Since long before the beginning of the Industrial Age, oceans were effectively absorbing carbon dioxide. Recent studies show that in addition to rising sea levels and a 0.5 degree Fahrenheit increase in ocean surface temperature, excess Carbon dioxide in the atmosphere is increasing ocean pH levels, a process known as ocean acidification. This change in chemistry appears to be affecting the ability of some mollusks and other marine organisms to build calciferous shells, and threatens to shift the ranges of marine organisms and alter entire food webs in ways not yet fully understood. Several species of marine birds that depend on California habitats—including already endangered breeding populations of ashy storm-petrels, Craveri’s murrelets and black-vented shearwaters—are expected to experience declines as rising sea levels inundate the rocky areas where they breed on and near the coast. In addition, critical foraging for at least 70 bird species could be disrupted by the effects of rising sea temperatures on the food chain. Corals, calcareous phytoplankton, mussels, snails, sea urchins and other marine organisms use calcium and carbonate in seawater to construct their calcium carbonate shells or skeletons. As the pH decreases, carbonate becomes less available, which makes it more difficult for organisms to secrete Calcium carbonate to form their skeletal material. For animals in general, including invertebrates and some fish, carbon dioxide accumulation and lowered pH may result in acidosis, or a buildup of carbonic acid in the organism's body fluids. This can lead to lowered immune response, metabolic depression, behavioral depression affecting physical activity and reproduction, and asphyxiation. Since the oceans have never experienced such a rapid acidification, it is not clear if ecosystems have the ability to adapt to these changes. Effects of ocean acidification on organisms and ecosystems are still poorly understood. Over the last few years, research has intensified significantly to fill the many knowledge gaps. Nearly 500 million people depend on healthy coral reefs for sustenance, coastal protection, renewable resources, and tourism, with an estimated 30 million of the world's poorest people depending entirely on the reefs for food. Coral reefs face two challenges from increasing atmospheric carbon dioxide. First, higher carbon dioxide concentrations in the atmosphere are linked to warmer global temperatures, which in turn lead to warmer water temperatures. Corals are very sensitive to temperature change: a 1–2º Celsius change in local temperature above their normal summer maximum can lead to a phenomenon called ‘bleaching’, whereby the corals expel their vital algal symbionts (algae which live in the cells of the coral), leaving the coral tissues translucent. In 1998, a single bleaching event led to the loss of almost 20% of the world’s living coral. Corals can recover from these events but repeated episodes are likely to weaken the coral ecosystem, making them more susceptible to disease and causing a loss of biodiversity. The second challenge faced by corals is the increasing acidity of the water caused by higher carbon dioxide concentrations. Lowered calcification rates affect the reef’s ability to grow its carbonate skeleton, leading to slower growth of the reef and a more fragile structural support, which makes it more vulnerable to erosion. By the middle of this century, the estimated reduction in calcification rates may lead to more reef area erosion than can be rebuilt through new calcification.