The release of excess CO2 from the combustion of fossil fuels, deforestation, and other processes doesn’t just affect our air; it also affects our oceans. The oceans absorb as much as 30 percent of the excess carbon dioxide in the atmosphere, which lowers their pH. Thus, our emissions have two large consequences for our oceans: warmer temperatures and increased acidity.

These changes may have a profound effect on coral growth, since corals are sensitive to both temperature and pH. There is mounting evidence that coral health has been declining in recent years. But what, exactly, is affecting coral? A new study in Science shows that current changes in coral growth may have more to do with the ocean’s temperature than its pH.

Corals, which gain nutrients through a symbiotic process with algae, pull calcium from the ocean to create protective layers, a process known as “calcification.” Over time, these layers accumulate and grow into what we call coral reefs. Scientists have hypothesized that the recent declines in the coral health at many well-known reefs were caused by the decreasing pH of the oceans, which can make it harder for coral to use calcium from the surrounding water. If ocean acidification is the driving force behind these declines, corals in cold, high-latitude areas should be affected first, since lower-temperature waters are more subject to acidification.

To examine coral growth in the reefs inhabiting the waters off the western coast of Australia, a team of researchers took 27 core samples from six populations of coral. The samples spanned 11 degrees of latitude in the Indian Ocean; some of the populations lived in cold, high-latitude waters, while others lived in warmer parts of the ocean closer to the equator. From the cores, the scientists could look at various aspects of the coral’s growth over the last 110 years.

The scientists found that the calcification rate of the corals did depend on the location, but not in the direction that the ocean acidification hypothesis would predict. The two southernmost populations showed definite increases in calcification since 1900, up to a 23.7 percent increase in one location. Among the more northern populations, four showed no change in calcification over time, and two showed decreases in calcification rates. If ocean acidification were driving these trends, the southernmost waters would be more prone to lowered pH in that region, and corals there would show decreased calcification.

Instead, the calcification rates have more to do with temperature than acidification. The scientists compared changes in the sea surface temperature at each location to the changes in calcification rates. Coral populations where the sea had warmed the most (those living in the coldest water) actually had the largest increases in calcification rates, and populations with smaller increases in sea surface temperature (where the sea was warmer to begin with) either had no change in calcification or underwent declines.

The results show that temperature may have a larger impact on coral growth than acidification does, at least in this set of samples. Rather than be adversely affected by warming oceans, coral reefs in cold water take advantage of higher temperatures that bring them closer to optimal growth conditions. However, temperature increases in other locations may put the water over the ideal temperature for coral growth. This may either cause declines in calcification directly, or leave them susceptible to acidification.

Due to the diversity of coral species and the large geographic range in which they live, corals aren’t responding uniformly to climate change. Furthermore, the effects are likely to change over time; even though some populations are benefitting initially from changes in temperature, these advantages will likely disappear as the warming process continues.

Science, 2012. DOI: 10.1126/science.1214570 (About DOIs).