Hot, muggy weather created dangerous conditions for residents of the South and Midwest on Tuesday, and there were reports of heat-related deaths. We all know that it's impossible to link any particular heat wave to the phenomenon of global warming, but those of us suffering in humid areas have to be wondering—is the Earth getting wetter, too?
Most climatologists think so. The planet's total humidity seems likely to rise in the coming years. But there's a difference between that figure—which represents the mass of all the water vapor in the air—and the planet's relative humidity, which describes how close the air is, on average, to its saturation point at a given temperature. Total humidity is the more important metric for the planet, because water vapor is itself a greenhouse gas. Relative humidity, on the other hand, is more closely associated with human comfort, because it affects your ability to cool off by sweating. Few scientists profess to know with certainty what's going to happen to either measure over the next few decades or centuries. There's very little global data (PDF) on the issue, and those that do exist are in dispute. The majority view appears to be that relative humidity will remain more or less stable,and most climate change models are based on this assumption. If relative humidity holds constant while the temperature rises, there will be an increase in absolute humidity.
In recent years, a few well-known scientists have rejected the assumption of stable relative humidity, however, and we're now in the middle of a dust-up in the field. On one side, there is evidence that relative humidity can change significantly (PDF) over time, particularly at higher altitudes. There's even some indication that it has declined over the last half-century. On the other, scientists point to data that absolute humidity at ground level rose by about 2.2 percent (PDF) overall between 1973 and 2003. The increases were particularly significant in the tropics and the northern hemisphere. (Some parts of the globe dried out over that period, too.)
It's possible for both sides to be correct. If relative humidity declines modestly, significant increases in temperature would still lead to a rise in absolute humidity.
This isn't just a dispute over how sweaty your grandchildren are going to be. Absolute humidity levels have a powerful effect on temperature projections. If scientists are wrong about humidity, they could have the temperature projections wrong as well. Water vapor can create a feedback loop that accelerates the effects of other greenhouse gases like carbon dioxide and methane. As the climate warms, the air soaks up more moisture. The moisture then prevents heat from radiating through the atmosphere and into space, which warms the air further, enabling it to hold still more water. Most climate change models take this cycle into account.
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Explainer thanks Brian Soden of the University of Miami.