Global Acidification or Warming
The issues of global ocean acidification and global warming are two symptoms of the same disease, too much carbon in the atmosphere. And they have 1 solution, put less carbon into the atmosphere. It doesn't really matter which aspect is the more harmful one, you can't treat one without treating the other.

If reducing carbon output were simple, we would have done it by now. However, it will most likely require radical changes to our energy infrastructure, with moves away from coal, oil, and natural gas and towards solar, wind, and nuclear power. It will probably require changes to our agricultural practices, and you should really watch Allan Savory's TED Talk for how those changes should be implemented. 

People are very slow to change their habits, and short term profit is a powerful motivator against change, but I believe these we can and must solve these problems if humanity is to continue to thrive on Earth.