Urs Schenker

Companies are increasingly setting greenhouse gas (GHG) emission reduction targets to align with the 1.5 °C goal of the Paris Agreement. Currently, companies set these science-based targets (SBTs) for aggregate GHGs expressed in CO₂-equivalent emissions. This approach does not specify which gases will be reduced and risk misalignment with ambitious mitigation scenarios in which individual gas emissions are mitigated at different rates. We propose that companies instead set reduction targets for separate baskets of GHGs, defined according to the atmospheric lifetimes and global mitigation potentials of GHGs. We use a sector-level analysis to approximate the average impact of this proposal on company SBTs. We apply a multiregional environmentally extended input output model and a range of 1.5 °C emissions scenarios to compare 1-, 2- and 3-basket approaches for calculating sector-level SBTs for direct (scope 1) and indirect (scope 2 and upstream scope 3) emissions for all major global sectors. The multi-basket approaches lead to higher reduction requirements for scope 1 and 2 emissions than the current single-basket approach for most sectors, because these emission sources are usually dominated by CO₂, which is typically mitigated faster than other gases in 1.5 °C scenarios. Exceptions are scope 1 emissions for fossil and biological raw material production and waste management, which are dominated by other GHGs (mainly CH₄and N₂O). On the other hand, upstream scope 3 reduction targets at the sector level often become less ambitious with a multi-basket approach, owing mainly to substantial shares of CH₄and, in some cases, non-CO₂long-lived emissions. Our results indicate that a shift to a multi-basket approach would improve the alignment of SBTs with the Paris temperature goal and would require most of the current set of companies with approved SBTs to increase the ambition of their scope 1 and scope 2 targets. More research on the implications of a multi-basket approach on company-level SBTs for all scope 3 activities (downstream, as well as upstream) is needed.

Recently, increasing concentrations of persistent organic pollutants (POPs) have been observed in the sediment of glacier-fed Lake Oberaar, Switzerland. Melting glaciers have been suggested as a secondary source of POPs released to Alpine lakes. Here we further investigate whether climate warming may accelerate the release of POPs previously deposited to Alpine glaciers (“glacier hypothesis”). To test this hypothesis, a dynamic multimedia mass balance model is developed for the catchment area of Lake Oberaar and is applied to polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and dichlorodiphenyl trichloroethane (DDT). This lake model is combined with two other models. The first is a dynamic multimedia mass balance model parametrized for the Swiss lowlands that is used to calculate (on the basis of historical emission data) the atmospheric concentrations that are an advective input into the model of the lake catchment. The second is a flow model of Oberaar Glacier that determines the residence time of persistent chemicals in the glacier after their deposition to the glacier surface. According to results from these three models in combination, the release of POPs by the glacier is currently increasing and accounts for the observed increase in concentrations in the lake sediment. The models indicate that approximately half of the amount of PCBs, PCDD/Fs, and DDT initially incorporated into the glacier ice is still stored in the glacier. Under the assumption that the climate is warming, accelerated release of POPs is to be expected for the future; in a model run where no climate warming is assumed, the period of time required for release of the same amount of chemicals is longer by several decades than in the scenario with a changing climate.

A global-scale fate and transport model was applied to investigate the historic and future trends in ambient concentrations of perfluorooctane sulfonate (PFOS) and volatile perfluorooctane sulfonyl fluoride (POSF)-based precursor compounds in the environment. First, a global emission inventory for PFOS and its precursor compounds was estimated for the period 1957−2010. We used this inventory as input to a global-scale contaminant fate model and compared modeled concentrations with field data. The main focus of the simulations was to examine how modeled concentrations of PFOS and volatile precursor compounds respond to the major production phase-out that occurred in 2000−2002. Modeled concentrations of PFOS in surface ocean waters are generally within a factor of 5 of field data and are dominated by direct emissions of this substance. In contrast, modeled concentrations of the precursor compounds considered in this study are lower than measured concentrations both before and after the production phase-out. Modeled surface ocean water concentrations of PFOS in source regions decline slowly in response to the production phase-out while concentrations in remote regions continue to increase until 2030. In contrast, modeled concentrations of precursor compounds in both the atmosphere and surface ocean water compartment in all regions respond rapidly to the production phase-out (i.e., decline quickly to much lower levels). With respect to wildlife biomonitoring data, since precursor compounds are bioavailable and degrade to PFOS in vivo, it is at least plausible that declining trends in PFOS body burdens observed in some marine organisms are attributable to this exposure pathway. The continued increases in PFOS body burdens observed in marine organisms inhabiting other regions may reflect exposure primarily to PFOS itself, present in the environment due to production and use of this compound as well as degradation of precursor compounds.