This chapter shows how successive UK governments have applied a first-best-world neoclassical approach to climate policy: one that understands risk as something that can be calculated, modelled, and integrated within market mechanisms by essentially rational market actors. This was not just the economics that gave us the Global Financial Crisis, the chapter shows that the underlying Rational Expectations and Efficient Markets hypotheses are the market analogues to Soviet theories of optimal planning, and built on the same ontological and epistemological fallacies. The chapter explores how dependence on closed-system reasoning condemns government to ‘write out’ the uncertain dynamics of the climate emergency and the precautionary principle that should follow. The empirical section shows how successive governments of New Left and New Right have implemented regulatory and market-making policies built on the assumption that market agents not only can but will behave rationally in the face of future risk. They have also relied on carbon budgeting, forecasting and audit as dependable methods of risk management, though such methodologies are only coherent in a closed-system world.