The Crucial Role of Water Feedbacks in Global Warming

One of the most important features of our climate system, and the computer models developed to represent it, is feedbacks. Most people don’t know that without positive feedbacks, the climate would be so insensitive to CO₂ and other greenhouse gases that global warming wouldn’t be a concern. Positive feedbacks amplify global warming, while negative feedbacks tamp it down.

A doubling of CO₂, acting entirely on its own, would raise global temperatures only by a modest 1.1 degrees Celsius (2.0 degrees Fahrenheit). In climate models, it’s positive feedback from water vapor – by far the most abundant greenhouse gas – and, to a lesser extent, feedback from clouds, snow and ice, that boosts the warming effect of doubled CO₂ alone to the predicted very likely range of 2 degrees Celsius (3.6 degrees Fahrenheit) to 5 degrees Celsius (9 degrees Fahrenheit).

Contributions of the various greenhouse gases to global warming can be surmised from the figure below, which depicts the wavelength spectrum of thermal radiation transmitted through the atmosphere, where wavelength is measured in micrometers. Greenhouse gases cause warming by absorbing a substantial portion of the cooling longwave radiation emitted upwards by the earth. The lower panels of the figure show how water vapor absorbs strongly in several wavelength bands that don’t overlap CO₂.

The assumption that water vapor feedback is positive and not negative was originally made by the Swedish chemist Svante Arrhenius over a century ago. The feedback arises when slight CO₂-induced warming of the earth causes more water to evaporate from oceans and lakes, and the extra moisture then adds to the heat-trapping water vapor already in the atmosphere. This amplifies the warming even more.

The magnitude of the feedback is critically dependent on how much of the extra water vapor ends up in the upper atmosphere as the planet warms, because that’s where heat escapes to outer space. An increase in moisture there means stronger, more positive water vapor feedback and thus more heat trapping.

The concentration of water vapor in the atmosphere declines steeply with altitude, more than 95% of it being within 5 kilometers of the earth’s surface. Limited data do show that upper atmosphere humidity strengthened slightly in the tropics during the 30-year period from 1979 to 2009, during which the globe warmed by about 0.5 degrees Celsius (0.9 degrees Fahrenheit). However, the humidity diminished in the subtropics and possibly at higher latitudes also during this time.

But the predicted warming of 2 degrees Celsius (3.6 degrees Fahrenheit) to 5 degrees Celsius (9 degrees Fahrenheit) for doubled CO₂ assumes that the water vapor concentration in the upper atmosphere increases at all latitudes as it heats up. In the absence of observational evidence for this assumption, we can’t be at all sure that the water vapor feedback is strong enough to produce temperatures in the predicted range.

The uncertainty over CO₂ warming is exacerbated by lack of knowledge about another water feedback, from clouds. As I’ve discussed in another post, cloud feedback can be either positive or negative.

Positive cloud feedback is normally associated with an increase in high-level clouds such as cirrus clouds, which allow most of the sun’s incoming shortwave radiation to penetrate, but also act as a blanket inhibiting the escape of longwave heat radiation to space. More high-level clouds amplify warming that in turn evaporates more water and produces yet more clouds.

Negative cloud feedback can arise from a warming-induced increase in low-level clouds such as cumulus and stratus clouds. These clouds reflect 30-60% of the sun’s radiation back into space, acting like a parasol and thus cooling the earth’s surface. The cooling results in less evaporation that then reduces new cloud formation.

Conversely, a decrease in high-level clouds would imply negative cloud feedback, while a decrease in low-level clouds would imply positive feedback. Because of all these possibilities, together with the paucity of empirical data, it’s simply not known whether net cloud feedback in the earth’s climate system is one or the other – positive or negative.

If overall cloud feedback is negative, rather than positive as the computer models suggest, it’s possible that negative feedbacks in the climate system from the lapse rate (the rate of temperature decrease with altitude in the lower atmosphere) and clouds dominate the positive feedbacks from water vapor, and from snow and ice. This would mean that the response of the climate to added CO₂ in the atmosphere is to lessen, rather than magnify, the temperature increase from CO₂ acting alone, the opposite of what climate models tell us. Most feedbacks in nature are negative, keeping the natural world stable.

So until water feedbacks are better understood, there’s little scientific justification for any political action on CO₂ emissions.

Next: El Niño and La Niña May Influence the Climate More than Greenhouse Gases