Correct Response: D. Scientists have long predicted that increasing carbon dioxide levels in the atmosphere will change Earth's climate. One of the greatest scientific challenges, however, is to determine which changes are related to the climate system's natural variability and which changes are caused by human activities. The effect of water vapor on climate change (A) is complex, but it is accounted for in most climate models. Most climate data is generated by countries with data standards considered acceptable by the scientific community (B). Measuring these changes (C) is not challenging, but has been ongoing and has provided some of the best evidence for global warming.
