evidence: 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. [...] Water vapour is the most abundant GHG in the atmosphere and is the biggest overall contributor to the greenhouse effect. However, almost all the water vapour in the atmosphere comes from natural processes. Human emissions are very small and thus relatively less impactful. [...] 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.