EUSTACH: SUMMARY RATIONALE

Significant changes related to human activities are occurring in Amazonia which could have global effects on the carbon balance, the concentrations of greenhouse gases and aerosol particles, and on the oxidising power of the atmosphere. Present scientific knowledge is inadequate to assess these changes and to determine their impacts on global change processes with the required degree of reliability. This presents a serious drawback for formulating possible outreach of the European Environmental and Climate Change policies.

The Amazon basin constitutes a large global store of carbon which exchanges with the atmosphere. This exchange is influenced by changes in land use and in relation to variations in climate and atmospheric chemistry. Both types of change introduce uncertainties in the global carbon balance and both may influence the carbon dioxide concentration of the atmosphere and thus interact with the climate system. Forest clearing and agricultural development in tropical forests are often cited as a large net source of CO2 to the atmosphere, with a net release estimated at 25-40% of the net release of CO2 from burning of fossil fuels. The experimental basis for quantitative estimates of the source strength is weak, however. Mature forests have been assumed to exist in a steady state, but this assumption has never been checked over long time periods and at a sufficiently large number of measurement sites. Recent short-term measurements in Amazonia indicate that undisturbed forests may be a net CO2 sink. Moreover, reconversion of abandoned farmland to secondary forest presents a considerable, but as yet unquantified CO2 sink.

Global atmospheric concentrations of N2O and CH4 have increased dramatically since pre-industrial times. The Amazonian tropical forest presents a significant source for these greenhouse gases, and it is widely believed to dominate current natural sources for both N2O and CH4. However, regional net emissions are not defined quantitatively, and the responses of emission rates to climate change or to forest clearing and associated agricultural development, land abandonment, and ecological succession, are not well understood.

The tropical troposphere is responsible for about 70% of the global atmospheric oxidation of long-lived gases including CH4, CO, HCFCs, and CH3Br, since the world's highest concentrations of OH are found there. Considerable uncertainty attaches to estimates of photochemical rates, however, because concentrations and sources of NOx, reactive hydrocarbons, and CO are not well-characterised. The forest soil and canopy are sources of NOx and hydrocarbons, respectively, and sinks for ozone, but our knowledge of the corresponding fluxes is poor.

Reactive species - NOx, CO, and hydrocarbons - may be exported from Amazonia to the global environment. Deep convection is a major mechanism for ventilation of the Amazon atmosphere. Reactive species such as NOx may thus be pumped to the upper troposphere where they have long lifetimes and hence large-scale effects on global atmosphere. However, the mass fluxes associated with convective processes are not well quantified. Lightning associated with deep convection is thought to represent a major global source of NOx, which can lead to substantial ozone production, but current estimates of this source vary by more than one order of magnitude.

Amazonia is one of the major direct sources of aerosols (primarily organic) to the global atmosphere. In addition, NH3 emitted from vegetation and soils can modify the content and phase of the atmospheric aerosol. The size and elemental composition of aerosol particles are important variables that influence their role as cloud condensation nuclei. Although aerosols and NH3 are efficiently scavenged by precipitation, export of only a small fraction could make a major contribution to aerosol budgets of the free troposphere.

To address most of these pressing questions, an international research initiative, called The Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) is planned to take place in the Amazon over 1997-2003. LBA is a multidisciplinary research effort, which has been jointly designed by Brazilian, European and North-American research communities in response to the world-wide concern about the fate of tropical rain forest and about the global implications of the changes, which the Amazon region has been undergoing. Moreover, the LBA-initiative is a direct response to the Climate Convention, which requires contributions to the global inventory of greenhouse gases.

The EUSTACH-LBA research project aims to contribute to LBA-initiative by focusing on following two overarching questions with global and European relevance:

• What is the contribution of Amazonia to the global atmospheric carbon balance?
• What is the contribution of Amazonia to the global atmospheric budgets of
  - radiatively active trace gases (H2O, CO2, CH4, N2O, O3) and aerosol particles?
  - other trace gases (NOx, NMHC's - esp. VOCs) which are chemically reactive in the global tropospheric ozone cycle ?

Furthermore, the project aims to contribute to understanding and quantification of the role of anthropogenic and natural processes in the tropics which have been shown to have global impacts and which may therefore alter the composition of the atmosphere over Europe. Moreover, information on trace gas fluxes and carbon stocks is required by Brazil under the terms of the Climate Convention, and is likely to influence national policy on land use, with possible global implications for conservation, biodiversity and economy. The EUSTACH-LBA project intends to broaden the scientific knowledge and data base on which the outreach of European Environmental and Climate Change policies can be assessed.