Assessment of spatio-temporal variation of net terrestrial ecosystem CO2 exchange over west Asia and around the Mediterranean sea (1982-2019)

The terrestrial ecosystem plays an important role in the carbon cycle and each year absorbs more than a quarter of human carbon emissions, which is called terrestrial carbon sinks. In the last few decades, carbon sources and sinks in West Asia and around the Mediterranean Sea have changed due to drastic changes in land cover due to economic development and urbanization growth. In the present study, using the observations-based CO2 flux data of Jena CarboScope Institute, temperature and precipitation data, the seasonal and annual pattern of CO2 exchanges during the period 1982-2019, has been investigated, as well as the role of teleconnection patterns on the CO2 flux fluctuations was analyzed. The results showed that on long term average, the land areas of the northern Mediterranean Sea were CO2 sinks with absorption up to -114 g.C/(m2.year) and the amount of CO2 absorption in this areas have decreased with a trend of up to 5 g.C/(m2.year.decade) during the study period. While the CO2 sinks in the western parts of India (up to -70 g.C/(m2.year)) increased with the trend of up to -8 g.C/(m2.year.decade). Also, the central regions (including Iran, Saudi Arabia and the northern parts of Africa), the northeast (including Afghanistan, Turkmenistan and Tajikistan) and the southwest (including the central regions of Africa) of the study area, which were the CO2 sources on average (between 0 to 50 g.C/(m2.year)), the intensity of CO2 emissions has been reduced with a trend of up to -10 g.C/(m2.year.decade). Based on the results, in the land areas of the northern Mediterranean Sea, Turkmenistan, Tajikistan and the coastal areas of the Caspian Sea in Iran (western parts of India, western Iran and central Africa) in the study area, seasonal changes in temperature (precipitation) played a more important role in the seasonal cycle of CO2 exchanges between the atmosphere and the terrestrial ecosystem. The positive correlation between the MEI.v2 index related to El Niño/Southern Oscillation (ENSO) and the variability of the atmosphere-land CO2 flux showed that in El Niño conditions (positive and warm phase of ENSO) the amount of CO2 absorption and sink in the land areas of the northern Mediterranean Sea and West India, Southeast Iran and Pakistan decreased, which can be due to the decrease in rainfall in El Niño conditions in these areas. In La Niño conditions (negative and cold phase of ENSO) in these areas, the amount of CO2 absorption increased and the sinks became stronger. The negative correlation of atmospheric-land CO2 flux variability with both El Niño/Southern Oscillation and Indian Ocean Dipole Oscillation showed that in El Niño and positive IOD conditions, CO2 emission decreased in the western and central parts of Africa in the study area, while in La Niño and negative IOD conditions CO2 emission increased in these regions. Also, in the positive phase of IOD, the CO2 sink in the eastern part of Iran, Afghanistan, Turkmenistan, Tajikistan, the southeastern regions of Iran, southern Pakistan, and western parts of India became weaker, and in the negative phase of IOD, the amount of CO2 absorption in these areas increased.