This paper takes a closer look at the potentially huge economic and fiscal transition costs of global decarbonization in economies dependent on fossil fuel exports. The document identifies 40 economies heavily dependent on fossil fuels. These countries are estimated to lose more than 60 percent in oil rents alone over the period 2023-2040 under a net-zero global decarbonization scenario by 2050, compared to a “business as usual” scenario reflecting established policies. Analysis of local projections provides evidence of potential large adverse impacts on growth, government revenues and debt from a rapid decline in global demand for fossil fuels in emerging markets and net-exporting developing economies. Finally, the paper discusses mitigating domestic and international policy options to assist countries.