Abstract
Abstract. Australia plays an important role in the global terrestrial carbon cycle on inter-annual timescales. While the Australian continent is included in global assessments of the carbon cycle such as the global carbon budget, the performance of dynamic global vegetation models (DGVMs) over Australia has rarely been evaluated. We assessed simulations of net biome production (NBP) and the carbon stored in vegetation between 1901 to 2018 from 13 DGVMs (TRENDY v8 ensemble). We focused our analysis on Australia's short-term (inter-annual) and long-term (decadal to centennial) terrestrial carbon dynamics. The TRENDY models simulated differing magnitudes of NBP on inter-annual timescales, and these differences resulted in significant differences in long-term vegetation carbon accumulation (−4.7 to 9.5 Pg C). We compared the TRENDY ensemble to several satellite-derived datasets and showed that the spread in the models' simulated carbon storage resulted from varying changes in carbon residence time rather than differences in net carbon uptake. Differences in simulated long-term accumulated NBP between models were mostly due to model responses to land-use change. The DGVMs also simulated different sensitivities to atmospheric carbon dioxide (CO2) concentration, although notably, the models with nutrient cycles did not simulate the smallest NBP response to CO2. Our results suggest that a change in the climate forcing did not have a large impact on the carbon cycle on long timescales. However, the inter-annual variability in precipitation drives the year-to-year variability in NBP. We analysed the impact of key modes of climate variability, including the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), on NBP. While the DGVMs agreed on sign of the response of NBP to El Niño and La Niña and to positive and negative IOD events, the magnitude of inter-annual variability in NBP differed strongly between models. In addition, we find that differences in the timing of simulated phenology and fire dynamics are associated with differences in simulated or prescribed vegetation cover and process representation. We further find model disagreement in simulated vegetation carbon, phenology, and apparent carbon residence time, indicating that the models have different types and coverage of vegetation across Australia (whether prescribed or emergent). Our study highlights the need to evaluate parameter assumptions and the key processes that drive vegetation dynamics, such as phenology, mortality, and fire, in an Australian context to reduce uncertainty across models.
Highlights
Decadal variability in the growth rate of atmospheric carbon dioxide (CO2) is strongly influenced by variability in the uptake and release of carbon by the oceans and the terrestrial biosphere (Ballantyne et al, 2012; Raupach et al, 2008)
This carbon uptake enhancement has been associated with the asymmetric response of gross primary production (GPP) to precipitation in combination with vegetation expansion linked to rainfall
We showed flux tower observations and simulations of net ecosystem exchange (NEE), GPP, and terrestrial ecosystem respiration (TER) together, we note that the spatial scales of observed ecosystem fluxes and simulations by a dynamic global vegetation models (DGVMs) are not directly comparable given that the flux tower footprint is ∼ 1 vs. ∼ 3000 km2 for a grid cell in a 0.5◦ grid
Summary
Decadal variability in the growth rate of atmospheric carbon dioxide (CO2) is strongly influenced by variability in the uptake and release of carbon by the oceans and the terrestrial biosphere (Ballantyne et al, 2012; Raupach et al, 2008). A recent study suggested that the Australian terrestrial carbon sink may be enhanced due to more extreme wet events projected for future decades (Ma et al, 2016). This carbon uptake enhancement has been associated with the asymmetric response of GPP to precipitation (i.e. positive GPP anomalies tend to be larger than negative ones; Haverd et al, 2017) in combination with vegetation expansion linked to rainfall (based on a single dynamic vegetation model; Poulter et al, 2014). A series of studies have identified evidence of rising CO2, leading to a marked greening of the Australian continent (Donohue et al, 2009, 2013; Ukkola et al, 2016b; Trancoso et al, 2017)
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