Opposition to collective action on climate change takes at least two forms. Some people deny that climate change is occurring or that it is due to human activity. Others maintain that, even if climate change is occurring, we have no duty to do anything about it because our efforts would be futile. This article rebuts the latter line of argument. I argue that: (1) everyone has a duty to do their share for the global common good, which includes doing one’s part to combat climate change; (2) the idea that taking action against climate change is futile should be treated with caution, because sometimes actions may seem to make no difference to climate change, when really they do; (3) in any event, the duty to do one’s share to combat climate change still applies, even if it is ultimately futile; and (4) this is because not doing one’s share for the common good harms oneself, regardless of whether it makes any difference to the wider outcome.