which statement is most accurate? which statement is most accurate? the greenhouse effect is a natural phenomenon. human activity has increased the amount of greenhouse gases in the atmosphere over the past 250,000 years. the greenhouse effect is a human-caused phenomenon. human activity has increased the amount of greenhouse gases in the atmosphere over the past 250,000 years. the greenhouse effect is a natural phenomenon. human activity has increased the amount of greenhouse gases in the atmosphere over the past 250 years. the greenhouse effect is a human-caused phenomenon. human activity has increased the amount of greenhouse gases in the atmosphere over the past 250 years.