Good article. Thanks for sharing.
Regardless of what you call it (global warming, climate change, etc.) increasing CO2 concentrations in the atmosphere leads to increasing air temperatures and consequently, warming of the oceans. Increasing CO2 is not only warming the ocean, but is also causing increased ocean acidification. Both of which have detrimental effects on hard corals.
Global temperature has risen dramatically over the last century (this takes into account seasonal and yearly variation).
Since this has already occurred some call it global warming. Climate change is also term that is used when looking at changes in climate that are linked to global temperature increases. I know the vernacular creates negative reactions in some people. But the fact is, global temperatures have and are still increasing. And it is not good for our ocean or our corals, since it is causing lots of issues.
In the atmosphere, CO2 is a greenhouse gases that absorb a wide range of energy, including infrared energy (heat) emitted by the Earth, and then re-emit it. The re-emitted energy travels out in all directions, but some returns to Earth, where it heats the surface. Thus, CO2 thru the greenhouse effect causes increasing air temperatures.
In the ocean, the sea surface CO2 partial pressure is always very close to the CO2 partial pressure in the atmosphere above it. The sea surface is always in equilibrium with the atmosphere. As CO2 has increased some is taken up by the land biosphere and the remainder CO2 is dissolved and added to the CO2 reservoir in the surface waters. The mixed layer in the ocean is the top 20 to 200 meters, depending on the amount of wave and current mixing. It contains roughly the same amount of CO2 as the atmosphere, as dissolved inorganic carbon (DIC). The difference, whether the ocean is emitting CO2 or absorbing it, is driven by temperature.
In addition, dissolving CO2 in the ocean creates carbonic acid, which increases the acidity of the water (ocean acidification). Or rather, a slightly alkaline ocean becomes a little less alkaline. Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate structures. With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile. Second, the more acidic water is, the better it dissolves calcium carbonate. In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean’s capacity to absorb carbon dioxide. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.
Both rising ocean temperatures and ocean acidification are having negative affects on hard corals (both on symbiotic zooxanthellae algae and for building hard coral structures).
Research is finding there are heat tolerant zooxanthellae in some corals, which may help them survive warming temps, but ocean acidification is also an issue. This problem is compounded by other issues as well, including both natural and man-made disturbances. Research is also finding that increases in temps on reefs are causing more coral disease outbreaks. And fish populations have been seen moving away from warming reefs which allows for more coral predators.
It is complicated and even though we are finding that some corals may be able to survive some of these issues, there is so much against them, and the domino effect has already started on some reefs ecosystems and is growing in proportion.
Interesting article though. It's sad to see no elkhorn in the Key's when I go diving there. My dad said the same reefs we dive used to be covered with elkhorn when he was younger. It's mostly button polyps, sponges, and gorgonias now with some stonies mixed in here and there.