Modeling anthropogenic impacts on the carbon cycle and climate: From land use to mitigation scenarios

Strassmann, Kuno M. (2008). Modeling anthropogenic impacts on the carbon cycle and climate: From land use to mitigation scenarios (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The overarching theme of this thesis is the anthropogenic interference with the climate system. It addresses the agents involved in anthropogenic climate change, their main sources, the options for their abatement, and the key to climate stabilization.
Chapter 2 (Strassmann et al., 2008a) analyzes the influence of land use as a driver of climate change. A land use model was implemented in the BernCC model framework to explicitly account for the impact of land use change on the global carbon cycle. We use scenarios combining estimates of cropland and pasture areas over the last 300 years with projections of future land use belonging to post-SRES integrated assessment scenarios developed at the International Institute of Applied Systems Analysis, Vienna. In agreement with earlier research we show that the historical impact of land use on the carbon cycle and the climate is on par with that of fossil emissions. We quantify the feedback of CO2 emissions caused by land use on land carbon uptake via CO2 fertilization, showing that it reduced land use impact in the past 300 years by about 25%. The future part of the simulations explores the potential of land use as a climate driver relative to that of fossil fuel burning in the absence of a mitigation policy regime. The uncertainty due to land use processes is compared with the uncertainty in carbon cycle parameters. The potential of land use emissions over the 21st century is shown to be dwarfed by that for fossil emissions. This conclusion is robust with respect to a possible more intense deforestation than in the scenarios used here. We demonstrate that under conditions of strongly rising CO2 concentrations as projected for the 21st century, the chief effect of land use on the global carbon cycle is to reduce the carbon uptake potential of the terrestrial carbon sink. As this is mainly a consequence of land use change in the past, we describe this effect by the concept of land use commitment.
In Chapter 3 (Strassmann et al., 2008b) and 4 (Van Vuuren et al., 2008), climate projections for recent multigas mitigation scenarios for the 21st century are discussed. We use a set of emission scenarios developed with Integrated Assessment Models within the Project on Multigas Mitigation and Climate Policy by the Stanford University’s Energy Modeling Forum. Each mitigation scenario is based on a reference scenario consistent with an assumed development of socio-economic drivers but excluding explicit policies for climate mitigation. Such policies are added to the economical framework to generate mitigation scenarios. Greenhouse Gas emissions in mitigation scenarios are constrained by a predefined radiative forcing target that must not be exceeded at the end of the century. A wide array of mitigation options is considered, encompassing all major radiative forcing agents.
In Chapter 3, we perform an attribution of anthropogenic global mean surface temperature change to individual radiative forcing agents. We use a pulse-response substitute formulation of the Bern2.5CC carbon cycle-climate model to project the temperature change caused by each of these forcings. The radiative forcing of CO2 is simulated with the Bern2.5CC model to capture the evolution of atmospheric CO2 as a result of emissions and interaction with the global carbon cycle. A simulation with climate sensitivity set to zero is used to separate the contribution due to the climate-carbon cycle feedback from the overall response. Non-CO2 radiative forcing is projected exogenously based on parametrizations of atmospheric chemistry to calculate atmospheric lifetimes, greenhouse gas concentrations, and radiative forcing.
In 2000, CO2 and non-CO2 Greenhouse Gases contribute by similar parts to anthropogenic temperature change. Aerosols, dominated by sulfate aerosol, offset about half of this warming. The contribution of CO2 to global warming is shown to increase over the century in reference and mitigation scenarios alike. The increasing importance of CO2 is explained by several factors. First, in the reference scenarios, CO2 emissions are tightly coupled to the rapidly expanding energy consumption, and show a similarly strong rise. Non-CO2 emissions are mainly coupled to agriculture, and consequently rise much less rapidly. An emphasis on non-CO2 mitigation options contributes to the dominance of CO2 as a greenhouse gas in the mitigation scenarios. Further, the accumulation of CO2 emissions in the atmosphere tends to increase the share of CO2 in global warming over time. Finally, the feedback of global warming on the carbon cycle leads to an additional rise of CO2 concentrations.
Non-CO2 abatement options account for a significant and economically beneficial contribution to the mitigation of global warming. The prominent position of non-CO2 abatement in the mitigation portfolio is, however, transitory, as CO2 emissions eventually have to be largely eliminated in order to achieve stabilization on the long term. Accordingly, the share of mitigation carried by CO2 increases when RF targets are lowered, and increases over the course of the century in all mitigation scenarios.
Mitigation rapidly reduces the sulfate aerosol loading and associated cooling. This potentially offsets a significant fraction of GHG mitigation. Inertia in the socio-economic and the climate system further delays the effect of mitigation efforts on global temperatures. In consequence, projected rates of temperature change are close to those of the references for decades. This slow start is followed by a rapidly unfolding and profound impact of mitigation efforts in the second half of the century. In comparison to the reference scenarios, rates of change in CO2 concentration, total radiative forcing, and temperature are drastically reduced. Trends in 2100 indicate that the gap between reference and mitigation scenarios is bound to widen further for projections beyond that year. By 2100 climate change progresses at slower rates than at present in all mitigation scenarios. In the scenarios with the most stringent forcing targets, temperature stabilization is achieved by the end of the century.
Chapter 4 gives an overview of the ranges of global temperature change projected for the mitigation scenarios, constrasting them to the no-policy reference scenarios. Global mean temperature increase until 2100 as projected for mitigation scenarios spans a range of 0.5 to 4.4 °C above the mean in 1990. This is 0.3–3.4 °C less than in the references scenarios. The range corresponds to a range of radiative forcing targets on the one hand, and to uncertainty in climate sensitivity and the strength of carbon cycle feedbacks on the other hand. The lower end of forcing targets (about 3 Wm−2) approaches the limit of what is feasible with currently implemented mitigation options and the current knowledge of available technologies as represented by the Integrated Assessment Models. A minimum warming of about 1.4 °C with respect to the year 1990 is projected for these scenarios with standard model setup. This central estimate is embedded in a range of 0.5–2.8 °C reflecting carbon cycle-climate uncertainties. The warming commitment from 20th century emissions corresponding to standard model settings is about 0.6 °C .An additional warming of 0.8 °C over the 21st century thus appears inevitable due to socio-economic and technological inertia and limitations. Consequently, while ambitious mitigation efforts can significantly reduce global warming, adaptation measures will still be needed.
Chapter 5 (Plattner et al., 2007) presents long-term simulations that were prepared for the recently published IPCC Fourth Assessment Report. Eight Earth System Models of Intermediate Complexity contributed to the study (Bern2.5CC, C-GOLDSTEIN, CLIMBER-2, CLIMBER-3, LOVECLIM, MIT-IGSM2.3, MoBidiC, and UVic 2.7). The simulations are based on the illustrative SRES scenarios, and implications of the 21st century emissions are explored using different idealized continuations beyond 2100. The simulations extend to the year 3000 and complement conceptually similar, but shorter simulations with Atmosphere-Ocean General Circulation Models.
Substantial climate change commitments for sea level rise and global mean surface temperature increase are identified. When atmospheric greenhouse gases and radiative forcing are kept constant after 2100, significant warming continues for about a century, and results in an additional 0.6 to 1.6 °C for the low-CO2 SRES B1 scenario and 1.3 to 2.2 °C for the high-CO2 SRES A2 scenario in the year 3000. In contrast, sea level rise due to thermal expansion continues for several centuries and reaches 0.3 to 1.1 m for SRES B1 and 0.5 to 2.2 m for SRES A2. When emissions are set to zero after 2100, the long-term impact of emissions over the 21st century is shown to affect atmospheric CO2 and climate even at year 3000. All models find that a substantial fraction (15 to 28 %) is still airborne even 900 years after carbon emissions have ceased.
21st century scenarios were extended by stabilization profiles for atmospheric CO2 to quantify allowable CO2 emissions corresponding to different stabilization levels. All EMICs agree that stabilization implies incisive emission reductions. Sensitivity simulations with the Bern2.5CC model indicate that carbon cycle and climate sensitivity related uncertainties propagate to a substantial uncertainty in allowable emissions.
In conclusion, all forcings are relevant for climate change, and have to be addressed in an effort to mitigate anthropogenic climate change. Only the eventual reduction of CO2 emissions, however, will clear the path from mitigation to stabilization. Due to the essential role of energy, and the present dependence on carbon-intensive energy sources, CO2 abatement poses the most daunting challenge. This challenge must be met if climate stabilization is to be achieved.

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