Model Structure and Climate Data Uncertainty in Historical Simulations of the Terrestrial Carbon Cycle (1850–2014)

Abstract

The divergence among Earth system models in the terrestrial carbon cycle has prompted interest in how to reduce uncertainty. Previous studies have identified model structural uncertainty arising from process parameterizations and parameter values. The current study highlights the importance of climate forcing in generating carbon cycle uncertainty. We use simulations in which three models (Community Land Model version 4 (CLM4), CLM4.5, CLM5) with substantially different carbon cycles are forced with two climate reconstructions (CRUNCEPv7, Global Soil Wetness Project 3 version 1 (GSWP3v1)) to examine the contributions of model structure and climate to uncertainty in the carbon cycle over the period 1850–2014. Climate uncertainty for global annual net biome production exceeds one third of total uncertainty (defined as the sum of climate and model structure uncertainty) in the first half of the twentieth century, but declines after the 1950s. Global annual gross primary productivity, net primary productivity, heterotrophic respiration, and vegetation and soil carbon stocks have substantial climate uncertainty (relative to total uncertainty) throughout the simulation period. Climate forcing contributes more than one half of total uncertainty for these carbon cycle fluxes and stocks throughout boreal North America and Eurasia, some midlatitude regions, and in eastern Amazonia and western equatorial Africa during the decade 2000–2009. Comparison with observationally based data sets of the carbon cycle using model benchmarking methods provides insight into strengths and deficiencies among models and climate forcings, but we caution against overreliance on benchmarking to discriminate among models. The conceptualization of uncertainty arising from this study implies embracing multiple feasible model simulations rather than focusing on which model or simulation is best.

The Community Land Model Version 5: Description of New Features, Benchmarking, and Impact of Forcing Uncertainty

“…Due to the breadth of model improvements and the scope of the model output, the assessment presented here is necessarily incomplete. Companion manuscripts focused on CLM5 for the CESM2 Special Issue provide more in‐depth assessment of specific aspects of the model (CO 2 and N‐additions response, Wieder et al, ; plant hydraulics, Kennedy et al, ; C‐N interactions and parameter uncertainty, Fisher et al, ; urban data sets, Oleson & Feddema, ; and terrestrial carbon cycle uncertainty, Bonan et al, ).…”

“…Furthermore and importantly, as noted above, Fisher et al () demonstrate that CLM5 responses to CO 2 and N fertilization exhibit strong sensitivity to several uncertain parameters. Nonetheless, the improvement in this important emergent behavior of the model is intriguing and is investigated in more depth in Bonan et al ().…”

High predictability of terrestrial carbon fluxes from an initialized decadal prediction system

“…Other terrestrial models, and perhaps even various versions of the same model, may represent terrestrial processes differently. The Community Land Model, for example, exhibits different sensitivity to meteorological forcings among versions 4 (used in this study) and versions 4.5 and 5 (Bonan et al 2019).…”

“…CESM-LE member #34) by comparing their statistical properties with a historical reconstruction. This reconstruction is a land-only simulation of the CLM4 that has been forced with the Global Soil Wetness Project (GWSP) version 3 reanalysis over 1850-2014 (Bonan et al 2019). Hereafter, we refer to this simulation as the 'GSWP reconstruction'.…”

The Community Earth System Model Version 2 (CESM2)

“…In CESM2(CAM6) and CESM2(WACCM6), the CO 2 fertilization response appears to be more reasonable and, when this is combined with updates to the land use and land-cover change carbon fluxes, results in a late 20th century accumulated carbon trend that agrees well with observationally based estimates. Further discussion on the source of improvement for this feature of CESM2, which is also seen in land-only simulations, is provided in Lawrence et al (2019), Wieder et al (2019), and Bonan et al (2019). In addition to the improved accumulated carbon, CESM2 simulations also show evidence of considerable improvements in the amplitude of the annual cycle of net ecosystem exchange, especially at northern high latitudes (see Carbon Dioxide variable metrics in Figure 19), which translate to improved annual cycle amplitudes of atmospheric CO 2 concentrations in emissions-driven simulations.…”