The United Nations should impose a moratorium on “geo-engineering” projects such as artificial volcanoes and vast cloud-seeding schemes to fight climate change, green groups say, fearing they could harm nature and mankind.

The risks were too great because the impacts of manipulating nature on a vast scale were not fully known, the groups said at a major U.N. meeting in Japan aimed at combating increasing losses of plant and animal species.

Envoys from nearly 200 countries are gathered in Nagoya, Japan, to agree targets to fight the destruction of forests, rivers and coral reefs that provide resources and services central to livelihoods and economies.