Research and analysis

Key points and questions: IPCC AR5 Synthesis Report

Published 3 November 2014

The UN’s Intergovernmental Panel on Climate Change (IPCC) published the final volume of its 5th Assessment Report (AR5) in Copenhagen on 2 November 2014, combining previously published volumes into a Synthesis Report. Following a six day meeting, the report was finalised and agreed upon by delegates from over 100 countries and a panel of the report’s leading scientific authors.

The Synthesis Report distils previous findings from the IPCC’s Working Group I report on the Physical Science Basis (published September 2013), the Working Group II report on Impacts, Adaptation, and Vulnerability (published March 2014), and the Working Group 3 report on Mitigation of Climate Change (published April 2014) to provide an integrated view of the issues surrounding climate change.

It reports that human influence on our climate is clear and growing, with impacts seen on all continents and in the oceans. Without significant reductions in greenhouse gas emissions, these changes will continue over the decades and centuries to come and could cause severe, widespread and irreversible impacts. Finally, the report provides robust information on the action we can take to avoid the risks from these changes.

This is the most significant report from the IPCC since its last major review, the 4th Assessment Report in 2007. Its assessment of the state of the climate is the most comprehensive ever written and it provides a strengthened body of evidence of man-made climate change. Over the entire process, 830 experts from over 80 countries reached these conclusions by reviewing over 30,000 published research papers. The findings are agreed by governments of the member countries.

1. What are the headline messages from this report?

The Synthesis Report found that:

• Human influence on the climate system is clear, and recent man-made emissions of greenhouse gases (carbon dioxide, methane, nitrous oxide and a number of industrial gases) are the highest in history.
• The atmosphere and oceans have warmed, the amounts of snow and ice have reduced, sea level has risen, the oceans have become more acidic and some extreme weather events have intensified.
• Without substantial efforts to curb greenhouse gas emissions, global temperatures by the end of the 21st century could be more than 4 °C above what they were before the industrial revolution.
• A change of that size would very likely lead to severe, widespread, and irreversible impacts on societies and the environment globally.
• We can adapt our societies to prepare for some of the risks of climate change and this is essential, but this alone is not enough. Therefore we need to significantly reduce greenhouse gas emissions to limit the scale of climate change.
• There are many options to reduce emissions, including the use of low carbon energy sources such as wind, solar and nuclear, removal and burial of carbon dioxide where fossil fuels are still used and more efficient use of energy. Nevertheless the scale of change required will pose significant technological, economic, social, and institutional challenges and the longer we wait the bigger these will be.

2. How is the climate changing and how is that affecting us?

Changes in the climate are widespread, and are affecting societies and the environment on in every country and across the oceans:

• Global temperatures have risen by about 0.8°C over the last century and sea levels have risen by about 20 cm.
• In many regions, snow and rainfall patterns have changed, affecting the water supplies of some communities.
• Snow, ice, permafrost and glaciers are melting at the poles and around the rest of the world.
• The oceans are becoming increasingly acidic as they absorb more and more carbon dioxide.
• Extreme weather events are changing. For example, heat waves are lasting for longer and becoming more intense, and heavy rainfall events are becoming heavier.
• Trees and forests have been seen to die back, corals are declining, and animals on land and in the sea have shifted from their natural habitats.

3. What does the report tell us about the causes of climate change?

A human fingerprint has been found in nearly all aspects of climate change:

• The report found that human influence, mainly from greenhouse gas emissions, is extremely likely to have been the major cause of the observed increase in global temperatures since the mid-20th century.
• Human influence has also been found in the warming oceans, in changes to rainfall patterns and in melting ice in the Arctic.
• Man-made emissions of greenhouse gases have risen enormously since the industrial revolution; CO2 levels are now at their highest in at least the last 800,000 years.
• Much of this rise in emissions has occurred in the last 40 years, with current emissions the highest in human history.

4. What does the report tell us about the future?

Without action to reduce emissions of greenhouse gases, we could see severe, widespread, and irreversible impacts on societies and the environment globally:

• Global temperatures could be more than 4 °C above what they were before the industrial revolution.
• It is very likely that heat waves will occur more often and last longer, increasing the risk of health problems particularly in vulnerable populations.
• Heavy rainfall events will become heavier and more frequent in many places, making flooding more likely.
• The ocean will continue to warm and acidify, negatively affecting marine wildlife and fisheries.
• Global average sea level will continue to rise, disrupting communities in low-lying areas and increasing the risk of coastal flooding from storm surges.
• Food security will be undermined as changes in the oceans affect fisheries and drought and rising temperatures reduce global crops yields.

5. How can we respond to climate change?

There are many options available to reduce the risks of harmful impacts from future climate change and to prepare for the change which is unavoidable.

• If we make substantial reductions in our emissions of greenhouse gases in the coming decades we will lower the risks of seeing harmful impacts later this century.
• The IPCC found that to have a good chance of keeping temperature change below 2 °C by the end of the century, global greenhouse emissions in 2050 need to be 40 to 70 % lower than in 2010, and emission levels near zero or below in 2100.
• This means we need total human emissions of CO2 to remain less than 2900 GtCO2. We have already emitted 1900 GtCO2, which is about two thirds of this limit.
• However, the longer we delay action, the harder and more expensive it will be.
• Recent extreme storms, flooding, heat-waves and droughts demonstrate that we are already vulnerable to climate and weather events. We must adapt our communities and lifestyles to manage these local risks from climate change, but this approach can only take us so far.
• Our options for reducing emissions include using energy more efficiently and switching to low-carbon energy sources such as renewables, nuclear energy, and fossil energy with carbon dioxide capture and storage options for reducing emissions include decarbonising the way we produce electricity, using energy more efficiently and trying to reduce the amount of energy that we all use.
• The Synthesis Report provides a strengthened case for international leaders to act now to reduce domestic carbon emissions and to secure an ambitious legally binding global agreement in 2015.

6. How was this report written?

The Synthesis Report is the culmination of six years of work. It combines and distils the main messages from the three previously published IPCC working groups (on climate science, climate impacts, and mitigating climate change) to provide

• Many experts have carefully considered the findings in the Synthesis Report. Compared to previous reports, the Synthesis Report assesses a substantially larger knowledge base of scientific, technical, and socioeconomic literature; the number of scientific publications more than doubled between 2005 and 2010.
• Over the entire process, including Working Groups I, II and III, 830 experts from over 80 countries reviewed over 30,000 published research paper to produce the different IPCC reports.
• The Synthesis Report was approved during a meeting in Copenhagen from the 26th October to 1st November 2014. Over 100 governments attended this meeting to discuss and approve the final report.