We show that an unexpected tightening of the EU Emissions Trading System led high-emission-intensity firms to cut emissions relative to low-intensity peers within the same industry, without reducing output, thereby improving emission efficiency. Effects are stronger for power producers than for manufacturing firms. Examining mergers and acquisitions (M&As), we find that high-intensity manufacturing firms acquire more green targets after the tightening than low-intensity firms, with no change in the overall number of acquisitions, indicating a shift in focus rather than activity. Finally, we show that these green M&As contributed to the observed emission reductions over the study period.