Resilient tree-planting under compounding climate and economic uncertainties

Paper abstract To meet decarbonisation targets, nations around the globe have made ambitious commitments to expand forested land. Operationalising these commitments requires choosing a planting strategy: how many trees should be planted, of which species, and where? Given those choices must be made now but have long term consequences, such decisions are plagued by uncertainty. For example, species that are well suited to present conditions may perform poorly under future climates, yet those future climates are themselves highly uncertain. Using the exemplar of the UK, a nation committed to achieving net zero emissions by mid-century, we quantify key uncertainties pertaining to co-evolving climate and economic conditions and examine how modern methods of decision-making under uncertainty can advise on planting choices. Our analysis reveals that the best planting strategy assuming a ‘high-emissions’ future is radically different to that for a future that remains on a ‘near-historic’ path. Planting for the former while experiencing the latter results in substantial net costs to UK society. Assimilating uncertainty into decision-making identifies planting strategies that diversify risk and significantly reduce the probability of high-cost outcomes. Importantly, our research reveals that the scope for mitigating risk through choice of planting strategy is relatively limited. Despite this persistent risk, we find that tree planting remains a highly cost-effective carbon removal solution when compared to alternative technologies, even when those alternatives are assumed to be riskless.

论文摘要：为实现脱碳目标，全球各国均已提出雄心勃勃的造林扩地承诺。要将这些承诺落地实施，需选定造林策略：即应种植多少树木、选用哪些树种，以及选址何处。由于此类决策需即刻作出，却会产生长期影响，因此饱受不确定性的困扰。例如，适配当前气候条件的树种，在未来气候下可能生长不佳，但未来气候本身亦存在高度不确定性。本研究以承诺于本世纪中叶实现净零排放的英国为研究范例，量化了与气候及经济状况协同演化相关的关键不确定性，并探讨了不确定性情境下的现代决策方法如何为造林选择提供参考。分析结果显示，适配"高排放"未来情景的最优造林策略，与适配"接近历史路径"未来情景的最优策略存在显著差异。若按适配高排放情景的策略造林，却遭遇接近历史路径的未来气候，将给英国社会带来巨额净成本。将不确定性纳入决策框架，可遴选出能够分散风险、大幅降低高成本后果发生概率的造林策略。值得注意的是，本研究发现，通过调整造林策略来降低风险的空间相对有限。尽管仍存在此类持续性风险，但研究表明，与其他碳移除技术相比，造林仍属于极具成本效益的碳移除方案——即便假设其他技术均无风险。