Is the atmosphere drying up?

A recent set of posts at Anthony Watt’s blog, particularly this one has sparked some interest over the internet as of late. From a quick glance, it looks like negative trends in specific humidity over the last half a century. Readers were quick to pick up on the connection to water vapor feedback, which is expected to at least double the sensitivity of climate to external perturbations (e.g., by human released CO2).

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U.S. Climate report- Weather and Climate Extremes in a Changing Climate

Although global temperature rise is widely discussed concerning the severity of climate change, a more societally relevant concern is how weather and climate will vary at the local and regional level. Temperature rises over very large spatial scales (e.g., a hemisphere or the globe) do not imply uniform changes of various climatic variables (hurricanes, droughts, storms, etc) but we should expect global inhomogenities in climate as greenhouse gases build up in the atmosphere. For example, changes in global mean precipitation (or evaporation) are not incredibly large in a warmer world; models tend to agree that there is a less than 2% increase per degree C global mean warming. However, changes in horizontal transport and increased precipitation gradients (drier areas getting drier, wetter areas getting wetter, droughts in areas, flooding in others, etc) are a big reason for concern. What’s more, people will inevitably be concerned about the likelihood of extremes, such as the possibility of more anomalous events like the European heatwave of 2003.

The first federal review of research on how global warming may affect extreme climate events in North America is available here. Regions of focus include North America, Hawaii, Caribbean, and U.S. Pacific Islands. A summary of findings below: