Populations around the world are increasingly beginning to suffer the effects of climate change, driven by carbon emissions resulting from the burning of fossil fuels. Atmospheric CO2 levels are 50% higher than they were before the start of the industrial revolution. Carbon dioxide is a greenhouse gas that traps heat in the atmosphere, thereby warming the planet. More than 90% of this trapped heat is absorbed by the oceans, which has helped to slow the rate of planet warming. But this heat hasn't magically disappeared, it has simply been transferred to the oceans, which are now becoming significantly warmer and more acidic as a result of rising atmospheric CO2 levels.
Ocean Warming Hotspots
Ocean warming is not occurring at the same rate everywhere. It tends to be more marked in the warm currents that flow along the east coasts of continents from the tropics to the polar regions. In the Southern Hemisphere, the warm currents that flow along the east coasts of Australia, Brazil and South Africa are warming faster than the global average, particularly at the southern margins where they form hotspots of ocean warming.
New research suggests that strong easterly tradewinds in the mid-latitudes are shifting further south, pushing the warmer currents southwards. This is causing ocean waters to warm two to three times faster here compared to elsewhere as more and more heat is carried southward. This warmer water displaces cold water leading to rapid warming. The extra heat energy also causes swirling eddies of water of up to 200 kilometers wide to form, which transports the heat further south and helps the warm water mix with cooler water, facilitating warming in deep ocean waters.
These warm currents play a critical role in distributing nutrients across the ocean and moderating climates of the coastal regions they flow past. Any shift in their path could affect ocean productivity as well as local weather patterns, which could in turn impact the lives and livelihoods of communities living along the east coasts of Australia, Brazil and South Africa.
Ocean Acidification
In the northern hemisphere, things are slightly different. While oceans in the northern hemisphere are not warming as fast as in the southern hemisphere, they are more prone to ocean acidification.
Oceans become acidic when carbon dioxide is absorbed from the atmosphere. Rising atmospheric CO2 levels have caused oceans to become 30% more acidic on average, resulting in significant impacts on marine ecosystems. While ocean acidification in most oceans tends to be closely aligned with rising atmospheric CO2 levels, ocean acidification is occurring three to four times faster in the western Arctic Ocean than anywhere else.
The Arctic is warming faster than anywhere else on Earth. As a result, there is a rapid loss of sea ice covering the Arctic Ocean. A recent study found that melting sea ice is amplifying the uptake of atmospheric CO2 in the surface waters of the Arctic Ocean, speeding up the rate of acidification. According to the authors, melting sea ice exposes seawater to the atmosphere, facilitating a rapid uptake of atmospheric CO2 and causes a decline in pH and aragonite — a mineral that organisms such as corals and shellfish need to build their exoskeletons.
According to the authors, meltwater also dilutes compounds that act as a buffer to absorbed CO2 and limits the amount of mixing that occurs between surface waters and the deep ocean.
"When you have sea ice melt you create a floating freshwater lake on the top of the ocean surface," says Wei-Jun Cai, Chair of Earth, Ocean, and Environment at the University of Delaware, and lead author of the study, who expects the spike in acidification to continue for as long as there is sea ice continues to melt in summer.
Cai predicts that the change in seawater chemistry will have a huge impact on marine life and marine ecosystems. "In lower latitudes, you have coral reefs and if you add carbon dioxide to the water, the carbon saturation rate will increase and the coral won't grow. Acidification reduces the carbonate ions needed to build coral skeletons," Cai explains.
"We are far from knowing what the cost is for biological systems. We don't know what organisms could be affected. This is something the biological community needs to look into," Cai cautions.
"So many things can be impacted by this changing chemistry, from plankton to the whales that feed on them," warns Hongjie Wang, Assistant Professor of Oceanography (Marine and Atmospheric Chemistry) at the University of Rhode Island. "Arctic ecosystems are especially sensitive, though the effects of acidification versus other changes like warming aren't yet clear."
Sources
https://www.science.org/doi/10.1126/science.abo0383
