Background
The authors indicate that the Amazon watershed is the largest on earth, that it carries about 20% of the globe's freshwater discharge, and that its tropical forests account for approximately 10% of the world's terrestrial productivity and biomass. And, therefore, in light of the Amazon's great significance to the planet ...

What was done
... Joetzjer et al. wrote that their study "aims at evaluating and comparing precipitation over the Amazon in two sets of historical and future climate simulations based on phase 3 (CMIP3) and 5 (CMIP5) of the Coupled Model Intercomparison Project," wherein they selected thirteen models to answer the following questions: (1) Is there any improvement in the models' ability to capture present-day precipitation in terms of its mean annual cycle, spatial distribution and inter-annual variability? and (2) Is there any change in the models' response to climate change and any reduction in the associated uncertainties? And in accomplishing these tasks, they say that they employed the Global Precipitation Climatology Center data set (Rudolf et al., 2011) and the Hadley Centre HadSST monthly SST [sea surface temperature] climatology (Rayner et al., 2003)," both of which provided data at 1° resolution for the 1901-2009 period.

What was learned
The four researchers report that while "significant improvements have been made from CMIP3 to CMIP5 to capture present-day precipitation over the Amazon basin," they say that "strong uncertainties remain in the climate projections" that "arise from contrasted anomalies in both moisture convergence and evapotranspiration," which "might be related to the diverse response of tropical SST and ENSO (El Niņo Southern Oscillation) variability, as well as to spurious behaviors among the models that show the most extreme response."

What it means
As Joetzjer et al. succinctly summarize the implications of their findings in the concluding sentence of their paper's abstract, "model improvements of present-day climate do not necessarily translate into more reliable projections and further efforts are needed for constraining the pattern of the SST response and the soil moisture feedback in global climate scenarios."

References
Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

Rudolf, B., Becker, A., Schneider, U., Meyer-Christoffer, A. and Ziese, M. 2011. New GPCC full data reanalysis version 5 provides high-quality gridded monthly precipitation data. GEWEX News 21: 4-5.