The costs of addressing climate change will be borne by firms through their investments, supply-chains, and products and services. Financial markets play a role in aggregating firm-level information on the costs of the transition but also on pricing these risks. We construct a carbon risk factor for 1,600 global firms with carbon risk data from four major ESG databases. This factor can be used as a straightforward measure of carbon beta absent firm-specific carbon emissions information. We compute the carbon beta of 39,000 global firms. Our factor can be used by firms, regulators and investors to better understand carbon risk.