In the Paris climate agreement, countries agreed to keep global warming below 2 °C above pre-industrial levels and to strive for the far less risky limit of 1.5 °C. However, their actual pledges set the world on track for a catastrophic 3 °C of warming, and, to make matters worse, major polluters like the US are backing away from their meager commitments. Now, a new report from the UN Intergovernmental Panel on Climate Change shows that current trends will take the world past the 1.5 °C goal in only a few decades. The difference between a 1.5 °C world and a 2 °C world (not to speak of a 3 °C world) is stark. An extra half degree of warming is likely to mean food scarcity and sea level rise affecting millions more people and severe habitat loss for countless species around the globe. Current trends need not continue: reforestation, escalated deployment of renewable energy, and more sustainable patterns of diet and transportation could eliminate carbon emissions by 2050. It’s past time for an unprecedented mass movement worldwide to demand action from recalcitrant governments. The clock keeps ticking.