Climate Change Could Push The Great Barrier Reef To Near Total Collapse

By Pooja Toshniwal PahariaReviewed by Lauren HardakerNov 13 2025

Intensifying marine heatwaves are leaving reefs too little time to recover. A new eco-evolutionary modelling study across 3,800 reefs reveals that coral survival hinges on keeping global warming below 2 °C.

Image credit: Cynthia A Jackson/Shutterstock.com

A recent study published in Nature Communications warns that the survival window for coral reefs is rapidly closing as global temperatures rise. Using an advanced ecosystem model of Australia’s Great Barrier Reef (GBR), researchers found that unless global warming is limited to below 2.0 °C over the course of this century, widespread coral loss is unavoidable.

While adaptation and targeted management in thermal refugia can provide some resilience, these measures are effective only under low-warming scenarios. They cannot fully counteract escalating heat stress, highlighting the urgent need for global climate action to preserve coral ecosystems.

How Heatwaves Threaten Reefs

Human-driven greenhouse gas (GHG) emissions have already warmed the planet by about 1 °C, triggering increasingly frequent and intense marine heatwaves that devastate coral reefs through mass bleaching and mortality. With less time to recover between bleaching events, coral communities are increasingly vulnerable, even if global emissions are sharply reduced.

While global projections help assess temperature thresholds, they often overlook the evolutionary and demographic dynamics that shape coral adaptive capacity. Understanding the interactions between adaptive evolution, larval dispersal, and climate refuges under escalating heat stress is crucial for predicting and managing the future of reef habitats.

Modeling reefs

In the present study, researchers modeled the future of Australia’s GBR by simulating coral eco-evolutionary dynamics across 3,806 reefs under multiple climate scenarios.

Using ReefMod-GBR, a spatially explicit, individual-based model, researchers projected coral persistence through the 21st century under five GHG emission pathways from the CMIP6 framework [Shared Socioeconomic Pathway (SSP)1-1.9 to SSP5-8.5]. The model tracks millions of individual coral colonies, representing six taxonomic groups, and simulates recruitment, growth, reproduction, and mortality at six-month intervals. It integrates key ecological processes such as larval dispersal, connectivity, and habitat structure derived from high-resolution spatial mapping.

The team modeled environmental stressors, including marine heatwaves, tropical cyclones, and outbreaks of the crown-of-thorns starfish (CoTS). They linked heat-induced bleaching mortality to degree heating weeks (DHW), with species-specific thresholds calibrated using data from the 2016 mass bleaching event. They simulated the impacts of cyclones and CoTS through intensity- and age-structured models, with suspended sediments affecting coral recovery and recruitment.

The researchers modeled thermal adaptation as an evolving quantitative trait (heat tolerance) shaped by natural selection and repeated bleaching events. They first validated the model against observed coral cover and satellite-derived heat-stress data from 2008 to 2023, then projected outcomes for 2024 to 2100.

To capture environmental variability, they downscaled daily sea surface temperatures from 10 CMIP6 models to 10-km resolution and generated 20 stochastic warming sequences per scenario, producing over half a million reef trajectories. Lastly, sensitivity analyses tested 13 adaptive response parameters, each varied by 20%, to identify the most influential drivers of coral survival under future climate conditions.

How Reefs are Responding to Climate Change

Model simulations predicted a pronounced and accelerating decline in coral cover across Australia’s GBR under all GHG emission trajectories. Starting from an estimated 27 % coral cover in 2008, projections indicate a decrease to 17 % by 2040, a 56 % reduction, and further to 16 % by 2050. By 2100, under the most probable intermediate-emission pathway (SSP2-4.5), mean coral cover would fall to 8.0 %, with more than 60 % of reefs retaining below 5.0 % coral cover. Under high-emission scenarios (SSP3-7.0, SSP5-8.5), the model predicted near-total reef collapse between 2080 and 2100.

Marine heatwaves are projected to intensify sharply. Under ~2.7 °C of warming, half of the GBR would face heat stress above 8 °C-weeks - enough to trigger mass bleaching - at least five times per decade. Under higher-warming scenarios, extreme heat stress (>16 °C-weeks) would occur almost every year by 2100.

Coral thermal tolerance increases modestly (1.10 to 1.40 °C-weeks per decade) during the early 21st century before slowing as the model’s upper limits on heat tolerance are approached, but plateaus as adaptation reaches physiological limits. Recovery is possible only if global warming remains below 2.0 °C (SSP1-1.9 or SSP1-2.6), allowing sea surface temperatures to stabilize and reproductive ability to be maintained.

The recovery potential appears to be concentrated in temperature refuges, particularly in the cooler southern (Pompey and Swain reef complexes) and far-northern regions, where localized upwelling supports gradual adaptation and a greater diversity of thermal phenotypes. However, these refugia would diminish as warming intensifies. Larval connectivity further enhances viability: reefs functioning as larval “hubs” maintain higher coral cover through demographic rescue, with no evidence that dispersal of cooler-adapted corals hinders local thermal adaptation (i.e., no evidence of gene swamping).

Action is Critical

The study highlights that the survival of coral reefs critically depends on limiting global warming to below 2.0 °C, with warming trajectories needing to flatten before mid-century, while reinforcing local adaptive capacity and conservation measures. Without rapid emission reductions, widespread degradation and near-total coral loss are likely by the end of the century, threatening biodiversity, fisheries, and coastal protection.

However, protecting climate refuges, enhancing larval connectivity, and mitigating local stressors such as nutrient pollution, overfishing, and starfish outbreaks could help sustain ecosystem function and genetic diversity. A modest recovery is possible even as temperatures continue to rise, provided that warming rates remain slow enough for coral adaptation to keep pace.

Integrating global climate action with targeted reef management offers the most effective path to preserve coral viability and ensure the long-term stability of reef habitats in an increasingly warming ocean.

Journal Reference

Bozec, Y. et al. (2025). A rapidly closing window for coral persistence under global warming. Nature Communications, 16(1):1-15. DOI: 10.1038/s41467-025-65015-4. https://www.nature.com/articles/s41467-025-65015-4