A recently published paper appearing in the PNAS authored by et al reveals that calculations of water storage in many river basins from commonly used global computer models differ markedly from storage estimates from GRACE satellites.
Source: PNAS, et al
Because we increasingly rely on models to project the impacts of humans and climate on water resources, it’s crucial we know how reliable these models really are.
In total the authors evaluated model reliability based on a comparison of decadal trends (2002–2014) in land water storage from seven global models (WGHM, PCR-GLOBWB, GLDAS NOAH, MOSAIC, VIC, CLM, and CLSM) to trends from three Gravity Recovery and Climate Experiment (GRACE) satellite solutions in 186 river basins, representing roughly 60% of global land area.
The GRACE satellites are operated by NASA and the German Aerospace Center and measure changes in the force of gravity across the Earth, which is influenced by changes in the water storage in an area.
They found that the models underestimate the large decadal (2002–2014) trends in water storage relative to GRACE satellites. The authors summarized: “The poor agreement between models and GRACE underscores the challenges remaining for global models to capture human or climate impacts on global water storage trends.”
In summary, the model results calculated a decline in global water storage during the study period, but GRACE data showed an increase!
For example, in the Amazon, GRACE estimates a large increasing trend of ∼43 km3/y, whereas most models estimate decreasing trends (−71 to 11 km3/y). Land water storage trends, summed over all basins, are positive for GRACE (∼71–82 km3/y) but negative for models (−450 to −12 km3/y), contributing opposing trends to global mean sea level change. Impacts of climate forcing on decadal land water storage trends exceed those of modeled human intervention by about a factor of 2.
As the authors note, policymakers and planning experts increasingly rely on models for impacts of humans and climate on water resources, so their accuracy is important. But the paper study shows that relying on the models would be totally unwise and would lead to totally bad decision making.
It’s time for policymakers to realize that climate models of any type cannot be used to prepare for the future.