Final answer:
The correct statement regarding Earth's natural climate variations is that natural factors such as volcanic eruptions and solar radiation contribute to climate variations. Human activities, particularly the burning of fossil fuels, have significantly enhanced the greenhouse effect and are the main cause of recent warming trends.
Step-by-step explanation:
Among the statements regarding Earth's natural climate variations, the correct one is c) Natural factors, such as volcanic eruptions and solar radiation, contribute to climate variations. Historically, Earth's climate has not remained constant but has experienced various natural changes. Natural influences on climate include events like volcanic activity, which can inject large amounts of ash and sulfur dioxide into the atmosphere, blocking sunlight and leading to temporary cooling. Changes in solar intensity, such as those that occurred during the 'Little Ice Age,' have also impacted global temperatures. Additionally, changes in Earth's orbit, known as Milankovitch cycles, influence climate on longer timescales.
While the greenhouse effect is a natural and significant factor in maintaining the habitable climate on Earth, human activities have altered the balance of this effect. Since the Industrial Revolution, the burning of fossil fuels has significantly increased the concentration of greenhouse gases, such as carbon dioxide, in the Earth's atmosphere, thereby enhancing the greenhouse effect and contributing to recent warming trends. This anthropogenic factor has been the dominant force in climate change since the mid-20th century.