For many years, scientists argued that variations in carbon dioxide levels being observed were natural, and that climate change was not a problem. At some point, however, many of these same scientists began to draw a different conclusion. Which of the following observations would have caused many scientists to conclude that climate change is occurring?

Current levels of CO 2 are higher than any observed over the last 400,000 years.

Current CO 2 level measurements are more accurate than they were 50 years ago.

Today scientists have more sophisticated computer programs for graphing CO 2 levels.

Today scientists have a better understanding of how CO 2 is distributed in the atmosphere.