As the climate changes, coral reefs will be affected by changes to the ocean's temperature and pH.
Individual corals will remain healthy only when water temperatures stay within a relatively narrow range. Coral polyps have a symbiotic relationship with zooxanthellae, single celled algae, that live in their tissues. The zooxanthellae provide food for the coral through photosynthesis, and get nutrients and a place to live in return. As temperatures increase, corals get stressed and may eventually expel the zooxanthellae, turning a white color. This is known as coral bleaching. The corals depend on the zooxanthellae for most of their food, and if the high temperatures continue for long enough and they stay bleached then they can die .
The usual way to measure the thermal stress on a coral is the degree heating week, or DHW. DHWs are calculated in relation to the maximum monthly mean temperature, or MMM, for the area being measured. The MMM is the mean temperature of the hottest month of the year, according to some set of historical data. One DHW is accumulated for each week and each degree Celsius spent above the MMM. For example, two DHW can be accumulated by either spending two weeks at 1°C above the MMM, or by spending one week at 2°C above the MMM. DHWs can be fractional, but are only accumulated if temperatures are at least 1°C above the MMM. Within a three month window, if corals in an area have undergone 4 DHW then there is some risk of bleaching, and if they have undergone 8 DHW then widespread bleaching and mortality is likely .
The DHW is used by NOAA's Coral Reef Watch program  to process satellite data and determine the areas that are at immediate risk of bleaching. The DHW can also be used in combination with data from climate models to make projections of future bleaching. As temperatures continue rising, areas will begin experiencing at least 8 DHW in a three month window every single year. This point is known as the onset of annual severe bleaching, or ASB. The extent to which reefs will be able to withstand such severe stress is unknown, but it's hard to imagine them surviving in anything like their current form.
Under RCP 8.5, the onset of ASB for the world's reefs has been projected to happen on average by 2043, for 89% of reefs by 2053, and for 99% of reefs by the end of the century .
Ocean acidification will reduce the concentration of ions and make it more difficult for corals to precipitate the aragonite needed to build their skeletons and grow. Corals need to grow in order for reefs to survive. They are under constant assault not just from anthropogenic sources but from natural sources as well. Crashing surf, the associated surge, burrowing animals, and corralivorous animals will both kill corals and erode the reef itself.
The effect of the aragonite saturation state on precipitation rates has been studied for corals as well as for inorganic precipitation.
Inorganic precipitation occurs when a sample of aragonite is placed in a supersaturated solution: additional aragonite crystals will form on the existing ones. The rate (unit: μmol m-2 h-1) at which this occurs has been measured and fit to the following equation in seawater :
In a saturated solution (), as expected. As the solution becomes increasingly supersaturated, the rate at which aragonite precipitates increases more than quadratically. Using this equation with the values for around Moorea at the start and end of the 21st century calculated here, a decrease of from 4.09 to 2.46 corresponds with a rate decrease from 176 to 30, an 83% decline.
The aragonite precipitation rate within corals is a more complicated topic. Corals build their skeletons far faster than they would naturally precipitate. The corals on a reef flat in Moorea have been measured to form 24.3 grams of aragonite per square meter of reef per day . This works out to a rate of 10,125 in the units for above, 58 times as fast as the natural rate of precipitation at an of 4.09, and equal to the natural rate of precipitation at an of 18.2 (note, though, that using the rate law this way isn't precise because coral calcification rates depend on the surface area of exposed live coral, which can be very different from the area of reef). Corals seem to be able to precipitate aragonite so effectively by altering the concentrations of ions at the sites where precipitation occurs, relative to the surrounding seawater. Recent work has measured the components of the carbonate system at these sites, finding elevated pH, higher concentrations of and ions, and an average of 12, far higher than in seawater .
While corals are able to create favorable conditions for precipitating aragonite, their effectiveness at doing so depends to a large degree on the chemistry of the surrounding seawater. Several studies have measured the rate of precipitation by corals at various saturation states. These studies generally find a strong correlation between the saturation state and precipitation rates, similar to the rate law for above. Together with projections of future changes to saturation state, these estimate a decline in coral precipitation rates of 60% by 2065 .
Adaptation to Warming and Acidification
Projections of future coral bleaching and reduced growth rates due to acidification generally assume that corals will not be able to adapt to the changing climate. Coral reefs exist in a narrow range of temperatures (21°C to 29.5°C, averaging 27.6°C) and aragonite saturation states (3.28 to 4.06, averaging 3.83) , with some locations experiencing saturation states under 3 . While these ranges do cover the conditions expected at many reefs during the 21st century, the corals in different areas have already adapted over time to the conditions local to their environment. Corals reproduce slowly and have a limited range when they do so, releasing free swimming larvae that settle down within a few days or weeks . This restricts their ability to disperse, but does not eliminate it. As the ocean warms and acidifies and individual coral colonies die off, is there enough local genetic diversity for reefs to regenerate with more tolerant individuals?
Some research has been done in this area. One study found that individuals of one coral species at a cool, high latitude location had some alleles associated with heat tolerance in individuals at a much warmer location. Population simulations were conducted using climate model data to see the effect of these alleles spreading out in the cooler location, but the simulated populations went extinct in both the RCP 8.5 and RCP 6 scenarios .
Given the limited amount of research in this area and the limited effects of adaptation that have been found, it seems best to be conservative and assume that adaptation will not have a significant effect on the ability of corals to survive in the short time frame being considered here.
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