Increasing confidence in projecting the Arctic ice-free year with emergent constraints
Increasing confidence in projecting the Arctic ice-free year with emergent constraints
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DOI：
10.1088/1748-9326/ac0b17
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发表时间：
2021-06
影响因子：
6.7
通讯作者：
Bin Wang;Xiao Zhou;Q. Ding;Jiping Liu
中科院分区：
文献类型：
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作者：
Bin Wang;Xiao Zhou;Q. Ding;Jiping Liu
北极无冰的夏季是全球变化的一个里程碑，对气候、环境和经济都有深远的影响。然而，耦合模型比较项目第6阶段模型预测的发生率仍然是出了名的不确定。寻找紧急约束以减少预测不确定性一直是首要挑战。为了建立约束条件的物理基础，我们首先通过数值实验证明，观测到的北极冰损失趋势主要是由北极近地表气温驱动的。因此，提出了两个约束条件：测量北极海冰对局部变暖响应的北极海冰敏感性，以及评估模式对人为强迫的响应程度和向北极地区分配热量的北极放大敏感性。这两个约束是互补的，并且几乎与场景无关。模型预测的第一个北极无冰年在很大程度上取决于模型的两个气候敏感性。因此，第一个北极无冰年可以通过两个北极敏感性测量的线性组合来预测。基于模型模拟的灵敏度技能，将20个CMIP模型分为两个数量相等的组。十个现实敏感性模型预测，在SSP2-4.5（中等排放）情景下，北极无冰的可能性为80%，比2019年水平或2040年之前全球变暖0.8°C。与10个低估敏感性模型的大价差相比，10个现实敏感性模型的价差减小了约70%。通过数值实验创建基于物理的紧急约束的策略可能有助于广泛应用于其他领域，以推进稳健的预测和理解不确定性来源。
An ice-free Arctic summer is a landmark of global change and has the far-reaching climate, environmental, and economic impacts. However, the Coupled Model Intercomparison Project Phase 6 models’ projected occurrence remains notoriously uncertain. Finding emergent constraints to reduce the projection uncertainties has been a foremost challenge. To establish a physical basis for the constraints, we first demonstrate, with numerical experiments, that the observed trend of Arctic ice loss is primarily driven by the Arctic near-surface air temperature. Thus, two constraints are proposed: the Arctic sea ice sensitivity that measures Arctic sea ice response to the local warming, and the Arctic amplification sensitivity that assesses how well the model responds to anthropogenic forcing and allocates heat to the Arctic region. The two constraints are complementary and nearly scenario-independent. The model-projected first Arctic ice-free year significantly depends on the model’s two climate sensitivities. Thus, the first Arctic ice-free year can be predicted by the linear combination of the two Arctic sensitivity measures. Based on model-simulated sensitivity skills, 20 CMIP models are divided into two equal number groups. The ten realistic-sensitivity models project, with a likelihood of 80%, the ice-free Arctic will occur by additional 0.8 °C global warming from 2019 level or before 2040 under the SSP2-4.5 (medium emission) scenario. The ten realistic-sensitivity models’ spread is reduced by about 70% compared to the ten underestimate-sensitivity models’ large spread. The strategy for creating physics-based emergent constraints through numerical experiments may be instrumental for broad application to other fields for advancing robust projection and understanding uncertainty sources.