We experimentally study strategic situations characterized by a large degree of uncertainty and the need for collective action in the form
of sufficient total contributions by members of a group. A theoretical model with max-min preferences towards ambiguity predicts higher
contributions under ambiguity than under risk. However, in a large representative sample of the Spanish population (1,500 participants)
we find that there is no effect of ambiguity on contributions. This suggests that policymakers concerned with ambiguous problems (like
climate change) do not need to worry excessively about ambiguity.