Young people's burden: requirement of negative CO<sub>2</sub> emissions

Abstract

Abstract. Global temperature is a fundamental climate metric highly correlated with sea level, which implies that keeping shorelines near their present location requires keeping global temperature within or close to its preindustrial Holocene range. However, global temperature excluding short-term variability now exceeds +1 • C relative to the 1880-1920 mean and annual 2016 global temperature was almost +1.3 • C. We show that global temperature has risen well out of the Holocene range and Earth is now as warm as it was during the prior (Eemian) interglacial period, when sea level reached 6-9 m higher than today. Further, Earth is out of energy balance with present atmospheric composition, implying that more warming is in the pipeline, and we show that the growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade. The rapidity of ice sheet and sea level response to global temperature is difficult to predict, but is dependent on the magnitude of warming. Targets for limiting global warming thus, at minimum, should aim to avoid leaving global temperature at Eemian or higher levels for centuries. Such targets now require "negative emissions", i.e., extraction of CO 2 from the air. If phasedown of fossil fuel emissions begins soon, improved agricultural and forestry practices, including reforestation and steps to improve soil fertility and increase its carbon content, may provide much of the necessary CO 2 extraction. In that case, the magnitude and duration of global temperature excursion above the natural range of the current interglacial (Holocene) could be limited and irreversible climate impacts could be minimized. In contrast, continued high fossil fuel emissions today place a burden on young people to undertake massive technological CO 2 extraction if they are to limit climate change and its consequences. Proposed methods of extraction such as bioenergy with carbon capture and storage (BECCS) or air capture of CO 2 have minimal Published by Copernicus Publications on behalf of the European Geosciences Union. 578 J. Hansen et al.: Young people's burden: requirement of negative CO 2 emissions estimated costs of USD 89-535 trillion this century and also have large risks and uncertain feasibility. Continued high fossil fuel emissions unarguably sentences young people to either a massive, implausible cleanup or growing deleterious climate impacts or both.

Differences between the last two glacial maxima and implications for ice-sheet, δ18O, and sea-level reconstructions

“…Such contrasting ice-mass distributions between successive glacial maxima highlight significant complexity in the processes that drive glaciation into different 'modes' (e.g., Liakka et al, 2016). The difference also has repercussions for glacioisostatic adjustment (GIA) studies of sea-level history during the LIG, which was about 1 C warmer than the Holocene (Clark and Huybers, 2009;Turney and Jones, 2010;McKay et al, 2011;Hoffman et al, 2017;Hansen et al, 2017), with sea levels that reached 4e10 m higher than today Dutton and Lambeck, 2012;Grant et al, 2012;Stocker et al, 2013;Dutton et al, 2015aDutton et al, , 2015b. Dendy et al (2017) investigated the sensitivity of the predictions of the last interglacial highstand to uncertainties in the configuration of the major northern hemisphere ice sheets during MIS 6.…”

“…Fourth, the size and spatial distribution of land ice during past glacials determines crustal rebound processes when ice masses melt, which in turn affects sea-level reconstructions for subsequent interglacials. The latter are key to investigations of sea-level changes above the present level during warmer-than-present interglacials (e.g., the Last Interglacial, LIG,~130-118 kyr ago (ka); Hibbert et al, 2016;Hoffman et al, 2017;Hansen et al, 2017), which can reveal ice-sheet disintegration processes of relevance to the future (e.g., Dutton and Lambeck, 2012;Dutton et al, 2015a,b;Yamane et al, 2015;DeConto and Pollard, 2016).…”

Only the instantaneous global warming potential is consistent with honest and responsible greenhouse gas accounting

“…The laws of nature allow humanity not to overspend this budget, but the required replacement of fossil fuels by renewables and both curves, the total CO 2 put into the atmosphere is 525 GtC, a number chosen because it happens to reproduce the rates of emission reduction contained in the Hansen et al paper, i.e. 3.5% in 2003Hansen et al , 6% in 2013Hansen et al , and 15% in 2020 As an important aside, beyond the scope of the this paper, is that Hansen et al have concluded, in view the industrialized 5 world's lack of action, that the climate can only be stabilized by "negative emissions," i.e., by extraction of CO 2 from the air (Hansen et al, 2017). This approach will be very expensive; it is also fraught with danger because of the difficulty of accurately predicting the instabilities associated with our human time scale irreversible disintegration of ice sheets and ice shelves Hansen (2005).…”

“…• C global mean temperature increase is a "guardrail" that protects life on Earth from the 15 essentially irreversible harm of run-away climate change (Geden, 2015;Friedman, 2015;Knutti et al, 2016) Hansen et al in (Hansen et al, 2008Hansen et al, 2017). This paper presented a simple model to keep track of how much of the global greenhouse gas budget is spent in carbon-equivalent units, a choice that is 5 consistent with the precautionary principle.…”

“…Reporting based on CO 2 equivalents using the 100-year horizon, which is standard practice (World Resoures Institute & World Business Council For Sustainable Development), is misleading and irreconcilable with the observed time-scale of developments of the climate system (National Oceanic Atmospheric Administration, a, b; Hansen et al, 2017). As the numerical thought experiments presented in this paper demonstrate, using more a realistic, dynamical approximation-one that is consistent with the climate time scale-is technically trivial and vital for responsible public policy.…”