To determine the main idea of the passage, we analyze the information presented. The passage states that climate change is "one of the biggest challenges facing humanity" and highlights its impact on the environment, economy, and health. It emphasizes the need for "immediate action" to "reduce greenhouse gas emissions." This clearly indicates that the main focus is on the urgency of addressing climate change.
Thus, the correct main idea from the given options is: Immediate action is needed to combat climate change.
Step 1: Identify key points
Climate change impacts environment, economy, and health.
Step 2: Note concluding statement
Immediate action is required to reduce emissions.
Step 3: Main idea
The passage emphasizes urgent measures against climate change.