I study atmospheric and oceanic dynamics in various paleoclimates. Currently I am working on the following intervals:
Green Sahara and the mid-Holocene
The Holocene is the current geological epoch and started 11,700 years ago. We are living in the Meghalayan stage (begun 4,200 years ago, also known as the late-Holocene) of the Holocene. Before the Meghalayan, during the stage known formally as the Northgrippian, and informally as the mid-Holocene, the Sahara was a green and lush landscape.
The reason for the existence of vegetation over Sahara was the increased precipitation, which was primarily brought via the West African Monsoon (WAM). Just like any other monsoonal system, the WAM is a product by the land-sea temperature contrast during the summer season which draws moisture laden air that is present off the coast towards the continent. During the mid-Holocene the strength of the monsoon was increased due to the greater summer insolation in the northern hemisphere owing to a different configuration of the Earth’s orbit.
It is reasonable to expect that the increased precipitation from the strengthened monsoon would be enough to maintain a Green Sahara. Nevertheless, simulations have shown that an insolation induced strengthening of the monsoon is not sufficient to sustain the greening that has been inferred. It has been found that feedbacks from the land surface are necessary for further amplification of the monsoon.
Recently, in a paper in GRL we show that the strength of feedbacks from the land surface has been generally underestimated in the literature and that a modern coupled-climate model forced with the most recent assessment of mid-Holocene boundary conditions is able to simulate a monsoon that is in very good agreement with reconstructions of mid-Holocene rainfall. The figure below shows the total annual precipitation difference between our ‘Green Sahara’ simulation and the preindustrial. Strong precipitation increases are found over the Sahel and the Sahara in response to the inclusion of land surface feedbacks from mid-Holocene vegetation, soil and lakes. The scatter marks show proxy reconstructed annual rainfall anomalies from Bartlein et al. 2011.
The mid-Pliocene Warm Period
The mid-Pliocene Warm Period (mPWP, ~3—3.3 Million years ago) was the most recent time period during which the globally averaged temperature was higher than present for an extended period of time. The world didn’t look very different from the present day (as shown below) because on such short time scales plate tectonics does not have any noticeably influence.
The figure below shows an interpretation by de Boer et al. 2010 of the mean NH continental temperature anomaly for the last 40 million years. Considering this as a proxy for the global mean temperature, we see that going back in time (right to left) the mPWP was the most recent interval during which the globally averaged temperatures were warmer than present for a sustained period of time.
Our investigation into the climate of the time period through numerical modelling using state-of-the-art climate models reveals the following picture of the warming that might have prevailed during this time. The figure shows the temperature anomaly with respect to the climate at the start of the Industrial Revolution (1850 CE). The widespread warming is very apparent.
