Six months of combined drought and heat hit 75% of tropical forests in the 2023-24 El Niño

In this study, "unprecedented" is a local measure. The team defined extremes with an anomaly-based approach that accounts for each region's baseline climate. An event counts as extreme against what typically happens in that place ^[6]. So the 75% figure ^[2] measures how far each forest moved from its own history. A forest that is usually wet and one that is usually seasonal can both qualify, at very different absolute temperatures and rainfall totals. The remaining quarter of tropical forest area stayed short of the six-month mark ^[14].

The corresponding author, Johanna Van Passel, is a scientist at Ghent University and a visiting researcher at the Institute of Biology of the State University of Campinas in Brazil ^[9]. "During the 2023-2024 El Niño, the area and duration of droughts and heat waves were the largest and most extensive in the past 25 years. When we apply the same models to the future, we see that temperatures are rising. We're concerned because, unfortunately, it could be even worse now," Van Passel said ^[8].

Both forward projections are multiples of a baseline likelihood. How many drought years per decade the Amazon multiple means depends on how often droughts that severe happened in the reference period ^[4]. Of the two, I would give more weight to the tropics-wide heat result. It holds on the lower-emissions path, the one in which the world reaches net-zero CO2 around 2075 ^[7]. That makes it a description of change that cutting emissions on that schedule does not prevent. The Amazon figure assumes emissions keep rising, and it applies to the end of the century ^[4].

The carbon claim has less direct support. According to the researchers, forests under thermal stress of this size undergo "functional collapse" and temporarily stop acting as sinks, emitting carbon instead ^[1]. The methods described in the account cover the climate the forests face: observed records, model projections and the local definition of extremes ^{[5] [6]}. The thing this doesn't tell you is how much carbon those months cost, or how long "temporarily" lasts. The account does not report a measured flux.

The authors frame the forest response as what comes next. Peter Groenendijk, a co-author and professor at IB-UNICAMP, studies ecological processes through tree growth rings ^[13]. "El Niño is one of the major modulators of the global climate. By seeking to understand its impacts, we can uncover various implications for forests, such as the mechanisms that lead to tree mortality," Groenendijk said ^[11]. He counts the comparison of one observed El Niño's drought and heat with future scenarios as the study's innovation, and in his view using multiple models made the results more robust ^[10]. Van Passel's own research looks at how heat and extreme drought affect tree growth and mortality, particularly in the Amazon ^[12].