Abstract
Terrestrial ecosystems are an important sink for atmospheric carbon dioxide (CO2), sequestering ~30% of annual anthropogenic emissions and slowing the rise of atmospheric CO2. However, the future direction and magnitude of the land sink is highly uncertain. We examined how historical and projected changes in climate, land use, and ecosystem disturbances affect the carbon balance of terrestrial ecosystems in California over the period 2001–2100. We modeled 32 unique scenarios, spanning 4 land use and 2 radiative forcing scenarios as simulated by four global climate models. Between 2001 and 2015, carbon storage in California's terrestrial ecosystems declined by −188.4 Tg C, with a mean annual flux ranging from a source of −89.8 Tg C/year to a sink of 60.1 Tg C/year. The large variability in the magnitude of the state's carbon source/sink was primarily attributable to interannual variability in weather and climate, which affected the rate of carbon uptake in vegetation and the rate of ecosystem respiration. Under nearly all future scenarios, carbon storage in terrestrial ecosystems was projected to decline, with an average loss of −9.4% (−432.3 Tg C) by the year 2100 from current stocks. However, uncertainty in the magnitude of carbon loss was high, with individual scenario projections ranging from −916.2 to 121.2 Tg C and was largely driven by differences in future climate conditions projected by climate models. Moving from a high to a low radiative forcing scenario reduced net ecosystem carbon loss by 21% and when combined with reductions in land‐use change (i.e., moving from a high to a low land‐use scenario), net carbon losses were reduced by 55% on average. However, reconciling large uncertainties associated with the effect of increasing atmospheric CO2 is needed to better constrain models used to establish baseline conditions from which ecosystem‐based climate mitigation strategies can be evaluated.
Highlights
Changes in land use have been a primary factor in the global rise of atmospheric carbon dioxide (CO2) and a major driver of global climate change (Le Quéré et al, 2017)
Under the ‘hot‐dry’ model, reducing global emissions resulted in similar rates of net primary production (NPP) and increases in net ecosystem productivity’ (NEP), indicating the avoided warming asso‐ ciated with the representative concentration pathway (RCP) 4.5 scenario decreases carbon losses associ‐ ated with ecosystem respiration; Net ecosystem carbon balance (NECB) was projected to increase by 28.3%
Compared to the reference scenario (β = 0), total carbon stored in California ecosystems was projected to increase by 12.5% under RCP 4.5 and 17.1% under RCP 8.5 assum‐ ing a moderate CO2 fertilization effect (CFE) (β = 0.082)
Summary
Changes in land use have been a primary factor in the global rise of atmospheric carbon dioxide (CO2) and a major driver of global climate change (Le Quéré et al, 2017). Since 1850, land‐ use change (LUC) has added nearly half as much carbon to the atmosphere as fossil fuel emissions and has exerted a dominant influence on the storage of carbon in terrestrial ecosystems (Houghton & Nassikas, 2017; Le Quéré et al, 2017). Land use and manage‐ ment have the potential to undermine the ability of ecosystems to produce a wide range of services (Foley et al, 2005), includ‐ ing the storage and sequestration of carbon to mitigate climate change (Houghton & Nassikas, 2017). Studies indicate that climate change is increasing the frequency and magnitude of extreme events (Mann et al, 2017) which can alter ecosystem carbon bal‐ ance by increasing gaseous emissions and through the transfer of carbon from live to DOM pools (Kurz et al, 2008). The direction and magnitude of these feedbacks will either hinder or facilitate the achievement of local to global‐scale greenhouse gas reduction targets
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