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

Jennifer K. McWhorter,Chris T. Perry,William J. Skirving,Paul R. Halloran,George Roff,Peter J. Mumby

doi:10.1111/gcb.15994

Abstract

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 Intergovernmental Panel on Climate Change 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 and 2°C global warming targets on coral thermal stress metrics for the Great Barrier Reef (GBR). 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 Degree Heating Weeks). Thermal stress under the more pessimistic pathways SSP3-7.0 and SSP5-8.5 is three to fourfold higher than the 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 GBR than the far north and northern GBR.

Highlights

Global average temperatures have increased by ~1°C since the 1880s, with the ocean surface warming by ~0.11°C per decade [CI 0.09 to 0.13]°C since the 1970s (IPCC, 2021; Stocker, 2014)
The magnitude of thermal stress upon Great Barrier Reef (GBR) corals intensifies dramatically over time, under scenarios which excludes strong international efforts to tackle climate change (SSP3-7.0) (Riahi et al, 2017) or assume an energy intensive fossil-based economy (SSP5-8.5) (Figure 1a) (O’Neill et al, 2016; Riahi et al, 2017). These scenarios lead to a 3- to 4-fold increase in the magnitude of thermal stress upon corals (Figure 1a) compared to the worst of recent bleaching events, which have already caused mass mortality on many GBR reefs (Bozec et al, In press; Hughes, Kerry, et al, 2018)
Note that while a Degree Heating Weeks (DHW) of 8 has been reached, and even exceeded, in some recent bleaching events, our analyses reveal the GBR-wide median DHW

Summary

Introduction

Global average temperatures have increased by ~1°C since the 1880s, with the ocean surface warming by ~0.11°C per decade [CI 0.09 to 0.13]°C since the 1970s (IPCC, 2021; Stocker, 2014). Periods of anomalously warm sea temperatures have increased in frequency (Eakin et al, 2010; Hughes et al, 2018b; Skirving et al, 2019) and severity, resulting in the deterioration of global coral ecosystems (Wilkinson & Souter, 2008). In the early 1980s, global severe coral bleaching was occurring once every 25-30 years, the frequency of severe bleaching has since increased to approximately once every 6 years in 2016 (Hughes, et al, 2018b)

Methods

Results

Conclusion