There are concerns regarding the proposed Earth system boundary
Comment on ‘Closed groundwater storage in the Earth’ [Am. J. Phys. 78 (1999) 93, 117-82]
Groundwater is widely used for domestic and agricultural purposes, but is subject to increasing risks from overexploitation. We’ve defined a safe and just earth system boundary for groundwater and used derived estimates of the amount of stored water from the Gravity Recovery and Climate Experiment (GRACE). In the accompanying Comment2, Cuthbert et al. agree that our effort to define a groundwater ESB is timely and important, but they assert that our approach is “flawed, unsafe and unjust”. Their concerns reflect misunderstandings of the definition and purpose of the boundary, a misunderstanding of the ‘safe’ and ‘just’ concepts3 that underpin our work, a lack of confidence in the use of GRACE data to calculate changes in GWS, and the possibility of confusion related to the use of some terms.
The supplementary information relates to the ref. 1 (pages 19–20), we followed methodologies to subtract all other water storage components via the Global Land Data Assimilation (GLDAS) National Oceanic and Atmospheric Administration Land Surface Model.
The trend analyses were done using previously used methodologies for country11 and global12 trend analyses. There are approaches to resolving GRACE solutions that show strong correlations between GRACE solutions at different scales.
Subglobal Groundwater Destruction Under a Dynamical Pumping Requires an Understanding of the Dynamics of the Groundwater System
When the water is pumped, there must be some decrease in GWS. Since a system is initially indynamic equilibrium over a similar timescale to the reference period, there isn’t going to be any new groundwater being created under the proposed subglobal ESB.
Historic pumping may have already reached a new dynamic equilibrium before the reference period for which a subglobal ESB is calculated. If so, previous pumping could have been devastating ecosystems, causing subsidence and inducing saline intrusion, and still be ascertained as ‘safe’ according to the proposed definition.
If there is a trend upwards (natural or otherwise) within the reference period for reasons that have nothing to do with pumping, or the error in the GWS estimation causes an incorrect upwards trend, a subglobal ESB might not be transgressed despite sufficient pumping occurring to cause significant harm.
If long-term pumping has led to an improvement of industrial productivity in wealthier nations, for instance, some over-abstracted aquifers may still remain within the proposed boundary. The paper3 argues that interspecies justice and future intergenerational justice are not met if local GWS declines over time, but without framing the boundary robustly within an appreciation of the groundwater system dynamics, this is potentially increasing environmental injustice instead.
