Evolving Assessments of Human and Natural Contributions to Climate Change

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As Congress continues to deliberate whether and how to address climate change, a key question has been the degree to which humans have influenced observed global climate change. Members of Congress sometimes stress that policies or actions “must be based on sound science.” Officials in the Trump Administration have expressed uncertainty about the human influence, and some have called for public debate on the topic.

To help inform policymaking, researchers and major scientific assessment processes have analyzed the attribution of observed climate change to various possible causes. Scientific assessments of both climate change and the extent to which humans have influenced it have varied in expressed confidence over time but have achieved greater scientific consensus. The latest major U.S. assessment, the Climate Science Special Report (CSSR), was released in October 2017 by the U.S. Global Change Research Program (USGCRP). It stated:

“It is extremely likely [>95% likelihood] that human influence has been the dominant cause of the observed warming since the mid-20th Century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”

This CRS report provides context for the CSSR’s statement by tracing the evolution of scientific understanding and confidence regarding the drivers of recent global climate change.

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