Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond.
Projections of greenhouse gas emissions vary over a wide range, depending on both socio-economic development and climate policy. …
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.
Surface temperature is projected to rise over the 21st century under all assessed emission scenarios.
It is very likely that heat waves will occur more often and last longer, and that extreme precipitation events will become more intense and frequent in many regions.
The ocean will continue to warm and acidify, and global mean sea level to rise.
Warming caused by CO2 emissions is effectively irreversible over multi-century timescales unless measures are taken to remove CO2 from the atmosphere. …
[Limiting] total human-induced warming … to less than 2°C relative to the period 1861-1880 with a probability of >66% would require total CO2 emissions from all anthropogenic sources since 1870 to be limited to about 2,900 GtCO2 …
About 1,900 … GtCO2 were emitted by 2011, leaving about 1,000 GtCO2 to be consistent with this temperature goal.
Estimated total fossil carbon reserves exceed this remaining amount by a factor of 4 to 7 …
Climate change will amplify existing risks and create new risks for natural and human systems.
Risks are unevenly distributed and are generally greater for disadvantaged people and communities in countries at all levels of development.
Increasing magnitudes of warming increase the likelihood of severe, pervasive, and irreversible impacts for people, species and ecosystems.
Continued high emissions would lead to mostly negative impacts for biodiversity, ecosystem services, and economic development and amplify risks for livelihoods and for food and human security.
Many aspects of climate change and its impacts will continue for centuries, even if anthropogenic emissions of greenhouse gases are stopped.
The risks of abrupt or irreversible changes increase as the magnitude of the warming increases.
Advances, confidence and uncertainty in modelling the Earth’s climate system
Improvements in climate models since the AR4 are evident in simulations of
- continental-scale surface temperature,
- large-scale precipitation,
- the monsoon,
- Arctic sea ice,
- ocean heat content,
- some extreme events,
- the carbon cycle,
- atmospheric chemistry and aerosols,
- the effects of stratospheric ozone, and
- the El Niño-Southern Oscillation.
Confidence in the representation of processes involving clouds and aerosols remains low.
(Box 2.1, p 18)
Yellow indicates that associated impacts are both detectable and attributable to climate change with at least medium confidence.
Red indicates severe and widespread impacts.
Purple … shows that very high risk is indicated by all key risk criteria.
Reasons for concern regarding climate change
Five ‘reasons for concern’ have provided a framework for summarizing key risks since the Third Assessment Report. …
All warming levels in the text … are relative to the 1986–2005 period.
Adding ~0.6°C to these warming levels roughly gives warming relative to the 1850–1900 period, used here as a proxy for pre-industrial times (right-hand scale in figure 1).
The five reasons for concern are:
- Unique and threatened systems
- Extreme weather events
Climate-change-related risks from extreme events, such as heat waves, heavy precipitation and coastal flooding, are already moderate (high confidence). …
- Distribution of impacts
Risks are unevenly distributed between groups of people and between regions; risks are generally greater for disadvantaged people and communities everywhere.
Risks are already moderate … particularly for crop production (medium to high confidence). …
- Global aggregate impacts
Extensive biodiversity loss, with associated loss of ecosystem goods and services, leads to high risks at around 3°C additional warming (high confidence). …
- Large-scale singular events
[The risks of] abrupt and/or irreversible changes … are moderate between 0 and 1°C additional warming, since there are signs that both warm-water coral reefs and Arctic ecosystems are already experiencing irreversible regime shifts (medium confidence).
(p 29, emphasis added)
The ‘Representative Concentration Pathways’ (RCPs)
The RCPs describe four different 21st century pathways of greenhouse gas emissions and atmospheric concentrations, air pollutant emissions and land use. …
- a stringent mitigation scenario (RCP2.6),
- two intermediate scenarios (RCP4.5 and RCP6.0), and
- one scenario with very high greenhouse gas emissions (RCP8.5).
RCP2.6 is representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures.
The majority of models indicate that scenarios meeting forcing levels similar to RCP2.6 are characterized by substantial net negative emissions by 2100, on average around 2 GtCO2/yr. …
The RCPs cover a wider range than the scenarios from the Special Report on Emissions Scenarios (SRES) used in previous assessments, as they also represent scenarios with climate policy.