Climate Change

World’s Leading Climate Change Authority Releases its Fifth Assessment

By Amaury Laporte

September 30, 2013

On September 30, the Intergovernmental Panel on Climate Change (IPCC) published its fifth report assessing climate change science, Climate Change 2013: The Physical Science Basis. As the scientific intergovernmental body responsible for publishing information relevant to the implementation of the United Nations Framework Convention on Climate Change (UNFCCC), the IPCC's reports are considered the global benchmark in climate change understanding. According to the report, "Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased." Critically, the report concludes, "It is extremely likely [95 percent confidence] that human influence has been the dominant cause of the observed warming since the mid-20th century." This is the strongest, most emphatic language yet used by the IPCC to describe its confidence in the human causes of climate change.

U.S. Secretary of State John Kerry responded to the report’s release stating, “Once again, the science grows clearer, the case grows more compelling, and the costs of inaction grow beyond anything that anyone with conscience [...] should be willing to even contemplate. The science itself [is] demanding action from all of us." EESI Executive Director Carol Werner agreed , "It is time to wean ourselves off fossil fuels. It is time to match our policies and our actions to what science is telling us. It is time to listen to the voices of local leaders who already are facing the impacts of climate change on their vulnerable communities."

Climate Change 2013: The Physical Science Basis reflects the work of 259 climate scientists from 39 different countries, with input from thousands of additional government officials and experts, and draws on 9,200 peer-reviewed studies and publications regarding climate change. The report's summary for policymakers , released on Friday, September 27, was approved line-by-line by 195 governments at the United Nations last week. The report was drafted by Working Group I (WRI) , which is primarily concerned with the physical science of climate change and contains detailed information on projections and changes to the atmosphere, lithosphere, oceans and the poles due to climate change. Two additional sections of the report, Working Group II (WRII) and Working Group III (WRIII), will be published in March and April 2014. These later portions of the report will contain information on climate change impacts, adaptation, vulnerability, and mitigation.

Major findings of the WRI report include the determination that warming will likely reach at least 2 degrees Celsius by the end of century, leading to increased ice sheet melting, sea level rise, droughts and heat waves, additional ocean acidification as well as changes to precipitation amounts and patterns. The IPCC expects sea levels to rise 26 to 82cm (10-32 inches) by the end of the century, more than its last forecast (7-23 inches). It is important to note that these predictions are conservative. The IPCC's hundreds of volunteer scientists review thousands of published papers about climate change, and then reach their conclusions based on consensus. Such consensus-seeking eliminates the more alarmist studies.

For the first time, the IPCC assigns a global carbon budget to keep warming below an internationally agreed upon target of 2 degrees Celsius (3.6 degrees Fahrenheit), below which the worst effects of climate change should be avoided. The panel estimates that, to meet the 2-degree target, we must not burn more than 1 trillion tons of carbon from fossil fuels, and we have already burned more than half that since the beginning of the Industrial Revolution. If energy consumption rates remain unchanged, we will reach the 1 trillion mark in about 25 years, according to Oxford University's Myles R. Allen, one of the report's authors.


Authors: Jessie Stolark, Amaury Laporte


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