How are uncertainties handled in this report?
The IPCC uses a very specific language when it comes to expressing the degree
of uncertainty or agreement for each statement in the fifth assessment report.
For an overview of the specific meaning of each qualifier, you can read the
relevant section in our summary of the Working Group I report.
What is projected by emission scenarios?
For scenarios without mitigation efforts beyond those in place today,
greenhouse gas concentrations
would reach 750 to over 1300 ppm CO2eq by 2100 (versus 400 ppm
now). Global surface temperature would increase by 2.5 to 7.8 °C over
pre-industrial levels (high confidence).
Greenhouse gases emissions are expected
to continue to grow in all sectors except agriculture, forestry and other
land uses (robust evidence,
medium agreement). By 2050, the emissions from the energy supply sector
are projected to triple compared to 2010, mainly from the electricity used in
buildings and industry. Emissions from transport and buildings are projected to
almost double, unless improvements in energy efficiency are accelerated
(medium evidence, medium agreement).
Scenarios limiting CO2eq concentrations to about 450 ppm by
2100 – necessary to limit global warming to 2°C above pre-industrial levels
– require greenhouse gas emissions that
are 40% to 70% lower in 2050 than in 2010, and near zero in 2100.
This requires large‐scale global changes in the energy supply sector (robust
evidence, high agreement).
Scenarios that exceed 650 ppm CO2eq by 2100 are
unlikely to limit global warming to 2°C above
How can climate impact mitigations objectives best be reached?
If mitigation efforts are delayed, it would be more difficult to limit global
warming to 2°C and the range of options would be more limited (high
confidence). Infrastructure development and long-lived products can
lock a society into a pathway of high greenhouse
gas emissions, something that can be difficult and costly to change.
This reinforces the importance of early action for ambitious mitigation
(robust evidence, high agreement).
Since most greenhouse gases accumulate
over time and mix globally, climate change
can only be effectively mitigated if collective actions are taken at the
global scale. International
cooperation is needed to help developing and spreading environmentally sound
In order to accurately estimate the benefits of mitigation, the full range of
possible impacts of climate change need to
be taken into account by social, economic and ethical analyses.
What are the possible mitigation measures?
Major options needed to reach the maximum concentration objective of 450 ppm
- Replacing coal‐fired power plants with modern, highly
efficient natural gas power plants, provided that natural gas
is available and that gas leaks are kept low during extraction and
- More efficient energy use, as well as tripling to nearly
quadrupling the share of zero‐ and low‐carbon energy sources by the year
2050; this includes renewables, nuclear energy,
fossil energy with
CO2 capture and storage (CCS), and
bioenergy with CCS (BECCS).
- Reducing CO2 emissions in all transport
modes, through technical improvements, behavioural changes, as well
as new infrastructure and urban redevelopment investments.
Would decarbonising and CO2 storage technologies be effective mitigation tools?
The energy supply is currently largely dominated by carbon intensive
fossil fuels. Decarbonising it,
i.e. reducing the amount of carbon it releases, is a key requirement to
stabilize emissions below 580 ppm CO2eq by 2100 (robust evidence,
high agreement). However in the near-term, there may be more to be
gained by increasing energy efficiency than by decarbonising energy
Carbon Storage technologies currently exist, but there is a need for
regulatory incentives to deploy them on a large scale. Combining
bioenergy with carbon capture and
storage (BECCS) can bring net negative emissions.
How can individuals contribute to the reduction of greenhouse gas emissions?
Individuals could substantially lower emissions by changing their diet,
reducing food waste, and modifying consumption patterns such as mobility demand
and modes, energy use in households, choice of longer‐lasting products. Such
changes in behaviour may improve energy efficiency by 20 to 30 % in 2030
(medium evidence, medium agreement).
In developed countries, lifestyle and behavioural changes could reduce energy
demand in buildings by up to 20% in the short term and by up to 50% by
What would be the cost of climate change mitigation?
If all countries would begin mitigation measures immediately and on the same
basis, reaching concentrations of about 450ppm CO2eq by 2100 entails
losses in global consumption of 1 to 4% in 2030, 2 to 6% in 2050, and 3 to 11%
in 2100. This is quite small considering that global consumption should grow
anywhere from 300% to over 900% during this century. Delaying mitigation would
increase the costs.
The distribution of the costs would vary: the majority of mitigation efforts
would take place in countries where future emissions are expected to be highest,
as well as in sectors where key mitigation technologies are effective.
Mitigation measures entail a wide range of possible adverse
side‐effects as well as co‐benefits, which have not been well
quantified. For instance, the land used to plant trees as
carbon sink cannot be used for human
food, animal feed or bioenergy
How will mitigation affect the energy industry?
- Revenues from the export of coal and
oil are expected to
decrease (high confidence). The effect on natural
gas exports is more uncertain.
- Nuclear energy could make an increasing contribution to
low‐carbon energy supply, but a variety of barriers and risks
exist (robust evidence, high agreement). New technologies
addressing some of these issues are being investigated and progress has been
made in nuclear safety and waste disposal.
- Methane, biofuels and
electricity produced from low‐carbon sources are already
increasing their share in the transport sector; hydrogen
fuels from low‐carbon sources are an option for the longer term.