最新研究表明,为了实现《巴黎协定》制定的1.5或2℃低温升目标,温室气体浓度需要在增长到某个峰值后逐渐下降.利用第五次国际间耦合模式比较计划(CMIP5)中气候模式和理想的一维两盒模型的模拟结果,研究了全球变暖下海洋混合层的快速响应和深层海洋的缓慢响应及其对低温升目标的影响.多模式平均结果显示,在辐射强迫先增长后稳定情景(RCP4.5)下,全球表面平均温度(global mean surface temperature, GMST)会以先快后慢两种速率增长;而在辐射强迫先增长后减弱情景(RCP2.6)下, GMST会先快速增长,然后缓慢下降,且在2050~2100年间基本保持不变.这是由于不同情景下, GMST的变化特征由海洋快、慢响应在各个阶段的贡献比例所决定. RCP2.6情景下, GMST在2100年的温升值为1.83℃,对应辐射强迫下降阶段;而在RCP4.5情景下, GMST同样达到该温升值的时间为2033年,对应辐射强迫增长阶段.虽然两个时刻的GMST温升相同,气候系统在两种情景下的响应却有很大区别.其中,由热膨胀导致的全球海平面平均升高幅度在RCP2.6中要远高于RCP4.5,表面增温的空间结构也存在重要差异.在CMIP5使用的大多数未来情景中,多模式平均预估的1.5和2℃温升目标到达时间都远远早于2100年,这意味着如果利用这些情景下的结果来类比21世纪末低温升目标下的情况,会严重低估海洋慢响应过程的气候效应.
英文摘要:
Recent studies reveal that to achieve the 1.5 or 2°C warming target proposed in the 2015 Paris Agreement, greenhouse gas (GHGs) concentration is required to decrease after reaching a peak around the middle of this century, distinct from most scenarios without configuring GHGs concentration decrease. Climate response to external radiative forcing under two different scenarios is then investigated in this study by simulations from 8 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Based on an idealized two-box (ocean mixed layer and the deeper layer) model, global-mean surface temperature (GMST) change is further divided into components due to the fast response of the ocean mixed-layer (fast contribution) and the deeper ocean slow evolution (slow contribution), respectively, to explain the GMST trajectory in CMIP5 simulations. The response timescale is 3-5 a for the fast contribution and decades to centuries for the slow contribution. Under the scenario with radiative forcing (RF) firstly increases and then levels off towards a constant after 2070 (RCP4.5), GMST rises rapidly at the first stage and slowly after RF levels off. In comparison, under the scenario with RF firstly increases and then decreases after 2045 (RCP2.6), GMST rises rapidly to a peak below 2°C and then declines at a very slow rate, following a nearly flat trajectory between 2050 and 2100. Corresponding to the GMST change, the deeper ocean warms faster than the upper ocean after the RF levels off in RCP4.5 while decreasing in RCP2.6. In fact, the GMST trajectory depends on the ratio between the fast and slow contributions from the ocean at different stages under different RF pathways. During the RF increase period, GMST mainly follows the RF pathway because the fast contribution dominates. After RF holds constant or decreases, slow contribution due to the deeper ocean warming increases persistently, causing the GMST change to deviate from the RF pathway when RF holds constant or decreases. As a result, contribution from the deeper ocean slow warming to the magnitude and trajectory of GMST change cannot be neglected in scenarios for the 1.5 and 2°C low warming target. For RCP2.6, GMST increase is 1.83°C in 2100, the time when RF is in decrease. In contrast, for RCP4.5, GMST reaches the same increase in 2033, the time when RF is still increasing. Despite the same GMST increase, climate responses in 2033 under RCP4.5 are distinct from that in 2100 under RCP2.6. For example, ocean stratification weakens in 50-300 m in 2100 under RCP2.6 but strengthens at all depths in 2033 under RCP4.5 due to the persistent heat accumulation in the deeper ocean. Global-mean thermosteric sea-level rise due to thermal expansion of seawater warming is much higher in RCP2.6 (16.39 cm) than that in RCP4.5 (9.01 cm). Moreover, surface warming pattern displays substantial structural differences between RCP2.6 and RCP4.5, especially over regions with strong ocean dynamics and hence large deeper ocean feedback to surface warming. Specifically, surface warming is notably larger over the North Pacific Ocean and Southern Ocean in 2100 under RCP2.6 than that in 2033 under RCP4.5, while the opposite is true over the subtropical and mid-latitude land regions in the Northern Hemisphere. Under most CMIP5 scenarios without a RF decrease, the 1.5 or 2°C GMST warming target is projected to reach much earlier than 2100, which are used in some studies to represent the climate responses for the Paris targets at the end of this century. These results underestimate the effect of the deeper ocean slow contribution and are biased in projecting climate response for the Paris targets.
1.中国科学院南海海洋研究所 2.Scripps Institution of Oceanography, University of San Diego 3.中国海洋大学海洋与大气学院, 热带海洋环境国家重点实验室 4.教育部物理海洋重点实验室, 广州 5.La Jolla 6.青岛, 7.USA 8.510301 9.92093-0206 10.266100 11.Scripps Institution of Oceanography, University of San Diego 12.中国海洋大学海洋与大气学院, 13.教育部物理海洋重点实验室, La Jolla 14.青岛, USA 15.92093-0206 16.266100 17.中国海洋大学海洋与大气学院, 教育部物理海洋重点实验室, 青岛, 山东 266100, 中国 18.中国科学院大气物理研究所, 大气科学和地球流体力学数值模拟国家重点实验室, 北京 100029, 中国 19.中国科学院南海海洋研究所, 热带海洋环境国家重点实验室, 广州, 广东 510301, 中国