We know this from climate models, statistical predictions of how fossil fuel use will impact our climate. And they say that if we carry on as we have, temperatures will rise to 5°C, with catastrophic consequences. Already for a 4°C rise, a 2013 World Bank report concluded there is no certainty that humans can adapt. To avoid this, we have established a consensus to limit global warming to 2°C by 2100, with a high probability that humans can adapt. But associated with the 2° target is a fixed limit as to how much total CO2 we can emit. This is our carbon budget.