This first lecture of a series entitled 'Climate Connections' is presented by John Broome, White's Professor of Moral Philosophy and Fellow of Corpus Christi College, Oxford. The progress of climate change places moral demands on all of us to do something about it. It makes moral demands on governments and the international community, and also on each of us as private individuals. The public and private morality of climate change derives from moral duties of two different sorts. Firstly, it derives from the general duty of beneficence to make things better. Secondly, it derives from duties of justice, and in particular from the duty not to harm other people except in specific circumstances. It turns out that the private morality of climate change is entirely governed by the duty of justice not to harm. Emitting greenhouse gas harms other people in a way that is not morally permitted, so we should not do it. Fortunately, at present we can satisfy this moral demand by offsetting our emissions. The public morality of climate change is partly governed by the duty of justice, but more strongly by the duty to make things better. To carry out this duty properly requires goods and harms to be weighed against each other; it requires cost-benefit analysis in a broad sense. Because the problem is so complex, it requires the technical methods of economics. But the economic methods must be properly founded on ethical principles.