Comparing Climate Forcers on a Common Scale

Licentiatavhandling, 2015

The climate is changing at a rapid pace. Through the United Nations Framework Convention on Climate Change (UNFCCC), the world has agreed to hold the on-going temperature increase below 2 °C. Climate change is caused by emissions of different atmospheric species (climate forcers). In order to meet the UNFCCC objective, major emission abatement measures are needed. To compare the climate effects of different measures, emissions of different climate forcers need to be compared on a common scale. Emission metrics are used for this purpose. In Paper I, we develop and analyse two new emission metrics based on the Sea Level Rise (SLR) that emissions of a given climate forcer cause. One of them is the Global Sea level rise Potential (GSP). The metrics are compared with the commonly used Global Warming Potential (GWP) and Global Temperature change Potential (GTP) metrics. Climate forcers with different atmospheric lifetimes are evaluated using an upwelling-diffusion energy balance model. All climate forcers, including short-lived forcers, have long-term influences on SLR. If we only account for the thermosteric part of SLR, GSP values fall in between GWP and GTP values. In Paper II, we compare two different approaches to including climate-carbon cycle feedbacks (CCF) for emission metrics. The IPCC AR5 approach to including CCF is based on Linear Feedback Analysis (LFA). The second approach is based on a coupled climate-carbon cycle model in which CCF is modelled by explicitly making the biosphere and ocean carbon reservoirs temperature dependent. We find that including CCF for non-CO2 climate forcers through the Explicit CCF (ECCF) approach gives higher GWP and GTP values than using the LFA approach, for short time horizons. While the opposite is true for long time horizons. With the LFA approach, a fraction of the indirectly induced atmospheric CO2, caused by an emission pulse of a non-CO2 forcer, stays in the atmosphere basically forever, while with the ECCF approach it eventually returns back to the unperturbed levels when the direct warming is gone. In Paper III, we develop and analyse a spatially explicit model of multiple independent villagers engaged in forest extraction. A spatial Non-Cooperative Equilibrium (NCE) of extraction patterns is analysed and compared to an equilibrium with coordinated villagers, for a range of spatial landscapes and model assumptions. Each villager chooses from where, and how much, to extract and whether to perform non-forest wage work part or full-time instead. We investigate the model assumptions, commonly adopted by earlier research, which include the use of a representative villager and only allowing the villager to extract from one location. We find a priori identical villagers to behave differently in equilibrium and show that forest extraction and degradation patterns depend on the model assumptions used.