IPCC Fourth Assessment Report (AR4)

Category: Science & Technology Key figures: Rajendra K. Pachauri (IPCC Chair, 2002–2015), Susan Solomon (WG I Co-Chair), Martin Parry (WG II Co-Chair), Bert Metz (WG III Co-Chair), Al Gore (co-recipient, 2007 Nobel Peace Prize), 450+ coordinating lead authors and 800+ contributing authors from 130+ countries

Summary

The Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report (AR4) in four installments across 2007, representing the most comprehensive international scientific evaluation of climate change produced to that date. Developed over six years by 450+ lead authors, 800+ contributing authors, and reviewed by 2,500+ expert reviewers from 130+ countries, the AR4 synthesized more than 6,000 peer-reviewed scientific studies. The report’s central conclusion—stated with greater certainty than any predecessor—was that “Warming of the climate system is unequivocal,” with a 90% or greater probability (“very likely” in IPCC confidence language) that most observed global warming since the mid-20th century is caused by human greenhouse gas emissions. The IPCC shared the Nobel Peace Prize with Al Gore on October 12, 2007, in recognition of the report’s contribution to building international scientific consensus on climate change.

Structure and Release Timeline

The AR4 was organized across three working groups and a synthesis report, each released separately:

• Working Group I (“The Physical Science Basis”), released February 2, 2007 in Paris, covered the basic physics of the climate system and the observed temperature, precipitation, and sea-level trends. This was the most-cited installment and the one that contained the key “very likely” attribution statement.
• Working Group II (“Impacts, Adaptation and Vulnerability”), released April 6, 2007 in Brussels, assessed projected impacts on natural and human systems across regions, as well as the scope for adaptation. It documented risk to water supply, agriculture, biodiversity, and coastal infrastructure.
• Working Group III (“Mitigation of Climate Change”), released May 4, 2007 in Bangkok, evaluated emission reduction options and their costs, including scenarios for limiting warming to 2°C and the technological and policy pathways to achieve them.
• Synthesis Report, released November 17, 2007 in Valencia, Spain, integrated findings from all three working groups into a coherent policy-relevant summary for the world’s governments.

Key Scientific Findings

Attribution of Warming

The report’s most consequential scientific statement was the AR4’s attribution finding: “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG [greenhouse gas] concentrations.” The phrase “very likely” corresponds to at least 90% probability in IPCC’s calibrated uncertainty language—a step up from the Third Assessment Report (AR3, 2001), which had used “likely” (66% confidence). This shift reflected nearly two decades of additional data, improved climate models, and the detection-and-attribution methodology that had matured since the 1990s.

Observed Temperature Trend

WG I documented that global mean surface temperature had increased by approximately 0.74°C (±0.18°C) over the 1906–2005 period. Eleven of the twelve years from 1995 to 2006 ranked among the twelve warmest years since instrumental records began in 1850, demonstrating the statistical robustness of the warming trend rather than attributing it to any single anomalous year.

Sea Level Rise

Global average sea level had risen at an average rate of 1.8 mm per year from 1961 to 2003, and at 3.1 mm per year from 1993 to 2003—suggesting acceleration. The 20th century total rise was estimated at 0.17 m (17 cm). Importantly, the AR4 acknowledged a major uncertainty gap: it could not reliably model dynamic ice sheet contributions from Greenland and Antarctica, meaning its 2100 sea level projections (0.18–0.59 m) were likely conservative. Subsequent research confirmed this.

Future Temperature Projections

The AR4 presented six emission scenarios (from the IPCC Special Report on Emissions Scenarios), producing a range of projected 2100 temperature increases:

This range—1.1°C to 6.4°C—became the framework cited in international climate policy discussions through the 2010s and established the scientific basis for the Copenhagen Accord’s 2°C target (2009) and the Paris Agreement’s 1.5°C and 2°C goals (2015).

Arctic Findings

The WG I report noted that Arctic temperatures had increased at almost twice the global average rate over the prior 100 years. Projections suggested continued Arctic amplification. These findings were strikingly confirmed in the same year by the September 2007 Arctic sea ice minimum, which fell approximately 24% below the previous satellite-era record—exceeding even the AR4’s most pessimistic near-term projections for ice loss.

Nobel Peace Prize

On October 12, 2007, the Norwegian Nobel Committee announced that the 2007 Nobel Peace Prize would be shared equally between the IPCC and Al Gore “for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change.” The formal award ceremony was held in Oslo on December 10, 2007; IPCC Chair Rajendra Pachauri delivered the Nobel Lecture on behalf of the IPCC.

Awarding the Peace Prize—rather than a physics or chemistry prize—was deliberately chosen to frame climate change as a security threat rather than solely a scientific or environmental concern. The Nobel Committee cited the risk that “climate change may induce large-scale migration and lead to greater competition for the earth’s resources,” making it a source of armed conflict and humanitarian catastrophe. This framing was significant in the same year that the Darfur conflict in Sudan—partly rooted in climate-linked desertification and resource competition—was designated “the world’s worst humanitarian crisis.” The IPCC award was the first time climate science was formally recognized in the Nobel Peace Prize’s history.

Policy Influence

The AR4 was the definitive scientific reference for international climate negotiations from 2007 onward. Its findings directly informed:

• Bali Road Map (December 2007): The UN Climate Change Conference (COP13) in Bali, Indonesia, opened in December 2007. The AR4 provided the scientific baseline for the Bali Road Map, which set a two-year timeline for negotiating a successor to the Kyoto Protocol—a process that eventually led to the Copenhagen Accord (2009) and the Paris Agreement (2015).
• Copenhagen Accord (2009): The Accord’s 2°C temperature limit was derived directly from the AR4’s scenarios and the cost-benefit analysis of WG III.
• National Climate Legislation: Multiple countries adopted or revised national climate legislation citing AR4 findings throughout 2008–2012, including the European Union’s 2009 Climate and Energy Package (setting 20-20-20 targets).

Scientific and Methodological Significance

The AR4’s methodology—international author teams, expert review cycles, consensus language calibrated to probability levels, and government approval of summary documents—established a template for how scientific consensus on a contested, high-stakes empirical question could be formally assessed and communicated to policymakers. Its three-tier confidence language (likely/very likely/virtually certain) allowed scientists to communicate degrees of uncertainty without either overstating certainty or creating false ambiguity.

The report’s separation into working groups also established a structural norm: physical science (WG I) as the empirical foundation, impact assessment (WG II) as the translation to human consequences, and mitigation analysis (WG III) as the policy options layer. This tripartite structure influenced how governments, media, and civil society organized their responses to the science.

Context: Concurrent Events of 2007

The AR4’s release coincided with a convergence of climate-relevant events in 2007 that amplified its impact:

• Arctic sea ice record low (September 2007): The observed 24% drop below the previous record provided real-time empirical validation of AR4’s Arctic findings, accelerating the report’s political impact (see Arctic Sea Ice Record Low).
• Live Earth concerts (July 7, 2007): The global climate awareness concerts co-produced by Al Gore—held across 12 venues and all seven continents—brought AR4’s message to a popular audience (see Live Earth 2007).
• Oil price surge: The simultaneous spike toward $100/barrel made energy costs and supply central to public consciousness, creating a brief convergence of economic and environmental concerns.
• Darfur humanitarian crisis: The same year’s “world’s worst humanitarian crisis” in Sudan illustrated the security consequences of resource competition linked to climate-driven desertification (see Darfur Conflict in 2007).

Sources

• IPCC Fourth Assessment Report (AR4) — IPCC — Official IPCC repository with all working group reports, synthesis, and summary documents including release dates.
• 2007 Nobel Peace Prize — NobelPrize.org — Official Nobel Prize website including citation language and Nobel Lectures by Pachauri and Gore.
• Wikipedia: IPCC Fourth Assessment Report — Comprehensive overview of the report’s structure, findings, authors, and policy influence.