Sea Level Rise: Climate Change Inescapable and Accelerating
Sea level rise, due to climate change, is inexorable and accelerating, but regional impacts are sometimes difficult to separate from seasonal trends. A new report has estimated the impacts of rising seas on coral reefs, tidal marshes and mangroves around the world – ecosystems on which millions of people depend for their livelihoods and safety. Dr Hirotada Moki and colleagues of the Coastal and Estuarine Environment Group at Japan’s National Institute of Maritime, Port and Aviation Technology (NIMPAT) estimated the marine losses by the year 2100. in a paper just published in the open-access journal PLOS Climate. Cosmos spoke to Moki via video conference. Moki and his team forecast the catastrophic loss of up to three-quarters of our global coral habitat, including the Great Barrier Reef. And blue carbon sinks such as tidal marshes and mangroves could shrink by about 92% and 74% percent respectively. What do we lose? Coastal habitats are critical to carbon storage and climate mitigation. The International Union for the Conservation of Nature says 83% of global carbon circulates through the oceans with about half of that sequestered in coastal habitats, a mere 2% of the total ocean area. These and seagrass meadows are important ‘blue carbon’ stores – carbon locked up in coastal and marine habitats. Most carbon (green carbon) stored in a forest tree is lost to the atmosphere when that tree dies, but in coastal ecosystems, dead plant material, and terrestrial detritus, often get buried – potentially storing the carbon for thousands of years. Mangroves, for example, may store 3-5 times as much carbon as other tropical forests. Because these ecosystems are important carbon sinks, their loss is an important driver of climate change, says the Massachusetts Institute of Technology (MIT). Coral reefs are not blue carbon stores, as carbon dioxide is released in the making of their limestone (calcium carbonate) skeletons, but “they contribute an astonishing $34 billion annually to the global economy, despite only covering 0.2% of the seafloor”, says Dr Manuel Gonzalez-Rivero from the Australian Institute of Marine Science. United Nations Climate Action says coral reefs support more than 25% of the globe’s marine biodiversity and provide about a billion people with coastal protection, fisheries, sources of medicine, recreation and tourism. “Losing reef habitats can mean entire countries become uninhabitable due to saltwater intrusion, flooding, and lack of food resources,” says Gonzalez-Rivero, “in addition to loss of non-quantifiable values including culture and identity.” Unfortunately, conservation of such ecosystems often clashes with the priorities of 40% of the global population (87% in Australia) living within 50km of the coast. Moki and his NIMPAT researchers used topographic data and an MIT climate model to predict how such shallow water ecosystems might look out to 2100, using data including projected sea level rises, water temperatures, and photosynthesis and sedimentation rates. Climate change effects on tidal marsh, mangrove, seagrass meadow, macroalgal bed, and coral reef areas were estimated, using two hypothetical scenarios, called “representative concentration pathways” (RCPs), for the present day, the 2050s and the 2090s.
The two RCPs represented the best and the worst possible outcomes. The best, RCP2.6, includes all the sensible strategies that the scientific community have been recommending for years – major efforts to curb emissions, 100% renewables, carbon capture, use of electric vehicles, bicycles and public transport. For this scenario, the projection is 1oC global temperature rise by 2100 over pre-industrial levels (we are approaching 1.5oC), and small increases in sea level (0.4m) and extreme weather.
The worst outcome —RRCP8.5 – predicts increases in sea level by 0.6m and in global temperature by about 3.7oC, almost double the critical threshold, 2oC, at which NASA says, “dangerous and cascading effects of human-generated climate change will occur.” The authors say that global coral habitat could shrink by up to 74% by 2100 under RCP8.5 and 25% under RCP2.6. But Moki says it could be worse as the model did not include data on potential effects of ocean acidification on marine organisms, including corals. “Models, are imperfect by default”, says Gonzalez-Rivero. “Ocean acidification is already affecting the capacity of coral reefs to recover and adapt to other impacts from climate change, such as rising sea levels, therefore exacerbating the effects of ocean warming.” Under RCP8.5, tidal marshes and mangroves were also predicted to shrink by 91.9% and 74.3% respectively by 2100, assuming coastal development and land use prevented their migration inland with sea level rise. Shallow water ecosystem (SWE) survival, as sea levels rise, is a matter of appropriate coastal management, Moki concluded, which should include prevention of unnecessary development and a compromise between man-made infrastructure and SWE conservation to mitigate coastal erosion. The end of the century may be 63 years away but sea-level rise impacts are very real, right now, in many parts of our region. The US National Oceanic and Atmospheric Administration says the global average sea level has risen by 21-24 cm since 1880, largely due to a combination of increased meltwater from glaciers and ice sheets and thermal expansion of water as it warms. Half of this rise has happened since 1970, says CSIRO, and the rate is accelerating – the 2022 global average sea level was at 10.1 cm above 1993 levels, the highest since satellite records began. Sea level rise on the east coast of Australia is close to the global average, at 2-4mm/year, but that in the north of the country is much higher, at 4-6mm/year. “Local communities and First Nation peoples will likely see the impacts of coral reef degradation much earlier [than 2100] due to the reliance on the ecological services coral reefs provide,” says Gonzalez-Rivero. For Tuvalu, rising seas are an existential threat to this nation of 11,900 people, on 9 reef islands and atolls, midway between Australia and Hawaii. Perhaps fated to be the first nation to witness their home islands disappear beneath the waves due to climate change, Tuvaluans are currently battling coastal erosion, saltwater intrusion and increasing disease outbreaks caused by sea level rise.
Australia’s critical northern coastal ecosystems are also under real threat, says Dr Norman Duke of TropWATER at James Cook University in Cairns, with “upper tidal wetlands (mangroves and saltmarshes) already responding to sea level rise”. “So, when I think of future changes, I think of these in terms of the even greater acceleration of already changing shoreline ecosystems,” Duke adds. Northern Australian mangroves inhabit a semi-arid environment and are sensitive to changes in sea levels during ENSO events. Reduced sea levels brought about by El Niño events and made more extreme by climate change effects have already killed mangroves, says Duke. Localised sea level reductions – effectively extreme droughts – in tidal ecosystems caused catastrophic mass dieback of mangroves in 1982 and 2015, across swathes of Northern Australia, he adds. Damage caused to mangroves by severe tropical cyclones has also increased tenfold in the last decade.
