Climate change detection and attribution have proven unexpectedly challenging during the 21st century. Earth’s global surface temperature increased less rapidly from 2000 to 2015 than during the last half of the 20th century, even though greenhouse gas concentrations continued to increase. A probable explanation is the mitigation of anthropogenic warming by La Niña cooling and declining solar irradiance. Physical climate models overestimated recent global warming because they did not generate the observed phase of La Niña cooling and may also have underestimated cooling by declining solar irradiance. Ongoing scientific investigations continue to seek alternative explanations to account for the divergence of simulated and observed climate change in the early 21st century, which IPCC termed a “global warming hiatus.” Amplified by media commentary, the suggestions by these studies that “missing” mechanisms may be influencing climate exacerbates confusion among policy makers, the public and other stakeholders about the causes and reality of modern climate change.

Understanding and communicating the causes of climate change in the next 20 years may be equally challenging. Predictions of the modulation of projected anthropogenic warming by natural processes have limited skill. The rapid warming at the end of 2015, for example, is not a resumption of anthropogenic warming but rather an amplification of ongoing warming by El Niño. Furthermore, emerging feedbacks and tipping points precipitated by, for example, melting summer Arctic sea ice may alter Earth’s global temperature in ways that even the most sophisticated physical climate models do not yet replicate.

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