This digital document is a journal article from Journal of Environmental Economics and Management, published by Elsevier in . The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.Description: Recent research has shown that perfect household mobility can serve as a disciplinary mechanism inducing national policy-makers to internalize interregional externalities caused by transboundary pollution. This paper develops a differential game to illustrate that the result is not necessarily robust when migration costs and explicit dynamics are introduced. The paper shows that if governments constantly reoptimize, imperfect household mobility leads to a tragedy of the commons, and individual countries overemit even when pollution is purely local. Moreover, these dynamic externalities get worse as the degree of household mobility increases, and there is a qualitative difference between almost-perfect and perfect mobilty, that is, a discontinuity at zero migration costs.