The recent quantification of the reservoir of carbon held in permafrost soils has rekindled the concern that the terrestrial biosphere will transition from a carbon sink to a carbon source during the 21st century. This dissertation is a compilation of four modelling studies that investigate the permafrost carbon feedback, its consequences for the projected future behaviour of the carbon cycle, and the origins of the proportionally between cumulative CO$_2$ emissions and near surface temperature change. The dissertation is centred around five questions: 1) what is the strength and timing of the permafrost carbon feedback to climate change? 2) If anthropogenic CO2 emissions cease, will atmospheric CO2 concentration continue to increase? 3) Can climate warming be reversed using artificial atmospheric carbon-dioxide removal? 4) What are the underlying physical mechanisms that explain the existence in Earth system models of the proportionality between cumulative CO2 emissions and mean global near surface temperature change? And 5) can strong terrestrial carbon cycle feedbacks, such as the permafrost carbon feedback, disrupt this proportionality? By investigating the these questions using the University of Victoria Earth System Climate Model (UVic ESCM) and analytical mathematics the following conclusions are drawn: 1) The permafrost carbon feedback to climate change is simulated to have a strength of 0.25 C (0.1 to 0.75)C by the year 2100 CE independent of emission pathway followed in the 21st century. This range is contingent on the size of the permafrost carbon pool and the simulated model climate sensitivity. 2) If CO2 emissions were to suddenly cease, the UVic ESCM suggests that whether or not CO2 would continue to build up in the atmosphere is contingent on climate sensitivity and the concentration of non-CO2 greenhouse gasses in the atmosphere. For a given model climate sensitivity there is a threshold value of radiative forcing from non-CO2 greenhouse gasses above which CO2 will continue to build up in the atmosphere for centuries after cessation of anthropogenic CO2 emissions. For a UVic ESCM the threshold value for the Representative Concentration Pathway (RCP) derived emission scenarios is approximately 0.6 Wm^-2 of non-CO2 greenhouse gas radiative forcing. The consequences of being above this threshold value are mild, with the model projecting a further 11-22 ppmv rise in atmosphere CO2 concentration after emissions cease. 3) If technologies were developed and deployed to remove carbon from the atmosphere simulations with the UVic ESCM suggest that a Holocene-like climate could be restored by the end of the present millennium (except under a high climate sensitivity and high emission scenario). However, more carbon must be removed from the atmosphere than was originally emitted to it. 4) The proportionality between cumulative CO2 emissions and global mean temperature change seen in most Earth system model simulations appears to arises from two factors: I) the stability of the airborne fraction of emitted carbon provided by the ocean uptake of carbon begin nearly a function of CO2 emission rate; and II) the diminishing heat uptake by the oceans compensating for the reduced radiative forcing per unit mass CO2 at high atmospheric CO2 concentrations. 5) Strong terrestrial carbon cycle feedbacks can disrupt the proportionality between cumulative CO2 emissions and global mean temperature change. However, within the range of emission rates project for the RCPs the permafrost carbon feedback is not strong enough to disrupt the relationship. Overall, the addition of the permafrost carbon pool to the UVic ESCM alters model behaviour in ways that if representative of the natural world will make stabilizing climate or reversing climate change more difficult than has previously been foreseen. / Graduate / 0768 / 0373 / andrewhughmacdougall@gmail.com

The recent quantification of the reservoir of carbon held in permafrost soils has rekindled the concern that the terrestrial biosphere will transition from a carbon sink to a carbon source during the 21st century. This dissertation is a compilation of four modelling studies that investigate the permafrost carbon feedback, its consequences for the projected future behaviour of the carbon cycle, and the origins of the proportionally between cumulative CO$_2$ emissions and near surface temperature change. The dissertation is centred around five questions: 1) what is the strength and timing of the permafrost carbon feedback to climate change? 2) If anthropogenic CO2 emissions cease, will atmospheric CO2 concentration continue to increase? 3) Can climate warming be reversed using artificial atmospheric carbon-dioxide removal? 4) What are the underlying physical mechanisms that explain the existence in Earth system models of the proportionality between cumulative CO2 emissions and mean global near surface temperature change? And 5) can strong terrestrial carbon cycle feedbacks, such as the permafrost carbon feedback, disrupt this proportionality? By investigating the these questions using the University of Victoria Earth System Climate Model (UVic ESCM) and analytical mathematics the following conclusions are drawn: 1) The permafrost carbon feedback to climate change is simulated to have a strength of 0.25 C (0.1 to 0.75)C by the year 2100 CE independent of emission pathway followed in the 21st century. This range is contingent on the size of the permafrost carbon pool and the simulated model climate sensitivity. 2) If CO2 emissions were to suddenly cease, the UVic ESCM suggests that whether or not CO2 would continue to build up in the atmosphere is contingent on climate sensitivity and the concentration of non-CO2 greenhouse gasses in the atmosphere. For a given model climate sensitivity there is a threshold value of radiative forcing from non-CO2 greenhouse gasses above which CO2 will continue to build up in the atmosphere for centuries after cessation of anthropogenic CO2 emissions. For a UVic ESCM the threshold value for the Representative Concentration Pathway (RCP) derived emission scenarios is approximately 0.6 Wm^-2 of non-CO2 greenhouse gas radiative forcing. The consequences of being above this threshold value are mild, with the model projecting a further 11-22 ppmv rise in atmosphere CO2 concentration after emissions cease. 3) If technologies were developed and deployed to remove carbon from the atmosphere simulations with the UVic ESCM suggest that a Holocene-like climate could be restored by the end of the present millennium (except under a high climate sensitivity and high emission scenario). However, more carbon must be removed from the atmosphere than was originally emitted to it. 4) The proportionality between cumulative CO2 emissions and global mean temperature change seen in most Earth system model simulations appears to arises from two factors: I) the stability of the airborne fraction of emitted carbon provided by the ocean uptake of carbon begin nearly a function of CO2 emission rate; and II) the diminishing heat uptake by the oceans compensating for the reduced radiative forcing per unit mass CO2 at high atmospheric CO2 concentrations. 5) Strong terrestrial carbon cycle feedbacks can disrupt the proportionality between cumulative CO2 emissions and global mean temperature change. However, within the range of emission rates project for the RCPs the permafrost carbon feedback is not strong enough to disrupt the relationship. Overall, the addition of the permafrost carbon pool to the UVic ESCM alters model behaviour in ways that if representative of the natural world will make stabilizing climate or reversing climate change more difficult than has previously been foreseen. / Graduate / 2015-05-01 / 0768 / 0373 / andrewhughmacdougall@gmail.com

L’objet de cette thèse est l’analyse des effets de la variabilité du taux de change réel effectif (TCRE) sur le commerce extérieur de la Tunisie global et sectoriel sur la période de 1975-2009. La variabilité du TCRE signifie la volatilité et le mésalignement. La volatilité est mesurée par l’écart type mobile (ETM) et le modèle GARCH(1,1). Cependant le mésalignement du TCRE est mesurée par la déviation par rapport aux taux d’équilibre donnés par trois théories du taux de change d’équilibre à savoir le modèle d’Edwards, de BEER et du NATREX. A l’aide des estimations des modèles d’exportations et d’importations sans/avec ruptures, les résultats ont montré que les effets de la volatilité sur le commerce extérieur ne sont pas très pertinents à long terme. Quant aux effets du mésalignement du taux de change, ceux-ci apparaissent très pertinents essentiellement lorsque il s’agit des flux commerciaux sectoriels. / The purpose of this thesis is the analysis of the variability of the real effective exchange rate (REER) on foreign trade total and sectoral of Tunisia over the period 1975-2009. Variability REER means volatility and misalignment. Volatility is measured by the mobile standard deviation (ETM) and the GARCH (1, 1). However, the misalignment of REER is measured by the deviation from the equilibrium rate given by three theories of exchange rate equilibrium namely the Edwards model, BEER and NATREX. Using estimates of exports and imports models with / without breaks, the results showed that the effects of volatility on trade are not very relevant in the long term. But the effects of the exchange rate misalignment appear very relevant mainly when the sectoral trade flows are used.