Evaluation of the terrestrial carbon cycle, future plant geography and climate‐carbon cycle feedbacks using five Dynamic Global Vegetation Models (DGVMs)

Abstract

This study tests the ability of five Dynamic Global Vegetation Models (DGVMs), forced with observed climatology and atmospheric CO 2 , to model the contemporary global carbon cycle. The DGVMs are also coupled to a fast 'climate analogue model', based on the Hadley Centre General Circulation Model (GCM), and run into the future for four Special Report Emission Scenarios (SRES): A1FI, A2, B1, B2. Results show that all DGVMs are consistent with the contemporary global land carbon budget. Under the more extreme projections of future environmental change, the responses of the DGVMs diverge markedly. In particular, large uncertainties are associated with the response of tropical vegetation to drought and boreal ecosystems to elevated temperatures and changing soil moisture status. The DGVMs show more divergence in their response to regional changes in climate than to increases in atmospheric CO 2 content. All models simulate a release of land carbon in response to climate, when physiological effects of elevated atmospheric CO 2 on plant production are not considered, implying a positive terrestrial climate-carbon cycle feedback. All DGVMs simulate a reduction in global net primary production (NPP) and a decrease in soil residence time in the tropics and extratropics in response to future climate. When both counteracting effects of climate and atmospheric CO 2 on ecosystem function are considered, all the DGVMs simulate cumulative net land carbon uptake over the 21st century for the four SRES emission scenarios. However, for the most extreme A1FI emissions scenario, three out of five DGVMs simulate an annual net source of CO 2 from the land to the atmosphere in the final decades of the 21st century. For this scenario, cumulative land uptake differs by 494 Pg C among DGVMs over the 21st century. This uncertainty is equivalent to over 50 years of anthropogenic emissions at current levels.

Using model‐data fusion to interpret past trends, and quantify uncertainties in future projections, of terrestrial ecosystem carbon cycling

“…Models of forest carbon cycling, such as the one used here, have been coupled with earth system models to project terrestrial carbon sinks and sources (e.g., Sitch et al, 2008) and feedbacks to climate change in the 21st century (Cox et al, 2000;Fung et al, 2005;Friedlingstein et al, 2006). Results have been incorporated into the assessment reports of the Intergovernmental Panel on Climate Change (IPCC, 2007) to guide mitigation efforts by governments and public (Solomon et al, 2007), though models diverge largely when projecting the future responses to climate change (Friedlingstein et al, 2006;IPCC, 2007).…”

“…Such models are often shown to reproduce observations 'reasonably well' (e.g., Williams et al, 2005;Braswell et al, 2005). However, model intercomparisons and model-data comparison studies show tremendous variations among models for both short-and long-term projections (e.g., Friedlingstein et al, 2006;Siqueira et al 2006;Sitch et al, 2008;Schwalm et al, 2010;Dietze et al, 2012;Keenan et al, in pressGCB).…”

Predicting the future of forests in the Mediterranean under climate change, with niche- and process-based models: CO2 matters!

“…Various types of process-based models are used and under development, such as gap models (Pacala et al, 1993;Bugmann, 2001), landscape models (Lischke et al, 2006;Scheller & Mladenoff, 2007), fitness-based models (Chuine & Beaubien, 2001), or sophisticated 'hybrid' dynamic vegetation models (e.g., Sitch et al, 2008), which focus on achieving a balance between realism, accuracy and complexity. The suite of available models represents a range from very detailed species-specific models which describe stand structure and hourly plant physiological processes (i.e.…”

Arctic Climate Tipping Points

“…A warmer future climate should enable northward expansion of the boreal forest into tundra regions (Scholze et al 2006;Sitch et al 2008). This typically occurs when regions exceed 1000 growing degree days (GDD) above zero, and it initiates a positive feedback whereby the trees obscure snow thus amplifying warming, as happened in the early Holocene (Foley et al 1994).…”