Northeast Brazil (NEB) is a semiarid area located in the tropical zone, and has been undergoing an anthropogenic desertification process. Currently, about 22% of NEB area shows a high degree of environmental degradation. The climatic impacts of a large scale desertification in NEB are assessed using the CPTEC/COLA GCM (T62L28). Two 5-year integrations are performed. In the control case, NEB is covered by its natural vegetation, known as "caatinga" (broadleaf shrubs with perennial groundcover); in the desertification case, "caatinga" is replaced by desert (bare soil) over all NEB area. The results for the NEB wet season (March to May) are analysed.

Desertification leads to a statistically significant decrease in precipitation over the western part of NEB. Regionally (i.e., areal average over NEB), the hydrological cycle weakens: precipitation, evapotranspiration, moisture convergence, runoff and soil water storage decrease. The surface net radiation decreases due to albedo and surface temperature increase. Both sensible and latent heat fluxes decrease; this leads to a drier and cooler reference level (lowest GCM level). At the reference level, the easterly flow over NEB accelerates due to roughness length decrease. Bowen ratio decreases; this unexpected result happens due to the large increase in aerodynamic resistance.

The decrease of the surface net radiation is transferred to the top of the atmosphere; this leads to little changes in the atmosphere radiative cooling. Therefore, impacts in diabatic heating comes from changes in sensible heat and latent heat release; and since both decrease, diabatic heating also decreases. To keep atmospheric energy balance, there is an increase in adiabatic warming. This is related to a subsidence anomaly which is larger at the lower levels, and reaches a maximum at around 850 hPa: therefore, there are atmospheric divergence anomalies below 850 hPa, and convergence anomalies above it. These anomalies are responsible for the vertically integrated moisture convergence decrease. The positive feedback mechanism between precipitation and evapotranspiration, and the Charney's albedo mechanism, seem to be responsible for the regional climate changes.

On a larger scale, changes are not confined to NEB; precipitation increases over the ocean close to the northernmost part of NEB (N-NEB). At the lower levels (850 and 700 hPa), wind anomalies resemble the linear baroclinic response of a shallow atmospheric layer (850-700 hPa) to a tropical heat sink placed at the layer middle level; it means divergence at 850 and convergence at 700 hPa. The more meridionally confined structure of the precipitation anomaly dipole between NEB and N-NEB is related to the relatively shallow diabatic heating vertical profile that has a maximum around 850 hPa. The heat sink at NEB generates a mixed wave structure at 850 hPa, and this leads to a weak convergence over the ZCN; by positive feedback mechanisms, this convergence anomaly increases and gives rise to the positive precipitation anomaly.

Therefore, this study shows the possibility of significant climate impacts, both regional and on a larger scale, if the environmental degradation continues to take place in NEB.