The idea of runaway ocean acidification has now joined the idea of runaway global warming as a threat so large that it stands almost co-equal in its danger.
Part of the problem with ocean acidification is that geoengineering schemes for lowering Earth's temperature by reducing the sunlight that reaches the Earth's surface won't affect ocean acidification. And recent research from the Massachusetts Institute of Technology suggests that there is a tipping point in acidification beyond which the process becomes self-reinforcing and could lead to a mass extinction.
The idea of runaway global warming has been around for a while. In its original form it was speculation about whether the Earth could enter an unstoppable process that appears to have occurred on Venus billions of years ago and boiled its oceans away—leaving a planet so hot that surface temperatures today are high enough to melt lead.