COPENHAGEN (AFP) - Crowning a landmark review, the Intergovernmental Panel on Climate Change (IPCC) said emissions of three key greenhouse gases were at their highest in more than 800,000 years.
Following are the main points from the summary of the vast overview on global warming.
The situation today
Levels of carbon dioxide (CO2), methane and nitrous oxide are the highest in at least 800,000 years.
Since the start of the Industrial Revolution in 1750, around 2,000 billion tonnes of carbon dioxide (CO2) have been emitted. Half of manmade emissions have occurred in the last 40 years.
From 1880-2012, the global average surface temperature rose by 0.85 deg C, while the global mean sea level rose by 19cm from 1901-2010.
Each of the last three decades has been successively warmer than any preceding decade since 1850. In the northern hemisphere, 1983-2012 was likely the warmest period of the last 1,400 years.
The report uses four scenarios called Representative Concentration Pathways (RCP) based on greenhouse gas levels.
Under RCP 2.6, the lowest scenario, global average temperatures over this century are likely to rise by 0.3-1.7 C, leading to between 26-55 cm in sea-level rise.
Under RCP 8.5, the highest scenario, warming would be 2.6-4.8 C, causing sea-level rise of 45-82 cm.
Without additional measures to curb greenhouse gases "warming is more likely than not to exceed 4C" by 2100 over pre-industrial levels, the report says.
On present emissions trends, "warming by the end of the 21st century will lead to very high risk of severe, widespread and irreversible impacts globally."
- Worse food security, with impacts on wheat, rice and maize harvests and fish catches
- Accelerating species extinction and damage to ecosystems on which mankind depends
- Migration caused by climate-related economic damage and loss of land from rising seas and storm surges - greater water stress, especially in sub-tropical regions, but also a greater risk of flooding in northerly latitudes and the equatorial Pacific
- Risk of conflict over scarce resources and worsening health caused by heatwaves and spread of mosquito-borne disease If CO2 emissions continue over the long term, ocean acidification and sea-level rise will continue for centuries to come
- "Abrupt and irreversible ice loss from the Antarctic ice sheet is possible, but current evidence and understanding is insufficient to make a quantitative assessment," the report says.
The 2C target
UN members have vowed to agree an emissions-curbing pact next year to limit warming to 2 C over pre-industrial levels.
For a "likely" chance (at least 66 per cent or more) of achieving 2 C, atmospheric CO2 should not exceed about 450 parts per million by 2100. This means humans can emit only around another 1,000 billion tonnes of CO2.
The report points to one pathway to reach 2 C: cut annual greenhouse gas emissions by 40-70 per cent by 2050 compared with 2010, and to near zero or even below by 2100.
"No single option is sufficient by itself," the report says, advocating a mix of measures.
Achieving the 2 C target will require a massive change in energy habits, led by a switch out of high-polluting fossil fuels.
By 2030, investment in low-carbon energy and energy efficiency in transport, industry and buildings will need to rise by several hundred billion dollars per year.
Countries can shore up defences by reducing water waste and encouraging recycling, preventing settlement in climate-prone areas and conserving wetlands and mangroves, which are shields against climate stress. Preventing deforestation and encouraging afforestation are also a carbon buffer.
SOURCE: Intergovernmental Panel on Climate Change (IPCC), Fifth Assessment Report.