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Q&A: IPCC 5th Assessment Report

AR5 Working Group I Final Report
Climate 2013: The Physical Science Basis



What is the IPCC?

The Intergovernmental Panel on Climate Change (IPCC) is an international body responsible for assessing the science related to climate change. It was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to provide policy makers with reports detailing the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation. More information about IPCC and the IPCC process is available here.

What is the most recent report released by the IPCC?

The most recent report released is Climate Change 2013: The Physical Science Basis, which is the final report of Working Group 1 (WG1) of the Fifth Assessment Report (AR5) of the IPCC. The final draft was released in September 2013 and the final edited publication is scheduled to be released in January 2014.

The Working Group 1 contribution to AR5 provides a comprehensive assessment of the physical science basis of climate change, drawing on the scientific literature accepted for publication up to March 15, 2013. It includes contributions from more than 600 authors from 32 countries and includes citations to more than 9,200 scientific publications. The review process elicited nearly 55,000 comments from 1,089 expert reviewers and 38 governments. Subsequent reports that are part of AR5 will include Impacts, Adaptation, and Vulnerabilities in March 2014; Mitigation of Climate Change in April 2014; and a Synthesis Report in October 2014. More information about IPCC AR5 is available here.

How is this report different from others released by the IPCC?

Unlike previous Assessment Reports, WG1 AR5 includes chapters on sea level change, the carbon cycle, and climate phenomena such as monsoons and their relevance to future, regional climate change. In addition, the report includes coverage of the science of clouds and aerosols and extended analysis of climate change predictions based on an assessment of both near- and long-term projections. More information about the IPCC WG1 AR5 report is available here (pdf).

Who at WHOI was involved in the report?

Scientists from around the world were involved in the research, writing, reviewing, and editing process of the WG1. In addition to the many authors and co-authors of peer-reviewed research cited throughout the report, the following individuals served as Contributing Authors on individual chapters:

Physical oceanographer Ray Schmitt: Chapter 3, Observations: Ocean (pdf)
Paleoclimatologist Kevin Anchukaitis: Chapter 5, Information from Paleoclimate Archives (pdf)
Marine chemist Scott Doney: Chapter 6, Carbon and Other Biogeochemical Cycles (pdf)
Physical oceanographer Caroline Ummenhofer: Chapter 14, Climate Phenomena and their Relevance for Future Regional Climate Change (pdf)

What are the main findings that pertain to ocean systems?

According to the report, warming of the climate system is unequivocal and Earth’s surface has become successively warmer in each succeeding decade since 1850. As the atmosphere and ocean have warmed, concentrations of atmospheric greenhouse gases have increased, the amounts of snow and ice have diminished, and the sea level has risen.

Human influence on the climate system has been made clear by the increasing greenhouse gas concentrations in the atmosphere and positive radiative forcing. The atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have increased to levels unprecedented in at least the last 800,000 years. Carbon dioxide concentrations, in particular, have increased by 40 percent over pre-industrial times, primarily due to fossil fuel emissions and, secondarily, from net land use change emissions. As a result, it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century and this evidence has grown since the previous IPCC report, AR4.

It is very likely* that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises. Global glacier volume will further decrease. The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. Over the period from 1901 to 2010, global mean sea level rose by 0.19m (0.17 to 0.21m).

It is virtually certain* that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely* warmed between the 1870s and 1971. The global ocean will continue to warm during the 21st century and this heat will penetrate from the surface to the deep ocean and affect ocean circulation.

Changes in the global water cycle in response to the warming over the 21st century will not be uniform. The observed, long-term trends in sea-surface salinity show an intensification of both evaporation and precipitation in the ocean over the past 50 years--salty areas are getting saltier and fresh areas fresher. Similar trends on land are expected to occur, such that the contrast in precipitation between wet and dry regions and between wet and dry seasons will increase. Moreover, because a warmer atmosphere can carry more moisture, evaporation and rainfall rates will intensify leading to more frequent floods in wet areas and more severe droughts in dry areas.

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90 percent of the energy accumulated between 1971 and 2010. The ocean has also absorbed about 30 percent of the emitted anthropogenic carbon dioxide, causing ocean acidification.

Climate models have improved since AR4. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions. Combined observational and model studies of temperature change, climate feedbacks, and changes in the Earth’s energy budget provide confidence in the magnitude of global warming in response to past and future forcing.

Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.

Source: Adapted from Headline Statements from the Summary of Policymakers (pdf)

*NOTE: The following terms are used here to indicate the assessed likelihood of the outcomes described:

Term                    Likelihood of outcome
Virtually certain     99–100% probability
Very likely             90–100% probability
Likely                    66–100% probability

Additional terms that appear throughout the AR5 WG1 report are used to indicate varying levels of likelihood associated with other specific outcomes. In addition, the report assigns confidence levels to statements about the likelihood of outcomes based on the nature and agreement of lines of evidence. For more information, refer to the source below.

Source: Working Group 1 Contribution to the IPCC Fifth Assessment Report (AR5), Climate Change 2013: The Physical Science Basis; Technical Summary (pdf).

Last updated: October 25, 2013