On a cold morning, when my home heater kicks on, natural gas burns and a plume of carbon dioxide streams out of my chimney and into the sky. When I make my morning toast, there is a power plant somewhere providing electricity to my toaster that is burning gas or coal and releasing giant plumes of carbon dioxide into the air. Much of this CO₂ will remain in the atmosphere for many thousands of years. This pollution has physical consequences for our climate system, but it also has chemical consequences for our oceans. In the atmosphere, too much CO₂ can make the world uncomfortably warm and melt the giant ice sheets and so on. Bad stuff. This year has been just about the worst ever for coral bleaching. CO₂ in the atmosphere has been making Earth hotter and this is causing coral reefs to bleach, causing many colonies to die. Most of the carbon dioxide we release to the atmosphere will eventually be absorbed by the oceans. They have already absorbed about one-quarter of the CO₂ produced by industrial civilisation over the past couple of centuries, and this process is making them more acidic. When CO₂ reacts with seawater, it forms an acid. In high enough concentrations, this acid can corrode shells and skeletons of many marine organisms. In lower concentrations, it can make it harder for an organism to produce its shell or skeleton, and this could in some cases tip the balance between life and death for it. There has been plenty of carbon dioxide in the atmosphere in the ancient past, and corals and clams and so on did just fine. There are natural processes involving rivers and sediments that buffer ocean chemistry over periods of thousands or tens of thousands of years. But right now, the ocean chemistry is changing way too fast to be buffered by Earth's natural cycles. The problem for marine organisms isn't so much where we are going but how fast we are getting there. If we do not dramatically reduce our rate of CO₂ emission, we will make the biggest and most rapid change in ocean chemistry the world has seen in many tens of millions of years – long before there were any humans on this planet. We recently conducted experimental work in the Great Barrier Reef, providing evidence that ocean acidification is already slowing growth of the coral reef we examined. Increased stress from ocean acidification is probably making corals less able to stand up to heat stress and so contributing to coral bleaching. As a scientist, I work to better understand the relationships between our actions and what happens in the real world. On a chilly morning, I heat my home and make myself some toast and coffee, and sink into my easychair. I understand that in doing this I am contributing to killing off coral reefs and that I am affecting other marine ecosystems in unimagined ways. We know that protecting the oceans means we need, among other things, to stop using the atmosphere as a waste dump for our carbon dioxide pollution. We need to stop building things with smokestacks or tailpipes. Scientific understanding is the cornerstone of good public policy. Science can never tell us what to do, but it can give us an idea of what might happen if we choose different courses of action. An intensified effort to develop the scientific knowledge can help us better understand the consequences of our choices. And we, as a society, can choose wisely. Ken Caldeira is a climate scientist at the Carnegie Institution for Science.

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