Final answer:
Natural processes affecting greenhouse gas levels include volcanic activity and respiration, while human processes include fossil fuel combustion and deforestation. The continued rise in emissions, particularly from fossil fuels, poses significant risks to global climate stability.
Step-by-step explanation:
Two natural processes that can increase or decrease the amount of greenhouse gases in the atmosphere are volcanic eruptions and biological respiration. Volcanic eruptions release carbon dioxide, a greenhouse gas, while biological respiration both releases and absorbs CO₂ depending on whether it is occurring in producers (like plants during photosynthesis) or consumers. Conversely, two human processes that affect greenhouse gas levels are burning of fossil fuels and deforestation. The burning of fossil fuels, such as coal, oil, and natural gas for energy, drastically increases the amount of CO₂ and methane in the atmosphere, while deforestation reduces the number of trees that can absorb CO₂, increasing the concentration of greenhouse gases.
If the current trend of rising carbon emissions from the burning of fossil fuels continues, the consequences can be severe, including accelerated global warming, extreme weather conditions, and significant ecological and socioeconomic impacts. Agriculture, including animal husbandry practices, also contributes to the increased levels of greenhouse gases like methane and nitrous oxide.