In hot water

For centuries, we thought that our vast ocean was limitless and immune to human impacts. The death of 100,000 marine mammals and 1 million seabirds per year, globally, has taught us otherwise. As per record, the decade leading to 2019 has been confirmed as the warmest decade in memory. Last year, ocean temperatures were 0.075 C above the 1981-2010 average. As the planet warms, ice sheets and glaciers melt and warming seawater expands, leading to increases in the volume of the world's oceans. As sea levels continue to rise throughout the century, chronic flooding will spread and more land will be permanently lost to the ocean.

Human activities are the major cause of endangering our most valuable ocean ecosystems. For supplementing this view, some examples are stated. The discarded plastics and other residential waste, discharge from industries and agricultural areas without adopting any effective cleaning practices eventually find their way into the sea with devastating consequences for marine life and their habitats. The amount of discarded plastics will outweigh the amount of fish in our oceans by 2050 as reported. About 25 major accidents and many smaller ones have caused spillage of more than 700 tons of crude oil into the sea. In addition to this, deliberate dumping from vessels (mainly during the illegal cleaning of tankers) on the high seas further aggravates this problem. Some 140,000 tons of ballast water carried by merchant vessels, which is often taken up in one port and released in another far-away from the point of origin, is ultimately dumped into the sea. Some of the species (including pathogens) in ballast water may be introduced in a new ecosystem leading to serious damage to the local flora and fauna.

Oxygen in the oceans is being lost at an unprecedented rate leading to proliferation of 'dead zones' on account of the climate emergency and intensive farming. Overfishing endangers ocean ecosystems and the billions of people who rely on seafood as a key source of protein. If fossil-fuel emissions continue to rise rapidly, for instance, the maximum amount of fish in the ocean that can be sustainably caught could decrease by as much as a quarter by century's end. In addition, rapid urbanisation along the world's coastlines has seen the growth of coastal 'megacities' like Mumbai. Many of these populations put pressure on infrastructure where urban waste and sewage management is poor.

Contrary to popular belief, sea plants produce 70 per cent of the oxygen (phytoplankton and marine plants exchange some 200,000 million tons of CO2/O2) whereas rainforests produce 28 per cent of the oxygen. Phytoplankton, the tiny little organism which spends its life being carried by oceanic currents, act in the same way as tree leaves do on land. Everyone loves the ocean but few realise the function of these tiny organisms as the Earth's lungs and their role in creating the ozone layer that protects us from harmful ultraviolet-B rays. Furthermore, evaporated seawater forms clouds which empty their contents over the earth to create rivers, lakes and other ecosystems as well as provide the levels of moisture necessary to sustain life.

The deep waters are home to wildlife and some of the biggest creatures on earth that provides us with food, jobs, life, entertainment, sailing and key services like climate regulation, through the energy budget, carbon cycle and nutrient cycle. For decades, the oceans have served as a crucial buffer against global warming, soaking up roughly a quarter of the carbon dioxide emitted from anthropogenic activities and absorbing more than 90 per cent of the excess heat trapped on Earth by carbon dioxide and other greenhouse gases. Without that protection, the land would be heating much more rapidly.

Despite its significant importance to humanity, the governments in many countries simply violate the norms of probity and overlook the acts of unethical practices that cause the destruction of the oceans' richest areas. Almost 80 per cent of the pollutants dumped into the sea come from inland operations, either through rivers, direct dumping and coastal drainage or transported through the atmosphere. The remainder comes from accidental or deliberate spills from vessels and marine facilities.

There are now close to 500 dead zones covering more than 245,000 km² globally. One such zone is present off the west coast of India, in the Arabian Sea. The bright green colour of the water in winter is evidence of the excessive delivery of nutrients from agriculture and urban centres that stimulates algal productivity and the subsequent microbial degradation of this organic matter reduces oxygen levels, contributing towards hypoxia. In reality, low oxygen waters are also related to acidified waters. So hypoxia and acidification are increasingly co-occurring in the ocean and have additive and synergistic negative effects on the growth, survival and metamorphosis of early life stage bivalves. Rising levels of atmospheric CO2 due to the burning of fossil fuels are exacerbating the ocean acidification. Since the industrial revolution, the average pH of the ocean has been found to have fallen from 8.2 to 8.1, which may seem small but corresponds to an increase in acidity of about 26 per cent. Many marine creatures have shells and skeletons of calcium carbonate (basically chalk), which can erode as pH falls. Ocean acidification prompts some plankton species to grow faster, while slowing growth for several others, disrupting the natural linkages and competition between species and putting entire ocean ecosystems at risk. As acidification worsens and the sea surface warms, the ocean becomes less effective in absorbing CO2. The fate and behaviour of contaminants in the environment, particularly their persistence, their ability to be taken up by organisms and how they behave once absorbed, is strongly driven by environmental factors such as salinity, pH and temperature and these are all subject to change under climate change scenarios. This means that organisms may be more or

less susceptible to pollutants; the degree of change will depend on the specific pollutants and the organism species involved.

As fish populations are already declining in many regions as warming waters throw marine ecosystems into disarray, fishery managers will need to crack down on unsustainable fishing practices to prevent seafood stocks from collapsing otherwise we face a food crisis. Nations could also expand protected areas of the ocean to help marine ecosystems stay resilient against shifting conditions. The crisis in our fisheries and in our oceans and climate are not mutually exclusive problems to be addressed separately.

Dr Debapriya Mukherjee is a former Senior Scientist, Central Pollution Control Board. Views expressed are strictly personal