Global climate-driven sea surface temperature and chlorophyll dynamics

Herein we study long-term changes in global sea surface temperature (SST) and chlorophyll-a concentration (CHL) in order to evaluate possible effects of climate change on the global marine ecosystems. Our approach is to analyze multi-model ensemble-means from global numerical-simulations available through the Coupled Model Intercomparison Project Phase 6 (CMIP6). A 250-year span consisting of the 1850-2014 historical period and the 2015-2099 climate-change projection was considered, where the Shared Socioeconomic Pathways (SSPs) 2.45 and 5.85 were selected as the projected climate-change scenarios. In the historical period, global linear trends show an SST increasing at 0.0024 °C year-1 and a CHL decreasing at -2.35x10-5 mg m-3 year-1, but by the last years (1985-2014) these changes become more abrupt: SST rising at 0.0146 °C year-1 and CHL declining at -1.49x10-4 mg m-3 year-1. During the intense climate-change scenario (SSP-5.85), SST increases at 0.0341 °C year-1 and CHL decreases at -0.0002 mg m-3 year-1, but in the last years (2070-2099) the warming is stronger (0.045 °C year-1) and the CHL decline is weaker (-0.0001 mg m-3 year-1). Additionally, Empirical Orthogonal Function (EOF) and dual Self-Organizing Maps (SOM) analyses on the model-data ensembles show: 1) significant correlations between SST and CHL patterns and climate teleconnection indices, 2) contracting polar and high latitude seascapes, 3) rising SST range (both high and low temperatures), 4) declining CHL in warming tropical seascapes, and 5) global expansion of low CHL levels. On the other hand, recent (2022-2023) global observed-SST anomalies mirror end-of-century projections, with extreme anomalies in tropical and subtropical regions and significant changes in near-polar regions. Thus, our findings emphasize the need to curb fossil fuel emissions in order to avoid irreparable consequences for the marine environment.