The influence of the North Atlantic on vegetation greening patterns in the northern high latitudes

Rising CO₂ concentrations due to anthropogenic carbon emissions and the resulting warming raise expectations of an increase in biospheric activity in temperature-limited ecosystems. Early satellite observations since the 1980s confirm this expectation, revealing so-called "greening" trends of the high northern vegetation. However, since the early 2000s, these observational records show these greening trends have stagnated in high-latitude Eurasia (HLE), with many regions even reversing to browning trends. We propose here that decadal variations of the North Atlantic ocean could have contributed to these HLE browning trends. 

Our analysis shows that roughly 80% of HLE area has become drier in the last two decades compared to the previous decades. It is mainly in these drying regions that the vegetation exhibits browning trends. Satellite observations of vegetation and the ERA5 reanalysis show HLE browning to be concomitant with a stagnation of North Atlantic sea surface temperature (SST). North Atlantic SST was previously shown to potentially influence remote climate by modulating a circumglobal atmospheric Rossby wave train. Indeed, we find a precipitation decrease over Eurasia to potentially originate from this North Atlantic teleconnection, linking SST stagnation to the observed browning trend.

Next, we turn to fully-coupled Earth system models to assess the plausibility of the proposed cause-and-effect chain. We employ a pattern matching algorithm to select realizations with similar-to-observed North Atlantic SST variations from three large ensembles (MPI-GE, IPSL-LE, and CanESM5). These ensembles enable a clean separation of the unforced signal (internal variability) from the forced vegetation response (CO₂ forcing). Our results show that realizations that closely resemble the observed North Atlantic spatio-temporal SST pattern also simulate the respective wave-train and associated precipitation patterns over Eurasia that cause HLE vegetation to change. Thus, the models confirm that unforced decadal variations of HLE vegetation can be modulated by North Atlantic SST via changes in precipitation patterns. In addition, model simulations suggest that the relative decrease in vegetation greenness is accompanied by a reduction in land carbon uptake, such that changes in North Atlantic SST ultimately affect the global carbon balance.

This study therefore demonstrates that the recently observed trend in HLE browning may well be due to an unforced signal originating from the North Atlantic. This implies that even decades-long trends in biospheric variables can emerge from natural climate variability and thus could be incorrectly attributed to an external forcing. This has major implications for the understanding of biospheric dynamics, including carbon uptake and release processes.