This thesis investigates the ‘Little Ice Age’ (LIA) glacial maximum in southeast Iceland through integration of the glaciological, climatological and historical records of change in order to enhance current understanding about the timing, expression and consequences of glacier-climate interactions in the North Atlantic. Iceland’s proximity to large scale climatic drivers, such as the thermohaline circulation and North Atlantic Oscillation (NAO), indicates that its glaciers may provide one of the clearest signals of oceanic-atmospheric interactions at a range of time-scales. Previous opinions have differed regarding the timing of the LIA maximum extensive of Icelandic glaciers with possibilities generally ranging from the mid-/late- 18^th to late 19^th centuries. Moraines along the margins of Skálafellsjökull and Heinabergsjökull, two Vatnajökull outlet glaciers in southeast Iceland, were re-dated in order to examine whether disparities may have arisen due to differing glacier response rates, selective preservation of evidence or unreliable dating techniques. Approximately 12,000 lichens were measured on 40 moraine fragments to provide surface age proxies. The population gradient lichenometric technique yields late 18^th to early 19^th century moraine dates, whereas the conventional ‘average of the five largest’ method clusters moraine dates to the late 19^th century. Subsequently, an updated tephrochonology for southeast Iceland (geochemistry and tephrastratigraphy) based on 25 reference soil profiles was used to identify tephra layers within 15 additional soil profiles dug around the LIA moraines. Tephrochonology confirms that the LIA maximum extent for both glaciers occurred between AD 1755 and 1873. A late 18^th to early 19^th century LIA maximum at Skálafellsjökull and Heinabergsjökull accords with widespread glacial maxima (extent and ice volume) across Iceland, indicative of a pervasive response to climatic deterioration. A positive degree-day mass balance model was implemented for Skálafellsjökull, Heinabergsjökull and Lambatungnajökull to assess the linkages between spatial expression of LIA glacier maxima with potential climatic envelopes during the late 18^th to early 19^th centuries. Reconstructions suggest that glacier mass balance was at least 1.5 to 2 mwe a^-1 greater than at present induced by a ~1-1.5°C temperature reduction associated with severe sea-ice years during the 1780s. Equilibrium line altitudes may have lowered by ~140 m during the LIA maximum. The model also indicates that glacier hypsometry significantly affects differential response of the three Vatnajökull outlet glaciers under various climatic regimes.

Various improvements were made to a state-of-the-art aerosol-cloud model and it was validated against observations from field campaigns. The robustness of this aerosol-cloud model is in its ability to explicitly resolve all the known modes of heterogeneous cloud droplet activation and ice crystal nucleation. The model links cloud particle activation with the aerosol loading and chemistry of seven different aerosol species. Continental and maritime cases were simulated for the purposes of validating the aerosol-cloud model, and investigating the salient microphysical and dynamical mechanisms of aerosol indirect effects (AIE) from anthropogenic solute and solid aerosols, focusing mainly on glaciated clouds. The results showed that increased solute aerosols reduced cloud particle sizes and inhibited warm rain processes, thus, enhancing chances of homogeneous cloud droplet and aerosol freezing. Cloud fractions and their optical thicknesses increased quite substantially in both cases. Although liquid mixing ratios were boosted, there was however a substantial reduction of ice mixing ratios in the upper troposphere owing to the increase in snow production aloft. Convective updrafts became weaker mainly in the continental case, while weak vertical velocities strengthened slightly in the upper troposphere. With an increase in solid aerosols, clouds became slightly more extensive over the continents, while the cloudiness diminished over the oceans. The total AIE of clouds from solute aerosols was two times higher in the oceanic than in the continental case, because the sensitivity of the cloud properties to perturbation in aerosol concentrations diminishes with increasing background aerosol concentrations. Also, the AIEs of glaciated clouds were greater than those of water-only clouds by a factor of two in the continental case while both cloud types were equally important in the maritime case. The radiative importance of glaciated clouds lied in their large collective spatial extent and existence above water-only clouds. The glaciation AIE from solid aerosols had a cooling effect in continental clouds because of an increase in cloud fraction and a warming effect in maritime clouds because of a decrease in cloud fraction. In addition to the traditional AIEs (glaciation, riming and thermodynamic), sedimentation, aggregation and coalescence were new AIEs identified. Importantly, it was discovered that these individual AIEs interact, compensate and buffer each other, hence, the relative importance of contributions from responses of various processes vary during the climate change. Finally, meteorology was identified to have little effect on the mechanisms of aerosol-cloud interaction.

This narrative is a history of the contributions made to graph theory in the United States of America by American mathematicians and others who supported the growth of scholarship in that country, between the years 1876 and 1950. The beginning of this period coincided with the opening of the first research university in the United States of America, The Johns Hopkins University (although undergraduates were also taught), providing the facilities and impetus for the development of new ideas. The hiring, from England, of one of the foremost mathematicians of the time provided the necessary motivation for research and development for a new generation of American scholars. In addition, it was at this time that home-grown research mathematicians were first coming to prominence. At the beginning of the twentieth century European interest in graph theory, and to some extent the four-colour problem, began to wane. Over three decades, American mathematicians took up this field of study - notably, Oswald Veblen, George Birkhoff, Philip Franklin, and Hassler Whitney. It is necessary to stress that these four mathematicians and all the other scholars mentioned in this history were not just graph theorists but worked in many other disciplines. Indeed, they not only made significant contributions to diverse fields but, in some cases, they created those fields themselves and set the standards for others to follow. Moreover, whilst they made considerable contributions to graph theory in general, two of them developed important ideas in connection with the four-colour problem. Grounded in a paper by Alfred Bray Kempe that was notorious for its fallacious 'proof' of the four-colour theorem, these ideas were the concepts of an unavoidable set and a reducible configuration. To place the story of these scholars within the history of mathematics, America, and graph theory, brief accounts are presented of the early years of graph theory, the early years of mathematics and graph theory in the USA, and the effects of the founding of the first institute for postgraduate study in America. Additionally, information has been included on other influences by such global events as the two world wars, the depression, the influx of European scholars into the United States of America, mainly during the 1930s, and the parallel development of graph theory in Europe. Until the end of the nineteenth century, graph theory had been almost entirely the prerogative of European mathematicians. Perhaps the first work in graph theory carried out in America was by Charles Sanders Peirce, arguably America's greatest logician and philosopher at the time. In the 1860s, he studied the four-colour conjecture and claimed to have written at least two papers on the subject during that decade, but unfortunately neither of these has survived. William Edward Story entered the field in 1879, with unfortunate consequences, but it was not until 1897 that an American mathematician presented a lecture on the subject, albeit only to have the paper disappear. Paul Wernicke presented a lecture on the four-colour problem to the American Mathematician Society, but again the paper has not survived. However, his 1904 paper has survived and added to the story of graph theory, and particularly the four-colour conjecture. The year 1912 saw the real beginning of American graph theory with Veblen and Birkhoff publishing major contributions to the subject. It was around this time that European mathematicians appeared to lose interest in graph theory. In the period 1912 to 1950 much of the progress made in the subject was from America and by 1950 not only had the United States of America become the foremost country for mathematics, it was the leading centre for graph theory.

Plant processes affect fluxes of energy, moisture and CO2 between the land and the atmosphere. Land surface models need to correctly represent the vegetation functioning and its response to environmental conditions. Due to anthropogenic carbon emissions rising, and global warming, plant processes are being affected and in turn modulate the terrestrial carbon sink. However, models still disagree on the response of plants to changing conditions. This work analyses how vegetation is treated in two land surface models: the Joint UK Land Environment Simulator (JULES) and Carbon Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land (CTESSEL). The aim is to analyse how environmental variables control the vegetation processes at daily and seasonal timescales at present day climate and the changes that arise in a scenario of double atmospheric CO2 and higher temperature. The analyses are carried out at the leaf level and at the canopy level. To investigate the responses at the leaf level, the photosynthesis scheme used in each model was extracted, thereby providing a submodel that can be run in stand alone mode. The photosynthesis submodel provides a means to analyse the leaf level response of each photosynthesis model to environment variables as well as the internal model parameters that characterise each plant type. In JULES the environmental controls on photosynthesis are explicitly introduced by three limiting regimes: light, rubisco (carbon) or export limiting regime. In CTESSEL the carbon and light limitations are implicitly represented but there is no export limitation. Due to the lack of export limiting regime, CTESSEL presents higher sensitivity to CO2 concentration resulting in a stronger CO2 fertilization effect. The carbon and energy fluxes produced by the full land surface models were tested and compared at 10 European FLUXNET sites. The main differences between modellled carbon fluxes were found to be the treatment of soil moisture stress and the lack of export limiting regime in CTESSEL. The optimum temperature for photosynthesis in models is the result of model parameters’ dependence on temperature and the combination of limiting regimes. The optimum temperature for photosynthesis was found to be a determining element in the strength and sign of the vegetation modelled feedback to climate change.

No description available.

No description available.

The Arctic is undergoing big changes, there has been a reduction in sea ice extent and an increase in the amount of thinner, newer ice. This changing surface causes challenges to numerical weather prediction and climate simulation, due to the very interconnected nature of the surface and atmosphere in the Arctic. Changes to the surface may impact on the air temperature, the surface fluxes of heat and moisture and the microphysics of clouds amongst others. These areas could also, in turn, change the surface. The Marginal Ice Zone (MIZ) is the area of the sea ice which is between the open water and solid ice, and is characterised by a complex and constantly changing ice surface. The impact of thinning ice in the MIZ was investigated using a series of idealised modelling experiments, and it was found that thinner sea ice in the MIZ would increase the surface temperature and fluxes of heat. In turn this causes the clouds, which formover the open water, to formcloser to the ice edge. The clouds were found to be thinner for the experiments with thinner ice, which resulted in them allowing more shortwave radiation to reach the surface. This result implies that thinning sea ice would act to further thin sea ice. Using observations from the Aerosol- Clouds Coupling And Climate Interactions in the Arctic (ACCACIA) campaign, two sets of model sensitivity studies were undertaken to test which boundary layer parametrisation schemes would performbetter. It was discovered that the boundary layer in the model is more sensitive to the surface representation rather than the choice of boundary layer scheme. These results point to the need for more work, both observations and modelling, on the sea ice and its impact on the atmosphere in order to better predict the changes the Arctic and the planet will undergo with a changing climate.

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Mesoscale jet formation due to the Coriolis Effect is well understood over sharp changes in surface roughness such as coastlines. This sharp change in surface roughness is experienced by the atmosphere flowing over, and ocean flowing under, a compacted sea ice edge. Sea ice edge jets have been observed. This thesis presents a study of a dynamic sea ice edge responding to atmospheric and oceanic jet formation during various wind and ocean current conditions. An idealised analytical model of sea ice drift is created using a momentum balance and the viscous plastic rheology. This is compared to an ice edge in the Los Alamos sea ice climate model (CICE) run on an idealised domain. A scheme has been developed which analyses sea ice concentration and adds jets to the CICE model forcing data. The response of the model to jet formation is tested at various resolutions. The formation of atmospheric jets at the sea ice edge is shown to increase the wind speed parallel to the sea ice edge and results in the formation of a sea ice edge jet. The increase is dependent upon the angle between the ice and wind and results in an increase in ice transport along the sea ice edge of 40%. Observa- tions and climate model data of the polar oceans has been analysed to show areas of likely atmospheric jet formation with the Fram Strait being of particular interest. The possibility of oceanic jet formation and the resultant effect upon the sea ice edge is less conclusive. The coupling between the components of climate models is currently crude and does not allow for jet formation. Most climate model also misrepresent the ice drift through the Fram Strait leading to errors in the prediction of Arctic sea ice extent.

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