Surface Temperature Changes Projected by FGOALS Models under Low Warming Scenarios in CMIP5 and CMIP6

Abstract

To meet the low warming targets proposed in the 2015 Paris Agreement, substantial reduction in carbon emissions is needed in the future. It is important to know how surface climates respond under low warming targets. The present study investigates the surface temperature changes under the low-forcing scenario of Representative Concentration Pathways (RCP2.6) and its updated version (Shared Socioeconomic Pathways, SSP1-2.6) by the Flexible Global Ocean-Atmosphere-Land System (FGOALS) models participating in phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively). In both scenarios, radiative forcing (RF) first increases to a peak of 3 W m⁻² around 2045 and then decreases to 2.6 W m⁻² by 2100. Global mean surface air temperature rises in all FGOALS models when RF increases (RF increasing stage) and declines or holds nearly constant when RF decreases (RF decreasing stage). The surface temperature change is distinct in its sign and magnitude between the RF increasing and decreasing stages over the land, Arctic, North Atlantic subpolar region, and Southern Ocean. Besides, the regional surface temperature change pattern displays pronounced model-to-model spread during both the RF increasing and decreasing stages, mainly due to large intermodel differences in climatological surface temperature, ice-albedo feedback, natural variability, and Atlantic Meridional Overturning Circulation change. The pattern of tropical precipitation change is generally anchored by the spatial variations of relative surface temperature change (deviations from the tropical mean value) in the FGOALS models. Moreover, the projected changes in the updated FGOALS models are closer to the multi-model ensemble mean results than their predecessors, suggesting that there are noticeable improvements in the future projections of FGOALS models from CMIP5 to CMIP6.

摘要

为了实现2015年巴黎协定提出的低增温目标,人类社会需要在未来大量地减少碳排放量,相应的未来情景称之为低增温情景。全球表面温度在低增温背景下如何演变是气候变化中的重要问题。本文研究了在CMIP5中低强迫情景(RCP2.6)和CMIP6中相应升级的低强迫情景(SSP1-2.6)下,4个FGOALS模式预估的未来全球表面温度的变化趋势及相应机制。在RCP2.6和SSP1-2.6两种情景下,外辐射强迫(RF)都是先不断增长,在2045年左右达到3W m-2的峰值,然后在2100年下降到2.6W m-2。在4个FGOALS模式中,全球平均表面气温都是在RF增长的时候(RF上升阶段)不断上升,而在RF下降的时候(RF下降阶段)保持稳定不变或者下降。在RF上升和下降两个不同阶段,表面温度的变化符号和变化强度在陆地、北冰洋、北大西洋副极地地区和南极都有非常显著的差异。此外,表面温度变化的区域分布特征在这两个阶段则都表现出显著的模式间差异性,这主要是因为不同模式在模拟气候态表面温度、冰-反照率反馈机制、自然变率和北大西洋经向翻转环流的变化方面存在非常大的区别。在FGOALS模式中,热带降水的空间分布特征大都受表面温度相对变化(即与热带平均增温强度的偏差)的空间结构所调控。FGOALS模式在低增温情景下得到的预估结果,在CMIP6中相对于在CMIP5中更接近于多模式集合平均的结果。这表明从CMIP5到CMIP6,FGOALS模式对低增温情景下的未来预估模拟有显著提高。