The Synthesis Report is based on the reports of the three Working Groups of the Intergovernmental Panel on Climate Change (IPCC), including relevant Special Reports. It provides an integrated view of climate change as the final part of the IPCC’s Fifth Assessment Report (AR5).

The report addresses observed changes and their causes; future climate change, risks and impacts; future pathways for adaptation, mitigation and sustainable development; adaptation and mitigation.

The report asserts that continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions which, together with adaptation, can limit climate change risks.

As it is, the report said, warming will continue beyond 2100 under all RCP (Representative Concentration Pathways, the modeling scenarios based on projected population size, economic activity, lifestyle, energy use, land-use patterns, technology and climate policy), except RCP 2.6 (stringent mitigation).

Global mean surface temperature increase as a function of cumulative total global CO2 emissions. Multi-model results from a hierarchy of climate carbon-cycle models for each RCP until 2100 are shown (colored lines). Model results over the historical period (1860 to 2010) are indicated in black. The colored plume illustrates the multi-model spread over the four RCP scenarios and fades with the decreasing number of available models in RCP8.5. Dots indicate decadal averages, with selected decades labelled. Ellipses show total anthropogenic warming in 2100 versus cumulative CO2 emissions from 1870 to 2100 from a simple climate model (median climate response) under the scenario categories used in WGIII. Temperature values are always given relative to the 1861–1880 period, and emissions are cumulative since 1870. The black filled ellipse shows observed emissions to 2005 and observed temperatures in the decade 2000-2009 with associated uncertainties. Click to enlarge.

According to the models, surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO₂ emissions. A large fraction of anthropogenic climate change resulting from CO₂ emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO₂ from the atmosphere over a sustained period.

Mitigation pathways. There are multiple mitigation pathways to achieve the substantial emissions reductions over the next few decades necessary to limit, with a greater than 66% chance, the warming to 2 ˚C. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO₂ and other long-lived GHGs by the end of the century.

Potential co-benefits (blue text) and adverse side-effects (red text) of the main mitigation measures for the transportation sector. Co-benefits and adverse side-effects, and their overall positive or negative effect, all depend on local circumstances as well as on the implementation practice, pace and scale. The uncertainty qualifiers between brackets denote the level of evidence and agreement on the respective effect. Abbreviations for evidence: l= limited, m= medium, r= robust; for agreement: l=low, m=medium, h=high. Click to enlarge.

Without additional efforts to reduce GHG emissions beyond those in place today, global emissions growth is expected to persist, driven by growth in global population and economic activities. Modeled global mean surface temperature increases in 2100 in baseline scenarios—those without additional mitigation—range from 3.7 to 4.8 °C above the average for 1850-1900 for a median climate response. They range from 2.5°C to 7.8 °C when including climate uncertainty (5^th to 95^th percentile range).

Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2 °C relative to pre-industrial levels, the report said. It will require substantially higher rates of emissions reductions from 2030 to 2050; a much more rapid scale-up of low-carbon energy over this period; a larger reliance on CDR (carbon dioxide removal) in the long term; and higher transitional and long-term economic impacts. Estimated global emissions levels in 2020 based on the Cancún Pledges are not consistent with cost-effective mitigation trajectories that are at least about as likely as not to limit warming to below 2°C relative to pre-industrial levels, but they do not preclude the option to meet this goal.

Many adaptation and mitigation options can help address climate change, but no single option is sufficient by itself. Effective implementation depends on policies and cooperation at all scales, and can be enhanced through integrated responses that link adaptation and mitigation with other societal objectives.

—IPCC Fifth Assessment Synthesis Report

The Synthesis Report finds that mitigation cost estimates vary, but suggests that global economic growth would not be strongly affected. In business-as-usual scenarios, consumption—a proxy for economic growth—grows by 1.6 to 3% per year over the 21^st century. Ambitious mitigation would reduce this by about 0.06 percentage points. “Compared to the imminent risk of irreversible climate change impacts, the risks of mitigation are manageable,” said Youba Sokona, Co-Chair of IPCC Working Group III.

These economic estimates of mitigation costs do not account for the benefits of reduced climate change, nor do they account for the numerous co-benefits associated with human health, livelihoods, and development.

Resources

• IPCC AR5 documents