Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.6.2.2 Greening of the Sahara and the Sahel ====

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Greening of the Sahara and Sahel regions in North Africa, in response to an increase in precipitation, has long been considered an amplifying mechanism that can lead to abrupt change. Although the high surface albedo of the desert stabilizes the energy balance of the system ( [[#Charney--1975|Charney, 1975]] ), greening can induce strong, positive feedbacks between the land surface and precipitation that can shift the region into a ‘Green Sahara’ state. The fact that the transition phase between a Desert Sahara and Green Sahara is not theoretically stable (Brovkin et al., 1998) creates a tipping point and allows for the possibility of an abrupt shift between dry and wet climate regimes. Paleoclimate reconstructions provide evidence of past Green Sahara states (DeMenocal and Tierney, 2012), under which rainfall rates increased by an order of magnitude (Tierney et al., 2017), leading to a vegetated landscape (Jolly et al., 1998) with large lake basins (Gasse, 2000; [[#Drake--2006|Drake and Bristow, 2006]] ). The underlying driver of the Green Sahara is the periodic increase in summer insolation associated with the orbital precession cycle (Kutzbach, 1981). In this sense, Green Saharas are not direct analogues for a response to anthropogenic greenhouse gas emissions (GHGs), as these past states were forced by natural, seasonal changes in solar radiation. However, the climate dynamics of Green Sahara periods (which have global impacts, [[#Pausata--2020|Pausata et al., 2020]] ), and the speed of the transitions between Desert Saharas and Green Saharas, are relevant for future projections.

Since AR5, paleoclimatic studies have improved our view of the timing, spatial extent, and speed of transitions associated with the early Holocene (11,000–5,000 years ago) Green Sahara. Observed transitions into and out of Green Sahara states are always faster than the underlying forcing, in agreement with theoretical considerations ( ''high confidence'' ) (Tierney and DeMenocal, 2013; [[#Shanahan--2015|Shanahan et al., 2015]] ; [[#Tierney--2017|Tierney et al., 2017]] ). However, there is ''low confidence'' in the duration of the transition because sedimentary records cannot typically resolve changes on decadal to multi-decadal time scales (Tierney and DeMenocal, 2013). Both paleoclimate data and modelling experiments suggest that the timing and speed of the transition was spatially heterogeneous ( ''high confidence'' ), with northern Saharan locations becoming drier thousands of years before more equatorial locations (Shanahan et al., 2015; [[#Tierney--2017|Tierney et al., 2017]] ; [[#Dallmeyer--2020|Dallmeyer et al., 2020]] ). These observations are consistent with theoretical studies suggesting that spatial heterogeneity and diversity in ecosystems can mitigate the probability of catastrophic change (Van Nes and Scheffer, 2005; [[#Bathiany--2013|Bathiany et al., 2013]] ). Conversely, low ecosystem diversity can produce local or regional ‘hot spots’ of abrupt change such as those seen in some paleoclimate records (Claussen et al., 2013).

CMIP5 and CMIP6 models ''',''' some of which include dynamic vegetation schemes, cannot simulate the magnitude, nor the spatial extent, of greening and precipitation change associated with the last Green Sahara under standard mid-Holocene (6,000 years ago) boundary conditions ( ''high confidence'' ) (Figure 3.11; Harrison et al. , 2014; Tierney et al. , 2017; Brierley et al. , 2020). This result remains unchanged since AR4 ( [[#Jansen--2007|Jansen et al., 2007]] ). This may be due to climatological biases in the models ( [[#Harrison--2015|Harrison et al., 2015]] ) or could imply that the strength of the feedbacks between vegetation and the water cycle in the models is too weak (Hopcroft et al., 2017). To date, climate models still only produce the amount and spatial extent of rainfall that is needed to sustain a Green Sahara if they are given prescribed changes in the land surface, such as albedo, soil moisture, vegetation cover and/or dust emissions (Pausata et al., 2016; [[#Skinner--2016|Skinner and Poulsen, 2016]] ; [[#Tierney--2017|Tierney et al., 2017]] ).

Some climate model simulations suggest that under future high-emissions scenarios, CO <sub>2</sub> radiative forcing causes rapid greening in the Sahel and Sahara regions via precipitation change (Claussen et al., 2003; [[#Drijfhout--2015|Drijfhout et al., 2015]] ). For example, in the BNU-ESM RCP8.5 simulation, the change is abrupt with the percentage of bare soil dropping from 45% to 15%, and percentage of tree cover rising from 50% to 75%, within 10 years (2050–2060; Drijfhout et al., 2015). However, other modelling results suggest that this may be a short-lived response to CO <sub>2</sub> fertilization (Bathiany et al., 2014).

In summary, given outstanding uncertainties in how well the current generation of climate models capture land surface feedbacks in the Sahel and Sahara, there is ''low confidence'' that an abrupt change to a greener state will occur in these regions before 2100 or 2300.

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