Limited effect of ozone reductions on the 20-year photosynthesis trend at Harvard forest.

Ozone (O3 ) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O3 concentrations ([O3 ]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long-term changes in ambient [O3 ] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O3 ] as a predictor was slight, and independent of O3 concentrations, which suggests limited high-frequency O3 inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long-term O3 inhibition of 10.4% for 1992-2011. A decline of [O3 ] over the measurement period resulted in moderate predicted GPP trends of 0.02-0.04 µmol C m-2  s-1  yr-1 , which is negligible relative to the total observed GPP trend of 0.41 µmol C m-2  s-1  yr-1 . A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O3 ]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O3 ] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O3 ], thus helping to buffer the changes of total photosynthesis due to varied [O3 ]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.

The shift of decadal trend in Middle East dust activities attributed to North Tropical Atlantic variability.

The Middle East, as the world's second-largest dust source region, has dust emissions that significantly impact numerous populated areas, extending from North America to South Asia. Over the past two decades, dust activity in the Middle East has exhibited pronounced variability, with a notable trend shift from positive to negative around 2010. The underlying cause of this trend shift remains elusive. In this study, we employ multi-source datasets and global climate model simulations to demonstrate that the variability of Middle East dust activities is closely tied to changes in North Tropical Atlantic (NTA) sea surface temperature (SST). Specifically, a warm NTA SST anomaly generates an anomalous regional zonal cell characterized by ascending air motion above the NTA and descending air surrounding the Middle East. The associated surface high pressures around the Middle East subsequently induce hot and dry conditions accompanied by intensified Shamal winds in the north, which are favorable for dust emission and transport. The shift in SST trends from positive to negative in the NTA around 2010 is therefore responsible for the observed dust trend shift in the Middle East. This mechanism holds vital implications for predicting decadal dust variability over the Middle East region and further the project of global environment.