Global warming is not caused by the expansion of oil palm plantations but is due to increases in the intensity of GHG emissions affecting the earth’s atmosphere. Naturally the earth’s atmosphere is filled with GHGs especially water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrogen (N2) in certain natural concentrations. Their function forms the mechanism of the natural greenhouse effect to protect and maintain the temperature of the earth’s atmosphere to be compatible with life. Through the mechanism of the natural GHG effect, a part of the sun’s energy is trapped in the earth’s atmosphere with the other part being reflected into outer space (Figure 1). Without the natural greenhouse effect, all solar energy would be reflected into outer space so that the temperature of the earth’s atmosphere would be very low and not compatible with life.
The intensity of the natural greenhouse effect increases when the GHG concentration in the earth’s atmosphere increases above its natural concentration. This is caused by rising CHG emissions from human activities on the earth and the emergence of man-made gases such as chlorofluorocarbons (CFCs) and halogen creating a human-enhanced greenhouse effect.
With the rising intensity of the greenhouse effect, the amount of radiation/solar energy that is trapped in the earth’s atmosphere also rises (Soemarwoto, 1992) from its natural condition, thereby making the earth’s temperature hotter. The increase in the temperature of the earth’s atmosphere, which we know as global warming is caused by the rising intensity of the greenhouse effect on the earth’s atmosphere.
According to the Intergovernmental Panel on Climate Change (IPCC, 1991) in the pre-industrial period up to the year 1990, CO2 in the earth’s atmosphere increased from 280 to 353 partsper million volume (ppmv). Meanwhile CH4 increased from 0.8 to 1.72 ppmv; N2O rose from 288 to 310 parts per billion volume (ppbv). And the CFC concentration increased from zero to 280-484 parts per trillion volume (pptv). And according to data from the International Energy Agency (IEA), the CO2 concentration in the earth’s atmosphere, which in 2005 reached 379 ppmv, increased to 396 ppmv in 2013 and to 399 ppmv in 2015 (IEA, 2016).
The increase in GHG concentrations in the earth’s atmosphere has been related to the activities of the global community since the pre-industrial era up to the present. According to the IEA (2016), the global GHG emission source is based on the GHG gas types, the largest of which is (Figure 2) from CO2 emissions (90 percent), followed by CH4 (9 percent) and N2O (1 percent).