Final answer:
The primary source of major greenhouse gas increases over the past century is the burning of fossil fuels, with deforestation also contributing significantly.
Step-by-step explanation:
The main source of the three major greenhouse gases that have increased significantly over the past 100 years is the burning of fossil fuels (coal, gas, oil). This human activity is responsible for the majority of carbon dioxide (CO2), methane, and nitrous oxide emissions that contribute to the greenhouse effect and global warming. Large-scale deforestation also plays a significant role in increasing these gases in our atmosphere.
Since the Industrial Revolution, there has been a marked increase in greenhouse gas concentrations due to the use of fossil fuels as a source of energy by humankind and widespread deforestation. Carbon dioxide and similar gases act similarly to the glass panels of a greenhouse, by allowing short-wave radiation from the sun to enter but preventing the long-wave radiation of heat from escaping into space. This leads to an increase in global temperatures and has various climatic disturbances.
Compared to carbon dioxide, methane is in much smaller quantities in the atmosphere but is more effective per molecule. Human activities, including the use of fossil fuels, digestion processes in cattle, and decay of organic matter in landfills, have resulted in a 2.5 fold increase in atmospheric methane levels since the pre-industrial era. Both these gases, along with nitrous oxide, contribute significantly to the accelerated climate change we are witnessing today.