Global Warming & Rising Sea Temperature

In my last post, I examined the increased concentration of CO2 and other greenhouse gases in the atmosphere elevate the average and extreme temperatures.  In this post, I want to discuss the effects of global warming on our oceans. Seventy-one percent (71%) of our planet is covered by the ocean. In fact of all that extra heat that's being absorbed by the Earth system, 93% of it goes into the oceans. What happens in the oceans does not stay in the oceans.

This extra energy is warming the oceans, especially in the water closest to the surface. Globally, from 1971–2010, the upper 75 meters of ocean warmed an average of 0.11 degrees Celsius per decade. Scientists project that the oceans will continue to warm during the 21st century, with the strongest ocean warming forecasted for the surface in tropical and Northern Hemisphere subtropical regions.  In deeper waters, the warming will be most pronounced in the Southern Ocean.

Oceans circulate heat around the world through massive surface and deep-water currents – one of the largest of which is the Atlantic Meridional Overturning Circulation (AMOC) – which help regulate global climate and weather. Warmer, less dense water can slow down ocean heat circulation, as can salinity changes (a notable consequence of runoff from melting land-based freshwater ice).  According to the Intergovernmental Panel on Climate Change (IPCC), it is very likely (90–100% confidence) that the AMOC could weaken between 11 and 34 percent, depending on future emissions levels, over the 21st century.  This slowdown could mean cooling across the entire Northern Hemisphere while parts of the Southern Hemisphere become hotter. While cooler temperatures might sound good in the face of global warming, it could mean massive sea-level rise in eastern North America and shifting rainfall patterns that could dry up Europe’s rivers.

Warming oceans are devastating the world’s coral reefs. Coral reefs are some of the most biodiverse habitats on the planet, home to nearly one-quarter of all ocean species, yet occur in less than 1% of the world’s oceans. Prolonged high water temperatures (among other factors) can cause coral polyps to expel their symbiotic algae partners (zooxanthellae) that help them produce food. The result is coral bleaching, which puts the health of the whole reef system at risk. The world’s largest reef, the Great Barrier Reef off the Australian coast, frequently suffers from catastrophic bleaching events in response to regular heat stress. During the nine-month marine heatwave of 2016, an estimated 30% of the Great Barrier Reef’s corals died. Higher levels of CO2 in the atmosphere are making oceans increasingly acidic, disrupting natural processes and entire ecosystems. The ocean plays a critical role in the storage of carbon, as it holds about 50 times more carbon than the atmosphere. The ocean absorbs this carbon largely through a chemical reaction at its surface: CO2 combines with sea water to form carbonic acid, which then breaks down into bicarbonate ions and hydrogen ions. This increase in hydrogen ions results in increased ocean acidity. Since 1750, the global ocean has absorbed about 28 percent of the CO2 from burning fossil fuels. Ocean acidification makes it more difficult for creatures – like plankton, corals, and shellfish – to produce calcium carbonate, the main ingredient in their hard skeletons or shells. This can lead to broader changes in the overall structure of ocean and coastal ecosystems, which can affect fish populations and the people that depend on them. The degradation of coral reef ecosystems due to warming and acidification will significantly affect communities in tropical developing nations that heavily depend on them for both nutrition and livelihoods.

(Picture Credit: AP)

Warmer oceans can encourage dangerous algae overgrowths that produce toxins and lower oxygen levels in the water. Increased ocean temperature combined with excessive nutrients like phosphorous and nitrogen (often from agricultural fertilizer runoff) is conducive to rapid algae growth known as algal blooms. Some types of algal blooms can produce extremely dangerous toxins that can cause eye and lung irritation and worsen asthma. These blooms can even kill people and animals. The growth of the algae depletes oxygen content necessary for marine organisms to live, forcing species to flee or perish. From 1960–2010, the oceans lost an estimated 77 billion metric tons of oxygen, affecting an accumulated area approximately the size of the European Union. According to the National Oceanic and Atmospheric Administration (NOAA), 65% of studied estuaries and coastal water bodies in the contiguous US are moderately to severely degraded by excessive nutrients such as phosphorous from agricultural fertilizer runoff, treated sewage wastewater, and atmospheric fallout from burning fossil fuels.

Pollution has severe impacts on ocean ecosystems and communities that depend on them.  Plastic pollution in oceans has become an increasing concern, with 4.8–12.7 million metric tons entering the ocean in 2010 according to UN Environment. Marine mammals, seabirds, turtles, fish, and crustaceans can become entangled in debris or ingest plastic materials, causing them to drown, starve, or incur wounds. The plastic can also cover phototropic organisms, cutting off access to light and oxygen. Oil spills in the ocean interfere with the insulating ability of fur-bearing mammals and the water repellency of birds, exposing them to the elements and raising the risk of death by hypothermia. Oil can also poison birds and animals that accidentally ingest it while trying to clean themselves. Exposure to oil can cause reduced growth, enlarged livers, eroded fins, altered heart and respiration rates, and reduced egg and larval survival in fish. The economic impacts of oil spills include the cost of cleanup as well as losses from fisheries, aquaculture, tourism, recreation, and industries that depend on clean water such as nuclear power plants and desalination plants.  Poor and vulnerable communities are disproportionately affected by mismanaged plastic waste, oil spills, and other sources of harmful pollution. 

Beside less ocean species (a substantial population depends on seafood for subsistence) due to increasing sea temperature, the world’s weather patterns are significantly altered. In the my next post, I will explain further.