Blue Carbon

Our planet is now about 1.2°C warmer, on average, compared with pre-industrial times some 140 years ago. The underlying cause of this warming trend is unequivocal; human activities have released vast amounts of greenhouse gases (mostly carbon dioxide) into the atmosphere, which then blanket the Earth and trap heat from the sun, once reflected from our planet’s surface. Perhaps fortuitously for humankind, however, the oceans cover about 70% of the Earth’s surface and water is extremely effective at absorbing heat and carbon dioxide. In fact, the oceans have soaked up about 90% of the excess heat and about one-quarter of the additional carbon dioxide in our atmosphere. Without this vital transfer of heat and gas into the oceans, the atmospheric climate we experience today would be unrecognisable.

Once carbon enters the oceans it may be assimilated by living organisms through photosynthesis before entering foodwebs or sediments and remaining within marine ecosystems for many years or even centuries. Whilst phytoplankton (single-celled algae) are by far the most abundant type of photosynthetic organisms in the sea, other marine ‘plants’ such as seagrasses, seaweeds, salt marshes and mangroves are extremely efficient at capturing and storing carbon. When the carbon captured by these organisms is stored in seafloor sediments it can be locked away and removed from the carbon cycle for long periods of time. This process is referred to as natural carbon sequestration, or ‘Blue Carbon’. For thousands of years, Blue Carbon habitats have been steadily locking away carbon into marine sediments, and continue to do so today.

The protective nature of the oceans has come at a cost, however. The global ocean is now significantly hotter and more acidic. Increased temperatures, in particular, pose a major threat to marine life and the integrity of entire ecosystems. On average, the global ocean has warmed at a rate of about 0.15°C per decade over the past half-century, although several hotspots of change – including waters off southeast Australia, eastern Canada, southern Brazil and Uruguay, and Arctic coastlines – have experienced much faster rates of warming. As well as longer-term decadal warming trends, the frequency and intensity of extreme temperature events, termed ‘marine heatwaves’, has increased significantly. Marine heatwaves are periods of days-to-months when sea temperatures are much greater than expected for that particular time or place, and are comparable to the atmospheric heatwaves that we experience on land.

Our scientific team at the Marine Biological Association of the UK, based in Plymouth, has been working on several projects that aim to better understand how marine ecosystems are impacted by climate change, in terms of their responses to both longer term warming trends and increased marine heatwave activity. In collaboration with an international network of scientists, we showed that marine heatwaves have intensified across most of the global ocean, having become both more frequent and longer in duration. We then showed that marine heatwaves have had catastrophic consequences on a range of species and ecosystems. For example, widespread bleaching and death of coral reefs, large-scale declines in seagrass meadows, losses of vast kelp forests, and mass die-offs of fish, shellfish, seabirds and even mammals have been linked to marine heatwaves in recent decades. Put simply, extreme sea temperatures during marine heatwaves are too hot for some marine life to cope with, leading to reduced growth and reproduction, poor condition or even death, with ramifications for the wider foodweb. In turn, this affects people that depend on coastal marine ecosystems for livelihoods and sustenance, through losses in commercial and small-scale fisheries, lower aquaculture yields, and reduced recreation and tourism activities.

To exacerbate this already alarming situation, marine ecosystems are engaged in a feedback loop between climate change and carbon sequestration. As the oceans warm, many key Blue Carbon players become less productive and capture and store less carbon. For example, when sea temperatures become stressful, seaweeds grow more slowly, reaching a smaller size and holding less carbon. Even worse, in Australia, the Mediterranean and elsewhere, vast areas of seagrass meadows have been lost due to ocean warming, releasing centuries-old carbon from underlying sediments. Blue Carbon habitats are also threatened by bottom-trawling the seafloor, changes in coastal land use and increased pollution in many regions. The net result is reduced efficiency of the biological carbon pump, leaving more carbon in the upper layers of the ocean and in the atmosphere, where it further contributes to global heating.

So what does the future hold? Over the next few decades at least, the Earth’s atmosphere and oceans will continue to warm in response to greenhouse gases already emitted. The oceans will become hotter and more acidic and marine heatwaves will intensify, with grave consequences for marine life and the human societies that depend on these marine ecosystems. There will, of course, be ‘winners and losers’ as some species will thrive in a warmer world, but our research has shown that the overall impacts of ocean warming will be negative. However, conservation and management efforts across the world are beginning to value and protect marine life, especially Blue Carbon habitats, as understanding grows of the importance of natural carbon sequestration for climate change mitigation. In many regions, Blue Carbon habitats are being restored towards their former glory, in attempts to promote carbon storage in the oceans. In other regions, seafloor habitats are protected from dredging and other destructive activities to protect the carbon-rich sediments. There is also ever-increasing talk of harnessing the incredible growth rates of seaweeds to capture and lock away carbon, although this remains in its infancy. It is clear that better management and protection of marine ecosystems will increase the efficiency of the biological carbon pump and buy some time.

The real challenge remains, however. Only through significant global reductions in greenhouse gas emissions can we avoid catastrophic climate change this century. As the dust settles on the COP26 meeting in Glasgow last November, the jury is still out on whether the agreements reached will lead to the avoidance of dangerous levels of warming. World leaders will meet again in 2022 at COP27 in Egypt; ambitious, binding, meaningful targets on greenhouse gas reductions are imperative. The oceans will help us enormously in our self-imposed battle with the climate, but only up to a point. That breaking point draws ever nearer.

By Dr Dan Smale, Marine Biological Association, Plymouth