That study was a really good read.
Other knock on effects:
The importance of stratospheric H2O is well established; it affects stratospheric chemistry and dynamics as well as atmospheric radiation. For example, excess stratospheric H2O could lead to enhanced OH concentrations, slightly enhancing O3 production through the CH4 oxidation cycle but worsening O3 depletion through the HOx cycle, leading to a net decrease in O3 (e.g., Dvortsov & Solomon, 2001; Stenke & Grewe, 2005). The enhanced OH concentrations could also increase the loss of CH4, resulting in a decrease in its lifetime (e.g., Ko et al., 2013; Stevenson et al., 2020) and thus reducing its long-term effect on climate. In addition, if enhanced H2O concentrations were to be entrained into the developing Antarctic vortex to an extent sufficient to raise the formation temperature of polar stratospheric clouds, then the earlier onset of heterogeneous processing would exacerbate cumulative chemical O3 loss. In terms of transport, a study of the dynamical response to a uniform doubling of stratospheric H2O concluded that such moistening could reduce stratospheric temperature and increase the strength of the BDC; it could also result in the tropospheric westerly jets becoming stronger and storm tracks shifting poleward (Maycock et al., 2013). Since the HT-HH injection is ∼10% of the stratospheric H2O burden, a dynamical response of lesser magnitude than that found by Maycock et al. (2013) would be expected.
H2O enters the stratosphere primarily in the tropics, where it freeze-dries at the cold point tropopause (Brewer, 1949). This mechanism gives rise to the “tape recorder,” whereby the annual cycle in tropopause temperatures is imprinted in alternating bands of dry and moist air rising in the tropical stratosphere (Mote et al., 1996). By short-circuiting the pathway through the cold point, HT-HH has disrupted this “heartbeat” signal (Figure 5a).
Consistent with the freeze-drying mechanism, unusually low tropopause temperatures around 2001 led to a sharp drop in the amount of H2O entering the stratosphere (e.g., Randel et al., 2006; Rosenlof & Reid, 2008; Figure 5). This dry anomaly propagated via the BDC (Randel et al., 2006; Urban et al., 2014), slowly rising through the stratosphere and moving toward the poles. Using the propagation of the 2001 H2O drop as described by Brinkop et al. (2016) as an analog for the transport of the HT-HH plume, we expect that ascent could carry volcanic H2O to 10 hPa within ∼9 months. The excess H2O could arrive in northern and southern midlatitudes in ∼18 and ∼24 months, respectively, over a broad domain in the upper stratosphere. Since part of the plume has entered the lower branch of the BDC, the elevated H2O may reach lower stratospheric midlatitudes within a few months. The timescale for complete dissipation of the plume may be 5–10 years (Hall & Waugh, 1997).
Radiative calculations of the sudden drop in H2O of ∼0.4 ppmv (at 100 hPa) in 2001 (Figure 5b) demonstrated that the radiative forcing from even small variations in lower stratospheric H2O could induce decadal-scale changes in global-mean surface temperature (e.g., Solomon et al., 2010). The unprecedented HT-HH enhancement would correspond to ∼1.5 ppmv (at 31 hPa) if averaged over 60°S–60°N.
Previous studies of the radiative effects of stratospheric H2O perturbations, including direct volcanic injection, have shown that they can cause surface warming (e.g., Joshi & Jones, 2009; Rind & Lonergan, 1995). As established in Section 3, the HT-HH eruption was unusual in that it injected extremely large amounts of H2O. Preliminary climate model simulations (see Supporting Information S1 for details) suggest an effective radiative forcing (e.g., Forster et al., 2001; Myhre et al., 2013; Smith et al., 2020; Wang et al., 2017) at the tropopause of +0.15 Wm−2 due to the stratospheric H2O enhancement (Figure S3b in Supporting Information S1). For comparison, the radiative forcing increase due to the CO2 growth from 1996 to 2005 was about +0.26 Wm−2 (Solomon et al., 2010).
The HT-HH H2O enhancement will exert a positive radiative forcing on the surface, offsetting the surface cooling caused by the aerosol radiative forcing (e.g., Sellitto et al., 2022; Zhang et al., 2022). Given the extraordinary magnitude of the HT-HH H2O injection and the fact that its anticipated stratospheric residence time exceeds the typical 2–3 years timescale for sulfate aerosols to fall out of the stratosphere (Joshi & Jones, 2009), HT-HH may be the first volcanic eruption observed to impact climate not through surface cooling caused by volcanic sulfate aerosols, but rather through surface warming caused by excess H2O radiative forcing.
In summary, MLS measurements indicate that an exceptional amount of H2O was injected directly into the stratosphere by the HT-HH eruption. We estimate that the magnitude of the injection constituted at least 10% of the total stratospheric H2O burden. On the day of the eruption, the H2O plume reached ∼53 km altitude. The H2O injection bypassed the cold point tropopause, disrupted the H2O tape recorder signal, set a new record for H2O injection height in the 17-year MLS record, and could alter stratospheric chemistry and dynamics as the long-lived H2O plume propagates through the stratosphere in the BDC. Unlike previous strong eruptions in the satellite era, HT-HH could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the excess stratospheric H2O forcing. Given the potential high-impact consequences of the HT-HH H2O injection, it is critical to continue monitoring volcanic gases from this eruption and future ones to better quantify their varying roles in climate.
