There was a time when things started rolling on with advent of photosynthesis by cyano-bacteria or green algae which began preparing its food from carbon dioxide and water which were the only substances available in plenty. They paved the way for introduction of oxygen into the atmosphere and for beginning of life dependent on oxygen.
In fact, both the products made by photosynthesis namely sugar and oxygen are used by animals including humans for sustaining their life. We animals are thus the species who learned to make use of the leg work of others to their own advantage. Of course, in today’s world, human beings who do this are termed smarter than the others.
So what the plants and algae used for making their food is being recycled back to the atmosphere in the form of green house gases. Plants use CO2 and H2O and took energy from the Sun to convert these to sugars and animals extract that energy from the food synthesized by plants and algae and return the CO2 and H2O back to the atmosphere.
CO2 is in fact one of the end products of burning the fossil fuels like coal and petroleum which are the preserved forms of energy of Sun due to being buried away from the reach of oxygen. Since all industrial activities use extensively these fuels, levels of CO2 are rising in the atmosphere to alarming levels.
Trees breathe the CO2 through the stomatal pores present in their leaves. They also lose the water through these pores during the day. They are smart enough to adjust the size of their leaves so as to optimize the intake of CO2 and loss of H2O for proper growth.
One such tree was born in 1948 in a isolated place on the edge of a pit 30 kilometers from Dutch city called Eindhoven. By 1990 it was a big majestic tree as most of the Birch family trees are. Scientists from the laboratory of Paleo-botany and Palynology named it fondly “Big Betty”.
During each autumn, the tree will shed a carpet of dry leaves into the pit. So layer after layer is deposited inside the pit. But scientists noticed a peculiar thing about the leave size. Each year the leave size began becoming smaller thus reducing the stomatal index. This was due to the fact that as levels of CO2 rose, the tree needed fewer number of stomata to inhale the same quantity as required and thus avoided the undue loss of moisture from the leaves. In fact when the levels of CO2 were plotted against the stomatal indices of leaves over these years, they matched the patterns of levels of CO2 variations. So the stomatal index was used to calibrate and interpolate the results to guess the levels of CO2 in the past.