evidence: Greenhouse gas (GHG) emissions – the atmospheric gases responsible for causing global warming and climatic change – are critical to understanding and addressing the climate crisis. Despite an initial dip in global GHG emissions due to COVID-19, the United Nations Environment Programme’s latest Emissions Gap Report (EGR) expects a strong rebound in 2021, when emissions are expected to be only slightly lower than the record levels of 2019. While most GHGs are naturally occurring, human activities have also been leading to a problematic increase in the amount of GHG emitted and their concentration in the atmosphere. This increased concentration, in turn, can lead to adverse effects on climate. Effects include increases in the frequency and intensity of extreme weather events – including flooding, droughts, wildfires and hurricanes – that affect millions of people and cause trillions in economic losses. [...] However, GHGs, unlike other atmospheric gases such as oxygen and nitrogen, are opaque to outgoing infrared radiation. As the concentration of GHGs in the atmosphere increases due to human-caused emissions, energy radiated from the surface becomes trapped in the atmosphere, unable to escape the planet. This energy returns to the surface, where it is reabsorbed.