Changes in soil organic carbon storage predicted by Earth system models during the 21st century

Abstract

Abstract. Soil is currently thought to be a sink for carbon; however, the response of this sink to increasing levels of atmospheric carbon dioxide and climate change is uncertain. In this study, we analyzed soil organic carbon (SOC) changes from 11 Earth system models (ESMs) contributing simulations to the Coupled Model Intercomparison Project Phase 5 (CMIP5). We used a reduced complexity model based on temperature and moisture sensitivities to analyze the drivers of SOC change for the historical and high radiative forcing (RCP 8.5) scenarios between 1850 and 2100. ESM estimates of SOC changed over the 21st century (2090–2099 minus 1997–2006) ranging from a loss of 72 Pg C to a gain of 253 Pg C with a multi-model mean gain of 65 Pg C. Many ESMs simulated large changes in high-latitude SOC that ranged from losses of 37 Pg C to gains of 146 Pg C with a multi-model mean gain of 39 Pg C across tundra and boreal biomes. All ESMs showed cumulative increases in global NPP (11 to 59%) and decreases in SOC turnover times (15 to 28%) over the 21st century. Most of the model-to-model variation in SOC change was explained by initial SOC stocks combined with the relative changes in soil inputs and decomposition rates (R2 = 0.89, p < 0.01). Between models, increases in decomposition rate were well explained by a combination of initial decomposition rate, ESM-specific Q10-factors, and changes in soil temperature (R2 = 0.80, p < 0.01). All SOC changes depended on sustained increases in NPP with global change (primarily driven by increasing CO2). Many ESMs simulated large accumulations of SOC in high-latitude biomes that are not consistent with empirical studies. Most ESMs poorly represented permafrost dynamics and omitted potential constraints on SOC storage, such as priming effects, nutrient availability, mineral surface stabilization, and aggregate formation. Future models that represent these constraints are likely to estimate smaller increases in SOC storage over the 21st century.

ORCHIDEE-SOM: modeling soil organic carbon (SOC) and dissolved organic carbon (DOC) dynamics along vertical soil profiles in Europe

“…The soil is the largest terrestrial carbon pool and its response to global warming is thus crucial for the global carbon (C) cycle and its feedback to climate change (Jobbagy and Jackson, 2000;Todd-Brown et al, 2014). Among other things, Earth system models aim to predict the vulnerability of the global carbon cycle to climate change.…”

“…However, to date the response of the soil organic carbon (SOC) pool and fluxes to climate change remains highly uncertain, mainly because the mechanistic understanding of soil processes remains imperfect and because new knowledge is not rapidly incorporated into these global models (Bradford et al, 2016;Schmidt et al, 2011;Todd-Brown et al, 2013;Wieder et al, 2015). In this sense, many authors have called for a more realistic representation of what governs SOC dynamics and transport within land surface models (LSMs), which are the land component of Earth system models (Battin et al, 2009;Nishina et al, 2014;Schmidt et al, 2011;Todd-Brown et al, 2014;Wieder et al, 2015). Among the suggested model improvements, the representation of the vertical SOC distribution and dissolved organic carbon (DOC) in the soil and the lateral export of carbon out of the soil are most likely to considerably improve model simulations.…”

Field-warmed soil carbon changes imply high 21st century modeled uncertainty

“…The CMIP5 models, representing our best coupled climate models to date, have a wide range of soil carbon responses over the 21 st century (Todd-Brown et al, 2014). While it would be nice to have all the models agree on a tightly bound answer, 15 the question we should be asking scientifically is: Does the variation in the models reflect our best scientific understanding?…”

“…However, these studies addressed the impact of warming in isolation, and it remains unclear how this process will interact with the variety of other global change drivers to affect the global soil carbon stock over the rest of this century. Reflective of such uncertainty, soil carbon changes projected for 2100 under business-asusual scenario for Coupled Model Intercomparison Project Phase 5 (CMIP5) range from -70 to 250 Pg-carbon across different Earth system models (Todd-Brown et al, 2014), making the land-carbon feedback one of the largest sources of uncertainty in 5 future climate projections (Friedlingstein et al, 2014). Improving the soil carbon component of the Earth system models is essential to predicting the future evolution of the Earth system and thus establishing meaningful greenhouse gas emissions targets.…”

“…Estimating Q10 directly from 25 warming-induced changes in soil carbon stocks could be a valuable approach to address these limitations, but the variability and relative imprecision of soil carbon stock data necessitates a large sample size to adequately describe variation at the global scale ). Yet, results from a recent Earth system model meta-analysis indirectly suggests that, with enough sample coverage it may be possible to infer Q10 directly from changes in soil carbon stocks (Todd-Brown et al, 2014).…”