Final answer:
The composition of the atmosphere is influenced by a combination of natural processes and human activities. Human contributions have markedly increased the levels of greenhouse gases like CO2 and methane since the industrial era. Water vapor, while the most abundant greenhouse gas, is mainly controlled by temperature and generally falls outside direct human influence on a global scale.
Step-by-step explanation:
The composition of the atmosphere is controlled by C) a combination of natural processes and human activities. Over Earth's history, the concentrations of atmospheric gases have changed due to natural processes, like volcanic eruptions, biological activity, and changes in solar intensity. However, since the industrial revolution, human activities have significantly altered these ratios, with the burning of fossil fuels and deforestation leading to a marked increase in carbon dioxide (CO2) levels, and activities such as livestock farming and landfill decomposition increasing methane (CH4) concentrations.
The increase in greenhouse gas concentrations due to human activity is clearly visible when we look at historic data from ice cores, which show that the atmospheric concentration of CO2 has risen from about 280 parts per million before the industrial era to 400 parts per million by 2016. Methane has increased 2.5 times since the industrial era and is now responsible for about 20 percent of the enhanced greenhouse effect. This significant impact of human activities on the atmosphere is an element that cannot be overlooked in discussions about climate change and global warming.
It's important to understand that while water vapor is the most abundant greenhouse gas, its global concentration is mainly controlled by temperature rather than direct human emissions, and thus is largely a product of natural processes.