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Evolving Assessments of Human and Natural Contributions to Climate Change

CRS report via – Evolving Assessments of Human and Natural Contributions to Climate Change, February 1, 2018.

“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. Climate change science can be traced back to the early 1800s. Through the 20th century, academic institutions, federal and state agencies, foreign governments, and other entities invested significant time and billions of dollars in climate research. This investment has led to substantial advances in empirical observations, atmospheric and ocean physics and chemistry, climate and economic simulation models, statistical methods, and other achievements. As a result, scientists have increased their confidence in their detection and understanding of climate change and attribution of observed changes to their causes. There is now high scientific confidence that the global climate is warming, primarily as a result of increased human-related greenhouse gas (GHG) emissions and other activities. This confidence has evolved from nearly two centuries of research and assessments. This report describes a chronology (in the Appendix) of 200 years of major scientific statements, selected to represent views at each time, regarding the human contribution to global climate change. The chronology demonstrates how scientific views and confidence in those views evolved over time. That GHGs, including carbon dioxide (CO2), water vapor, and other gases, warm the Earth’s climate is not a recent concept. The greenhouse effect, as it is sometimes called, was deduced as early as 18279 with relatively little dispute since the 19th century among scientists about the role of GHGs: Some level of GHGs in the atmosphere is necessary for maintaining a temperate climate on Earth. Instead, the debate that unfolded involved whether the climate had been warming overall10 and, if so, to what the changes may be attributable (such as industrial releases of GHGs, volcanoes, solar activity, or other natural variations).

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