The oceans act as “the lungs of the earth” – with algae and cyanobacteria in seawater providing up to 80 percent of the atmospheric oxygen which we rely on to breathe. They are essential to life. However, climate change, in tandem with other human impacts, such as pollution and overfishing, threaten the very resource that life on Earth depends on. These threats will continue to intensify as the global population grows, placing an ever-increasing strain on the world’s marine ecosystems.
The oceans have a two-way relationship with the Earth – the oceans influence climate, weather and coastal landscapes, and the Earth’s climate directly alters the oceans physical and chemical conditions. Consequently, the increasing temperatures on Earth, owing to global warming over the past 50 years, have also resulted in warmer surface waters and greater heat-storage in the world’s oceans.
The oceans have a high latent heat capacity, which means they are very good at storing energy – so efficient, in fact, that they have absorbed an estimated 93 percent of the additional energy created from the greenhouse effect. This, combined with the slow mixing time of the world’s oceans, means that it can take up to a decade for changes in climate to alter ocean temperatures.
A temperature increase makes sea-levels to rise: it causes seawater to physically expand (known as thermosteric sea level rise) and melts ice and glaciers, which adds volume to the oceans (known as eustatic sea level rise). Since the mid-19th century, sea level has risen at a greater rate than the mean values from the last two millennia. As a result, glacial habitats are diminishing and coastal habitats are being flooded. Scientific estimates suggest that sea-level is rising at a rate of 3.5 millimetres per year, a trend that threatens coastal communities globally and could mean low-lying islands, such as the Maldives, are lost to the sea.
Melting ice and glaciers also transfers freshwater into the oceans, which changes the salinity (how much salt is in the water) of seawater. Over the last 50-odd years, changes in ocean salinity have corresponded with shifts in rainfall patterns and an acceleration in the evaporation and rainfall cycle. This has profound repercussions on crop production and food security.
Variations in rainfall patterns and freshwater input, along with elevated temperatures, also threaten to disrupt ocean currents. The oceans are in constant movement, resulting from surface wind-driven currents and deep water thermohaline currents (thermo meaning temperature; haline meaning salinity). Colder and more saline seawater sinks and is replaced by warmer surface waters – creating the Great Ocean Conveyor Belt. Disruption to the oceans’ currents from climate change has the potential to alter global weather patterns, as well as the migration and dispersal of many marine organisms. Already, changes in thermal stratification (heat-layering in the ocean) have been detected, resulting in reduced mixing of seawater in the deep ocean. This can decrease nutrient availability, limiting the fundamental building blocks needed by marine organisms to grow and sustain life.
As unpredictable as the weather
Changes in ocean currents and precipitation patterns also contribute to the frequency and intensity of extreme weather events (predicted to become more common). Large storms such as hurricanes and cyclones can cause significant habitat loss, with increased storm surges causing dramatic coastal erosion. The impact of large storms can be devastating on both the environment and local communities.
El Niño events are also becoming more common, warming the eastern and central Pacific above their normal seasonal averages. El Niño events alter global weather patterns, which affect extreme weather systems worldwide. The potential increase in severe El Niño events threatens ecosystems and could have large socio-economic consequences. For example, 2016 saw the third mass global coral bleaching event, resulting from warmer than normal seawaters which was attributed to it being an El Niño year. Bleaching is a stress response of corals, resulting in the loss of their microscopic algae that they depend on for energy production. Scientists believe that up to a third of the northern Great Barrier Reef was lost from El Niño in 2016. With predictions that El Niño events will become more frequent, the ability for the reef to recover is worrying, especially when other climate impacts, such as ocean acidification, are intensifying.
The Osteoporosis of the Sea
The oceans absorb atmospheric carbon dioxide, which initiates chemical reactions that reduce seawater pH and carbonate ions in seawater. pH is the scale used to measure how acidic something is. The scale ranges from 0-14, with the lower values measuring that which is more acidic. Seawater is slightly basic (higher than seven on the pH scale) meaning that the process of ocean acidification is a shift towards pH neutral (pH equals seven) rather than acidic conditions. The shift in chemistry also reduces the carbonate ions in seawater, which are the fundamental building blocks needed for marine organisms that have a calcium carbonate shell or skeleton. The greater acidity also increases the risk of dissolution, making ocean acidification “the osteoporosis of the sea” – compromising the structural integrity of organisms made of calcium carbonate.
The impact of climate change on the world’s oceans is extensive. Collectively, the effects threaten to disrupt the balance of the ocean’s ecosystems. Already, there are reports of the pole-ward migration of marine organisms to cooler waters where conditions are more optimal. Unfortunately, not all species are capable of re-locating; many species face increasing risk of extinction. As scientists continue to understand and uncover the planet’s responsive processes, we are seeing how intricately balanced life on Earth is. Optimistically, scientists do not believe the tipping-point of irreversible change has yet occurred. There is, however, an urgent need to implement policies and practices to reduce carbon dioxide emissions, to ensure the effects of climate change do not negatively change our oceans.