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The Paris Agreement has opened debate on whether limiting warming to 21 1.5°C is compatible with current emission pledges and warming of about 22 0.9°C from the mid-19 th -century to the present decade. We show that limiting 23 cumulative post-2015 CO2 emissions to about 200 GtC would limit post-2015 24 warming to less than 0.6°C in 66% of Earth System Model members of the 25 CMIP5 ensemble with no mitigation of other climate drivers, increasing to 26 240GtC with ambitious non-CO2 mitigation. We combine a simple climate-27 carbon-cycle model with estimated ranges for key climate system properties 28 from the IPCC 5 th Assessment Report. Assuming emissions peak and decline 29 to below current levels by 2030 and continue thereafter on a much steeper 30 decline, historically unprecedented but consistent with a standard ambitious 31 mitigation scenario (RCP2.6), gives a likely range of peak warming of 1.2-32 2.0°C above the mid-19 th -century. If CO2 emissions are continuously adjusted 33 over time to limit 2100 warming to 1.5°C , with ambitious non-CO2 mitigation, 34 net future cumulative CO2 emissions are unlikely to prove less than 250 GtC 35 and unlikely greater than 540GtC. Hence limiting warming to 1.5°C is not yet a 36 geophysical impossibility, but likely requires delivery on strengthened pledges 37 for 2030 followed by challengingly deep and rapid mitigation. Strengthening 38 near-term emissions reductions would hedge against a high climate response 39 or subsequent reduction-rates proving economically, technically or politically 40 unfeasible. 41 42 Main text: 43The aim of Paris Agreement is "holding the increase in global average 44 temperature to well below 2°C above pre-industrial levels and pursuing efforts 45 to limit the temperature increase to 1.5°C " 1 . The Parties also undertook to 46 achieve this goal by reducing net emissions "to achieve a balance between 47 anthropogenic sources and removals by sinks of greenhouse gases in the 48 second half of this century", and hence implicitly not by geo-engineering 49 Long-term anthropogenic warming is determined primarily by cumulative 64 emissions of CO2 7-10 : the IPCC 5 th Assessment Report (IPCC-AR5) found that 65 cumulative CO2 emissions from 1870 had to remain below 615GtC for total 66 anthropogenic warming to remain below 1.5°C in more than 66% of members 67 of the CMIP5 ensemble of Earth System Models (ESMs) 11 (see Fig. 1a). 68Accounting for the 545GtC that had been emitted by the end of 2014 12 , this 69 would indicate a remaining budget from 2015 of less than 7 years' current 70 emissions, while current commitments under the Nationally Determined 71Contributions (NDCs) indicate 2030 emissions close to current levels 13 . 72 73The scenarios and simulations on which these carbon budgets were based, 74 however, were designed to assess futures in absence of CO2 mitigation, not 75 the very ambitious mitigation scenarios and correspondingly small amounts of 76 additional warming above present that are here of interest. cumulative carbon emissions (5...

This paper summarizes results from ten global economy-energyenvironment models implementing mechanisms of endogenous technological change (ETC