In hot water: how climate change is affecting the oceans

Perhaps as a result of the nation being gripped recently by the stunning images of Blue Planet II, more and more attention is rightly being afforded to the world’s oceans – and the environmental problems which afflict them. Given the vastness of the oceans, the challenges they face are numerous. Yet one threat, climate change, appears to be particularly acute.

Our species’ centuries-long reliance on fossil fuels to produce energy, as well as trends in animal agriculture and other polluting industries, had emitted greenhouses gases into the atmosphere and changed the planet’s climate. These greenhouse gasses, like carbon dioxide and methane, trap heat close to the Earth’s surface, which have been behind the steady rise in global temperatures.

The impact on marine ecosystems

A changing climate has exhibited itself in several forms across various habitats. One serious manifestation for oceans, however, has been the problem of acidification. This refers to the steady alteration of the chemical composition of seawater, triggered largely by more and more carbon dissolving into the oceans. Since the beginning of the industrial revolution, the pH balance of surface ocean waters has fallen by 0.1. Whilst this may seem insignificant, because the pH scale is logarithmic it actually represents a 30% increase in acidity.

One of the most widely understood impacts of ever more acidic oceans is the effect it can have on organisms known as ‘calcifiers’. These are animals such as crustaceans, molluscs, and corals which use calcium and carbonate ions to build shells and exoskeletons around themselves. But acidic oceans dissolve calcium carbonate, so when the pH level drops, the ability for calcifiers to maintain themselves – let alone grow and prosper – becomes all the more difficult. In addition to this, research has found that successful fertilisation rates for some calcifiers decrease in acidic waters.

Coral reefs are some of the most biodiverse ecosystems in the world. Yet warmer waters present an existential threat to them – with experts claiming that the Paris Agreement’s commitment to 1.5℃ being the only way to save coral reefs. When the waters in which corals are found get too warm, they expel the algae which give them their renowned vibrancy, and turn white. This ‘bleaching’ does not mean the coral is dead, but without the algae corals find it more challenging to survive, and consequently often do die off as a result. Moreover, scientists have observed numbers of certain organisms which predate on corals – such as crown-of-thorns starfish – unsustainably flourishing in warmer waters, which further adds to the struggle to survive for corals.

As the climate warms, and the ice caps melt, sea levels inevitably rise. Indeed, over the past century, it is thought that the global mean sea level has risen by between four and eight inches. Even more worryingly, the rate at which it is has been rising over the past 20 years is double that of the preceding 80.

The consequences of rising sea levels for marine wildlife are multifarious, and will almost certainly result in the destruction of habitats vital for semiaquatic animals. Sea turtles, for instance, which depend on beaches to lay their eggs are one particularly vulnerable species. But one report claims that as many as 233 already endangered species will become further threatened by rising sea levels.

A lesser appreciated consequence of increased ocean temperatures is the effect it has on ocean currents. These currents influence aquatic animals’ migratory patterns, which can disturb ecosystems, as well as dispersing the nutrients vital for life below the waves. Changes in them, therefore, may starve areas of biodiversity of the nourishment necessary to sustain life.

The human cost

Whilst the impact of climate change on the globe’s seas and oceans is tragic enough in its own right, it also poses significant costs which will be borne more directly by humankind, too.

Coral reefs, for instance, are valuable sources of medicinal learning – with drugs to treat cancer, arthritis, and asthma having already been developed from resources found in corals and surrounding ecosystems. Evidence has shown that coral reefs also act as barriers to ocean waves and storms, which provides protection to millions of people the world over. Of course, pristine coral reefs also attract tourists – and are estimated to be worth over £25 billion a year globally, with that money often going into some of the most economically challenged parts of the world. Climate change, and ocean acidification, therefore, imperils all of that. Indeed, it is thought that the Great Barrier Reef is now beyond repair because of this combination of threats.

Changing ocean temperatures which alter currents can also have negative ramifications for people. Ocean currents have two significant roles in the global ecosystem. It has already been considered how they serve as a nutrient dispersal mechanism, shifting food for aquatic species up from the depths and then around the oceans. Warming waters which can decelerate these currents, therefore, could spell bad news for fishing communities who find themselves with fewer fish to catch.

Ocean currents also have an influencing role in local climates. Whilst they do not effect global temperatures per se, currents do facilitate the movement of heat – such as from the warm equator to temperate Britain, East America, and Europe. This partly explains why certain regions on the same latitude experience different temperatures. Were this process to slow down or cease, such aforementioned locations could see their localised temperatures change. 

For many good reasons, some of the most populous cities can be found next to the sea – Shanghai, Miami, and Rio de Janeiro to name but a few. Yet, faced with rising sea levels, their coastal locations could be their very downfall. One estimate places the figure at risk from rising sea levels, and the increased flooding and intensified storms associated with global warming, just shy of two billion individuals.


Seas and oceans cover almost three-quarters of the Earth’s surface. Though often thought to be harsh and uncompromising environments, they contain fragile ecosystems, and have intimately felt the effect of anthropogenic climate change. The future consequences of this are hard to predict, however signs are already beginning to show the costs – for humans and wildlife – of climate change for our oceans.

Eamonn Ives is a researcher at Bright Blue