evidence: Water vapour is the most abundant greenhouse gas in the atmosphere, yet other greenhouse gases (such as carbon dioxide and methane) are often portrayed as the main drivers of climate change. Why is that? Water vapour is the most abundant greenhouse gas in the atmosphere, both by weight and by volume (1), (2). Water vapour is also an effective greenhouse gas, as it does absorb longwave radiation and radiates it back to the surface, thus contributing to warming. When compared to other greenhouse gases, water vapour stays in the atmosphere for a much shorter period of time. Water vapour will generally stay in the atmosphere for days (before precipitating out) while other greenhouse gases, such as carbon dioxide or methane, will stay in the atmosphere for a much longer period of time (ranging from years to centuries) thus contributing to warming for an extended period of time.