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p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 107 Methods and Models for Costing Carbon Mitigation Jayant Sathaye and P.R. Shukla p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 137 On the Sustainability of Renewable Energy Sources Ottmar Edenhofer, Kristin Seyboth, Felix Creutzig, and Steffen Schlömer p p p p p p p p p p p p p 169

Abstract. Anthropogenic air pollutant emissions have been increasing rapidly in China, leading to worsening air quality. Modelers use emissions inventories to represent the temporal and spatial distribution of these emissions needed to estimate their impacts on regional and global air quality.

The Paris Agreement is based on emission scenarios that move from a sluggish phase-out of fossil fuels to large-scale late-century negative emissions. Alternative pathways of early deployment of negative emission technologies need to be considered to ensure that climate targets are reached safely and sustainably. The historic climate summit in Paris in 2015 galvanized global commitments to an ambitious yet vaguely defined goal of climate stabilization. At the same time some scientists argue that the model-based scenarios with 1.5-and even 2-degree temperature change targets seem unattainable and detached from current political realities 1,2. Here we scrutinize the dominant climate mitigation scenario archetype that projects low global decarbonization rates in the first half of this century followed by large negative emissions in the second half, thanks to Carbon Dioxide Removal (CDR) technologies 3. We call this approach to mitigation timing the "Late Century CDR" scenario archetype (Figure 1a). This archetype is consistent with nearly all of 2-degree scenarios covered by the Fifth Assessment Report (AR5) by the Intergovernmental Panel on Climate Change (IPCC) 4 , 87% of which deploy CDR technologies in the second half of the century 5. Following this predominant archetype might not only turn out to be a risky strategy, but also lead to significant environmental damages and may be economically inefficient. In "Late Century CDR" scenarios, CDR mostly in the form of bioenergy with Carbon Capture and Storage (BECCS) typically removes the equivalent of 20 years of current GHG emissions to reverse the temporary GHG budget overshoot that is tolerated earlier on 6. The challenges and uncertainties associated with CDR are well described in the scientific literature 5,7 , yet the scientific and political debate addressing the consequences of large-scale and late deployment of CDR as a "backstop" strategy is only at an early stage. We argue that a new set of scenarios needs to be generated and analyzed to inform the policy process on robust timing of climate mitigation with the aim of avoiding negative side effects. Essentially, three attributes characterize such budget-constrained scenarios: the timing and magnitude of global peak net emissions and its speed of decline thereafter; the maximum amount of allowable deployment of biomass-based CDRs; and an admissible risk threshold associated with a temperature overshoot.

Integrated assessment models (IAMs) have emerged as key tools for building and assessing long term climate mitigation scenarios. Due to their central role in the recent IPCC assessments, and international climate policy analyses more generally, and the high uncertainties related to future projections, IAMs have been critically assessed by scholars from different fields receiving various critiques ranging from adequacy of their methods to how their results are used and communicated. Although IAMs are conceptually diverse and evolved in very different directions, they tend to be criticised under the umbrella of ‘IAMs’. Here we first briefly summarise the IAM landscape and how models differ from each other. We then proceed to discuss six prominent critiques emerging from the recent literature, reflect and respond to them in the light of IAM diversity and ongoing work and suggest ways forward. The six critiques relate to (a) representation of heterogeneous actors in the models, (b) modelling of technology diffusion and dynamics, (c) representation of capital markets, (d) energy-economy feedbacks, (e) policy scenarios, and (f) interpretation and use of model results.

Solar photovoltaic (PV) electricity generation is expanding rapidly in China, with total capacity projected to be 400 GW by 2030. However, severe aerosol pollution over China reduces solar radiation reaching the surface. We estimate the aerosol impact on solar PV electricity generation at the provincial and regional grid levels in China. Our approach is to examine the 12-year (2003-2014) average reduction in point-of-array irradiance (POAI) caused by aerosols in the atmosphere. We apply satellite-derived surface irradiance data from the NASA Clouds and the Earth's Radiant Energy System (CERES) with a PV performance model (PVLIB-Python) to calculate the impact of aerosols and clouds on POAI. Our findings reveal that aerosols over northern and eastern China, the most polluted regions, reduce annual average POAI by up to 1.5 kWh/m per day relative to pollution-free conditions, a decrease of up to 35%. Annual average reductions of POAI over both northern and eastern China are about 20-25%. We also evaluate the seasonal variability of the impact and find that aerosols in this region are as important as clouds in winter. Furthermore, we find that aerosols decrease electricity output of tracking PV systems more than those with fixed arrays: over eastern China, POAI is reduced by 21% for fixed systems at optimal angle and 34% for two-axis tracking systems. We conclude that PV system performance in northern and eastern China will benefit from improvements in air quality and will facilitate that improvement by providing emission-free electricity.