David Medvigy (Duke University)

Amazon in Perspective:Integrated Science for a Sustainable Future

Manaus, Brazil20 November 2008

Hydroclimatic consequences of land coverchange in the Amazon

Deforestation and the Amazon

--- Six major watersheds will loseat least 2/3 of their forest cover

The Scaling Challenge

1) In global simulations, the size of a grid cell (2-5 degrees) is much larger than the typical scale of deforestation (10s of km or smaller). Furthermore, the size of a grid cell is too large to represent important mesoscale effects.

2) Mesoscale simulations solve these resolution-related problems. However, they rely on boundary conditions from a global model.

months

years

decades

10s of km 4 degree continental global

deforestation

Most studies(GCMs)

Mesoscale models

3) One solution: The problems of resolution and boundary conditions can simultaneously be addressed using a variable-resolution global model.

OLAM

4) Ultimate objective: simulate the full, time-dependent mesoscale and global responses to deforestation.

The Ocean-Land-Atmosphere Model (OLAM):An expansion of RAMS into an Earth System Model

OLAM solves a finite-volume analog of the full compressible Navier-Stokes equations in conservation form, and exactly conserves mass and momentum.

Icosahedron

Unstructured Grid; No overlapping grid cell; No special nest communication; Each cell communicates directly with its neighbor independently of resolution

StaggeredC Grid;Shaved,

Cartesiancoordinates

Simulation Strategy1. Carried out 2 simulations from October 1996 through December 20002. Both used 50 km resolution over South America, expanding to 400 km for the rest of the world.3. One simulation used current land cover. The other used the projected land cover for 2050 from the Soares-Filho et al. (2006) business-as-usual (BAU) scenario.

Crop/mixed farmingEvergreen broadleaf forest

Wooded grassland

Deciduous shrub

Short grassDesert, bare soil

Other land cover

Water

CURRENT LAND COVER BAU 2050 PROJECTED LAND COVER

Simulation Average Precipitation [mm month-1] Current LC 154 BAU 2050 LC 156

Impact of deforestation on regional (15S – 5N; 75W – 50W) precipitation

Difference in average precipitation [mm y-1]2050 land cover minus current land cover

Impact of deforestation on precipitation

1. Increase in northeast part of basin2. Increase along eastern slopes of Andes3. Decrease in central Amazon

CURRENT > BAU CURRENT < BAU

Why is OLAM break from the standard paradigm?

2. The representation of topography is critical. Deforestation reduces simulated latent+sensible heat fluxes, but this has the effect of enhancing the importance of orographic forcing for lifting air parcels.

1. While deforested areas indeed have reduced latent heat flux, this is partially compensated for by increased latent heat flux in non-deforested areas. Because most GCM studies have assumed 100% deforestation, they miss this affect.

Latent Heat [W m-2],2050 – current land cover

Not deforested

3. In fact, when OLAM was run with a “total deforestation” scenario and coarse topography, the deforested case had 12% less precipitation. This is comparable to results obtained with the RAMS regional model.

Conclusions and next steps1. OLAM simulates only a small impact of deforestation on Amazon hydroclimate. This

contrasts with the results from other GCMs.

2. While most of the basin experiences decreased evapotranspiration, some (non-deforested) areas experience increased evapotranspiration. There is an overall increase in moisture convergence due to changes in roughness and land-sea pressure gradient.

3. A regional re-distribution of precipitation was simulated. Precipitation decreases in the central basin, but increases in non-deforested areas and near topography.

4. Previous, coarser-resolution studies differ (in part) because of assumptions of total deforestation and coarser representation of topography.

Current work:

-- In reality, the transition between “current” and deforested vegetation is gradual, playing out over decades. -- Furthermore, there are likely to be feedbacks between intact forest, the atmosphere, and deforested regions. -- Therefore, we are carrying out 50-100 yr simulations with OLAM coupled to the ED2 dynamic vegetation model.