It's often said that we know as much about Mars and the moon as we do about our oceans. Considering that Earth is 71% ocean, this should be cause for concern. At the very least, we should be doing more to protect our oceans from the negative effects of human activities, even if we don't fully understand all that is happening under the seas.

One thing we do know is that oceans are changing - and not for the better. For centuries, we've thought of our oceans as stable. But ocean currents, upwellings, oxygen levels, acidity and temperature are changing in ways we haven't seen before. We've always assumed that oceans would provide us with an endless bounty of food. We rely on our oceans for transportation, recreation and numerous resources. And oceans provide almost half the oxygen we breathe.

Some threats to our oceans are easier to pinpoint than others. Swirling masses of plastic garbage in the oceans - one of them in the North Pacific is estimated to be bigger than Quebec - are obvious artifacts of our disposable societies. "Dead zones," or areas where oceans are starved of oxygen because of a nitrogen overdose from agricultural runoff, are showing up around the globe.

On top of the many direct threats to ocean health, we also have climate change to contend with. We know that global warming is causing the oceans to become more acidic. This is a worrisome trend.

As with our atmosphere, too much carbon is having dangerous effects. Carbon dioxide is necessary for photosynthesis, which is how plants grow and develop. But when we burn fossil fuels or clear-cut forests, we release too much carbon dioxide into the atmosphere, upsetting the balance.

The oceans absorb carbon dioxide, keeping some of it from the atmosphere. But while oceans help slow the pace of global warming, they, too, are absorbing too much carbon dioxide, resulting in disruption of the ocean's pH balance. This increasing acidity causes calcium carbonate to dissolve, affecting life forms including corals, shellfish and several species of plankton that rely on calcium for their structure.

Science is confirming that our old assumptions are no longer valid, and we find ourselves in a situation of escalating risk. We need to look at our oceans in an entirely new way. We can't continue to exploit ocean resources on false assumptions. That means investing in science that will help explain the interactions between changing ocean conditions and the species that depend on the seas.

We need a new way to manage our oceans in the face of uncertainty and elevated risk facing marine life. A comprehensive marine-planning initiative that considers new and evolving science, and the evidence of what is actually happening to marine ecosystems would be a good start. This process must be based on a precautionary approach that recognizes increased uncertainty and the fact that our oceans will continue to change.

We can't rely on governments alone to protect the health of our oceans. Industry, nongovernmental organizations, First Nations, coastal communities and governments at all levels must come together to plan and monitor conservation efforts based on science and local community knowledge.

After all, one thing we've learned about Mars and the moon is that we can't move there if we destroy our home on this beautiful and generous planet. Neglecting the health of our oceans, where all known life began, is a risk we cannot afford to take.

Take David Suzuki's Nature Challenge and learn more at www.davidsuzuki.org.