NOAA/WDS Paleoclimatology - Lea 2004 Eastern Tropical Pacific Foraminiferal Mg/Ca and SST Reconstruction

The key scientific uncertainty in the global warming debate is the equilibrium climate sensitivity. Coupled atmosphere-ocean general circulation models predict a wide range of equilibrium climate sensitivities, with a consequently large spread of societal implications. Comparison of models with instrumental data has not been able to reduce the uncertainty in climate sensitivity. An alternative way to gauge equilibrium climate sensitivity is to use paleoclimatic data. Two recent advances, the development and application of proxy recorders of tropical sea surface temperature (SST) and the synchronization of the deep-sea and Antarctic ice-core time scales, make it possible to directly relate past changes in tropical SST to atmospheric carbon dioxide (CO2) levels. The strong correspondence of a proxy SST record from the eastern equatorial Pacific and the Vostok CO2 record suggests that varying atmospheric carbon dioxide is the dominant control on tropical climate on orbital time scales. This effect is especially pronounced at the 100,000-yr cycle. Calibration of the CO2 influence via tropical SST variability indicates a tropical climate sensitivity of 4.4°-5.6°C (errors estimated at ą 1.0°C) for a doubling of atmospheric CO2 concentration. This result suggests that the equilibrium response of tropical climate to atmospheric CO2 changes is likely to be similar to the upper end of available global predictions from coupled models.

全球变暖议题中的核心科学不确定性，是平衡气候敏感度（equilibrium climate sensitivity）。大气-海洋耦合全球环流模式（coupled atmosphere-ocean general circulation models）所预测的平衡气候敏感度跨度极大，由此带来的社会影响差异也十分显著。将模式与器测数据进行对比，无法缩小气候敏感度的不确定性区间。另一种估算平衡气候敏感度的途径，是借助古气候数据。近年来两项关键进展为相关研究提供了可行条件：一是热带海表温度（sea surface temperature, SST）代用记录手段的开发与应用，二是深海与南极冰芯年代标尺的同步校准。这两项进展使得我们能够直接将过去的热带SST变化与大气二氧化碳（CO₂）浓度水平建立关联。东赤道太平洋的SST代用记录与沃斯托克（Vostok）冰芯的CO₂记录之间存在极强的对应关系，这表明在轨道时间尺度上，大气二氧化碳浓度变化是调控热带气候的主导因素，该效应在10万年周期中尤为显著。通过热带SST变率对二氧化碳的影响进行校准后，结果显示：当大气CO₂浓度翻倍时，热带气候敏感度为4.4℃~5.6℃（误差估算值为±1.0℃）。该结果表明，热带气候对大气CO₂浓度变化的平衡响应，大概率处于耦合模式所给出的全球预测结果的上限区间。