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.
(p 18)
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.
(pp 20-1)
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.
(p 24)
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.
(p 31)
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)
Figure 1
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. …
[They include:]
- 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.
(p 19)
Contents
Key drivers of future climate and the basis on which projections are made
Projected changes in the climate system
Future risks and impacts caused by a changing climate
Climate change beyond 2100, irreversibility and abrupt changes
Intergovernmental Panel On Climate Change
- AR5 Synthesis Report — Longer Report, 1 November 2014.
Key drivers of future climate and the basis on which projections are made
Key factors driving changes in anthropogenic greenhouse gas emissions are:- economic and population growth,
- lifestyle and behavioural changes,
- associated changes in energy use and land use,
- technology, and
- climate policy …
For some aspects of climate change and climate-change impacts, uncertainty about future outcomes has narrowed.
For others, uncertainty will persist. …
The combination of persistent uncertainty in key mechanisms plus the prospect of complex interactions motivates a focus on risk in this report.
Because risk involves both probability and consequence, it is important to consider the full range of possible outcomes, including low-probability, high-consequence impacts that are difficult to simulate.
(p 19)
Risk of climate-related impacts results from the interaction between- climate-related hazards (including hazardous events and trends) and
- the vulnerability and exposure of human and natural systems. …
Experiments, observations, and models used to estimate future impacts and risks have improved since the AR4, with increasing understanding across sectors and regions.
Models and methods for estimating climate change risks, vulnerability and impacts
Future climate-related risks, vulnerabilities and impacts are estimated in the AR5 through experiments, analogies, and models, as in previous assessments. Risks are evaluated based on the interaction of projected changes in the Earth system with the many dimensions of vulnerability in societies and ecosystems.
Projected changes in the climate system
Air Temperature
The global mean surface temperature change for the period 2016–2035 relative to 1986–2005 is similar for the four RCPs, and will likely be in the range 0.3 °C to 0.7 °C (medium confidence). …
Relative to 1850–1900, global surface temperature change for the end of the 21st century (2081–2100) is projected to likely exceed 1.5 °C for RCP4.5, RCP6.0 and RCP8.5 (high confidence).
Warming is- likely to exceed 2 °C for RCP6.0 and RCP8.5 (high confidence),
- more likely than not to exceed 2 °C for RCP4.5 (medium confidence), but
- unlikely to exceed 2 °C for RCP2.6 (medium confidence). …
It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales, as global mean surface temperature increases.
(pp 21)
Water cycle
Changes in precipitation in a warming world will not be uniform. …
Ocean, cryosphere and sea level
The global ocean will continue to warm during the 21st century. …
It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century …
Year-round reductions in Arctic sea ice are projected for all RCP scenarios.
(p 22)
The global glacier volume … is projected to decrease- by 15 to 55% for RCP2.6, and
- by 35 to 85% for RCP8.5
Global mean sea level will continue to rise during the 21st century. …
Sea-level rise will not be uniform across regions. …
Sea-level rise depends on the pathway of CO2 emissions, not only on the cumulative total; reducing emissions earlier rather than later, for the same cumulative total, leads to a larger mitigation of sea-level rise. …
Carbon cycle and biogeochemistry
Ocean uptake of anthropogenic CO2 will continue under all four RCPs through to 2100, with higher uptake for higher concentration pathways (very high confidence). …
Based on Earth System Models, there is high confidence that the feedback between climate change and the carbon cycle will amplify global warming. …
The decrease in surface ocean pH is in the range of- 0.06 to 0.07 (15–17% increase in acidity) for RCP2.6,
- 0.14 to 0.15 (38–41%) for RCP4.5,
- 0.20 to 0.21 (58–62%) for RCP6.0, and
- 0.30 to 0.32 (100–109%) for RCP8.5.
It is very likely that the dissolved oxygen content of the ocean will decrease by a few per cent during the 21st century …
Climate system responses
Climate system properties that determine the response to external forcing have been estimated both from climate models and from analysis of past and recent climate change.
The equilibrium climate sensitivity (ECS) is likely in the range 1.5 °C–4.5 °C …
Cumulative emissions of CO2 largely determine global mean surface warming by the late 21st century and beyond.
(p 23)
The global mean peak surface temperature change per trillion tonnes of carbon (1000 GtC) emitted as CO2 is likely in the range of 0.8 °C to 2.5 °C. …
(p 24)
Future risks and impacts caused by a changing climate
Key risks are potentially severe impacts relevant to understanding dangerous anthropogenic interference with the climate system. …
Their identification is based on- large magnitude or high probability of impacts;
- irreversibility or timing of impacts;
- persistent vulnerability or exposure; or
- limited potential to reduce risks. …
Key risks that span sectors and regions include the following (high confidence):
- Risk of severe ill-health and disrupted livelihoods resulting from
- storm surges, sea-level rise, and coastal flooding;
- inland flooding in some urban regions; and
- periods of extreme heat.
- Systemic risks due to extreme weather events leading to breakdown of infrastructure networks and critical services.
- Risk of food and water insecurity and loss of rural livelihoods and income, particularly for poorer populations.
- Risk of loss of ecosystems, biodiversity, and ecosystem goods, functions, and services. …
The overall risks of future climate change impacts can be reduced by limiting the rate and magnitude of climate change, including ocean acidification. …
Adaptation can substantially reduce the risks of climate change impacts, but greater rates and magnitude of climate change [the greater] the likelihood of exceeding adaptation limits (high confidence). …
Ecosystems and their services in the oceans, along coasts, on land and in freshwater
Risks of harmful impacts on ecosystems and human systems increase with the rates and magnitudes of warming, ocean acidification, sea-level rise and other dimensions of climate change (high confidence).
(p 25)
A large fraction of terrestrial, freshwater and marine species faces increased extinction risk due to climate change during and beyond the 21st century, especially as climate change interacts with other stressors (high confidence). …
Global marine species redistribution and marine biodiversity reduction in sensitive regions, under climate change, will challenge the sustained provision of fisheries productivity and other ecosystem services, especially at low latitudes (high confidence). …
Open-ocean net primary production is projected to … decrease globally, by 2100, under all RCP scenarios (medium confidence). …
Marine ecosystems, especially coral reefs and polar ecosystems, are at risk from ocean acidification (medium to high confidence). …
The risks from [Ocean Acidification] increase with warming because OA lowers the tolerated levels of heat exposure, as seen in corals and crustaceans. …
A 10- to more than 100-fold increase in the frequency of floods in many places would result from a 0.5 m rise in sea level in the absence of adaptation.
(p 26)
Carbon stored in the terrestrial biosphere is susceptible to loss to the atmosphere as a result of climate change, deforestation, and ecosystem degradation (high confidence).
Increased tree mortality and associated forest dieback is projected to occur in many regions over the 21st century (medium confidence) …
Coastal systems and low-lying areas will increasingly experience submergence, flooding and erosion throughout the 21st century and beyond, due to sea-level rise (very high confidence).
Water, food and urban systems, human health, security and livelihoods
The fractions of the global population that will experience water scarcity and be affected by major river floods are projected to increase with the level of warming in the 21st century (robust evidence, high agreement).
Climate change over the 21st century is projected to reduce renewable surface water and groundwater resources in most dry subtropical regions (robust evidence, high agreement), intensifying competition for water among sectors (limited evidence, medium agreement).
In presently dry regions, the frequency of droughts will likely increase by the end of the 21st century under RCP8.5 (medium confidence).
In contrast, water resources are projected to increase at high latitudes (robust evidence, high agreement).
The interaction of- increased temperature;
- increased sediment, nutrient, and pollutant loadings from heavy rainfall;
- increased concentrations of pollutants during droughts; and
- disruption of treatment facilities during floods
All aspects of food security are potentially affected by climate change, including food- production,
- access,
- use, and
- price stability
Global temperature increases of ~4°C or more above late 20th century levels … would pose large risks to food security … (high confidence).
(p 27)
Until mid-century, projected climate change will impact human health mainly by exacerbating [existing] health problems … (very high confidence).
Throughout the 21st century, climate change is expected to lead to increases in ill-health in many regions … (high confidence).
Health impacts include- greater likelihood of injury and death due to more intense heat waves and fires,
- increased risks from foodborne and waterborne diseases, and
- loss of work capacity and reduced labour productivity in vulnerable populations
Risks of undernutrition in poor regions will increase (high confidence).
Risks from vector-borne diseases are projected to generally increase with warming … despite reductions in some areas that become too hot for disease vectors (medium confidence).
Globally, the magnitude and severity of negative impacts will increasingly outweigh positive impacts (high confidence). …
In urban areas, climate change is projected to increase risks for people, assets, economies and ecosystems, including risks from- heat stress,
- storms and extreme precipitation,
- inland and coastal flooding,
- landslides,
- air pollution,
- drought,
- water scarcity,
- sea-level rise and
- storm surges
Rural areas are expected to experience major impacts on- water availability and supply,
- food security,
- infrastructure, and
- agricultural incomes …
These impacts will disproportionately affect the welfare of the poor in rural areas …
Aggregate economic losses accelerate with increasing temperature (limited evidence, high agreement) but global economic impacts from climate change are currently difficult to estimate.
[Incomplete] estimates of global annual economic losses for warming of ~2.5 °C above pre-industrial levels are 0.2% to 2.0% of income (medium evidence, medium agreement).
Changes in population, age structure, income, technology, relative prices, lifestyle, regulation, and governance are projected to have relatively larger impacts than climate change, for most economic sectors (medium evidence, high agreement). …
From a poverty perspective, climate change impacts are projected to- slow down economic growth,
- make poverty reduction more difficult,
- further erode food security, and
- prolong existing poverty traps and create new ones
Climate change is projected to increase displacement of people (medium evidence, high agreement).
(p 28)
Climate change can indirectly increase risks of violent conflict by amplifying well documented drivers of these conflicts, such as poverty and economic shocks (medium confidence). …
Table 2.3
Examples of global key risks for different sectors, including the potential for risk reduction through adaptation and mitigation, as well as limits to adaptation.
Each key risk is assessed as very low, low, medium, high, or very high.
Risk levels are presented for three time frames:- present,
- near term (here, for 2030–2040), and
- long term (here, for 2080–2100).
For the long term, risk levels are presented for two possible futures (2°C and 4°C global mean temperature increase above pre-industrial levels).
For each time frame, risk levels are indicated for a continuation of current adaptation and assuming high levels of current or future adaptation. …
Climate change beyond 2100, irreversibility and abrupt changes
Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. …
Stabilisation of global average surface temperature does not imply stabilization for all aspects of the climate system. …
Ocean acidification will continue for centuries if CO2 emissions continue, will strongly affect marine ecosystems (high confidence), and the impact will be exacerbated by rising temperature extremes. …
Global mean sea-level rise will continue for many centuries beyond 2100 (virtually certain).
(p 30)
Sustained mass loss by ice sheets would cause larger sea-level rise, and part of the mass loss might be irreversible. …
Within the 21st century, magnitudes and rates of climate change associated with medium to high emission scenarios (RCP4.5, 6.0, and 8.5) pose a high risk of abrupt and irreversible regional-scale change in the composition, structure, and function of marine, terrestrial and freshwater ecosystems, including wetlands (medium confidence), as well as warm water coral reefs (high confidence). …
A reduction in permafrost extent is virtually certain with continued rise in global temperatures.
Current permafrost areas are projected to become a net emitter of carbon (CO2 and CH4) … under RCP8.5 over the 21st century (low confidence).
(p 31, emphasis added)
