Climate change communication campaigns can reach many audiences cost-effectively. However, some climate messages may not work universally as there may be heterogeneity in message effects across audiences. An online experiment (N = 57,968) across 23 countries found that three climate messages had modest positive effects on support for climate action. An “Urgency & Generational” message had the strongest effect overall and had, on average, stronger effects in countries with lower baseline support for climate action (e.g., developed countries, democratic countries). While the size of this message’s positive effects varied across countries, effects were positive across all audience subgroups investigated and there was no evidence of backfire effects. For instance, this message had positive effects across the political spectrum and effects were marginally stronger among the political Right. Although the average message effects were small, the results indicate that, when deployed at a large scale, climate change messages have the potential to strengthen public support for climate action.