Sahara and boreal forests natural vegetation: from mid-Holocene to future

As anthropogenic forcing increases, there is a rising concern about crossing tipping points. A key information is the critical thresholds at which the so-called tipping elements could undergo abrupt changes, and the associated early-warning signals. This is true, in particular, for boreal forests and the possible greening of the Sahara. In that perspective, testing climate models against paleoclimate allows for exploring the climate response over multi-millennial time series, while offering the possibility to compare it with past vegetation reconstructions.

We consider a transient simulation from the mid-Holocene (6,000 years before present) to 2100 obtained using the IPSL general circulation model including dynamical natural-only vegetation [1]. The simulation is forced with changes in orbital parameters and trace gases, transitioning from the paleo- to the historical period, and then to the scenario SSP4.5. We focus on two terrestrial ecosystems, both identified as tipping elements: the Sahara/Sahel and boreal forests in northern Europe.

We analyze the evolution of vegetation patterns and extents in the two regions along the Holocene with the objective to determine if their response to forcing conditions can be assimilated to the crossing of a tipping point. Thus, we isolate rapid shifts and investigate whether they correspond to tipping points or to centennial variability. We link them to changes in regional vegetation drivers, including vegetation feedbacks, or to AMOC variability. Then, we explore possible analogies between changes during the Holocene and in projections, allowing for the identification of sensitive regions, which may help to detect regional thresholds for tipping points.

References:

[1] Braconnot, P., Viovy, N., and Marti, O. (2025). Dynamic vegetation highlights first-order climate feedbacks and their dependence on climate mean state. Earth System Dynamics, 16, 2113–2136.