Climate dreaming 213 ‘the ocean never forgets’.26 Popular writing about climate change routinely points out that significant percentages of emitted carbon dioxide remain in the atmosphere for centuries, while emitted methane persists for only a decade or two. Accordingly, carbon dioxide continues to produce its bad effects for a much longer period of time than methane does. The implication is that if we had to choose, we ought to choose to cut carbon dioxide rather than cutting methane.27 But we do not have to choose: it is perfectly possible to work on both fronts simultaneously, especially since fossil fuels are one source of large amounts of methane as well as of most anthropogenic carbon dioxide. And, thanks to the oceans, the indirect effects of methane – the effects of its effects – are very long-lasting. The greenhouse effect of methane is short and sharp. The methane does not remain in the atmosphere for very many years, but during its atmospheric residence it powerfully forces temperature upwards. A constant process of equilibration occurs between atmospheric temperature and ocean temperature. When atmospheric methane drives atmospheric temperature up, some of the heat moves into the ocean. But because of the complex processes sketched by Zickfeld, Solomon, and Gilford, the heat only slowly circulates through the various depths of the world’s interconnected oceans, remaining out of the atmosphere for centuries but meanwhile driving thermal expansion of the ocean water – water expands as it is heated. This thermal expansion will exacerbate sea-level rise. And the amount of heat added to the oceans by human activity is turning out to be far greater than previously suspected.28 In addition, as we saw above, warming oceans will speed basal melt of marine-based ice sheets, which exist in Greenland as well as in West and East Antarctica, providing a stunning illustration of reverberating positive feedbacks allowing climate change to feed upon itself. And when, after centuries, the process of equilibrating temperatures eventually releases some of the heat from the ocean into a cooler atmosphere, it will again heat the atmosphere. Thus, the brief but sharp initial rise in atmospheric temperature produced directly by the greenhouse effect of the methane translates, via the slow grinding of oceanic processes, into (i) atmospheric temperature rise centuries later, as well as earlier (ii) thermal expansion of ocean water and (iii) basal melt of ice sheets. Negative emissions produced decades after any particular methane emissions – irrespective of whether the NET is BECCS in particular – leave the effects, direct and indirect, of that methane untouched. With regard to methane emissions in the 26. C Harvey, ‘Methane May Not Last Long in the Atmosphere – but It Drives Sea Level Rise for Centuries’ (2017) Washington Post, 13 January, <https://www.washingtonpost.com/news/ energy-environment/wp/2017/01/09/methane-may-not-last-long-in-the-atmosphere-but-itdrives-rising-seas-for-hundreds-of-years/?utm_term=.774974544c34> accessed 10 May 2017. See K Zickfeld, S Solomon, and D Gilford, ‘Centuries of Thermal Sea-level Rise Due to Anthropogenic Emissions of Short-lived Greenhouse Gases’ Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1612066114. 27. H Shue, ‘Mitigation: First Imperative of Environmental Ethics’ in S Gardiner and A Thompson (eds), The Oxford Handbook of Environmental Ethics (Oxford University Press, New York 2017), 465–73. 28. L Cheng, K Trenberth, J Fasullo, et al., ‘Improved Estimates of Ocean Heat Content from 1960 to 2015’ (2017) Science Advances 3, e1601545 (10 March 2017). For an appreciation of the significance of these estimates, see C Harvey, ‘The World’s Oceans are Storing Up Staggering Amounts of Heat – and It’s Even More Than We Thought’ (2017) Washington Post, 10 March 2017, <https://www.washingtonpost.com/news/energy-environment/wp/2017/03/10/ the-worlds-oceans-are-storing-up-staggering-amounts-of-heat-and-its-even-more-than-wethought/?nid&utm_term=.957e4a297ccd> accessed 10 May 2017. © 2017 The Author Journal compilation © 2017 Edward Elgar Publishing Ltd