The UN Intergovernmental Panel on Climate Change (IPCC) released the Synthesis Report that distills and integrates the findings of the full IPCC Fifth Assessment Report (AR5) released over the past 13 months. The report expresses with greater certainty than in previous assessments that emissions of greenhouse gases and other anthropogenic drivers have been the dominant cause of observed warming since the mid-20th century.
The report expressed with medium confidence that the period from 1983 to 2012 was likely the warmest 30-year period of the last 1,400 years in the Northern Hemisphere, where such assessment is possible. The report also noted that the globally averaged surface temperature exhibits substantial decadal and inter-annual variability. Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade). [The much discussed “pause”.]
|GHG growth since 2010|
|Total annual anthropogenic GHG emissions have increased by about 10 GtCO2eq between 2000 and 2010. According to the report, this increase directly came from the energy (47%), industry (30%), transport (11%) and building (3%) sectors (medium confidence). Accounting for indirect emissions raises the contributions by the building and industry sectors (high confidence).|
|Since 2000, GHG emissions have been growing in all sectors, except in agriculture, forestry and other land use (AFOLU). In 2010, 35% of GHG emissions were released by the energy sector, 24% (net emissions) from AFOLU, 21% by industry, 14% by transport and 6.4 % by the building sector.|
|When emissions from electricity and heat production are attributed to the sectors that use the final energy (i.e. indirect emissions), the shares of the industry and building sectors in global GHG emissions are increased to 31% and 19%, respectively.|
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).
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.
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 (5th to 95th 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 21st 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.
IPCC AR5 documents