Metaphors are prevalent in environmental science communication, because they describe complex topics in more familiar terms. Yet, little research has investigated whether metaphors contribute to comprehension in such communication. This experiment (N=510) disentangles the effects of different metaphor types on comprehension-related outcomes for three environmental concepts (greenhouse effect, carbon footprint, greenwashing). Results showed small but statistically significant effects of some metaphors on perceived text comprehensibility and perceived comprehension, but no effects on actual comprehension. No mediation effects were attested. Science communication could thus benefit from metaphor, but communicators should be careful not to overdo it, nor to overestimate its effectiveness.