Which of the following statements in their research paper would need a citation

1. "The average temperature in Antarctica has increased by 2.5 degrees Celsius over the past two decades."

2. "Previous studies have shown a correlation between air pollution and respiratory illness."
3. "The use of renewable energy sources has been proven to reduce carbon emissions."
4. "According to a survey conducted by Smith et al. in 2015, 80% of respondents reported feeling stressed at work."