Fig. 4. Partial Least Square Regression models to evaluate the potential association between trends from Essential Climatic Variables (EVCs) and trends of low-cloud fraction (ΔCF). a. Describe the number of latent components required to optimally explain the ΔCF variability, b. illustrates observed and predicted relationships, c. displays the Root Mean Squared Error (RMSE) of 5000 iterative observed-predicted models, and d. presents the Variable of Importance of Prediction (VIP) of each ECVs. Error bars represent the standard deviation in all panels.\ref{339889}\ref{788330}

Discussion

The cloudiness of TMCF is vital to the functioning of these ecosystems. It has been predicted that the cloudiness of these ecosystems will likely decline (Helmer et al., 2019), but until now, recent trends have not been formally quantified. Using state-of-the-art climatic databases, our study assessed trends of low CF in these ecosystems, revealing that most of the evaluated TMCF already present low-cloud reductions. We further unpack the association of the observed trends with other ECVs globally and regionally at the biographical realm level. The following sections highlight the key aspects of our findings and offer broad insights into future TMCF.

Trends of low-cloud fraction

Our results based on ERA5 reanalysis indicate that TMCFs face reductions in low-clouds at higher rates than tropical landmasses. Our estimates and their uncertainties at the biogeographic realm level also reveal that the decline in low-clouds might be associated with regional drivers; thus, it cannot be concluded that all TMCFs are experiencing cloudiness decreases. In this sense, Neotropical TMCFs are among the sites with the highest reductions in low-clouds. Neotropical TMCFs are also likely to be the most affected, given their substantial increases in surface temperature, dew point, pressure, and PET, and their significant reductions in precipitation and VSWC. Predictions by Helmer et al. (2019) suggest that 57% to 86% of the existing Neotropical TMCF zone area will experience decreases in cloud immersion. Similarly, our results indicate that 79.1% of the evaluated Neotropical TMCFs already showed reductions in low-clouds. Furthermore, our perception that regional factors may be responsible for cloudiness declines in some TMCF is supported by Los et al. (2021), who observed an increase in cloud base height in America’s TMCFs over the past four decades, but a decrease in Asia’s TMCFs. Helmer et al. (2019) predictions, Los et al. (2021) findings, and our results support the idea that Neotropical TMCFs are under threat, and thus species and essential ecosystem services that these ecosystems provide (Mayer et al., 2022; Mulligan, 2021).