Drivers of late Quaternary African humid/arid phases

Palaeoclimate records from Africa imply rapid and spatially complex shifts between wet and dry conditions over the late Quaternary. There is substantial evidence for the existence of 'mega' lakes in both northern and southern African regions occupied by drylands currently (e.g. Lake Chad Fig. 1). For example, the dry arid Sahara of today was transformed into a "green" and wet Sahara during some periods in the past, with lakes larger than the current Great Lakes. Such wet phases may have been critical to human dispersal out of Africa and hence may have fundamentally influenced the evolution of modern humans. The project will attempt to explain the causes of these climate changes and the possible feedbacks between Africa and the rest of the globe.

Figure 1. Evidence that Lake Chad has been much larger in the past.

The reason for the changes between wet and dry phases over the late Quaternary has been the subject of much interest. Previous work has shown that wetter conditions in Northern Africa during early Holocene may have been due to changes in the Earth's orbital parameters, resulting in an increased seasonality, and increase in precipitation over the region. It has been suggested that there is antiphasing between the Southern and Northern hemispheres timing of maximum surface wet conditions. This has also been used to infer that changes in solar insolation are the key forcing mechanism. However, recent work has found that the patterns are much more complex, particularly in the southern and eastern African palaeorecords. Several records show significant correlations with the timing of high latitude Heinrich iceberg discharges, which cause rapid changes in large-scale ocean circulation. Preliminary studies with a climate model have demonstrated the possibility of high latitude forcing of African environments on millennial time-scales.

There are also important climatic feedbacks when extensive lakes and wetlands are present. The presence of substantial surface water, in the form of lakes and wetlands, impacts on regional climate through changing the surface albedo, surface energy and moisture budgets, and modified heat capacity. Previous studies have confirmed the importance of surface water in modelling African palaeoclimates, the North African monsoon has been found to be particularly sensitive to land surface changes. Key unresolved questions concern the current lack of consensus on the forcing mechanisms of arid/humid transitions, partly because the timing is difficult to ascertain from palaeorecords, and partly that while modelling studies have demonstrated the importance of including surface water changes, detailed examination of mechanisms of transient changes and feedbacks have yet to be undertaken. This NERC funded PhD project seeks to examine these mechanisms using the Hadley Centre climate model and interactive vegetation and surface hydrology models. [started October 2008]

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Bio-geoengineering crops for global warming mitigation

Geoengineering is the manipulation of either the Earth's albedo or atmospheric composition to mitigate global warming. This project involves the investigation of the idea of increasing the albedo of agricultural crops via choosing species with higher natural albedos or changing leaf glaucousness. Initial investigations have involved the assessment of this concept with the Hadley Centre ocean-atmosphere-vegetation model in which the albedo of C3 and C4 plants in agriculatural regions has been increased (See publication list: Ridgwell et al., 2009 and Singarayer et al., 2010). Further examination will include physical assessment of the range of albedos or crop sub-species at leaf and canopy level.

Figure 2. Fraction of HadCM3 model grid cells designated as agricultural land, in which C3 and C4 plant functional type albedo is increased.

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Glacial cycle climate variability

This is an ongoing project which was initially funded by the BBC as part of the series, The Incredible Human Journey. We have run several large model experiments to reconstruct the last glacial cycle (120kyr to present). These are currently being used to investigate mechanisms and timings of orbital scale changes in the Earth system. Specific projects underway are glacial cycle changes in Polar Regions (Singarayer and Valdes, 2010), wetland and methane emissions changes, influences on the ITCZ and monsoons, and expression of millennial vs orbital scale changes in the hydrological cycle over Africa (in prep).

Figure 3. Average HadCM3 seasonal ITCZ position at maximum (red) and minimum (green) boreal summer insolation, mostly influenced by orbital precession.

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Patagonian Ice-sheet impacts during the deglaciation

Research grants provided to Glasser (NERC) and Jansson (Swedish Research Council). The aims are to reconstruct the evolution of the southern South America glacial lakes and evaluate the impact of associated meltwater on the Southern Hemisphere thermohaline circulation during the last deglaciation. Meltwater from the Laurentide Ice Sheet and Antarctica are known to have had a critical impact on Late-glacial thermohaline circulation and climate. Recent research on South Pacific proxy data stresses the important relationship between the thermohaline circulation and meltwater released during the deglaciation of the Patagonian icefields. The project will combine a variety of methods including remote sensing, DEM analysis, sedimentological field studies, novel dating methods and numerical model simulations of ocean circulation which will be provided performed here at Bristol. [Start date for fieldwork Jan 2010]

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Understanding radiocarbon gradients

Variations in the ratio of carbon-14 to carbon-12 isotopes (∆14C) have the potential to provide valuable information concerning millennial-scale climate changes, carbon cycle, solar activity and geomagnetic strength. However, deconvolving these different factors is difficult from data alone, even when using complementary records. Modelling ∆14C has the potential to play a crucial role in extracting maximum understanding from the isotope record. We are using a recently developed intermediate-complexity 3D Earth System Model (Grid Enabled Integrated Earth system model, GENIE) to quantitatively examine the causes of ∆14C variability, and hence improve our understanding of the mechanisms of natural Earth system change. The model is unique because of its fully dynamic representation of the spatial and temporal patterns of long-term (multi-millennial) Earth system variability, and by including all major components of the Earth system. It includes a fully dynamic atmosphere and ocean, a dynamic terrestrial carbon cycle, and a detailed representation of the ocean carbon cycle, and is capable of multi-millennial simulations. We are using this model to quantitatively investigate the possible causes of variations in ∆14C during the last 50,000 years and combine the modelling studies with data in order to advance our understanding of Earth System History. One example is the approach of data-model comparisons of radiocarbon distribution during the Younger Dryas to investigate the cause of the increase in atmospheric radiocarbon concentrations (Singarayer et al., GRL, 2008).

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The role of sea-ice in Arctic climate variability

This was a NERC COAPEC (Coupled Ocean-Atmosphere Processes for European Climate) theme funded project to investigate the role of Arctic sea-ice in European and North Atlantic climate variability on interannual to decadal time scales. Sea-ice concentration and area are important variables in the climate system and in climate modelling due to their effect on surface albedo and because open-water areas within the ice pack are of major significance for ocean-atmosphere heat/ moisture exchanges. Consequently, accurate representations of these fields are essential both for driving atmospheric models and for evaluation of coupled GCMs (modelled sea-ice). The project included the assessment of satellite-based sea-ice datasets (Singarayer et al., 2005, Singarayer and Bamber, 2003), the impact of declining sea-ice concentrations on 21^st century climate (Singarayer et al., 2006), and coupled variability of the ocean-sea-ice-atmosphere system (Singarayer et al., in prep).

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