What the world does or does not do about climate change will have profound implications. If the world fails to act, we may cross a climate tipping point, with disastrous consequences all around. No country by itself can prevent this, but a substantial number of countries working together can avert disaster. Collectively, we know what we need to do. However, can we agree on what each of us should do? Can we be assured that, if we act, others will fulfill their pledges? An experimental paper in PNAS (1) provides reasons for optimism and despair, optimism because, with communication, outcomes improve; despair because intransigence by a few countries can block progress. A key impediment to success, this paper (1) shows, is overcoming historical inequities.