The state of climate science, the IPCC and its fifth report

The latest predictions of the World Meteorological Organization (WMO) and Météofrance for between now and 2050, with peak temperatures of over 40°C in France, are a dramatic illustration of the consequences that climate disruption will have if no measures are taken to limit global warming to 2°C.

In December2014, weather forecasts based on these predictions were broadcast by media channels worldwide, revealing to everyone the concrete stakes of the negotiations underway in Lima during COP20.

The main messages of the fifth report of the IPCC

The IPCC’s scientific conclusions are unquestionable: there can be no doubt that global warming is taking place and the IPCC considers it extremely likely (more than 95% probability, compared to 90% in 2007 and 66% in 2001) that human activity is responsible for the increase in mean global temperatures since the mid 20thcentury.

The IPCC Fifth Assessment Report (AR5) comprises three volumes and a Synthesis Report, published in October2014.

The content of the report will have to be fully taken into account in the drafting of the 2015 climate agreement, to ensure that it is sufficiently ambitious to address the challenge of climate change.

The first volume of the IPCC report is devoted to the scientific aspects of climate change. The main conclusions of this volume are as follows:

1. Global warming is a confirmed certainty.

The annual average temperature has already risen by 0.85°C since 1880 and should increase by 0.3-4.8°C by 2100, depending on greenhouse gas emissions. The globally averaged combined land and ocean surface temperature data shows an increase of 0.89°C over the period 1901–2012.

The energy accumulated by the increase in global warming deeply penetrates the oceans.

More than 90% of energy from global warming is stored in the oceans.

The oceans hold over 90% of the additional energy accumulated in the climate system between 1971 and 2010. While more than 60% of the net energy increase is stored in the upper ocean (0-700m), 30% is stored in the ocean below 700m.

The melting of the ice caps has become a major issue.

Over the period 1979 to 2012, Arctic sea ice extent decreased at a rate that was very likely in the range 3.5% to 4.1% per decade and is likely to continue decreasing in the 21stcentury, leaving the Arctic Ocean virtually free of ice by 2050. The volume of mountain glaciers is also set to continue shrinking, whatever scenario is considered.

Predictions for the rate of sea level rise have been revised upwards.

While in 2007, sea level rise was estimated at 18-59cm by 2100, the latest studies predict a probable rise of 26-82cm. The two primary causes of this sea level rise, which seems to have been accelerating over the past two decades, are thermal expansion (expansion of water due to warming) and ice loss. Global mean sea level already rose by 0.19m over the period 1901-2010.

The atmospheric concentrations of carbon dioxide, methane and nitrous oxide have risen to unprecedented levels.

The atmospheric concentrations of carbon dioxide, methane and nitrous oxide have risen to unprecedented levels.

+40%: anthropogenic carbon dioxide concentrations have increased by 40% since 1750.

Anthropogenic carbon dioxide concentrations have increased by 40% since pre-industrial times (1750) and by more than 20% since 1958. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification.

This rise is the result of human activity and can be explained mainly by fossil fuel combustion and deforestation, and to a lesser extent by cement production.

Concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) now exceed the highest concentrations recorded in ice cores during the past 800,000years.

2. It is still possible to limit global warming.

The goal to limit global warming to 2°C is still possible under certain conditions.

For global warming to be limited to 2°C between the end of the 19thcentury and the end of the 21stcentury, emissions would need to peak around 2020, then decline until 2100.

In order for global warming to be limited to 2°C by the end of the century compared with 1870, cumulative total anthropogenic carbon dioxide emissions must also not exceed 800gigatonnes of carbon. However, since 1870, human activity has already released 531 gigatonnes of carbon. The technical and societal hypotheses of such a scenario are presented in volume three of the IPCC report, published in April 2014.

The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)

Understanding climate change

Our understanding of the climate system is based on observations, theoretical studies and modelling. Compared to the Fourth Assessment Report, more detailed observations and improved climate models provide a clearer, sounder basis for attributing climate change to human causes.

Climate models have been continually improved since the Fourth Assessment Report.

Four scenarios, or profiles representing future concentrations of greenhouse gases, ozone and aerosol precursors, known as Representative Concentration Pathways (RCPs), are used for climate change projections. They correspond to four increasingly steep greenhouse gas emission trajectories over the course of the 21stcentury.

Intergovernmental Panel on Climate Change (IPCC)

The Intergovernmental Panel on Climate Change (IPCC) was set up in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). In 2007 it received the Nobel Peace Prize, jointly with Al Gore. Its Secretariat is based in Geneva, in the headquarters of the WMO.

Purpose of the IPCC

The IPCC has the role of assessing and presenting, clearly, objectively, methodically and without bias, the scientific, technical and socioeconomic information needed for a good understanding of the scientific basis of the risks linked to anthropogenic climate change. Specifically, it studies the possible consequences of this change and considers strategies for adaptation to climate change and mitigation of greenhouse gas emissions. Its assessments are based on scientific and technical publications that are widely recognized for their scientific value.

In France specifically, 35 scientists, including Jean Jouzel, Vice-Chair of Working GroupI on the Physical Science Basis, participate in drafting the Assessment Reports. The IPCC’s work fuels reflection on climate policies relating to mitigation, adaptation, development assistance and international negotiations. It is a basic working tool for several hundred people in France.

A guiding role in international climate negotiations

The IPCC’s major publications are its Assessment Reports (ARs).

The scientific information collated by the IPCC from publications that serve as references in the scientific community helps document international efforts to combat climate change.

The first report was published in 1990 and had a major influence on the content of the United Nations Framework Convention on Climate Change (UNFCCC) that was adopted in1992. This message from the world’s scientists in 1990 made the signature of the Convention possible. The second Assessment Report, published in 1995, had a decisive impact on the provisions of the Kyoto Protocol that was adopted in1997. A third report, in 2001, helped deepen study into the impact of climate change, reiterating the need for adaptation. The fourth report, in2007, established a solid basis for the negotiations seeking the conclusion of a universal agreement to follow on from the Kyoto Protocol. During the Conference of the Parties to the UNFCCC in Bali in2007, the conclusions of this report led to greater awareness of the need for more ambitious international action to combat climate change.

Generally speaking, the quality of the IPCC’s work and the scientific rigour of its methodology have provided negotiators with a sound scientific basis for the “shared vision”, that is, the major principles and goals of the fight against climate change which underpin the collective efforts of the Parties to the UNFCCC.

The goal of limiting the global mean temperature rise to 2°C relative to pre-industrial levels was recognized as central by the Parties to the UNFCCC in 2010, after the IPCC indicated that beyond that threshold, the consequences of climate change would be significant.

Working method

A very considerable portion of the global scientific community is involved in drafting IPCC Assessment Reports. The Group’s collegial working method makes it possible to cross-check information very thoroughly. For the fourth AR, published in 2007, over 500authors helped to write it and 600scientists were involved in the two-stage review process. The drafting of the fifth AR involved 831scientists.