Transforming our understanding of climate shifts in the North African dust belt and upskilling Earth System Models to simulate them

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As Earth heats up in the coming decades, global rainfall patterns will shift with uncertain regional outcomes. Nowhere is this problem more acute than in North Africa. The world's largest hot desert and biggest source of atmospheric dust, the Sahara, grew by >10% in the 20th century. In the semi-arid Sahel and Mediterranean borderlands, where annual rainfall amounts are low and inter-annual rainfall variability is high, tens of millions of people live in extreme water stress with sometimes devastating consequences as during the late 20th century Sahel drought. These are major incentives to understand the mechanisms driving past variability in the climate of North Africa. However, the instrumental record is short (~150 yrs) and geological and archaeological data reveal a capacity for far more extreme shifts in North African climate. Paced by Earth's gradually changing orbit around the Sun and millennial-scale variability in the climate system, North Africa's past climate has shifted repeatedly between drier and dustier conditions than today and humid green Sahara periods (GSPs) when the desert was transformed into a well vegetated landscape crosscut by rivers and lakes, populated by hippopotamuses and our early ancestors (most recently during the African Humid Period, AHP, ~14.5 to 5 thousand years ago, ka).What caused these past shifts? What do they imply for the future? We lack convincing answers to these questions because the extreme shifts indicated by the palaeo-data are not reproduced by the computer models used to predict future change. A main weakness of the existing palaeo-data is their limited diagnosis of mechanistic control. North Africa has two latitudinal and seasonally distinct rainfall regimes, the summer monsoon and winter westerly storm track, but we know too little of their temporal and spatial contributions to the profound changes in humidity-aridity and vegetation recorded in the data. A main weakness in the models is that we do not know whether their lack of skill reflects missing processes and feedbacks that dampen the response to climate forcing or whether they adequately represent the key processes but are inadvertently hard-wired for stability because uncertain parameter values are determined solely by evaluation against contemporary observational targets. We propose to transform our understanding of climate shifts in the North African dust belt and to upskill models to successfully simulate them by capitalizing on recent breakthroughs that we have made (see Track Record). We will develop proxy climate records for a bellwether Saharan region to tease apart past variability in summer monsoon and winter storm-track rainfall (see Objectives). And we will configure an IPCC-class Earth System model (ESM) that shows novel power to green the Sahara and is sufficiently computationally inexpensive to permit adequate sampling of parameter space to confront our new proxy records (see Objectives). This will allow us to develop a mechanistic understanding of extreme change and to translate our findings to those scientists who are developing the next generation of ESMs that will be used to predict future change over the coming decades (see Academic Beneficiaries).