Not so much the heat, but the humidity

The most prevalent greenhouse gas of all — water vapor — could become a little easier to represent in computer simulations thanks to a breakthrough observation by an international team of researchers.

The science team, which included Kei Yoshimura, a climate researcher at Scripps Institution of Oceanography, UC San Diego, created a map showing the movement of water vapor through the atmosphere.  By measuring the ratio of a certain water isotope using satellite date, they have found a way to follow vapor’s history of condensation, in essence, giving themselves a way to “age” water as it travels through the skies.

“It will help follow water’s fate in computer models and supplement direct observations of water,” said Yoshimura, a project scientist in Scripps’ Climate, Atmospheric Science and Physical Oceanography division.

The find is timely given that most climate change scenarios project an increase in levels of water vapor in the atmosphere as the planet warms and more evaporation takes place. Water vapor has a nuanced effect on climate, alternately making heat less bearable on humid days but also massing into clouds that cool Earth’s surface and reflect sunlight back into space. Researchers have sought to better characterize water vapor in the computer models they use to predict climate change.

The breakthrough that made the map possible was the researchers’ use of a sensor on board the European Space Agency (ESA)'s Environmental Satellite (ENVISAT) to look for ratios of “heavy water” — water molecules termed HDO that have the element deuterium in them  — to regular H₂O.  The ratio of the HDO isotope to H₂O is about 1 to 3,500.  Such inferences about atmospheric water content have been routinely made in the past but only by direct sampling of water vapor at limited regions and heights.  Other remote sensing measurements observed higher levels of the atmosphere by using far-infrared radiation, but because water vapor dominantly exists near the surface, scientists have sought to measure HDO at lower altitudes with wide spatial coverage.

The team, led by Christian Frankenberg of SRON-Netherlands Institute for Space Research, found a way to characterize water vapor near the surface by measuring for the first time with near infrared light bouncing to space from Earth’s surface. They did so through the use of an instrument employing a type of spectroscopy known as SCIAMACHY or scanning imaging absorption spectrometer for atmospheric cartography.

Variations in the measurement are an indication of how much HDO is mixed in with water. The map that results from this measurement will enable climate researchers to follow circulation of water in the atmosphere and identify trends in regional heating patterns.

“Water vapor in global climate models is still quite wrong right now,” Yoshimura said. “We want to validate water cycles in the atmosphere with some other metric and this can help.”

The team recently reported its findings in the journal Science.

• Robert Monroe