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Limiting warming to well below 2°C requires rapid and complete decarbonisation of energy systems. We compare economy-wide modelling of 1.5°C and 2°C scenarios with sector-focused analyses of four critical sectors that are difficult to decarbonise: aviation, shipping, road freight transport, and industry. We develop and apply a novel framework to analyse and track mitigation progress in these sectors. We find that emission reductions in the 1.5°C and 2°C scenarios of the IMAGE model come from deep cuts in CO 2 intensities and lower energy intensities, with minimal demand reductions in these sectors' activity. We identify a range of additional measures and policy levers that are not explicitly captured in modelled scenarios but could contribute significant emission reductions. These are demand reduction options, and include less air travel (aviation), reduced transportation of fossil fuels (shipping), more locally produced goods combined with high load factors (road freight), and a shift to a circular economy (industry). We discuss the challenges of reducing demand both for economy-wide modelling and for policy. Based on our sectoral analysis framework, we suggest modelling improvements and policy recommendations, calling on the relevant UN agencies to start tracking mitigation progress through monitoring key elements of the framework (CO 2 intensity, energy efficiency, and demand for sectoral activity, as well as the underlying drivers), as a matter of urgency. Key policy insights:. Four critical sectors (aviation, shipping, road freight, and industry) cannot cut their CO 2 emissions to zero rapidly with technological supply-side options alone. Without large-scale negative emissions, significant demand reductions for those sectors' activities are needed to meet the 1.5-2°C goal.. Policy priorities include affordable alternatives to frequent air travel; smooth connectivity between low-carbon travel modes; speed reductions in shipping and reduced demand for transporting fossil fuels; distributed manufacturing and local storage; and tightening standards for material use and product longevity.. The COVID-19 crisis presents a unique opportunity to enact lasting CO 2 emissions reductions, through switching from frequent air travel to other transport modes and online interactions.. Policies driving significant demand reductions for the critical sectors' activities would reduce reliance on carbon removal technologies that are unavailable at scale.

The latest scientific framing of climate change emphasizes the importance of limiting cumulative emissions and the need to urgently cut CO 2 . International agreements on avoiding a 2 °C global temperature rise make clear the scale of CO 2 reductions required across all sectors. Set against a context of urgent mitigation, the outlook for aviation's emissions is one of continued growth. Limited opportunities to further improve fuel efficiency, slow uptake of new innovations, coupled with anticipated rises in demand across continents collectively present a huge challenge to aviation in cutting emissions. While difficulties in decarbonizing aviation are recognized by industry and policymakers alike, the gap between what's necessary to avoid 2 °C and aviation's CO 2 projections has profound implications. Biofuel is one of the few innovations that could play a significant role in closing the gap, but with low anticipated penetration before 2020 its contribution would have little impact over the desired timeframe. If the aviation sector does not urgently address rising emissions, there is an increasing risk that investment in new aircraft and infrastructure could lead to stranded assets. This leaves it facing an uncomfortable reality. Either the sector acts urgently on climate change and curtails rising demand, or it will be failing to take responsibility for a considerable and growing portion of climate change impacts.