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

Abstract:

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