Because there is a maximum amount of greenhouse gases we can emit to ensure a safe future for human life, and that limit is fast approaching. We can’t afford to aim for any less. In Paris last year, the world agreed to act to keep global warming to “well below 2°C”, with a goal of staying under 1.5°C. We have already exceeded one degree of warming. The evidence says we need to reduce global greenhouse gas emissions to net zero by around 2070 for a high chance of keeping global warming under 2°C - let alone 1.5°C. Carbon dioxide emissions from fossil fuels need to reach zero sooner, by around 2050. As a well-off country with huge renewable energy potential and lots of land for tree planting, New Zealand can and should aim to achieve zero carbon by 2050.