Abstract
AbstractEcosystems continuously adapt to interacting environmental drivers that change over time. Consequently, the carbon balance of terrestrial ecosystem may presently still be affected by past anthropogenic disturbances (e.g., deforestation) and other environmental changes (e.g., climate change). However, even though such so‐called “legacy effects” are implicitly included in many carbon cycle modeling studies, they are typically not explicitly quantified and therefore scientists might not be aware of their long‐term importance. Here, we use the ecosystem model LPJ‐GUESS to quantify legacy effects for the 21st century and the respective contributions of the following environmental drivers: climate change, CO2 fertilization, land use change, wood harvest, nitrogen deposition, and nitrogen fertilization. According to our simulations, the combined legacy effects of historical (1850–2015) environmental changes result in a land carbon uptake of +126 Gt C over the future (2015–2099) period. This by far exceeds the impacts of future environmental changes (range −53 Gt C to +16 Gt C for three scenarios) and is comparable in magnitude to historical carbon losses (−154 Gt C). Legacy effects can mainly be attributed to ecosystems still adapting to historical increases in atmospheric CO2 (+65 Gt C) and nitrogen deposition (+33 Gt C), but long‐term vegetation regrowth following agricultural abandonment (+8 Gt C) and wood harvest (+19 Gt C) also play a role. The response of the biosphere to historical environmental changes dominates future terrestrial carbon cycling at least until midcentury. Legacy effects persist many decades after environmental changes occurred and need to be considered when interpreting changes and estimating terrestrial carbon uptake potentials.
Highlights
Terrestrial ecosystems and the atmosphere constantly exchange carbon via photosynthesis, respiration, and disturbances
The combined impacts from future environmental changes (+16 Gt C) and, despite the shorter time period, comparable in magnitude to historical losses (Figure 2). This can largely be explained by ecosystems still adapting to present‐day CO2 levels (+65 Gt C), but, maybe more surprisingly, by ecosystems not yet being in equilibrium with present‐day nitrogen deposition levels (+33 Gt C)
Legacy effects from historical climate change cause a net carbon release to the atmosphere (−11 Gt C) which is a result of enhanced soil carbon decomposition in the nowadays warmer climate (Figure S2b)
Summary
Terrestrial ecosystems and the atmosphere constantly exchange carbon via photosynthesis, respiration, and disturbances. Several modeling studies aimed to quantify the effects of specific environmental drivers on historical or future terrestrial carbon cycling (e.g., Friedlingstein et al, 2019; Huntzinger et al, 2017; McGuire et al, 2001; Tagesson et al, 2020). A review of recent modeling studies can be found in Tharammal, Bala, Devaraju, and Nemani (2019), who reported continued land carbon uptake in the 21st century across most scenarios and models, mainly due to CO2 fertilization. The review of Tharammal, Bala, Devaraju, and Nemani (2019) highlighted that often only the effects of one or two environmental drivers on the carbon cycle were investigated, making comprehensive assessments of the interplay of different drivers difficult. Studies often do not separate the isolated effects of KRAUSE ET AL
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