By Ethan Simpson
With the 2015 hurricane season upon us, it is more important than ever to understand how the most important greenhouse gas, water vapor, effects global climatological processes.
The last forty years have seen a marked rise in public and scientific interest in the global phenomenon known as climate change. Specifically, much research effort has focused on the increasing role of greenhouse gases as drivers of the change. These greenhouse gases (GHG’s) act in the atmosphere to absorb and trap heat in Earth’s atmosphere, thus keeping our planet warm. While some of the minor contributing GHG’s can be traced to human sources, the largest contributors are all found naturally in the atmosphere. Water vapor, and by extension clouds, account for the majority of the greenhouse effect, but seem to receive the least amount of attention in the public sphere. Recent work, however has attempted to rectify this by evaluating the proportional contribution of water vapor and clouds to the overall Greenhouse effect.
According to Gavin Schmidt and colleagues, the three largest contributors to the greenhouse effect are water vapor, clouds, and CO2, in decreasing order. Their environmental models reveal that water vapor contributes half of the total greenhouse effect globally, with cloud cover contributing another quarter and CO2 roughly one fifth. This revelation makes it apparent that to truly understand climate change, a better understanding of how water vapor functions in the atmosphere will be necessary. The true effect water vapor has on global temperature rise is difficult to imagine on its own, but when the other minor GHG’s are included, the picture becomes much clearer. As Schmidt points out in his paper, even marginal increases in global temperature due to the other GHG’s such as CO2, would lead to increased evaporative processes and add more water vapor to atmosphere, further increasing the overall greenhouse effect.1 However, one confounding effect of increased water vapor would be an increase in cloud production. Clouds are known to both reflect sunlight, decreasing global temperatures, and to in other circumstances retain heat in the atmosphere by absorbing heat. Complex processes such as these are difficult to accurately predict and may play very large, yet currently unknown roles in climate change.
What are the major sources of water vapor in our atmosphere? Well, the largest shouldn’t surprise you. Evaporative processes from the Oceans contribute the majority of atmospheric water vapor, and the production of this water vapor is a major driver of Earth’s weather systems and deep oceanic currents. While we understand some of the major effects water vapor has on a global scale, future studies must strive to disentangle the complex relationships water vapor has with other greenhouse gases in the atmosphere. So as we enter the peak of hurricane season, which is itself driven by the condensation of water vapor into storm clouds, take a moment to appreciate the immense power contained in atmospheric water vapor.
Schmidt, G. A., R. A. Ruedy, R. L. Miller, and A. A. Lacis (2010), Attribution of the present‐day total greenhouse effect, J. Geophys. Res., 115, D20106, doi:10.1029/2010JD014287.