps Institution of Oceanography, La Jolla, CA, United States, (3)Res. Inst. for Global Change, Yokohama, Japan, (4)National Center for Atmospheric Research, Boulder, CO, United States, (5)Harvard University, Cambridge, MA, United States, (6)Purdue University, EAPS, West Lafayette, IN, United States, (7)NOAA/Earth System Research Lab, Boulder, CO, United States, (8)University of Alaska Fairbanks, Anchorage, AK, United States, (9)University of Michigan Ann Arbor, Ann Arbor, MI, United States, (10)Imperial College London, London, United Kingdom, (11)Macquarie University, Sydney, Australia
Long-term measurements of atmospheric CO2 have revealed increasing amplitude in seasonal variations at Northern Hemisphere sites. In a recent paper1, we extended the analysis of seasonal CO2 amplitude using aircraft data from 1958-61 and 2009-11 and found large increases of 50% in the mid-troposphere north of 45°N. Changes in amplitude south of 45°N were less than 25%. The observations indicate that seasonal CO2 exchanges with northern terrestrial ecosystems must have increased by 30-60% over the past 50 years. The increased exchange is likely widespread over northern ecosystems but it must be focused in boreal forests to match the observed spatial pattern in the aircraft data. Small decreases in seasonal CO2 exchange of subtropical and tropical regions may also contribute to CO2 amplitude changes. The required increases in seasonal CO2 exchange in northern ecosystems are larger than simulated by terrestrial models, indicating the models do not capture substantial ecological changes occurring since 1960.

This presentation will give an overview of the recent paper1, highlighting the atmospheric evidence for a dominant influence from boreal forests and from the main growing season months. It will also expand on the investigation of modeled changes in seasonal CO2 flux using CMIP5 and other model intercomparisons, including the modeled influences of carbon vs climate drivers.

1. Graven et al. 2013, Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960, Science, 341, 6150, 1085-1089. DOI: 10.1126/science.1239207