Read the passage and find if the passage agrees, disagrees, or has a neutral stance on whether Global warming is caused by human activities. Answer only with keyword (a) agrees - if passage agrees with the target (b) disagrees - if passage disagrees with the target (c) neutral - if the given passage neither agrees nor disagrees with the target. You don't need to use external knowledge in this task, and you have to answer based on the given passage.

A warning sign that some scientists worry could mean global warming has passed an ominous tipping point.
agrees

Climate change is a likely threat to national security – though scientists can not yet predict particular climate effects of anticipated global warming with any degree of certainty.
agrees

An ice-free Arctic Ocean would not freeze again.

neutral