The importance of 1.5°C warming for the Great Barrier Reef

Tropical coral reefs are among the most sensitive ecosystems to climate change and will benefit from the more ambitious aims of the United Nations Framework Convention on Climate Change’s Paris Agreement, which proposed to limit global warming to 1.5° rather than 2°C above pre-industrial levels. Only in the latest IPCC focussed assessment, the Coupled Model Intercomparison Project phase 6 (CMIP6), have climate models been used to investigate the 1.5° warming scenario directly. Here, we combine the most recent model updates from CMIP6 with a semi-dynamic downscaling to evaluate the difference between the 1.5°C and 2°C global warming targets on coral thermal stress metrics for the Great Barrier Reef. By ~2080, severe bleaching events are expected to occur annually under intensifying emissions (Shared Socioeconomic Pathway SSP5-8.5). Adherence to 2° warming (SSP1-2.6) halves this frequency but the main benefit of confining warming to 1.5° (SSP1-1.9) is that bleaching events are reduced further to 3 events per decade. Attaining low emissions of 1.5° is also paramount to prevent the mean magnitude of thermal stress from stabilizing close to a critical thermal threshold (8 DHW). Thermal stress under the more pessimistic pathways SSP3-7.0 and SSP5-8.5 is 3- to 4-fold higher than present day, with grave implications for future reef ecosystem health. As global warming continues, our projections also indicate more regional warming in the central and southern Great Barrier Reef than the far north and northern Great Barrier Reef.