Slow ocean response and the 1.5 and 2°C warming targets

Abstract

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.