Final answer:
All of the above. Natural factors like Earth's orbital changes, solar radiation, and volcanic activity have influenced global temperatures, but the rapid and sustained increase in global warming is primarily due to human activities since the industrial era.
Step-by-step explanation:
The factors that have led to slow increases in global temperatures, except for human activities, include variations in the Earth's orbit, solar radiation changes, and volcanic activity. Variations in Earth's orbit and changes in solar intensity are long-term natural phenomena affecting the climate, although, in recent times, these factors have not been the primary cause of the observed warming trend. Volcanic activity can lead to temporary cooling due to the release of aerosols reflecting sunlight but does not account for the sustained rise in global temperatures. However, human activities, primarily through the burning of fossil fuels and altering the carbon cycle, have extensively contributed to the increase of greenhouse gases leading to the encroachment of the greenhouse effect and thus rapid and sustained global warming, especially since the mid-20th century.
Global temperatures have been influenced by various natural factors, including variations in the Earth's orbit, changes in solar radiation, and volcanic activity. However, human activities have not been a natural factor that has led to slow increases in global temperatures. Instead, human activities, particularly the combustion of fossil fuels, have significantly contributed to the recent rapid increases in global temperatures.