What this document is

This is the ‘Summary for Policymakers’ from Working Group I of the IPCC’s Fourth Assessment Report, released in February 2007. Working Group I covers the physical science of the climate, and its summary is a condensed, government-approved distillation of a far longer technical volume.

AR4 was, at the time, the most authoritative synthesis of climate science available, and this summary was its most widely read component. This page describes it from general knowledge; for the precise wording, figures and confidence terms, please consult the IPCC original.

What it covers

The summary concentrates on what was known about the climate system and its recent changes. Its broad themes include the following.

  • The conclusion that warming of the climate system is unequivocal;
  • The rise in atmospheric greenhouse gas concentrations since the industrial era;
  • The assessment that most recent warming was very likely driven by human activity;
  • Observed changes in temperature, sea level, snow and ice;
  • Projections of further warming under a range of future emission scenarios.

Like all IPCC summaries, it uses a careful vocabulary of likelihood and confidence, tying each statement to the strength of the supporting evidence.

Why it matters

AR4 marked a turning point in public understanding of climate change. Its clear language on human influence was widely reported, and the IPCC was awarded a share of the 2007 Nobel Peace Prize. The report framed climate debate for years and helped build the case for stronger policy.

It has been cited extensively ever since, so many links to the original PDF now fail. A stable summary such as this preserves the reference for readers arriving from those sources.

How it relates to UK energy

The science set out in AR4 is part of the reason the UK adopted binding carbon targets and began shifting its power system away from coal. Because energy use is the main source of emissions, the report’s findings connect directly to how electricity and heat are supplied at home — a link explored in our environment guides.

You can read the original on the IPCC’s official AR4 Working Group I page.

Frequently asked questions

What is AR4?

AR4 is the IPCC’s Fourth Assessment Report, published in 2007. It was the fourth in the series of periodic assessments of climate science, following earlier reports in 1990, 1995 and 2001, and was succeeded by AR5 in 2013 to 2014.

What did the Working Group I summary say?

Working Group I assesses the physical science. Its 2007 summary is widely remembered for concluding that warming of the climate system is unequivocal and that most of the observed warming was very likely due to human-caused greenhouse gas emissions.

Why was AR4 so prominent?

Its clear findings drew major public and political attention, and the IPCC shared the Nobel Peace Prize in 2007. AR4 shaped climate debate for years and informed subsequent negotiations.

Where can I read the original?

The report and its Summary for Policymakers are published openly by the IPCC. This page links to the official AR4 Working Group I pages.