It's not CO2... It's H2O!

The Overlooked Impact of Water Vapor on Climate Dynamics

Water vapor plays a crucial role in Earth's climate system, acting both as a natural greenhouse gas (GHG) and as a participant in the hydrologic cycle. It's the most abundant GHG in the atmosphere, surpassing carbon dioxide (CO2) in its greenhouse effect contribution. This significant role is primarily because water vapor absorbs infrared radiation emitted from Earth's surface, trapping heat in the atmosphere and warming the planet.

Water vapor's concentration in the atmosphere is not uniformly distributed; it varies greatly in space and time, primarily due to differences in temperature and availability of moisture sources. Warm air can hold more water vapor than cold air, which explains why tropical regions have higher humidity levels than polar regions. The global distribution and movement of water vapor are influenced by atmospheric circulation patterns, including wind currents and storms, which transport moisture around the globe.

The relationship between water vapor and climate is complex and involves numerous feedback mechanisms. One of the most important is thought to be the water vapor feedback loop, where an initial warming of the atmosphere (for example, due to increased CO2 emissions) causes more water to evaporate. Since water vapor itself is a potent GHG, this additional moisture further warms the atmosphere, leading to more evaporation and even higher concentrations of water vapor, thus amplifying the initial warming in a positive feedback loop.

The Intergovernmental Panel on Climate Change (IPCC) has detailed the significant role of water vapor in climate change across various reports, highlighting its impact on the water cycle and its feedback mechanisms within the climate system.

Schematic representation of fast and slow responses of the atmospheric energy balance and global precipitation to radiative forcing. Source: https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-8/

Water vapor is recognized as a key climate variable, the most important gaseous source of infrared opacity in the atmosphere, and accounts for about 60-80% of the natural greenhouse effect for clear skies. This makes it the largest positive feedback in model projections of climate change, accounting for a substantial amount of projected warming.

The detailed consideration of water vapor by the IPCC underscores its critical role in understanding and predicting climate change impacts. However, a new study published this month challenges many of the IPCC’s assumptions about the role of water vapor in the climate system.

The article explores the role of water vapor in global warming and questions the common assumption that CO2 and other nonaqueous GHGs are the primary drivers of climate change. It challenges contemporary climate models that largely consider water vapor within positive feedback loops initiated by CO2 and other gases, suggesting that direct anthropogenic emissions of water vapor might play a more critical role than previously acknowledged.