In our view, your headline 'Don't link carbon markets' is poor advice to policymakers (J. Green Nature 543, 484–486; 2017). To cut carbon pollution at the pace and scale that science demands, we must create linkages that can tap into the most cost-effective reductions.

Contrary to Jessica Green's claim that trading works only as a closed system, the US cap-and-trade programme for sulfur dioxide succeeded alongside an assortment of state and federal standards. The fact that sulfur allowances now trade for a few cents is more vindication than failure, given the deep emissions cuts achieved by the programme and subsequent regulations.

As for existing carbon-trading schemes, they are meeting their targets — and can be strengthened over time. California passed an ambitious 2030 target into law last year and the European Union is working to improve its system. The Regional Greenhouse Gas Initiative in the northeastern United States has tightened its carbon cap once and is reassessing it with a view to restricting it further.

However, linking markets is not a panacea and requires care. Emissions-trading systems should stand on their own before linking with other compatible systems, and countries involved in trading should adopt common standards and guidelines to ensure environmental integrity.