Considering intentional stratospheric dehydration for climate benefits

Abstract

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.

Fig. 1.. Schematic of the intentional stratospheric dehydration (ISD) concept to reduce water vapor transport across the tropical tropopause and into the stratosphere.

(A) A 22-day back trajectory for an air parcel ending at the solid circle and starting at the open circle; this is for an air mass that rose into the stratosphere near the WCP, as modeled by HYSPLIT and adjusted for clarity. (B) HYSPLIT altitude associated with the parcel. (C) Synthetic data to conceptually represent the temperature and relative humidity with respect to ice (RH_i) fluctuations experienced by an air parcel with few INP that does not experience high enough RH_i to initiate homogeneous nucleation of ice. An injection of INP either when the air mass exhibits a clear supersaturation or in advance of the supersaturation condition removes the WV generating the supersaturation with respect to ice at the low temperature of the TTL by forming ice that falls out of the air mass [shown in (D)]. After injection, the air mass continues its journey to the stratosphere with additional temperature fluctuations. Once in the stratosphere, temperature increases quickly with altitude, so relative humidity quickly decreases with altitude above the tropopause. The shaded areas represent the change in relative humidity and in WV content in the air mass due to the loss of ice generated by injection of INP, with the high-INP RH_i remaining a fixed fraction of the low-INP RH_i case after the ice has fallen from the air mass. Homogeneous nucleation occurs at RH_i of ~200% at the relevant temperatures (9).

Fig. 2.. Distribution of ice saturation ratios in the clear skies of the tropics.

Data from the Airborne Tropical TRopopause EXperiment (ATTREX) campaign. As air rises into lower temperature altitudes with fixed WV content, relative humidity with respect to ice (RH_i) increases, with increasing fractions of air showing supersaturations. The dashed line shows the relative humidity required for homogeneous nucleation from Schneider et al. (9).

Fig. 3.. Exploration of the length scales of supersaturated air masses in the tropical tropopause layer.

ATTREX observations of air parcels with >120% relative humidity with respect to ice were sorted and indexed according to their duration in the measurement record. Shown are cumulative volume (assuming a fixed depth for each observed region), and the duration in seconds of each transect. This dataset included air sampled below the altitudes that are more likely representative of final dehydration conditions.