Greenhouse gas (GHG) emissions are affecting a global common: the climate, and as a global environmental problem with a public good character it provides attractive opportunities for minimising control costs through the use of emission trading markets. This paper introduces cost and benefit principles that can be applied to the assessment of global markets for GHG emission reduction options and evaluates the scope for and the potential economic gains of such markets. (c) 2004 Elsevier Ltd. All rights reserved.