evidence: Back in the 1990s, the hole in the planet's ozone layer was a pressing global crisis – if we had ignored it, today there would be several. [...] And the future is not without risks. Major volcanic eruptions typically result in short-term ozone losses, while nitrous oxide, a powerful greenhouse gas emitted from fertiliser applications in agriculture, is also a potent ozone-depleting substance. However, it's not controlled by the Montreal Protocol, explains Revell – and emissions are growing. [...] There’s a tendency to compare the ozone hole to climate change, yet while the Montreal Protocol does demonstrate we can tackle large environmental problems the comparison only goes so far. CFCs were a replaceable component of a few products. The scope of climate change makes it considerably more difficult to address; fossil fuels are pervasive throughout our lifestyle, they cannot be replaced nearly as easily and most governments and industry have, thus far, resisted reducing fossil fuel emissions.