The state of the oceans

Overfishing and climate change

Professor Callum Roberts surveys our changing oceans, and asks what, if anything, can be done to control overfishing and the effects of climate change

Those who have spent long months and years at sea know well the immensity of the ocean. Those of us less familiar with the sea at first hand have our own views of its size, based on school atlases or long-haul plane journeys. Seen from outer space, one half of the planet – the Pacific side – is almost entirely ocean. The scale is breathtaking, humbling.

But these views are two-dimensional and miss the greater truth of the ocean – it is volume that counts, not area. The average depth of the sea is 3800 m, which, as a few simple calculations reveal, means that it occupies more than 95 per cent of the habitable space on planet Earth.

Whether you think of the sea in two dimensions or three, its colossal size is undisputed, a perception that has led us to three assumptions: (1) we can take what we want from the ocean, (2) we can put into it what we don’t want, with no comeback, and (3) we cannot change the sea. For most of human history, these assumptions have not strayed far from the truth. But the last century has seen an unprecedented acceleration of human influence on the earth. In this brave new world, the only thing than can be taken for granted is change. In this article, I summarise some of the changes under way in the sea.

Hunting and fishing

Illegal drift nets being set from a Moroccan fishing vessel.

Illegal drift nets being set from a Moroccan fishing vessel. Photo: © Oceana / Carlos Minguell

Hunting and fishing are the longest-standing effects that people have had on the sea. When whaling spread in the seventeenth and eighteenth centuries, it became one of the world’s first international industries, second only to slavery. When engines were added to boats in the late nineteenth century, fishing spread further afield, went deeper, and became ever more efficient. Governments encouraged fishing in the aftermath of World War 2 with subsidies, greatly expanding fishing fleets, a trend that continues to this day in countries like China and Myanmar. Lately, however, fishing has met with diminishing returns. A recent study shows that global fish landings peaked in 1996 at over 130 million tonnes. Since then, landings have declined by 1 million tonnes per year.

Better management in places like the US and Europe have turned fortunes here, as testified by recent headlines announcing the recovery of North Sea cod. But across much of the developing world there is poor management and the picture is bleak. Rapid expansion of fish farming has so far sheltered us from the consequences of overfishing, increasing supplies faster than the rate of population growth. But farming methods must change if productivity is to be sustained.

Climate change

Paradoxically, people are good at change and very bad at it too. Over the long course of time, our adaptability and ingenuity have served us well, bringing us to the planetary dominance we enjoy today. But over shorter timescales we prefer what we know and avoid change wherever possible. That means that climate change – as a supremely disruptive influence over all our lives – was always going to be difficult to address. Much easier to deny or belittle problems than to deal with them, which accounts for the noisy clamour of head-in-the-sand deniers and contrarians. But deal with it we must, because while we vacillate, the problems get worse.


The oceans are a giant air-conditioner for the planet. Without them, global warming would long ago have become intolerable. Greenhouse gases, such as carbon dioxide and methane, act like the glass of a greenhouse, trapping some of the sun’s incoming heat. Every year, the sea absorbs heat equivalent to 17 times all the energy generated by people today. Since the beginning of the industrial revolution, the oceans have taken up 93 per cent of the extra heat trapped by greenhouse gases.

Over the last fifty years or so, all that extra heat has increased the temperature of the ocean surface by an average of 0.6 °C. It doesn’t sound much, but some places have heated up by more than this – for example, the North Sea has warmed by more than 1.0 °C. The rise is limited because water has a huge specific heat capacity, much greater than that of the atmosphere. If all this extra heat had stayed in the atmosphere, the air would have warmed more than 36 °C by now!


For those of us who live in cold countries, a little bit of global warming might not seem amiss. Indeed, the first scientist to understand global warming was Swedish, Svante Arrhenius. In his famous book, Worlds in the Making, published in 1906, he said that with carbon dioxide emissions ‘we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind.’ If only it were that simple. In this world, changing one thing, like temperature, has far-reaching and sometimes unexpected consequences.

One simple physical property of increasing the temperature of water is that it can hold less dissolved oxygen. This has a surprising impact on marine life. With less oxygen in the water, fish will grow more slowly and reach smaller sizes than before. Smaller and slower-growing fish will produce fewer offspring and will support less productive fisheries. The tropics will experience the biggest losses, hitting developing countries hardest.


Life in the sea will take a second blow from warming. Over much of the tropics and warm temperate seas the ocean has two layers. Warmed by the sun, a layer at the surface floats on top of a cooler, denser layer beneath, with a sharp temperature gradient between them called the thermocline. The warm layer has most of the sunshine needed for photosynthesis but, away from coasts, is starved of the nutrients needed for plant growth. This is because nutrients sink from the surface in the form of dead bodies and faeces, passing below the thermocline. By contrast, the deep sea has lots of nutrients, but little or no sunlight, so plants cannot grow there.

For the sea to be productive, there must be upward mixing of nutrients, breaking through the thermocline. Storms and upwelling currents generate this mixing, but as the world heats up, the warm surface layer will get thicker and more stable. In a warmer world, fewer nutrients will be delivered to the surface and plant production will decline. Already, low-productivity deserts in the sea are growing. As almost all marine life depends ultimately on plants, the adverse effects will be inescapable.

Sea-level rise

A dwelling affected by coastal erosion on South Tarawa, Kiribati, Micronesia

A dwelling affected by coastal erosion on South Tarawa, Kiribati, Micronesia. The highest land on the atoll is barely 3 metres above sea level. Photo: Govt of Kiribati / CCA3.0U

Warm water takes up slightly more space than cold, so global warming is pushing up sea levels. Sea levels have increased by more than 20 centimetres since the beginning of the twentieth century, about a third of it from thermal expansion, the rest from ice melt. The rate of sea-level rise has doubled in the last 25 years and will more than double again in the next 25. Estimates of the rise by the end of the century keep going up. In 2007, the Intergovernmental Panel on Climate Change (IPCC) predicted a rise of up to 59 centimetres by 2100. Their most recent report in 2014 predicts a rise of up to 82 centimetres, and there are credible estimates that take rises above one metre.

Six hundred million people already live in low-lying coastal regions, vulnerable to sea-level rise and storm surges. Fertile deltas include some of the most productive agricultural lands on the planet and could be especially hard hit. The present flows of refugees seem large, but they are a trickle compared to what is to come.

What can be done?

The only good solution to the impacts of climate change is to urgently reduce greenhouse gas emissions. Even if the Paris Agreement is implemented to the letter, there will still be enormous disruption.

All the adverse effects I describe will grow for decades more before they peak and begin to fall. Most projections see sea levels continuing to rise for more than a century. So we must concentrate as well on increasing the resilience of marine ecosystems and coastal communities to the effects of climate change.

Falling ocean productivity could be offset by reducing fishing pressure and creating more marine protected areas. Greater protection will help ocean life to cope better with rapid environmental change. Sea-level rises can be ameliorated by protecting and replanting coastal wetlands such as mangrove forests and salt marshes, which trap sediments and grow upwards with rising seas. Coastal nations of the world have committed to setting aside 10 per cent of the oceans as marine protected areas by 2020. Recent announcements of enormous new protected areas suggest we might achieve this. But scientific research indicates we need to go further, with a new target of 30 per cent by 2030, including waters on the high seas where sovereignty is shared by all nations.

It is too late to avoid change, but we still have scope to steer the planetary ship to less inclement waters.

Callum Roberts is Professor of Marine Conservation at the University of York, and winner of the 2013 Mountbatten Literary Award for his book Ocean of Life: How our Seas are Changing (Penguin Books).