Bemnet Agata is a communications officer at the Tax Justice Network, where Alison Schultz is a research fellow.We are entering an age of permanent volatility.Climate change is making extreme weather more destructive. Geopolitical tensions are disrupting energy markets and supply chains. Governments are expected not only to decarbonise their economies, but to protect them against an increasingly unpredictable world. That requires sustained public investment at precisely the moment repeated shocks are placing ever greater pressure on public finances.Governments are rightly debating how to mobilise the trillions needed for the energy transition. Yet one of the largest untapped sources of climate finance requires neither higher corporate tax rates nor new international funds. It lies in correcting one of the oldest assumptions underpinning the international corporate tax system.One of the stranger features of the modern economy is that we no longer disagree about what a multinational corporation is—until the conversation turns to tax.