Physics Colloquium 23rd October 2015 delivered by Professor Myles Allen Physics is the foundation of current concerns about climate change, but climate policy sometimes appears like a baroque superstructure built with little reference to the foundations. For example, global temperatures depend on the accumulated stock of carbon dioxide emissions released into the atmosphere over all time, not the flow of emissions in any given year, but climate policy remains overwhelmingly pre-occupied with emission flows, not carbon stocks. Michal Kalecki once called economics “the science of confusing stocks with flows”, and while this is probably unfair on economics, it isn’t a bad characterisation of UN climate negotiations. As physicists, we are professionally concerned with the complexities of the climate system, so it may come as something of a shock to learn that many of the numbers that really matter for major policy decisions, like deciding on the right combination of prices and regulations to reduce greenhouse gas emissions or the “social cost of carbon” to use in evaluating investments, depend on models that are astonishingly simple compared to models of the general circulation of the atmosphere and oceans. I will introduce some of the ideas behind these ‘Integrated Assessment Models’ and show how, even though the units may be PetaDollars rather than ExaJoules, our basic physical intuition can be used to understand how they behave, and how they can give some rather surprising results. This talk should be accessible to anyone interested in the climate problem, and won’t assume any prior knowledge of either climate physics or economics. There will be some maths, but I’ll explain what I’m up to as I go along. Both physicists and economists welcome, to heckle the speaker or each other as they see fit.