Humans cause climate change. How do we know?

The document outlines how scientists use climate models, atmospheric records, and carbon isotope signatures to prove that human-induced greenhouse gas emissions are the primary driver of global warming since the mid-20th century. It highlights the consensus among the scientific community and major bodies like the IPCC, while noting that attribution science can now link specific extreme weather events to climate change.

• Atmospheric carbon dioxide (CO2) concentrations have risen from approximately 280 parts per million (ppm) in the pre-industrial era to over 410 ppm currently, reaching the highest levels seen in over 800,000 years according to ice core data.
• Scientists use a specific ratio of carbon isotopes, known as δ13C, as a 'smoking gun' to distinguish CO2 produced by burning fossil fuels from other sources, confirming that fossil fuel emissions are the main cause of rising CO2 levels since the 1880s.
• Observed warming patterns since the mid-20th century cannot be accurately reproduced by climate models using only natural drivers; however, they match reality when human-induced drivers are included. Human activity is responsible for more than half of the warming observed since 1951.
• Attribution science has enabled the quantification of human influence on specific extreme weather events. For example, climate change at least doubled the risk of the 2003 European summer heat wave and made the record-breaking precipitation of Hurricane Harvey in 2017 three times more likely and 15 percent more intense.
• There is a strong scientific consensus, with at least 97 percent of scientists agreeing that human activity is the primary cause of global warming, a position supported by the 2018 US National Climate Assessment and the Intergovernmental Panel on Climate Change (IPCC).

APA

Union of Concerned Scientists (2009). Humans cause climate change. How do we know? https://www.ucs.org/resources/are-humans-major-cause-global-warming

Chicago

Union of Concerned Scientists. Humans cause climate change. How do we know? 2009. https://www.ucs.org/resources/are-humans-major-cause-global-warming.

Wikipedia

{{cite report |author=Union of Concerned Scientists |title=Humans cause climate change. How do we know? |date=14 July 2009 |url=https://www.ucs.org/resources/are-humans-major-cause-global-warming |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{unionofconcernedscientists2009humans, author = {{Union of Concerned Scientists}}, title = {{Humans cause climate change. How do we know?}}, institution = {Union of Concerned Scientists}, year = {2009}, month = jul, url = {https://www.ucs.org/resources/are-humans-major-cause-global-warming}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }