The five threats
Threat 3: Climate change
"The current rate of ocean acidification is about a hundred times faster than the most rapid events" in the geologic past —William Howard, Antarctic Climate & Ecosystems CRC
The past decade has seen an explosion of interest in the phenomenon of modern climate change. The scientific consensus is that anthropogenic (human-caused) CO2 emissions are steadily warming the planet, and that the climate will change significantly as greenhouse gasses build up in the atmosphere. Less attention, however, has been given to the other main regulators of global climate: our oceans. Despite their central and complex role in the climate system, the oceans remain on the periphery of climate research and policymaking.
The oceans absorb up to half of all human CO2 emissions.1 2 Their heat capacity is nearly 1000 times that of the atmosphere; by absorbing heat in the summer and releasing it in winter, they are instrumental in moderating our climate and ensure that changes happen gradually. Anthropogenic climate change is making the oceans warmer and more acidic, and if these processes are allowed to continue unchecked, they will drastically alter our climate and the marine ecosystem.
In 2009, scientists recorded the warmest ocean temperatures in July since record-keeping began nearly 130 years ago.3 The most drastic temperature spikes occurred in the Arctic, where water temperatures reached 12 degrees above average. Multiple scientific studies using independent data have confirmed that the surface layer of the global ocean has become warmer over the past century. This trend has important repercussions for humans and sea life alike.
The ocean-related threat most often associated with climate change is the rise of sea levels, which has been occurring at an accelerating rate over the past century. This is caused by two factors: thermal expansion of water, and the melting of inland glaciers and continental ice sheets. Thermal expansion is currently taking place in the surface layer of the global ocean, only a few hundred meters deep; as the heat trapped in the surface layer slowly diffuses down to deeper waters – a process which can take centuries – thermal expansion of the deep ocean will occur, resulting in further sea level rises. Recent studies conclude that a sea level rise of 0.5m-0.8m by 2100 is likely, and that a rise of up to 2 metres is possible. This puts the existence of island nations like the Maldives at risk: no spot on the island chain is more than two metres above sea level. It also means an increase in storm surges and flooding, both in their frequency and intensity.
In addition, extreme weather events will be more common and destructive as the oceans warm. Heat is energy, and having more of it in the oceans boosts the intensity of hurricanes and typhoons.4
Less widely-known are the threats to sea life posed by warming oceans. Fish and shellfish thrive in specific water temperatures, and there are thresholds above which their ability to reproduce and grow are impacted. Warmer, fresher surface water also leads to stratification, where valuable nutrients become trapped in the deep ocean.4 This prevents phyloplankton, which live near the surface, from transforming those nutrients into energy and making that energy available to higher levels of the food web. When the base of the food web is disrupted in this way, all other species are affected, including those at the top: whales, sharks, seals, tuna, and people. Phyloplankton also have a more direct effect on terrestrial life, as they account for roughly half of all plant photosynthetic activity, consequently providing a large chunk of our breathable oxygen.
Ocean warming is also forcing many species to migrate towards the poles in search of cooler waters, where there may not be sufficient or suitable prey to sustain them. Those species which live in closed basins, such as the Red Sea and the Mediterranean, may not be able to migrate, and instead will simply die off.
Case study: “Flip of a switch” climate change?
When we think of climate change, what usually comes to mind is a gradual increase in global temperatures, only perceptible on a scale of decades. Recently, however, evidence has been accumulating which shows that Earth’s climate has repeatedly experienced abrupt and dramatic shifts in the past – shifts that happen within a decade – and that this could happen again in the near future.
While world leaders plan for the effects of warmer temperatures, such an abrupt climate shift could actually bring about major cooling in certain geographic regions, even while the Earth, as a whole, gets warmer. Evidence in the fossil record, supported by computer modeling, shows that our planet’s climate has more than one “mode of operation”; when certain thresholds are crossed, the climate system can rapidly jump from one equilibrium to another.
Ocean currents are the prime suspect for the mechanism that governs these rapid shifts: they have been described as the “Achilles’ heel of our climate system.” A collection of currents that spans the globe known as the Ocean Conveyor is instrumental in reducing temperature differences between equatorial and polar regions by moving warm, tropical water to higher latitudes, where it is released into the atmosphere as heat. It’s because of this circulation that areas adjacent to the North Atlantic – North America and Northern Europe – are uncharacteristically warm for their latitudes. And it’s because of this that the same areas would experience severe cooling if the Ocean Conveyor were to shut down, which has happened many times before – most recently 8,200 years ago.
The global ocean absorbs between 25% and 50% of all human CO2 emissions.1 2 While this process slows down the greenhouse effect, it also lowers the pH of seawater, making it more acidic. The effects of ocean acidification are particularly devastating to calcifying organisms, such as corals, shellfish, and sea snails; when exposed to elevated CO2 levels, the shells of these animals begin to dissolve. Once a certain level of acidity is reached, calcium carbonate shells and skeletons cannot form. But recent studies show that ocean acidification also threatens key non-calcifying organisms, such as kelp forests, which are among the most productive and dynamic ecosystems on Earth. While the kelp itself is unaffected by elevated CO2, increased acidity, combined with rising temperatures, stimulates the growth of turf-forming algae. These algae can inhibit the growth of, and even replace, kelp forests.5 Whales and dolphins will also be affected, as acidification reduces the sound absorption of seawater, causing ambient noise levels to rise significantly. A louder ocean will interfere with marine mammal communication, and could also affect essential behaviours like feeding and reproduction.
“Even the most optimistic estimates of future CO2 levels could mean the end of coral reefs.Ocean acidification is an inevitable byproduct of increasing levels of CO2 in the atmosphere. Current CO2 levels are so high that even if emissions were drastically cut tomorrow, some more acidification would still occur. But drastic cuts tomorrow are unlikely; a doubling of CO2 levels in the atmosphere is possible within the next 50 years, which, if it happens, could bring about extinction events of a magnitude the Earth hasn’t seen for 65 million years. Even the most optimistic estimates of future CO2 concentrations could mean the end of coral reefs.6 7
While some ocean warming and acidification are inevitable at this point, action is necessary to minimize their adverse effects and stabilize atmospheric CO2 levels to prevent major changes to the marine ecosystem. This means:
- Significantly and rapidly cutting worldwide CO2 emissions. While recent history has demonstrated that this will not be easy to accomplish, it is the only real solution to the problem.
- Setting targets for atmospheric CO2 levels that take into account the effects of ocean acidification.
- Further researching the effects of ocean warming and acidification. The Save Our Seas Foundation is helping to understand the effects of rising acidity by funding the pioneering research of Dr. Jason Hall-Spencer, whose work around volcanic undersea vents reveals what is in store for the oceans if acidification continues unabated.
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3. See In hot water: World sets ocean temperature record. Associated Press: August 20, 2009.