Tropical forests cover a huge area of South America, where they play key roles in the water and energy cycles of the atmosphere and also store enormous amounts of carbon. Some projections of climate change suggest that these forests may be threatened by increasing drought in the 21st Century. If the Amazon forest dies back the carbon now stored in the plants and soils would be released as dead vegetation decomposed, adding a lot of CO2 to the atmosphere and amplifying global warming. In addition, the loss of these forests would dramatically reduce the evaporation over the Amazon region, which might impact our climate in the Northern Hemisphere.

We will analyze the way that rainfall interacts with tropical forests using a new kind of model in which clouds and rainfall are computed on scales of a couple of miles. Most climate models use grid cells the size of Connecticut to compute rainfall over tropical forests. Spreading the rain out over these large areas makes it behave like constant drizzle that wets the leaves of the trees but rarely reaches the ground. By tracking the fate of rainwater on the scale of couple of miles, we get much more realistic weather: occasional heavy thunderstorms with dry periods in between. Less of the water evaporates from wet leaves and more of it gets into the soil where it can feed the roots of the trees.  This is a more realistic way to understand the impact of future warming and drying on these important forests.

We will use many kinds of real observations to better understand drought impacts on tropical forests. The northwest Amazon is very wet all the time, and rainfall decreases toward the southeast. In the southeast Amazon there is a pronounced dry season, but the forests are well adapted to drought of this kind. We will test different kinds of climate models by evaluating the way they respond to these seasonal droughts. Some colleagues have recently published results of experiments in which giant gutters were installed over a few acres of the Amazon for several years, as a way to subject the forest to artificial drought. We will use these measurements to test the way different climate models respond to prolonged droughts. We will also use new satellite data that can measure drought response of plants to evaluate model simulations of two very dry years in the Amazon forest. We will also study the way the carbon stored in trees and soils varies from the dry eastern Amazon to the much wetter western Amazon, to better estimate how much carbon might be released if the region dries out in the future.

Once we’ve thoroughly tested the way the new high-definition rainfall model works with existing tropical forests, we will then re-do climate model calculations for the 21st Century using the new model. We will study how the forests behave under a warming climate, and look for thresholds in rainfall and temperature that might trigger large-scale die-offs of these forests. We suspect that by representing the clouds and rainfall and their interaction with tropical forests over a few miles instead of hundreds of miles, the forest will turn out to be more resilient to climate change than it has been in existing models.