Coastal ecosystems have some capacity to adapt to sea-level rise due to rising temperatures, but their future depends on limiting global warming to less than two degrees Celsius, as stipulated in the Paris Agreement.
A study based on models published by Nature warns that warming above 1.5 or 2 degrees above pre-industrial levels will threaten the survival of mangroves, salt marshes, coral reefs, grasslands, and kelp forests that millions of people depend on.
The team, coordinated by Macquarie University (Australia), assessed the vulnerability of coastal ecosystems, including 190 mangroves, 477 tidal marshes, and 872 coral reef islands, to increasing rates of predicted sea level rise in various global warming scenarios of 4 to 10 millimeters per year.
A 2-degree increase in warming could double the area of tidal marshes exposed to a 4mm annual sea level rise over the period 2080–2100.
It is estimated that with 3 degrees of warming, almost all of the world’s mangroves and coral reef islands and 40% of mapped salt marshes will experience sea level rise of more than 7 millimeters per year.
That pace is likely to be reached in most parts of the world by 2100 unless major efforts are made to reduce greenhouse gas emissions, the authors note.
At this rate, coral reef islands could become destabilized by increased coastal erosion and wave inundation, and mudflats and mangroves would drown.
Tulane University, also involved in the study, noted that higher sea level rise is already being observed along the US Gulf Coast.
Previous studies by the same center showed that current sea-level rise could “flood” the Louisiana swamps and possibly other areas of the Gulf Coast in about 50 years.
“Right now, we are on track for between 2.4 and 3.5 degrees of warming by the end of the century, so a change of course is urgently needed. And that would have to happen very quickly,” warned Torbjörn Törnqvist from Tulane University.
In the short term, “Coastal ecosystems can play a crucial role in helping us humans mitigate climate change by removing carbon dioxide from the atmosphere and providing shelter from ocean storms, but we must help them too,” said Simon Albert, University of Queensland (Australia).
Without countermeasures, current climate change projections indicate that relative sea-level rise will exceed the adaptive capacity of areas such as mangroves and salt marshes, leading to instability and profound changes in coastal ecosystems, the study warns.
Nature also publishes a study on the impact of marine heat waves on the biomass of demersal fish, species that live close to the sea floor, such as cod, monkfish, or hake.
Research coordinated by the University of California, Santa Cruz (USA) shows that marine heat waves have “a limited impact” on the biomass of these fish (the total weight of specimens on a given surface).
Although there have been “significant declines” in some species after brief heat events, these have been the exception rather than the rule, while overall fish biomass has only been “minimally affected”.
The results raise questions about the causes of this variability and highlight the need to understand why these phenomena appear to affect some species more than others in order to sustain these ecosystems as global temperatures continue to rise.
The team analyzed the effects of 248 ocean floor heat waves from 1993 to 2019 on marine fish in Northern Hemisphere shelf ecosystems in climate zones ranging from the subtropics to the Arctic.
A marine heatwave has been defined as a period of at least five days with seafloor temperature anomalies above the seasonally varying 95th percentile for that region.
