The climate of the eastern section of the Sahelian latitude, especially over the Eritrean subdomain, is often associated with long drought episodes from which the atmospheric mechanisms are poorly understood. In an effort to improve our knowledge of weather and climate systems over this region, the PRECIS Regional Climate Model (RCM) from the United Kingdom (UK) was obtained and implemented. Such a climate model that is based upon the physical laws of nature has the ability to simulate regional-scale atmospheric patterns, and therefore, may significantly contribute to our understanding of local atmospheric processes. In this dissertation the assessment of past regional climate trends from both observations and model simulations, and the simulation of scenarios for possible future climate change were regarded as important. To investigate this, the PRECIS RCM was first nested over the Eritrean domain into the “atmosphere only” HadAM3H global General Circulation Model (GCM) and forced at its lateral boundaries by a 30-year present-day (1961-1990) integration of the same global model. Secondly, the PRECIS RCM was constrained at its lateral boundary by the “fully coupled” HadCM3 GCM (for Sea Surface Temperatures (SSTs) and sea-ice) and its improved atmospheric component (HadAM3H GCM). The latter simulations provided boundary conditions for the A2 and B2 future emission scenarios (Special Report on Emission Scenarios (SRES)) to simulate a 20-year (2070-2090) projection of future climate. These experiments allowed for verification of both spatial and temporal present-day climate simulations, as well as possible future climate trends as simulated by the PRECIS RCM over the Eritrean domain, with specific emphasis on temperature and moisture related variables. The study indicates that PRECIS RCM climate simulations are mostly in harmony with observed spatial patterns. This skill may be attributed to the full representation of the climatic system (land surface, sea, ice, atmosphere and atmospheric chemistry such as sulphur and greenhouse gasses) in the model configuration. However, when comparing PRECIS RCM results with the much coarser resolution (2.5ox2.5o) National Centre for Environmental Prediction (NCEP) reanalysis data, obvious differences do occur. These differences are not necessarily the result of poor model performance, but may be attributed to more detailed simulations over the finer RCM grid (0.44o x 0.44o). Future climate scenario simulation with the PRECIS RCM over Eritrea produce increased surface temperature in both the A2 and B2 SRES scenario integrations, relative to the present climatology. This temperature increase also appears in the driving GCM (HadCM3) as well as in other GCM results from the Inter Governmental Panel for Climate Change (IPCC) initiative. There are, however, mixed signals in rainfall projections. According to PRECIS RCM results, rainfall is expected to increase in most of the Eritrean region. Copyright 2005, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Beraki, A F 2005, Climate change scenario simulations over Eritrea by using a fine resolution limited area climate model : temperature and moisture sensitivity , MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-02102006-152327 / > / Dissertation (MSc)--University of Pretoria, 2007. / Geography, Geoinformatics and Meteorology / Unrestricted

Through the 21st century, global-mean stratospheric ozone abundances are projected to increase due to decreasing chlorine and bromine concentrations (as a consequence of the Montreal Protocol for Substances that Deplete the Ozone Layer), and continued CO₂-induced cooling of the stratosphere. Along with CO₂, anthropogenic emissions of the greenhouse gases N₂O and CH₄ are projected to increase, thus increasing their atmospheric concentrations. Consequently, reactive nitrogen species produced from N₂O and reactive hydrogen species produced from CH₄ are expected to play an increasingly important role in determining stratospheric ozone concentrations. Chemistry-climate model simulations were performed using the NIWA-SOCOL (National Institute of Water and Atmospheric Research - SOlar Climate Ozone Links) model, which tracks the contributions to ozone loss from a prescribed set of catalytic cycles, including the ozone-depleting nitrogen and hydrogen cycles, over latitude, longitude, pressure and time. The results provide a comprehensive picture of how stratospheric ozone may evolve through the 21st century under a range of greenhouse gas emissions scenarios, and quantitatively extend concepts that had previously been understood only qualitatively.

Warming over the past century has been greatest in high-latitudes over land and a number of environmental indicators suggest that the Arctic climate system is in the process of a major transition. Given the magnitude of observed and projected changes in the Arctic, it is essential that a better understanding of the characteristics of the Arctic climate system be achieved. In this work, I report on modifications to the UVic Earth System Climate model to allow it to represent regions of perennially-frozen ground, or permafrost. I examine the model’s representation of the Arctic climate during the 20th Century and show that it capably represents the distribution and thermal state of permafrost in the present-day climate system. I use Representative Concentration Pathways to examine a range of possible future permafrost states to the year 2500. A suite of sensitivity experiments is used to better understand controls on permafrost. I demonstrate the potential for radical environmental changes in the Arctic over the 21st Century including continued warming, enhanced precipitation and a reduction of between 29 and 54 % of the present-day permafrost area by 2100. Model projections show that widespread loss of high-latitude wetlands may accompany the loss of near surface permafrost. / Graduate

Ocean warming near the Antarctic ice shelves has critical implications for future ice sheet mass loss and global sea level rise. A global climate model with an eddying ocean is used to quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized 2xCO(2) experiment. The results indicate that relatively large warm anomalies occur both in the upper 100 m and at depths above the shelf floor, which are controlled by different mechanisms. The near-surface ocean warming is primarily a response to enhanced onshore advective heat transport across the shelf break. The deep shelf warming is initiated by onshore intrusions of relatively warm Circumpolar Deep Water (CDW), in density classes that access the shelf, as well as the reduction of the vertical mixing of heat. CO2-induced shelf freshening influences both warming mechanisms. The shelf freshening slows vertical mixing by limiting gravitational instabilities and the upward diffusion of heat associated with CDW, resulting in the buildup of heat at depth. Meanwhile, freshening near the shelf break enhances the lateral density gradient of the Antarctic Slope Front (ASF) and disconnect isopycnals between the shelf and CDW, making cross-ASF heat exchange more difficult. However, at several locations along the ASF, the cross-ASF heat transport is less inhibited and heat can move onshore. Once onshore, lateral and vertical heat advection work to disperse the heat anomalies across the shelf region. Understanding the inhomogeneous Antarctic shelf warming will lead to better projections of future ice sheet mass loss.

A 2°C increase in global mean temperature (ΔTg) has been widely adopted as a benchmark for dangerous climate change. However, there has been a lack of research into the implications of 2°C, or any other degree of warming, for Africa. In this thesis changes in African temperature and precipitation associated with 1°C, 2°C, 3°C, 4°C, and beyond are investigated for the first time, using output from 350 climate model experiments: a collection of simulations from international modelling centres (CMIP3), two Perturbed Physics Ensembles (PPEs), and a group of five regional models. The models project temperature and precipitation anomalies which increase in magnitude and spatial extent as global temperature rises, including a wet signal in East Africa, and drier conditions for African rainforests. The models consistently show that the evolution of change with global warming is gradual, even at 4°C and beyond; but the amplitude and direction of precipitation change at each ΔTg increment vary between models and between datasets. The PPEs project precipitation signals which are not represented by CMIP3, in particular a large drying (>0.5 mm day-1 °C-1) of western Africa. There are also important differences between global and regional models, especially in southern and West Africa (>1 mm day-1). Analysis of atmospheric circulation responses suggests that the higher resolution projections are no more credible in this case. Some of the variation between models can be understood as the result of untrustworthy simulations, leading to constraints on the PPEs, and casting doubt on the strong drying of west Sahel; but model evaluation is found to be limited by observations in the case of the Congo Basin. The implications of global warming are different depending on which models are consulted. The findings emphasise that caution should be exercised in the application of climate model data to inform mitigation debates.

A 'model-to-radiance' comparison of simulated brightness temperatures and radiances from the Hadley Centre Global Environmental Model 2 (HadGEM2-A) with longwave measurements from the High Resolution Infrared Radiation Sounder/4 (HIRS/4) and the Infrared Atmospheric Sounding Interfermeter (IASI) onboard the MetOp-A satellite is presented for all-sky and clear-sky global means. The fast Radiative Transfer model for TOVS 10 (RTTOV-10) is applied to HadGEM2 output to simulate observational-equivalent data. The results are compared with corresponding broadband analyses. A method is developed to extend hyperspectral IASI radiances to cover the whole outgoing terrestrial spectrum, in order to identify any compensating biases, and explore wavebands in the unobserved Far Infrared (FIR) region. For the all-sky HIRS analysis, the model overestimates brightness temperatures in the atmospheric window region with the greatest biases over areas associated with deep convective cloud. In contrast to many global climate models, much smaller clear-sky biases are found indicating that model clouds are the dominating source of error. Simulated values in upper atmospheric CO2 channels approximate observations better as a result of compensating cold biases at the poles and warm biases at lower latitudes, due to a poor representation of the Brewer Dobson circulation in the 38 level 'low-top' configuration of the model. Simulated all and clear-sky outgoing longwave radiation evaluated against the Clouds and the Earth's Radiant Energy System (CERES) and HIRS OLR products reveal good agreement, in part due to cancellation of positive and negative biases. Through physical arguments relating to the spectral energy balance within a cloud, it is suggested that broadband agreement could be the result of a balance between positive window biases and unseen negative biases originating from the water vapour rotational band in the FIR (not sampled by HIRS). Simple sensitivity tests show that dramatically altering existing cloud properties has little effect on the prominent window biases, however raising clouds a maximum of 5 atmospheric levels minimises the error in cloud contaminated channels, due to the introduction of spatially compensating errors. Sensitivities to the way ice clouds are parameterised in RTTOV-10 display a range of up to 2.5 K in window channels but absolute biases still exceed 3 K for all choices. Because of the lack of satellite based FIR observations due to a technological gap in the spectral region, an algorithm is created to 'fill in' the available data. Correlations between selected IASI channels and simulated unobserved wavelengths in the far infrared are used to estimate radiances between 25.25 - 644.75 cm-1 at 0.5 cm-1 intervals. The same method is used in the 2760 - 3000 cm-1 region. The spectrum is validated by comparing the Integrated Nadir Longwave Radiance (INLR) product (spanning the whole 25.25 - 3000 cm-1 range) with the corresponding broadband measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra and Aqua satellites at simultaneous nadir overpasses, revealing mean differences of 0.3 Wm-2sr-1 (0.5% relative difference) lower for IASI relative to CERES and significantly lower biases in nighttime only scenes. Averaged global data over a single month produces mean differences of about 1 Wm-2sr-1 in both the all and the clear-sky (1.2% relative difference). The new high resolution spectrum is presented for global mean clear and total skies where the far infrared is shown to contribute 44% and 47% to the total OLR respectively, which is consistent with previous estimates. In terms of spectral cloud radiative forcing, the FIR contributes 19% and in some subtropical instances appears to be negative, results that would go un-observed with a traditional broadband analysis. The equivalent complete IASI OLR model product is simulated from GCM data using RTTOV-10. The same process of applying predictors to the satellite measurements is applied to the model simulated radiances, with appropriate modifications, to produce a directly comparable model product. Annual mean all-sky radiances are still greatly overestimated at all wavenumbers with a total radiance bias of 4.52 Wm-2 across the whole range. Compensating negative biases outside of the HIRS coverage that were hypothesised are absent, with the far infrared contributing to the overall bias rather than cancelling it. Equivalent clear-sky biases are much lower overall at 0.39 Wm-2, in part due to spectral and spatial cancellation of errors. A flux-to-flux comparison is enabled by estimating the spatial distribution of anisotropic factors, using collated HIRS OLR fluxes and IASI OLR radiances, which yields global mean model fluxes in excess of 12 Wm-2 higher than observations in the all-sky. The difference between this and the fluxes calculated using the climate model's broadband radiation code (Edward-Slingo) are around 10 Wm-2 which is outside the range of uncertainty in the method used to estimate the flux. However, it is discussed that tuning of the climate model's broadband code to known flux values is a required practice to ensure global energy budgets balance but can produce inaccurate parameterised variables. An equivalent analysis adjusting the ice cloud parametrisation to reflect the radiances that have the biggest differences to the original configuration selected showed a bias reduction of 4.5 Wm-2, which is still not enough to completely explain its size, suggesting the existence of residual cloud problems. Finally, it is suggested that the way forward in separating and constraining cloud errors, in both radiative transfer codes, is a rigorous process of testing them with observation cloud properties and reanalysis data as inputs.

Ground source heat pumps (GSHPs) offer an efficient method for cooling and heating buildings, reducing energy usage and operating cost. In hot, arid regions such as Texas and the southwest United States, building load imbalance towards cooling causes design and performance challenges to GSHP systems in residential and commercial building applications. An integrated building load and GSHP model is developed in this thesis to test approaches to reduce GSHP cost, to properly size ground heat exchanger (GHEX) installations and to offer methods to improve GSHP performance in commercial buildings. The integrated model is comprised of a three-story office building, heat pumps, air handling system and a GHEX. These component models were integrated in the Matlab® Simulink® modeling environment, which allows for easy model modification and expansion. The building-load model was developed in HAMBASE, which simulates the thermal and hygric response of each zone in the building to external weather and internal loads. The building-load model was validated using the ASHRAE 140-2007 Standard Method of Test and with results from EnergyPlus. The heat pump model was developed as a performance map, based on data commonly provided by heat pump manufacturers. This approach allows for easy expansion of the number and type of heat pump models supported. The GHEX model was developed at Oklahoma State University and is based on Eskilson’s g-function model of vertical borehole operation. The GHEX model accurately represents the interaction between boreholes and the ground temperature response over short and long time-intervals. The GHEX model uses GLHEPRO files for parameter inputs. Long time-interval simulations of the integrated model are provided to assess the sensitivity of the GSHP system to various model parameters. These studies show that: small changes in the total GHEX length reduce system cost with minimal impact on performance; increased borehole spacing improves system performance with no additional cost; supplemental heat rejection reduces installation costs and improves system performance; industry-recommended design cutoff temperatures properly size the GHEX system; and, while cooling is the greatest contributor to operating cost in the southwest and southcentral United States, heating is the limiting design case for GHEX sizing. / text

There is much confidence in the global temperature change and its attribution to human activities. Global climate models have attained unprecedented complexity in representing the climate system and its response to external forcings. However, climate prediction remains a serious challenge and carries large uncertainty, particularly when the scale of interest becomes small. With the increasing interest in regional impact studies for decision-making, one of the urgent tasks is to make a systematic, quantitative evaluation of the expected skill of climate models over a range of spatiotemporal scales. The first part of this dissertation was devoted to this task, with focus on the predictive skill in the linear trend of surface air temperature. By evaluating the hindcasts for the last 120 year period in the form of deterministic and probabilistic predictions, it was found that the hindcasts can reproduce broad-scale changes in the surface air temperature, showing reliable skill at spatial scales larger than or equal to a few thousand kilometers (30° x 30°) and at temporal scales of 30 years or longer. However, their skill remains limited at smaller spatiotemporal scales, where we saw no significant improvement over climatology or a random guess. Over longer temporal scales, the feedbacks from the carbon cycle to atmospheric CO₂ concentration become important. Therefore the rest of the dissertation attempts to find key processes in the climate-carbon cycle feedback using one of the leading land-climate models, the National Center for Atmospheric Research Community Land Model. Evaluation of site-level simulations using field observations from the Amazon forest revealed that the current formulation for drought-related mortality, which lacks the ecophysiological link between short- and long-term drought stress, prevent the model from simulating realistic forest response. Global simulations showed that such dynamics of vegetation strongly influences the control of the nitrogen cycle on vegetation productivity, which then alters the sensitivity of the terrestrial biosphere to surface air temperature. This implies that if the state of the terrestrial biosphere is inconsistent with the simulated climate, either biased or prescribed, then its feedback to anthropogenic forcing could be also inconsistent.

Studies in paleoclimate are important because they give us knowledge about how the climate system works and puts the current climate change in necessary perspective. By studying (pre)historic periods we increase our knowledge not just about these periods, but also about the processes that are important for climatic variations and changes. This thesis deals mainly with the interaction between climate and vegetation. Vegetation changes can affect climate in many different ways. These effects can be divided into two main categories: biogeochemical and biogeophysical processes. This thesis studies the biogeophysical effects of vegetation changes on climate in climate models. Climate models are a necessary tool for investigating how climate responds to changes in the climate system, as well as for making predictions of future climate. The biogeophysical processes are strongly related to characteristics of the land surface. Vegetation changes alter the land surface’s albedo (ability to reflect incoming solar radiation), roughness and evapotranspiration (the sum of evaporation and tran-spiration), which in turn affects the energy fluxes between the land surface and the atmosphere and thereby the climate. It is not, however, evident in what way; denser vegetation (e.g. forest instead of grassland) gives decreased albedo, which results in higher temperature, but also increased evapotranspiration, which contrastingly results in lower temperature. Vegetation changes are in this thesis studied in four different (pre)historic periods: two very cold periods with no human influence (c. 44,000 and 21,000 years ago), one warm period with minor human influence (c. 6,000 years ago) and a cold period with substantial human influence (c. 200 years ago). In addition to that the present climate is studied. The combination of these periods gives an estimate of the effect of both natural and anthropogenic vegetation on climate in different climatic contexts. The results show that vegetation changes can change temperature with 1–3 °C depending on season and region. The response is not the same everywhere, but depends on local properties of the land surface. During the winter half of the year, the albedo effect is usually most important as the difference in albedo between forest and open land is very large. During the summer half of the year the evapotranspiration effect is usually most important as differences in albedo between different vegetation types are smaller. A prerequisite for differences in evapotranspiration is that there is sufficient amount of water available. In dry regions, evapotranspiration does not change much with changes in vegetation, which means that the albedo effect will dominate also in summer. The conclusion of these studies is that vegetation changes can have a considerable effect on climate, comparable to the effect of increasing amounts of greenhouse gases in scenarios of future climate. Thus, it is important to have an appropriate description of the vegetation in studies of past, present and future climate. This means that vegetation has the potential to work as a feedback mechanism to natural climatic variations, but also that man can alter climate by altering the vegetation. It also means that mankind may have influenced climate before we started to use fossil fuel. Consequently, vegetation changes can be used as a means to mitigate climate change locally. / Studiet av paleoklimat är viktigt för att det ger kunskap om hur klimatsystemet fungerar samt för att det sätter nuvarande klimatförändring i ett nödvändigt perspektiv. Genom att studera (för)historiska perioder ökar vi vår kunskap om dessa perioder, men också om vilka processer som har betydelse för klimatets variationer. Denna avhandling behandlar framförallt interaktionen mellan klimat och växtlighet. Förändringar i växtligheten kan påverka klimatet på flera olika sätt. Dessa kan delas in i två huvudgrupper: biogeokemiska och biogeofysikaliska processer. Denna avhandling studerar de biogeofysikaliska effekterna på klimatet i klimatmodeller. Klimatmodeller är ett nödvändigt verktyg för att studera hur klimatet svarar på förändringar i klimatsystemet, samt för att göra förutsägelser om framtidens klimat. De biogeofysikaliska processerna är förknippade med markytans egenskaper. Förändrad växtlighet förändrar markytans albedo (förmågan att reflektera inkommande soltrålning), skrovlighet och förmågan att transportera vatten från marken till atmosfären genom evapotranspiration (summan av avdunstning och transpiration), vilket i sin tur påverkar energiflödena mellan markytan och atmosfären. Dessa förändringar påverkar sedermera klimatet. Det är emellertid inte självklart på vilket sätt; tätare växtlighet (t.ex. skog i stället för äng) ger minskat albedo vilket ger högre temperatur, men också ökad evapotranspiration vilket däremot ger lägre temperatur. Växtlighetsförändringars påverkan på klimatet studeras i denna avhandling i fyra olika (för)historiska perioder: två väldigt kalla perioder utan mänsklig påverkan (ca 44 000 och 21 000 år sedan), en varm period med liten mänsklig påverkan (ca 6 000 år sedan) och en kall period med avsevärd mänsklig påverkan (ca 200 år sedan). I tillägg till det studeras också dagens klimat. Resultaten visar att förändringar i växtlighet lokalt kan ha en signifikant effekt på klimatet. Kombinationen av dessa perioder ger en uppskattning av effekten av både naturlig och antropogen växtlighet i olika klimatsammanhang. Förändrad växtlighet kan ändra temperaturen med 1-3 °C beroende på årstid och område. Responsen är inte densamma överallt utan beror på lokala egenskaper hos markytan. Under vinterhalvåret är oftast albedoeffekten viktigast eftersom skillnaden i albedo mellan skog och öppet landskap då är mycket stor. Under sommarhalvåret är evapotranspirationen oftast viktigast eftersom skillnaden i albedo mellan olika växtlighetstyper då oftast är små. En förutsättning för det är att det finns tillräckligt med vatten tillgängligt för evapotranspiration. I torra områden förändras evapotranspirationen inte särskilt mycket när växtligheten förändras, vilket gör att albedoeffekten dominerar även på sommaren.  Slutsatsen av dessa studier blir att förändrad växtlighet kan ha en betydande effekt på klimatet, jämförbar med den effekt som ökade halter av växthusgaser har i scenarier för framtida klimat. Alltså är det viktigt att ha en korrekt beskrivning av växtligheten i studier av (för)historiskt, nutida och framtida klimat. Det betyder att växtligheten har potentialen att fungera som en återkopplingsmekanism till naturliga klimatvariationer, men också att människan kan påverka klimatet genom att förändra växtligheten. Det betyder också att mänskligheten kan ha påverkat klimatet innan vi började använda fossilt bränsle. Följaktligen kan växtlighetsförändringar användas som ett sätt att lokalt begränsa klimatförändringar. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.</p>

Projected increasing temperatures and reduced summer precipitation in the UK raises questions about the sustainability of aspects of the agriculture industry and food security. This study investigates the potential impact of precipitation changes on sugar beet yield in Eastern England. Observations of precipitation was examined for the period 1971-2000 and the expected changes in precipitation were investigated using seven CMIP5 climate models for the historical phase (1971-2000) and RCP45 and RCP85 future scenarios (2021-2050). Three out of the seven models were found to show good agreement with observations but the MOHC ensemble mean was the closest to the observed means and was used for further precipitation analyses. Statistical analysis of the future precipitation changes were performed using the Met Office Hadley Centre (MOHC) model focused on changes between the historical phase and RCP45 and RCP85. Results showed a consistent and significant reduction in annual May-October precipitation under future scenarios. The study then investigated the impact of reduced future precipitation changes on sugar beet yield by applying controlled watering regimes informed by the CMIP5 projections to sugar beet plants in a greenhouse experiment over two seasons - the use of CMIP5 projections in this way is a first. In the first experiment carried out in 2014, a climatological watering regime (i.e. where the total seasonal rainfall for the different scenarios was applied in equal and regular watering events) was applied to the plants, which meant a 16% reduction in precipitation in the "future" category relative to a "control" category. Analysis of the yields indicated a statistically significant reduction in mean tuber wet mass: mean of 360g for the control and 319g for the future (p-value 0.03). This implies a potential yield reduction of 11% by 2050. In the second experiment carried out in 2015, a "realistic distribution" watering regime (i.e. where the total seasonal rainfall is applied in a series of watering events that reflect the analysed sizes and distribution of rainfall events in the different categories), this meant a reduction in precipitation in the months of June (-15.6%), July (-7.7%) and August (-3.7%). This resulted in statistically significant reduction in mean tuber wet mass between control (153g) and RCP85 (113g) with a p-value of 0.01. This implies a reduction of 26% in future yields under RCP85 by 2050. Results in this thesis further show how changes and variation in precipitation are intertwined with changes in soil moisture and yield of sugar beet plants. The findings will enable UK sugar beet farmers to identify potential areas of challenges in order to adapt their management practices to ensure maximum crop yield in future growing seasons. Moreover, from a global perspective, the findings here are also broadly applicable to a variety of agricultural crops in different parts of the world, where changes in yield may have important consequences to food security and food prices.