Here’s a test. Try to list five reasons why the oceans are important to life on earth. It may get a bit difficult when you are down to reason number four or five. Now, describe how the things you just listed would be affected by CO₂ emissions caused by human activities. You may think that I am expecting too much from you. I am not. These facts are as important as knowing why water is essential to the world and how polluting it would affect us.

The global ocean is vast and so is the diversity of organisms that call it home. It covers 71% of the Earth’s surface, housing hundreds of thousands of species. In fact, it is essential for the survival of humans and other organisms on land as it is critical in regulating the earth’s climate to make it hospitable for us. Marine ecosystems also provide other essential goods and services such as food and coastal protection and are an integral component of my culture.

The oceans contribute tremendously to moderating the effects of greenhouse gas emissions on the climate, although this is achieved with great costs. In a recent paper published in Science, my colleagues and I summarized these benefits and costs. While the Intergovernmental Panel on Climate Change (IPCC) concludes that the warming of the Earth’s surface is caused by human CO₂ emissions, the global ocean has already done a tremendous service to us by absorbing 93% of the additional heat caused by these emissions since the 1970s. The ocean also has captured 28% of CO₂ emitted from our (and our ancestor’s) activities since 1750. Thus, without these moderating effects from the oceans, climate change would have been much more severe by now.

The absorption of heat and CO₂ substantially alters the physical and chemical properties of the oceans, which in turn has large effects on marine organisms. The heat warms up the ocean, reducing the amount of oxygen context, and the CO₂ acidifies the oceans. This combination can be deadly to marine life and affects us through decreases in fisheries catches and diversity of species, loss of tourism attractions such as diving and snorkeling, and increases in exposure to storms and other extreme events. In addition, as the ocean absorbs heat and CO₂, its ability to moderate more CO₂ emissions actually reduces.

Many of the impacts of climate change on the oceans have already been recorded. Marine species ranging from plankton to fish, and marine mammals have shifted their distributions by tens to hundreds of kilometers per decade generally towards the poles, or to cooler waters, to find areas where they can survive as the ocean changes. Seasonality of biological events such as reproduction and migration are also altered. Ocean acidification has been shown to affect fish and shellfish from high latitudes to the tropics. These changes affect food web interactions between marine producers, grazers, and predators, creating new compositions of species that have not been seen in recent history. Additionally, marine biodiversity and ecosystems are already facing a multitude of threats from humans, including overfishing, habitat destruction, and pollution. These threats will add to, and even be exacerbated by, the impacts of CO₂ emissions.

Today, sensitive and vulnerable ecosystems such as tropical coral reefs are taking the brunt of these impacts. Tropical coral reefs are particularly sensitive to the combination of CO₂ related stressors. Under heat stress, corals “bleach” by losing symbiotic algae that is essential for their survival. Ocean acidification makes it more difficult for corals to build calcium carbonate exoskeletons, making them amongst the most sensitive groups of organisms to ocean acidification. In some locations in the tropical oceans, such as in Papua New Guinea, CO₂ is naturally seeping out from the ground, providing a window for us to see what coral reefs ecosystem are like in a high CO₂ ocean. Coral reefs closer to these sites have substantially lower diversity of both corals and their associated species. Coral reefs provide critical habitats for tens of thousands of species. They are also important tourist attractions, and their fisheries are an important source of food and livelihood for coastal communities. Loss of coral reefs will thus threaten biodiversity, income, livelihood, and food security.

Fishermen’s catches in general are affected when distribution and abundance of the fish stocks shift. A study, conducted by colleagues and myself, analyzed the world’s marine fisheries catches since the 1970s. We developed an indicator to use fish catches as a “thermometer” that is based on the average temperature preferences of species in the fisheries catches. We found that the world’s catches have been increasingly dominated by warmer water species in the last four decades at a rate that is related to ocean warming, even after the effects of changes in fishing intensity have been accounted for.

The oceans will look substantially different in the future if we make different decisions on limitations of greenhouse gas emissions and ocean management. Under the “business-as-usual” scenario, and without major efforts in limiting emissions, temperatures in the surface layer of the ocean are expected to increase by 2.7^o C from levels in the 1990s, and the ocean will acidify at a rate that is unprecedented over the past millions of years. Many marine organisms, from seagrass, coral reefs, and shellfish to fish and seabirds will be severely impacted. Fish stocks are projected to shift their distribution 65% faster towards the poles compared to a scenario in which we limit ocean warming below 1^o C relative to the 1990s.

Climate change is expected to impact the tropical marine ecosystems the hardest, through a combination of loss of biodiversity, decreases in potential fisheries catches, sea level rises, and the inherent high dependence of many tropical countries and communities on marine ecosystems. The capacity to adapt to these impacts is relatively low. However, areas outside the tropics will not be safe from climate change impacts. Warming, ocean acidification, and the loss of sea ice are threatening polar ecosystems. Shifts in distribution of commercially important fish stocks are expected to lead to international fisheries conflicts. Some countries and communities may lose their traditionally or culturally important fisheries. In addition, given the globalized networks of seafood supply, decreases in catches in the tropics, of tuna, for example, will also impact markets far away from the sources.

Opportunities still exist that would allow us to substantially reduce these impacts. Limiting emissions to a less than 1^o C warmth of the ocean going forward is necessary; otherwise, a substantially different ocean would result from any less-stringent emissions scenarios. Also, we need to implement all other measures to reduce the existing stresses on marine ecosystems, which will help reduce impacts from climate change. Efforts to rebuild ecosystems and biodiversity that have already been damaged (e.g., coral reefs) are needed. However, all these opportunities will disappear if we do not act quickly.

The ocean is a vital component of the Earth, supporting life in this planet and our own well-being. Any discussion about the sustainability of this planet, such as setting CO₂ emission limits or goals for sustainable development, must consider the oceans. Otherwise, such discussions will risk making inadequate decisions for the future of the planet.