Release of Carbon Dioxide by Terrestrial Ecosystems Over Indonesia During the 1997–1998 ENSO Warm Event

Abstract

El Nino-Southern Oscillation (ENSO) is a global climatic phenomenon causing redistribution of rainfall from Southeast Asia into the Pacific and to the western coast of South America. The climatic conditions during ENSO warm events in insular Southeast Asia are characterized by a decrease in precipitation which is assumed to contribute to a reduction of photosynthetic activity of vegetation during such periods. Against this background, the study analyses the release of carbon dioxide in terrestrial ecosystems over the Indonesian archipelago during the El Nino event of 1997–1998 using satellite-based measurements of net primary production (NPP) and an ENSO proxy. We used 10-day time series of the NPP product derived from Advanced Very High Resolution Radiometer (AVHRR) by the Global Photosynthesis Ecosystem Monitoring (GLOPEM) group and the Multiple ENSO Index (MEI). Respiratory processes were estimated from complex mathematical models incorporating biome-dependent constants and climatic data. Using temporal correlation analysis (TCA), we were able to estimate statistically significant relations between dynamics of carbon dioxide exchange and the MEI variable, both at the scale of vegetation types and at per-pixel scale. The results revealed that the release of carbon dioxide caused by the 1997–1998 El Nino event differs between vegetation types and varies from one locality to another. Natural tropical rainforest experienced the weakest relative loss of carbon dioxide (mean value is 14.26gC/m²/month). For cropland, mosaics, and shrubland, the relative loss of carbon dioxide was 18.67gC/m²/month, 18.31gC/m²/month, and 17.05gC/m²/month respectively. These results confirm the hypothesis that, in general, tropical rainforest is better adapted to ENSO droughts than other land use systems. However, because of the largest area covered by the tropical rainforest in Indonesia (1.03 millionkm²), the total loss of carbon dioxide for this land cover category was the greatest (17.23 million tonnes). For cropland (0.16 million km²), mosaics (0.49 million km²), and shrubland (0.18 million km²), the total carbon dioxide release was 3.33, 13.31 and 7.33 million tonnes, respectively. The results of the study improve our understanding of ENSO impacts on the carbon cycle in tropical regions and demonstrate that the current conversion of tropical rainforest to other land use systems significantly increases the total discharge of carbon dioxide during ENSO warm events.