Ocean-based mitigation of climate change warrants experimental evaluation because many “natural” terrestrial mitigation options are risky and may prove temporary

“Natural” mitigation of climate change via both carbon sequestration in, and reductions of greenhouse gas emissions from, biological reservoirs based on or near land, has been promoted as rapidly deployable and cost-effective. Many publications suggest that tropical forests, soils and wetlands offer natural mitigation, in order of decreasing capacity, while remediation involving the ocean has received less attention. Such natural ~terrestrial mitigation could keep post-industrialization warming below 1.5 degrees C, when coupled with reductions in fossil fuel emissions, as confirmed here with a numerical model of future emissions. However, such mitigation could cease in response to changes in future climate, land use or natural resource policies, which model simulations reveal could lead to cumulative emissions similar to those expected in the absence of any natural mitigation, negating much of the intended benefit. If we are to minimize climate change, low-risk natural mitigation (e.g. by reducing deforestation and minimizing fluxes from wetlands) should be considered, but additional options are needed. For this reason, mitigation options in the ocean warrant experimental evaluation to consider benefits and possible detrimental side effects. Briefly, marine mitigation options worthy of careful experimental evaluation include (but are not limited to): 1) storing carbon in the ocean by increasing ocean alkalinity, with fewer impacts than result from releasing CO2 to the atmosphere; 2) Fe fertilization of Fe-limited surface waters, which could enhance biological productivity and thereby reduce surface-water (and thus atmospheric) CO2 concentrations ; 3) development of marine-derived biofuels; 4) electricity generation from the ocean’s vertical temperature gradient. Given the risks and possibly short-term nature of many terrestrial-based “natural” mitigation options, marine-based options should be considered for their potential for safe, long-term mitigation.