To some scientists, the statements of US congressmen and senators about global warming can be baffling. Many legislators regularly deny that there is a scientific consensus, or even broad scientific support, for government action to address climate change. The public may wonder how some legislators come to views skeptical of any anthropogenic link to climate change—or even the presence of climate change itself. Could this resistance to climate science be due to the lack of a widely accepted scientific consensus presented in congressional hearings and elsewhere—an “information deficit”?

To answer this question, we recently assessed the content of congressional testimony related to either global warming or climate change from 1969 to 2007. For each piece of testimony, we recorded several characteristics about how the testimony discussed climate. For instance, we noted whether the testimony indicated that global warming or climate change was happening and whether any climate change was attributable (in part) to anthropogenic sources. The results suggest that Congress is not suffering from an information deficit. In fact, testimony to Congress—even under Republican reign—reflects the scientific consensus that humans are changing our planet’s climate.

What scientists told Congress. Among scientists who testified to Congress, 86 percent of those who had an identifiable stance indicated that climate change is happening. Their testimony accepts anthropogenic causes in 78 percent of cases where the matter is discussed. These numbers are similar to survey results of scientists from relevant disciplines.

More surprisingly, these majorities held up even under Republican-controlled congresses. Testimony under Republican-controlled congresses expressed a belief in anthropogenic sources of climate change in 75 percent of statements where any opinion on the matter was discussed. The similarity of opinion also held across different types of scientists’ testimony and across time generally.

Hearing is not believing. The results of our analysis have remarkable implications for how we understand the climate policy debate and how science can influence this process. While it is tempting to conclude that comments from legislators about the lack of consensus on climate can be addressed by additional testimony, the testimony has long had a strong majority supporting the presence of global warming and its link to human conduct. If legislators are not influenced by the testimony they heard, their positions must come from somewhere else. Some legislators are choosing not to accept that there is a consensus or need for aggressive policy change. The barrier is not an information deficit. While our research did not identify where the barrier may lie, future research would be well served to investigate the role of political ideology and lobbying carefully.

To many scientists, the large majorities of support for the existence of global warming may represent a scientific consensus. The same may be said of scientific opinion related to an anthropogenic link and the need for aggressive policy action. However, there is no scientific way to define consensus – a problem that has plagued democratic theorists for centuries. As a result, legislators always have the rhetorical option to say there is still no consensus—until the time that there is unanimity, which is an unlikely occurrence on any issue.

This leaves open the question of how scientists can better inform the congressional debate about climate. Merely providing more testimony related to anthropogenic causes of global warming, or calling for policy change, is not likely to persuade those legislators who are not already convinced of climate's importance as an issue. Greater understanding of the true barriers to acceptance of climate change as a pressing policy issue is essential. The problem is not as simple as ignorance—and not as easily addressed.