Principal Investigator

At a Glance

Assessments of alternative mitigation strategies to limit the impact of global change increasingly rely on simulations of Earth System Models (ESMs). In the tropics, a major biodiversity refuge and a net sink for anthropogenic carbon dioxide (CO2) emissions, ESMs consistently project that forests will thrive through the century due to CO2 fertilization. In contrast, ecological models warn about a potential catastrophic forest loss under future drying conditions. A team of CMI researchers from the Pacala group and NOAA-GFDL used a state-of-the-art ESM to assess the impact of global change on tropical forest dynamics under alternative emission scenarios. Their ESM accounts, for the first time, for complex ecological mechanisms and large-scale biophysical forcing of vegetation dynamics. This research has broad consequences for the monitoring and management of tropical forests and opens new avenues for the design and implementation of carbon mitigation strategies. Of particular relevance to bp’s natural climate solutions initiatives, this work informs the stability of carbon mitigated through avoided deforestation in tropical regions.


Land Model LM4.1 development and testing were guided by extensive analyses of global databases and of more than 30 years of continued study and monitoring at Barro Colorado Island (BCI), Panama.



Research Highlight

The scientific consensus about the future of tropical forests remains unsettled. One body of literature, rooted in field observations and ecological theory, predicts that the Amazon rainforest will cross a tipping point in a few decades. As climate change brings more frequent extremes such as droughts and fires, increases in tree mortality and arrested recovery may trigger a rapid transition to grass-dominated savanna ecosystems. On the other hand, modeling studies based on Earth System Models (ESMs) come to the opposite conclusion, and consistently predict that tropical forests will thrive through the century.

To reconcile these opposite views, researchers in the Pacala group working in collaboration with climate modelers from NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL), have developed an ESM with realistic fire, height-structured competition and subgrid-scale heterogeneity. Such an approach enables the model to simulate the successional mosaic in ecological models of savannas, coupled to largescale, climate-vegetation interactions. Simulation experiments with GFDL-ESM4 project that, under the extreme emission scenario SSP5-8.5, Amazon forests may begin to convert to savanna before mid-century as a consequence of increased forest fires. Projected fires resemble contemporary responses to dry conditions associated with El Niño Southern Oscillation and the Atlantic Multidecadal Oscillation, exacerbated by an overall decline in precipitation. Following the initial disturbance, grassland dominance promotes recurrent fires and competitive exclusion, which lead to an arrested successional state that prevents forest recovery and impairs the tropical carbon sink.

The effort of CMI and NOAA-GFDL researchers provides a unique tool to understanding the interaction of forest management with the dynamics of disturbance and successional recovery in forest ecosystems. Climate-induced tree mortality due to fires emerges as a potential key driver of forest damage during this century, prompting the inclusion of fire disturbances and height-structured competition in other ESMs. The team continues working to improve the ability of ESMs to assess the impact of meteorological extremes on global forests and the potential onset of abrupt transitions in land ecosystems.

Figure 11.1.
Emergence of alternative states and hysteresis in the structure of tropical vegetation along a gradient of water availability. The schematic highlights key mechanisms implemented in the dynamic land model LM4.1 embedded in ESM4.1. Low precipitation regimes favor the dominance of grasslands and savannas where seasonal fuel accumulation promotes recurrent fires that keep a state of arrested succession. At the other extreme, high precipitation regimes converge toward a high tree cover state where the closed tree canopy inhibits grasses, reduces evaporative water loss and increases transpiration to enhance moisture recycling at regional scales. Fire and humidity feedback mechanisms reinforce the resilience of each state and result in their coexistence at intermediate precipitation levels, where the dominant formation becomes contingent to past conditions. After a string of wet years, trees may be able to displace grasses, form a closed canopy and reach a new alternative equilibrium. As conditions become drier, a closed forest canopy resiliently keeps humidity and prevents its own collapse until disturbances like fires prompt an abrupt transition to the low cover state.