Seven in ten Americans think that global warming is happening, and six in ten think that, if it is happening, it is mostly owing to human activity, but only about one in five know that nearly all climate scientists agree that global warming is happening as a result of human activity. Those were among the key findings of Climate Change in the American Mind: December 2018 (PDF).
Presented with a definition of global warming as "the idea that the world's average temperature has been increasing over the past 150 years, may be increasing more in the future, and that the world's climate may change as a result" and asked whether they thought that global warming is happening, 73% of respondents said yes, 14% said no, and 13% indicated that they didn't know.
Asked about the cause of global warming, on the assumption that it is happening, 62% of respondents said that it is caused mostly by human activities, 23% said that it is caused mostly by natural changes in the environment, 8% opted for "neither because global warming isn't happening," and 6% volunteered that it was a mix of human activities and natural causes.
Asked to indicate "what percentage of climate scientists think that human-caused global warming is happening," only 20% selected a value between 91% and 100% — the correct range, as repeated independent studies have demonstrated. The mean of the values selected by the respondents was 72% (a new high); the median was 75%.
The study was conducted by the Yale Project on Climate Change Communication and the George Mason University Center for Climate Change Communication. The surveys were administered in a web-based environment from November 28 to December 11, 2018, using an on-line research panel of 1114 American adults.
According to the report, the sample "includes a representative cross-section of American adults — irrespective of whether they have Internet access, use only a cell phone, etc. Key demographic variables were weighted, post survey, to match U.S. Census Bureau norms." The margin of sampling error was plus or minus 3% at the 95% confidence level.