Legacy effects from historical environmental changes dominate future terrestrial carbon uptake

Andreas Krause,Almut Arneth,Anja Rammig

doi:10.5194/egusphere-egu2020-8031

Abstract

&lt;p&gt;The carbon balance of terrestrial ecosystems is determined by environmental drivers (chiefly related to climate and land use) which interact with each other and change over time. In particular, ecosystems are presently still affected by past environmental changes because they have not yet reached equilibrium with their environment. However, the magnitude and drivers of this legacy effect for the upcoming decades are still unclear. Here, we use the dynamic global vegetation model LPJ-GUESS to calculate the effects of historical (1850-2015) and future (2015-2099, exemplarily for the high emission/moderate deforestation scenario SSP5-8.5) environmental changes on historical and future terrestrial carbon cycling and to quantify the contributions of the following environmental drivers: climate change, CO&lt;sub&gt;2 &lt;/sub&gt;fertilization, agricultural expansion, shifting cultivation frequency, wood harvest, nitrogen deposition, and nitrogen fertilization.&lt;/p&gt;&lt;p&gt;According to our simulations, the land represented a cumulative net carbon source (-154 GtC) over the historical period mainly due to deforestation, wood harvest, and negative climate change impacts partly offset by carbon uptake via increased CO&lt;sub&gt;2&lt;/sub&gt; levels and nitrogen input. In contrast, the land is simulated to act as a net carbon sink (+118 GtC) over the 21&lt;sup&gt;st&lt;/sup&gt; century. This is mostly a result of historical environmental changes as ecosystems still adapt to present-day CO&lt;sub&gt;2&lt;/sub&gt; and nitrogen availability as well as long-term vegetation regrowth following agricultural abandonment and wood harvest. The net impact of future environmental changes on future carbon cycling is much smaller because effects from individual environmental drivers largely compensate. Historical environmental changes dominate future terrestrial carbon cycling at least until mid-century when legacy effects gradually diminish and future environmental changes start to trigger carbon accumulation. Our results suggest that legacy effects persist even many decades after environmental changes occurred and need to be considered when interpreting alterations of the terrestrial carbon cycle.&lt;/p&gt;

Highlights

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) play a role
Since the land acted as a net carbon sink and the terrestrial biosphere currently takes up around 30% of total anthropogenic CO2 emissions, thereby reducing the growth rate of atmospheric CO2 concentration (Friedlingstein et al, 2019)
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)

Summary

Introduction

Terrestrial ecosystems and the atmosphere constantly exchange carbon via photosynthesis, respiration, and disturbances. The interplay of changing environmental drivers will determine the fate of the land carbon sink in the future, but the relative importance of individual drivers is still under debate. 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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