FEATURE
Blue Carbon What is it and what is its role in mitigating climate change? 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 onequarter 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 6
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 halfcentury, 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
