Reports Showing Increasing Global Warning Signs in Oceans

The world's oceans are feeling the increasing effects of polluted runoff, warming water temperatures and rising levels of atmospheric carbon dioxide, scientists are finding. Their discoveries in the past few months alone are raising more alarm bells about the health of the Earth's seas, sea-life and even human life.

Among some of the more worrisome reports coming from the world's watery sides:

A possible glut of CO2. The world's oceans have already absorbed about one-third of all of human-caused carbon dioxide emissions, which is a good thing, because otherwise the levels of that greenhouse gas in the air would be much higher than they already are. On the other hand, scientists worry the oceans might be approaching their limit of how much more carbon dioxide they can take in. The North Atlantic, for example, is now absorbing only half as much CO2 as it did during the mid-90s, researchers from the U.K. reported this summer.

Faster climate change. If the oceans do start losing their ability to absorb carbon dioxide, the greenhouse gas stays in the atmosphere, where it can contribute to accelerated climate change. In fact, scientists say CO2 concentrations today would be much higher without the oceans' carbon sink abilities: possibly as high as 600 parts per million, compared to the current level of 380 ppm. Such concentrations could have devastating effects. At 550 ppm, for example, researchers warn average temperatures could rise by nearly 10 degrees Fahrenheit (5.5 Celsius). That would be enough to melt the ice covering Greenland and raise sea levels by more than 19 feet.

Widening dead zones. Polluted runoff from land, especially from fertilizer-loaded agricultural fields, eventually makes its way to the oceans, and the impact of that pollution is growing. Nitrogen-rich farm runoff fuels algae blooms in the ocean, and as that algae decays and dies, oxygen is sucked out of the water. The result is a so-called "dead zone" with not enough oxygen to support marine life. A new study reports that the number of ocean dead zones around the globe has risen to 405, compared to 49 four decades ago.

Impaired reproduction of marine life. As oceans keep soaking up carbon dioxide, they're becoming more acidic, which is already a known problem for creatures with shells (acidic water dissolves calcium carbonate). But researchers from Australia and Sweden recently discovered that rising acidity levels also make it harder for other sea life to reproduce. Higher acidity levels reduce fertilization rates for the eggs of sea urchins, lobsters, mussels and oysters, they found, and also make it harder for fertilized eggs to develop into larvae.

The rise of slime. That's what oceanography professor Jeremy Jackson sees as a threat to the world's oceans. The researcher at the Scripps Institution of Oceanography at the University of California, San Diego, says the combination of human-caused stresses to the oceans could drive waves of mass extinctions of marine life. As complex ecosystems of predators and prey, fish, corals, plants and invertebrates collapse, he says, a simpler, less-appealing web of life -- microbes, blooms of toxic algae and jellyfish (i.e., "slime") -- takes over.

Fewer sharks. While a reduced chance of encountering "Jaws" might seem like a good thing to the average snorkeler, it's bad news for sea life in general. A recent report from the organization Oceana warns that fewer sharks mean less food for scavengers, overpopulation of sick and weak prey species and a threat to marine biodiversity overall. That's a serious concern as humans globally now kill more than 100 million sharks a year.

Lost discoveries and benefits. Oceans cover a majority of the Earth's surface and hold many yet-undiscovered species of plants and animals, many of which could benefit humankind. Just this month, for instance, researchers at the University of Florida announced they had identified a compound derived from bacteria on coral reefs that appears to inhibit the growth of cancer cells in lab tests.