2024 News & Events

Could drying the stratosphere help cool the planet?

28 February 2024

Photo: Max Dollner, University of Vienna

Yes, but only to a small degree.

While human-caused carbon dioxide emissions are by far the most important driver of climate change, water vapor is actually the most abundant greenhouse gas, and is responsible for about half of Earth's natural greenhouse effect – the one that keeps our planet habitable.

Now, as scientists explore ways to address the impacts of climate change by removing excess heat-trapping carbon dioxide from the atmosphere and by reflecting sunshine back into space, one group of researchers has asked the question: Could removing some water vapor from the atmosphere also help mitigate climate change?

This is precisely the idea explored in a new research article published today in the journal Science Advances that authors have dubbed "intentional stratospheric dehydration" or ISD.

According to lead author Joshua Schwarz, a research physicist at NOAA's Chemical Sciences Laboratory, the ISD concept would involve dispersing small particles (known as ice nuclei) into high altitude regions of the atmosphere that are both very cold and super-saturated in water vapor. These nuclei would increase the formation of ice crystals that otherwise would not have formed.

"Pure water vapor doesn't readily form ice crystals. It helps to have a seed, a dust particle for example, for ice to form around," explained Schwarz.

If such seeds can be introduced into supersaturated air masses that are headed for the stratosphere, then some of the water vapor in that air will condense into ice and fall, thereby removing excess water vapor and dehydrating (at least partially) the stratosphere.

The basic physics behind the concept are straightforward in theory, but does such a region of the atmosphere exist? Indeed it does, and it has a name: the Western Pacific Cold Point (WCP) – a region of the atmosphere about the size of Australia.

The dominant route for water vapor to enter the stratosphere is through upward transport across the tropopause–the boundary separating the troposphere from the stratosphere – in the Tropics. The tropopause over the tropical western Pacific, at the WCP in particular, is known to be the decisive gateway for determining the amount of water vapor that is carried into the stratosphere.

The WCP is cold enough that it will naturally freeze-dry humid air by forming and raining out ice crystals. The problem goes back to the issue mentioned above – that pure water vapor doesn't readily form ice crystals – and the air in the WCP has few available ice nuclei. Without ice nuclei, the relative humidity of the air with respect to ice (RHi) has to be near 200% to spontaneously form ice crystals.

This means that although the WCP is very cold and frequently has air over 100% relative humidity, at times there is just not enough ice nuclei in the air to actually form ice. By adding ice nuclei, the ISD method would not seek to create a new mechanism to remove water vapor from the air – it would instead try to augment the one that already occurs in the WCP.

"In terms of effectiveness, the Western Pacific Cold Point is the ideal 'sweet spot.' That's why our focus was there," said Schwarz.

In their study, the researchers used a computer model to simulate the conditions of the WCP, driven by observations of temperature and motions of the tropical air near the stratosphere. This showed that the concept of ISD was effective in theory.

They also analyzed high-resolution measurements of water vapor and temperature collected during the 2014 NASA ATTREX (Airborne Tropical TRopopause EXperiment) mission. The ATTREX mission, based out of Guam, employed NASA's Global Hawk uncrewed aircraft to study the tropical tropopause and WCP region above the tropical Pacific.

The ATTREX measurements revealed that of the 550 regions of super-saturated air encountered by the Global Hawk, the available water vapor was heavily concentrated into a small fraction of them. Just 10% of the air parcels accounted for nearly all of the water vapor that could be potentially removed via ISD. If only small regions of the stratosphere were suitable targets, deploying ISD would be more feasible.

Based on this result, the authors estimated the climate effect of dispersing ice nuclei into only the most heavily supersaturated air in the WCP. The resulting scale of stratospheric water depletion equated to a reduction in radiative forcing of ~0.03 W/m2 , or about 1/70th of the 2.2 W/m2 induced by human-made CO2 emissions since 1750.

"It's a very small effect," said Schwarz, adding that ISD alone would not counteract a large fraction of the warming generated by CO2.

Nevertheless, ISD may be valuable as one element within a larger portfolio of climate intervention and mitigation strategies, since all of the methods being studied (e.g, stratospheric aerosol injection and marine cloud brightening) have different positive and negative outcomes and different timescales of effectiveness. These factors all go into deciding whether a method deserves further study.

If and when decisions about climate intervention are necessary, it will be critical for scientists to have adequately explored both the methods by which humanity might intentionally alter climate, and the wider implications of those methods. As Schwarz points out, research like this "helps distinguish the possible from the impossible."

Schwarz, J.P., R.S. Gao, T.D. Thornberry, A. Rollins, K.H. Rosenlof, R.W. Portmann, T.P. Bui, and E.A. Ray, Considering intentional dehydration of the stratosphere, Science Advances, doi:10.1126/sciadv.adk0593, 2024.


We introduce a climate intervention strategy focused on decreasing water vapor (WV) concentrations near the tropopause and in the stratosphere to increase outbound longwave radiation. The mechanism is the targeted injection of ice-nucleating particles (INP) in air supersaturated with respect to ice at high altitudes in the tropical entryway to the stratosphere. Ice formation in this region is a critical control of stratospheric WV. Recent airborne in situ data indicate that targeting only a small fraction of air parcels in the region would be sufficient to achieve substantial removal of water. This "intentional stratospheric dehydration" (ISD) strategy would not counteract a large fraction of the forcing from carbon dioxide but may contribute to a portfolio of climate interventions by acting with different time and length scales of impact and risk than other interventions that are already under consideration. We outline the idea, its plausibility, technical hurdles, and side effects to be considered.