Representative Concentration Pathway 6.0 (RCP6) is a pathway that describes trends in long-term, global emissions of greenhouse gases (GHGs), short-lived species, and land-use/land-cover change leading to a stabilisation of radiative forcing at 6.0 Watts per square meter (Wm −2 ) in the year 2100 without exceeding that value in prior years. Simulated with the Asia-Pacific Integrated Model (AIM), GHG emissions of RCP6 peak around 2060 and then decline through the rest of the century. The energy intensity improvement rates changes from 0.9% per year to 1.5% per year around 2060. Emissions are assumed to be reduced cost-effectively in any period through a global market for emissions permits. The exchange of CO 2 between the atmosphere and terrestrial ecosystem through photosynthesis and respiration are estimated with the ecosystem model. The regional emissions, except CO 2 and N 2 O, are downscaled to facilitate transfer to climate models.

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Mr. Castles and Mr. Henderson have criticized the Special Report on Emissions Scenarios (SRES) and other aspects of IPCC assessments. It is claimed that the methodology is “technically unsound” because market exchange rates (MER) are used instead of purchasing power parities (PPP) and that the scenarios themselves are flawed because the GDP growth in the developing regions is too high. The response is: The IPCC SRES reviews existing literature, most of which is MER based, including that from the World Bank, IEA and USDoE. Scenarios of GDP growth are typically expressed as MER (the preferred measure for GDP growth, as opposed to PPP which is a preferred measure for assessing differences in economic welfare). IPCC scenarios did include PPP-based scenarios, which Mr. Castles and Mr. Henderson have conveniently ignored. Contrary to what Mr. Castles and Mr. Henderson claim, IPCC scenarios are consistent with historical data, including that from 1990 to 2000, and with the most recent near term (up to 2020) projections of other agencies. Long-term emissions are based on multiple, interdependent driving forces, and not just economic growth. Mr. Castles and Mr. Henderson need to look beyond GDP. The IPCC scenarios provided information for only four world regions, and not for specific countries. Mr. Castles' and Mr. Henderson's critique is not of IPCC scenarios but of ongoing unpublished work in progress that is not part of SRES. We therefore show that Mr. Castles and Mr. Henderson have focused on constructing a “problem” that does not exist. SRES scenarios are sound and the IPCC has responded seriously and conscientiously. We detail our response below in nine sections. After an introduction (Section 1), we outline the SRES methodology for measuring economic output (Section 2). Section 3 compares SRES to long-historical economic development and provides five responses to the critics. Section 4 addresses the issue of country-level economic projections even if not part of SRES. Sections 5, 6 and 7 validate the SRES scenarios by comparing them with recent trends for economic and CO2 emission growth, as well as more recent scenarios available in the literature. Section 8 refutes the argument that lower economic growth in developing countries would lower GHG emissions correspondingly. Section 9 concludes.

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The Paris Agreement introduces long-term strategies as an instrument to inform progressively more ambitious emission reduction objectives, whilst holding development goals paramount in context of national circumstances. In the lead up to COP21, the Deep Decarbonization Pathways Project developed mid-century low-emission pathways for 16 countries, based on an innovative pathway design framework. In this Perspective we describe this framework and show how it can support the development of sectorally and technologically detailed and policy-relevant country-driven strategies consistent with the Paris Agreement climate goal. We also discuss how this framework can be used to engage stakeholder input and buy-in; design implementation policy packages; reveal necessary technological, financial and institutional enabling conditions; and support global stocktaking and ratcheting of ambition.

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