Ankle-tap is an expression I'd not heard previously, but read it in a discussion of climate change posted by someone from NZ. It seems to be in reference to the expression, "Achilles heel," based on the context. Is it?
Read the comment by Kiwiiano in this article: http://www.guardian.co.uk/envi...ange-wine-production

It's a reference to rugby football. An "ankle-tap" is used when the player carrying the ball is running at speed and a defending player is approaching from behind. Even if the defender is not able to get close enough to the ball-carrier to wrap his arms around him in a conventional tackle, he may still be able to dive at the other player's feet and, with an outstretched arm, deliver a tap or hook to the player's foot (or feet) causing the player to stumble. It's usually something of a desperation move - if it fails the attacker is free, often able to score a touchdown without any more problem.

In this case, he means that population pressure and resource depletion are likely to trip us up in our attempts to combat climate change.

Of course, in medicine we can tap an edematous ankle, which sometimes happens with gout.