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

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

The ocean is the largest sink of anthropogenic carbon from the atmosphere and therefore the magnitude of ocean carbon uptake largely determines the airborne fraction of emissions and the ultimate severity of surface climate change. However, climate-feedbacks on ocean carbon uptake over the historical period and in the future are uncertain. In particular, much uncertainty in the ocean carbon response hinges on the influence of wind-driven changes in the Southern Ocean, which is the most significant region of anthropogenic carbon uptake. Here I show that the Southern Hemisphere westerly winds simulated by the Coupled Model Intercomparison Project Phase 3 (CMIP3) and CMIP5 climate models have significant biases in their pre-industrial and satellite era-climatologies, relative to observationally based estimates. I also show that the models project the westerlies to intensify and shift poleward under anthropogenic forcing over the 20th and 21st centuries, but that they significantly underestimate the trends over the satellite era. I then use a novel experimental design, wherein I isolate the influence of the model’s pre-industrial wind bias on simulations of ocean carbon uptake and climate. I do this by using the UVic Earth System Climate Model (ESCM) with an ensemble of members, each forced by the winds from an individual CMIP model. I show here that the climate model pre-industrial wind bias can significantly increase ocean carbon uptake in transient climate change simulations, reducing the airborne fraction and projected climate change. By contrast, the simulated wind-changes over the 20th and 21st centuries reduce ocean carbon uptake, largely through an increase in outgassing from the Southern Ocean. However, I show that this transient- wind effect is i) smaller than the pre-industrial bias effect and ii) does not occur when using a variable formulation for the Gent-McWilliams coefficient of eddy diffusivity in the coarse resolution model, under simulated or observed wind-changes. I then go on to demonstrate that the simulated transient wind-changes significantly reduce the Antarctic sea-ice area simulated by the UVic ESCM. I also test the influence of fresh water input to the Southern Ocean from dynamic Antarctic Ice Sheet mass loss, which is a forcing absent from the CMIP5 models. The magnitude of the fresh water effect is small and has little influence on the sea-ice area trends simulated by the CMIP5 models over the historical era. These results have significant implications for previous model-based studies of the ocean carbon cycle, as well as for the quantification of the wind-induced uncertainty in future climate projections by current Earth System Models. / Graduate / 0725 / 0425 / 0415

No description available.

Diss. (sammanfattning). Uppsala : Sveriges lantbruksuniv., 2003. / Härtill 4 uppsatser.

Diss. (sammanfattning) Uppsala : Uppsala universitet, 2009. / Härtill 4 uppsatser.

Thesis (M.Sc.)--Victoria University of Wellington, 2008. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 136-142). Also available on the World Wide Web.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (p. 127-137).