Many people are by now familiar with the Keeling curve, a graph showing the steady increase in carbon dioxide in the atmosphere measured over decades by the Mauna Loa Observatory, the world's longest-running CO2 monitoring station. 

The research, started by renowned climate scientist Dave Keeling in 1958, is considered one of the pillars of the scientific consensus that human activity is the main driver of climate change. This year, the data revealed a troubling milestone: CO2 concentrations had passed 400 parts per million for the first time since the dawn of human civilization.

Less familiar, but every bit as troubling to climate scientists, is a parallel slope on a different track of climate data: the increase of CO2 in the world's oceans, which has been climbing almost in lockstep with the Keeling curve. The rising carbon level is cranking up ocean acidity with astonishing speed—probably 10 times faster than at any point in about 50 million years, according to scientists.

Among other concerns, scientists are now increasingly worried that the acidification of the oceans is likely to cause one of the first abrupt, severe and probably irreversible consequences of global climate change: the loss of tropical coral reefs.

Until recently, when ocean scientists thought about abrupt changes due to climate change they worried mainly about fundamental, catastrophic changes in the general state of the oceans—sudden and permanent changes to the Gulf Stream, perhaps; or rapid, calamitous melting of ice caps; or a sudden belching of greenhouse gases from the depths. Now, according to a comprehensive new report on abrupt climate changes from the National Academy of Sciences, careful study suggests that this kind of sudden disaster is not so likely any time soon.

What's more worrisome in the next few decades, the report said, is that important ocean ecosystems will collapse under the accumulating weight of slow, steady environmental changes.

The inexorable absorption of carbon by ocean waters—and the acidification that inevitably follows—would, at some point in the next few decades, deprive some species of the basic chemical conditions for survival. That could spell sudden death for some of them. 

The killing off of tropical coral reefs, which are at the very foundation of marine biodiversity, would be a case in point. It's not so much that acidification of the oceans would accelerate suddenly. Rather, the acidity is likely to upset the water's chemistry beyond the range in which coral reefs can thrive.

"Although ocean acidification is not an abrupt climate change," this report said, "the impacts of ocean acidification on ocean biology have the potential to cause rapid (over multiple decades) changes in ecosystems and to be irreversible when contributing to extinction events."

In other words, ocean scientists now agree that the creeping absorption of carbon by the oceans could easily lead to the complete disappearance of tropical coral reefs in just a few decades, according to another newly released document, a summary for policymakers on the acidification problem. The report represents the consensus of 540 scientific experts from 37 countries.

Only with a steep decline in the world's emissions of carbon dioxide can half those reefs be saved, it said.

"Reducing CO2 emissions is the only way to minimize long-term, large-scale risks," they warned.

The report is a good primer on the scope of the acidification crisis, which scientists like to call "the other CO2 problem."

"The other urgent CO2 problem," Carol Turley, a leading researcher in the field, emphasized at a seminar on the issue presented during the recent UN climate meeting in Warsaw, Poland. "Ocean acidification is a very big deal, it is happening now, and it is happening very rapidly."

The acidity of the oceans, scientists say, has increased as much as 30 percent since the start of the industrial era. At the current pace of pollution it is expected to go up anywhere from 100 to 170 percent by the end of the century.

Unless greenhouse gas emissions are brought down sharply, the worst is yet to come, scientists warn.

The future costs to people who depend on the oceans for their livelihoods could run into trillions of dollars. Some industries, notably in shellfish aquaculture, are already seeing the effects. 

One study by an international team of scientists suggests that economic damages from this one symptom of acidification could run to $1 trillion a year—although such predictions remain only tenuous.

Just as alarming, acidification in polar regions could badly hurt calcareous species like krill, which are at the very foundation of the whole ocean food chain. There are many other possibly dire consequences, some of them more fully understood than others. 

It's worth noting that one of the problems of acidification is that it reduces the capacity of the oceans to buffer atmospheric climate change. As the oceans turn more acid, they become less able to absorb carbon dioxide from the air—and that, in turn, would worsen the warming of the atmosphere.

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