Drivers of multi-century trends in the atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; mean annual cycle in a prognostic ESM

Jessica Liptak,Keith Lindsay,Gretchen Keppel-Aleks

doi:10.5194/bg-14-1383-2017

Abstract

Abstract. The amplitude of the mean annual cycle of atmospheric CO2 is a diagnostic of seasonal surface–atmosphere carbon exchange. Atmospheric observations show that this quantity has increased over most of the Northern Hemisphere (NH) extratropics during the last 3 decades, likely from a combination of enhanced atmospheric CO2, climate change, and anthropogenic land use change. Accurate climate prediction requires accounting for long-term interactions between the environment and carbon cycling; thus, analysis of the evolution of the mean annual cycle in a fully prognostic Earth system model may provide insight into the multi-decadal influence of environmental change on the carbon cycle. We analyzed the evolution of the mean annual cycle in atmospheric CO2 simulated by the Community Earth System Model (CESM) from 1950 to 2300 under three scenarios designed to separate the effects of climate change, atmospheric CO2 fertilization, and land use change. The NH CO2 seasonal amplitude increase in the CESM mainly reflected enhanced primary productivity during the growing season due to climate change and the combined effects of CO2 fertilization and nitrogen deposition over the mid- and high latitudes. However, the simulations revealed shifts in key climate drivers of the atmospheric CO2 seasonality that were not apparent before 2100. CO2 fertilization and nitrogen deposition in boreal and temperate ecosystems were the largest contributors to mean annual cycle amplification over the midlatitudes for the duration of the simulation (1950–2300). Climate change from boreal ecosystems was the main driver of Arctic CO2 annual cycle amplification between 1950 and 2100, but CO2 fertilization had a stronger effect on the Arctic CO2 annual cycle amplitude during 2100–2300. Prior to 2100, the NH CO2 annual cycle amplitude increased in conjunction with an increase in the NH land carbon sink. However, these trends decoupled after 2100, underscoring that an increasing atmospheric CO2 annual cycle amplitude does not necessarily imply a strengthened terrestrial carbon sink.

Highlights

The amplitude of the mean annual cycle of atmospheric CO2, an indicator of seasonal terrestrial and ocean carbon exchange, has increased over the Northern Hemisphere (NH) since observational records began in the late 1950s (Pearman and Hyson, 1981; Cleveland et al, 1983; Bacastow et al, 1985; Conway et al, 1994; Keeling et al, 1996; Randerson et al, 1997; Graven et al, 2013; Liu et al, 2015)
The Community Earth System Model (CESM) underestimated the magnitudes of AObs by roughly 50 % (Fig. 5b, c), and the 16 % relative increase in the hemispheric-average amplitude between 1985 and 2013 estimated by the CESM was lower than the observed increase of 24 %
By analyzing CESM simulations run to 2300 with the ECP boundary conditions, we identified notable carbon cycle interactions that were not apparent before 2100, the nominal end date for CMIP5 runs

Summary

Introduction

The amplitude of the mean annual cycle of atmospheric CO2, an indicator of seasonal terrestrial and ocean carbon exchange, has increased over the Northern Hemisphere (NH) since observational records began in the late 1950s (Pearman and Hyson, 1981; Cleveland et al, 1983; Bacastow et al, 1985; Conway et al, 1994; Keeling et al, 1996; Randerson et al, 1997; Graven et al, 2013; Liu et al, 2015). Because atmospheric CO2 observations are characterized by high precision and accuracy, the gradual, multi-decadal increase in the seasonal amplitude provides a unique observational target for Earth system models (ESMs) intended to predict the long-term coevolution of climate and the carbon. The mechanisms embedded in ESMs to predict future carbon–climate interactions have been identified as the likely drivers of the observed mean annual cycle amplitude increase as described in the following paragraphs

Methods

Results

Conclusion