GREENHOUSE GASES: THE EFFECTS ON CHEMICAL AND PHYSICAL PROCESSES OF THE OCEANS
The oceans have absorbed approximately 93% of the excess heat caused by global warming. Warming increases stratification, limiting the circulation of nutrients from deep waters to the surface. There is evidence that enhanced stratification and increasing temperature are causing a decline in dissolved oxygen concentration and expanding existing oxygen minimum zones (OMZs). Approximately 26% of anthropogenic CO2 is absorbed by the oceans, resulting in a reduction in pH and carbonate ion concentration, termed ocean acidification. Anthropogenic CO2 has caused global ocean pH to decrease by 0.1 units since the start of the Industrial Revolution.
CHANGING OCEANS: THE EFFECTS ON BIOLOGICAL PROCESSES
The ocean ecosystems are responding to the changing environment, but at different rates and magnitudes and with interspecific and geographic variation in responses. Warming causes shifts in species’ geographic distribution, abundance, migration patterns and phenology. Organisms that produce shells and skeletons from calcium carbonate are at most risk from ocean acidification as it lowers the saturation state of the mineral, favouring a dissolution reaction. To date, there are few observations of ocean acidification effects in natural communities; however, experimental evidence suggests that the risk to ecosystems will increase over the coming decades. Decreasing dissolved oxygen concentrations and expanding OMZs will favour anaerobic metabolisers such as bacteria and small microbes whilst reducing habitat for larger, oxygen dependant organisms.
WHAT DOES THE FUTURE HOLD FOR OCEAN ECOSYSTEMS?
The interaction of multiple drivers can amplify or alleviate each other’s effects. It is likely that marine organisms will experience a combination of warming, acidification and declining oxygen concentrations as well as regionally specific local stressors. This makes it difficult to predict the responses of individual species to multiple drivers, and species interactions make ecosystem- based projections challenging. Using the available evidence, projections have been constructed of the potential impacts on ocean ecosystems by 2100, under two the Representative Concentration Pathways RCP4.5 and 8.5.