Why do some scientists refute the claim that human activity is responsible for climate change?

Why do some scientists refute the claim that human activity is responsible for climate change?

4. What are the major human activities that contribute to carbon dioxide increase in the atmosphere?

5. Which of the many potential negative effects of rapid climate change do you believe will have the greatest economic costs in the U.S.? Which things will have the greatest economic costs in less developed regions of the world?