From global carbon budgets

to food security




Highlights of research into our changing climate



The latest observations continue to show a changing climate;

not only increases in global temperatures, but also rising sealevels,

shrinking glaciers and reducing Arctic sea-ice. All these

are consistent with a warming world driven by increases in

atmospheric greenhouse gas concentrations.


The effects of these changes are already evident during

extreme events, impacting on people, infrastructure and

natural systems. For example, heatwaves that would occur

twice a century during the early 2000s in Europe are now

expected to occur twice a decade.


The latest climate model simulations reproduce many

features of the historical climate, giving confidence in their

predictions of future conditions. Using plausible greenhouse

gas emission pathways these models show that restricting

global average warming to less than 2 °C above pre-industrial

levels is achievable, but challenging given the need for

unprecedented changes in global emission rates and



Increased natural greenhouse gas emissions from wetlands

and permafrost regions may add to the cumulative emissions

into the atmosphere. These effects could further reduce the

allowable anthropogenic emissions compatible with keeping

below the 2 °C limit by around 100 gigatonnes of carbon (GtC)

in the most pessimistic simulation. This corresponds to about

ten years of anthropogenic emissions at the current rate.


By including short-timescale variability into climate model

simulations, the range of seasonal conditions under different

levels of climate change can be investigated. Comparing

the average seasonal conditions by 2100 for a world with

2 °C of warming to one with 4 °C shows that they follow

the global annual average. However, cold winters and cool

summers are still possible, though these are less likely under

4 °C than 2 °C of warming. Similarly, warm winters and

hot summers are more likely under a 4 °C world. Reducing

greenhouse emissions will reduce the rate and long-term

effects of anthropogenic climate change, with extreme

events showing the largest changes.


Food security and climate interact in a complex way.

Many parts of the world are projected to experience

increased vulnerability to food insecurity by the middle

of the 21st century because of global warming.


Under high levels of greenhouse gas mitigation, vulnerability

to food insecurity can be stabilised at mid-21st century

levels. However under a high greenhouse gas emissions

scenario large increases are possible through until the end

of the century. This highlights the requirement for both

mitigation and adaptation.

A glossary of frequently used terms and acronyms can be found inside the back cover.


Our climate is changing and humans are responsible for at

least half of the warming observed since the mid-20th century.

Limiting future climate change requires reducing atmospheric

concentrations of greenhouse gases, chief amongst these are

carbon dioxide (CO 2

) and methane. Reductions will lead to a

range of benefits for people and ecosystems across the globe.

OBSERVATIONS Weather and climate research centres,

including the Met Office Hadley Centre, analyse observations

of the Earth’s climate. Measurements show that surface

temperatures, sea-levels and ocean heat content are

rising; glaciers and Arctic sea-ice are decreasing in extent

and volume. These, amongst many other measurements,

demonstrate increasing energy in the climate system and a

warming world. Studies combining observations with climate

model simulations show that it is extremely likely that humans

are responsible for at least half of the measured surface


PREDICTIONS Climate models can also be used to simulate

and predict how the Earth’s climate is likely to respond

under continuing increases in atmospheric greenhouse gas

concentrations. Limiting global average temperature rise

to less than 2 °C above pre-industrial conditions is a goal of

the Conference of the Parties (COP) to the United Nations

Framework Convention on Climate Change (UNFCCC).

This will require global emissions of greenhouse gases to be

limited or eliminated, and would avoid many severe impacts

of climate change and lead to a wide range of benefits for the

natural world and human society.

IMPACTS Extreme events, for example heatwaves in Europe

and the Indian sub-continent, or Hurricane Patricia in Mexico

during 2015, demonstrate the vulnerability of society to the

impacts of the current climate. Our present understanding

of the effects of climate change on regional climate extremes

and their subsequent impacts on human systems are

presented in this brochure.

MITIGATION Climate models indicate that it will be a

challenge to restrict climate change to a global average

temperature rise of less than 2 °C. Furthermore, alongside

human activities, natural processes release and absorb

greenhouse gases from the atmosphere, and the rate at which

they do so is linked to global temperature. We show how

these processes further restrict the amount of man-made

greenhouse gases that can be emitted if we are to avoid severe

impacts of climate change.

VULNERABILITY Food security is of concern to many people

in large parts of the world. Rising global temperatures along

with other aspects of climate change are projected to reduce

food security during the course of this century. Only with

large and sustained reductions in net emissions together

with adaptation to moderate levels of climate change can the

outlook be improved by the end of the century.

Here we present an update of recent research carried out by

the Met Office Hadley Centre in collaboration with partner

institutes across the globe.

The Met Office Hadley Centre Climate Programme is

funded by the UK Government through a three-year

focussed research programme, currently spanning

2015-2018. We work in collaboration with leading

academic and research organisations worldwide, as

well as governments and stakeholders across the globe,

to advance our understanding of climate science. We

also build capacity and enable knowledge transfer, for

example PRECIS (Providing Regional Climate for Impacts

Studies), and play a key role in many EU-funded projects,

including EUCLEIA (European Climate and weather

Events: Interpretation and Attribution) and EUSTACE (EU

Surface Temperature over All Corners of Earth).


Continuing evidence for a changing climate

Observations continue to

show a warming world

The Met Office Hadley Centre is one of a number of centres

around the world which monitor changes in the climate.

Land ice (in ice sheets and glaciers), sea-ice, sea-level,

ocean-heat content as well as global near-surface temperature

are among the observations which show long-term changes

consistent with a warming world.

A key result from the Intergovernmental Panel on

Climate Change Fifth Assessment Report (IPCC AR5)is

that most of the recent warming of the Earth’s surface

since the mid-20th century is likely to have been driven

by increasing concentrations of greenhouse gases

in the atmosphere. Measurements from the Mauna

Loa Observatory in Hawaii show this rise, with CO 2

concentrations reaching greater than 400 parts per

million (ppm) for the six months between February

and July 2015. This is around 120ppm (40%) above the

values in 1800 as determined from ice-core records.

Atmospheric CO 2

at Mauna Loa Observatory

Temperature difference (°C)




Met Office Hadley Centre and Climatic Research Unit

NOAA National Centers for Environmental Information

NASA Goddard Institute for Space Studies

Crosses for 2015 show January-September averages.

Whiskers show uncertainty in using the year-to-date

as an indicator of the full year average.

1850 1900 1950 2000


FIGURE 2: Average global near-surface temperature relative

to 1850-1900, including uncertainties (grey shading). Source:


In 2014 the global average temperature was

0.56±0.1 °C above the 1961-90 average. This was nominally

the joint-warmest year (with 2010) in records dating back to

1850. Uncertainties in the data, such as a lack of complete

coverage in the Arctic, mean it is not possible to say which of

several recent years was categorically the warmest.

Parts per million






Scripps Institute of Oceanography

NOAA Earth System Research Laboratory

1960 1970 1980 1990 2000 2010


October 2015

Currently 2015 looks set to be warmer again, with the global

average temperature running 0.70±0.1 °C above the 1961-

90 average up to September. Estimates of the 2015 annual

global temperature anomaly are that it will be very close to

1 °C above pre-industrial levels. This is close to the half-way

point of the 2 °C limit set by the UNFCCC. This year’s record

temperature is partly due to the influence of a strong El Niño

event taking place in the Pacific, but the long-term warming

trend due to human influence is by far the dominant cause.

A recent new analysis show that the much-discussed “slowdown”

in the rate of global temperature rise is no longer as

prominent in the observations 1 .

FIGURE 1: Monthly atmospheric CO 2

concentration at Mauna Loa

Observatory since 1958. Source: Scripps Institute of Oceanography

and NOAA Earth System Research Laboratory.


Karl, Thomas R., et al., 2015, Possible artifacts of data biases in

the recent global surface warming hiatus, Science 348.6242.









180 120W 60W 0 60E 120E 180

-10 -5 -3 -2 -1 -0.5 0 0.5 1 2 3 5 10

Anomaly (°C) relative to 1961-1990

FIGURE 3: Observed annual near-surface temperatures for 2014

relative to 1961-90. White boxes indicate areas with insufficient

data. Source: HadCRUT4.

The 2014 average European temperature was the warmest

on record, at around 0.3 °C above the previous record set

in 2007. There were no large-scale heatwaves; instead these

record values were the result of temperatures remaining

consistently above average for most of the year; 25% of

days were in the warmest 10% of the normal range.

So far during 2015 there have been heatwaves in Europe,

India, Pakistan, and around the Persian Gulf.

Warm days (% days per year)





Europe Australia Globe


1950 1960 1970 1980 1990 2000 2010 2020


FIGURE 4: Regionally averaged annual number of warm days (when

the maximum temperature exceeds the 90th percentile calculated

over the 1961-90 base period), for Europe, Australia and the globe;

the expected amount is shown by the dashed line. Source: GHCNDEX.

million km 2







Maximum (March)

Minimum (September)

1990 2000 2010


FIGURE 5: Change in monthly Arctic sea-ice extent from the

average over 1981-2010. Source: NSIDC.










FIGURE 6: Global sea-level rise (actual values) since 1993 from the

TOPEX (1993-2001), Jason-1 (2002-08) and Jason-2 (2008-13)

satellite altimeter experiments. Source: University of Colorado.


Maximum and minimum Arctic sea-ice extent and

global average sea-level rise show continued changes

consistent with increasing energy in the climate system.

During 2015 the tropical cyclone season in the

eastern and central North Pacific has been exceptionally

active. There have been 24 named storms so far this

year (to 27 October), which, joint with 1985, is the

largest number of storms recorded in a season.

The region has seen an Accumulated Cyclone Energy

(ACE) more than twice its usual level. It also saw the

development of Hurricane Patricia in October, which

became the strongest hurricane on record in the

western hemisphere. In contrast, the North Atlantic

has been relatively quiet with an ACE index 40% lower

than normal.


Human influence on climate extremes

Recent seasons and extremes fit into the picture of ongoing

human influence on the climate

Understanding how human influences on the climate are

affecting extremes helps inform decisions on how to adapt

to future conditions and highlights the potential benefits

of mitigation.

In 2013, the IPCC concluded that it is extremely likely that

human emissions and other activities have caused more

than half of the observed increase in global average surface

temperature from 1951 to 2010. Further to this assessment

of large scale global averages, it is also extremely likely

that human influence on the climate has contributed to

observed changes in the frequency and intensity of daily

temperature extremes across the globe since the middle

of the 20th century.

The number and intensity of climate extremes are very

sensitive to changes in the average conditions, and are also

the events which have the greatest impacts on society. In

many cases human influence can be shown to have affected

their severity or frequency. The relative differences in

probability of extreme events can be assessed by comparing

climate simulations of worlds with only natural influences to

those which also include human ones. These experiments

will allow the detection of any change in the characteristics of

extreme events and then attribute these changes to human

or natural causes. The Met Office Hadley Centre is leading on

a project to attribute extreme events shortly after they have

occurred (EUCLEIA).

UK winter of 2013/14

The winter of 2013/14 in the UK was dominated by an

exceptional clustering of intense low-pressure storms,

driven by a particularly strong North Atlantic jet stream.

These depressions caused tidal surge and wave damage

across coastal regions, and their rainfall led to saturated

ground and eventual flooding of river valleys and plains.

This resulted in damage to transport infrastructure, as well

as business and residential properties.

Climate simulations were used to model UK winter rainfall.

They show that the extreme level of total seasonal rainfall

experienced during the 2013/14 winter is about eight times

more likely when the large-scale atmospheric patterns

resemble those observed during that period. A human

influence can be identified in extreme rainfall events with

shorter durations. Anthropogenic climate change has made

extreme rainfall over 10 consecutive winter days around

seven times more likely. This new analysis is in agreement

with emerging evidence which suggests an increase in

the frequency and intensity of extreme UK rainfall. It is

also consistent with the detection of human influence on

changes in extreme rainfall over larger spatial scales.

Chinese spring 2014

In northern China, the spring of 2014 was the third

warmest since reliable records began in the late 1950s;

2.2 °C higher than the 1961-90 average. During late

May, 12 stations recorded temperatures over 40 °C,

breaking historical records. The number of hot spring

days (maximum temperature > 25 °C) is linked to the

mean spring-time temperature, and both these quantities

have increased over the last half-century. The likelihood

of hot spring mean temperatures as observed in 2014

has increased by around 11-fold as the result of human

influences on the climate system.






0 2 4 6 8 10

Temperature anomaly



of the average

European summer

temperature relative

to 1961-90 for the

end of the 21st

century under

two different CO 2


pathways. The

heatwave of 2003

is shown by the

vertical line. RCP2.6

results in a global

average temperature

change of around

2 °C by 2100, and

RCP8.5 by over 4 °C.

Increasing chances of hot European summers

A study in 2004 on the severe heatwave in western and

central Europe in summer 2003 showed that human

influence at least doubled the odds of such an event.

By accounting for the 0.81 °C rise in average summer

temperatures in Europe between the 1990s and 2003-12,

an updated study shows that moderate heatwaves that

would occur twice a century in the early 2000s are now

expected to occur twice every decade. Furthermore, in

little over a decade, the chances of more severe heatwaves

occurring, like that experienced in 2003, have increased

from less than once in a millennium in the late 1990s to

around once in a century. Observations of the summer

temperature across Europe over the years since 2003

suggest that we are continuing along a track where, by

the 2040s, more than half of summers are projected to be

warmer than that seen in 2003 if emissions of greenhouse

gases continue on their current rising path. The range of

summer European temperatures expected by 2100 under

two emissions futures are shown in Figure 7.


Carbon budgets and the link to global temperatures

Models indicate that limiting

global warming to 2 °C will require

large and rapid reductions of

near-term emissions sustained

well into the future

The amount of future warming depends not only on

current levels of greenhouse gas emissions, but also on

past emissions; in other words, the cumulative emissions

of greenhouse gases in the atmosphere. To have a 66%

chance of staying below a 2 °C rise in global average

temperature; compared to pre-industrial levels, total

cumulative emissions must remain less than 1000

gigatonnes of carbon (GtC). If not just CO 2

but also

other greenhouse gases and aerosols are included in this

calculation, the amount that can still be emitted (the

global carbon budget) becomes even smaller at 790 GtC

(equivalent to 2900 GtCO 2

). Total anthropogenic emissions

to date since 1870 have reached just over 545 GtC

(2000 GtCO 2


Different climate models, using the same pathways of

CO 2

concentrations, project a range of possible emissions

because they have different and plausible ways of

representing natural carbon sinks. However, not all natural

greenhouse-gas releasing mechanisms have been included

in these models. The possible effects of such a mechanism

are presented opposite.

Figure 8 shows the increase in global average temperatures

expected in two representative CO 2


pathways (RCPs), and Figure 9 the emissions which

correspond to these pathways. By mid century, despite

large and rapid near-term emission reductions in RCP2.6,

the global average temperatures are similar to the highemissions

in RCP8.5 because of inertia in the climate system.

Temperature difference relative to 1850-1899 (°C)









Global average surface temperature change





1850 1900 1950 2000 2050 2100


FIGURE 8: Globally averaged surface temperature increases over the 20th

and 21st century from historical observations and under two different CO 2

concentration pathways. Around half of the RCP2.6 models exceed 2 °C of

global warming even with sustained emission reductions, highlighted by

the dark blue shading. Following IPCC SPM Figure 7.

However, by 2100, emissions from RCP8.5 result in a global

average temperature rise of over 4 °C over pre-industrial

levels. Only with the sustained, significant emission

reductions through to the end of the century, captured

by the RCP2.6 scenario, is the global average temperature

rise restricted to around 2 °C. But some models indicate

temperature rises of over 2 °C by 2100 even under

RCP2.6. With RCP2.6, net global emissions of CO 2


to approximately zero by 2100 (Figure 9). Some models

simulate greater weakening of natural carbon sinks than

others meaning an increased requirement for emissions

reductions or even active CO 2

removal from

the atmosphere.


Feedback from natural carbon-releasing processes

Carbon emissions (GtCO 2












Fossil fuel emissions


1850 1900 1950 2000 2050 2100


FIGURE 9: Average and 5th-95th range of compatible emissions profiles

over the 20th and 21st century for the two different CO 2


pathways. Around half of the RCP2.6 models include net negative

emissions in the final years of the 21st century, highlighted by the dark

blue shading.

Additional emissions from wetlands

and permafrost regions reduce the

carbon budgets allowed to attain

global temperature thresholds

There are many processes, known as feedback mechanisms,

in the Earth system that both influence and are influenced

by the climate and atmospheric CO 2

concentration. Most

of the recent generation of climate models did not include

these effects from wetlands and permafrost regions,

because they are difficult to incorporate in an interactive

manner. The results of independent, offline experiments

show that these feedback mechanisms can have a

significant impact on the total amount of greenhouse

gases released over the 21st century.

Together, wetlands worldwide and permafrost regions

(Arctic and Antarctic tundra and high-altitude areas)

currently store vast quantities of carbon which could

be released as CO 2

and methane (CH 4

) if these regions

warm further. Any extra greenhouse gases emitted by

these natural sources will reduce the net anthropogenic

emissions allowable if global average temperature rise is

to be restricted to below 2 °C.

Including these natural greenhouse gas emission

mechanisms into projections from a high-emissions

scenario (RCP8.5) increase the annual amount of CH 4

and CO 2

emitted over time by an additional 20% of CO 2

and 4% CH 4

. Combining all these influences gives rise to

around an additional 100ppm of CO 2

in the atmosphere by

the end of the century, resulting in up to an extra degree

of warming by 2100.

The feedbacks from wetlands and permafrost regions can

be combined with other known processes to determine

their greenhouse gas input into the atmosphere. For a

global average temperature rise of 2 °C this reduces the

cumulative emissions that can be released by human

actions by around 100GtC (360 GtCO 2

) in the most

pessimistic simulation. This corresponds to about ten

years of anthropogenic emissions at the current rate.


Many benefits of emission reductions

Avoiding impacts from extreme seasonal temperatures and rainfall

Reducing greenhouse gas emissions will reduce the rate and

long-term effects of anthropogenic climate change. This

would result in smaller changes in the average conditions and

less severe extreme events. There are many benefits across a

wide range of sectors, from improvements in air quality and

human health to food and energy security. By comparing

impacts between greater or lesser amounts of global

temperature rise, the benefits of limiting anthropogenic

climate change can be demonstrated.

Adding simulated short-timescale variability into longterm

projections, illustrates the range of seasonal average

temperature and rainfall that regions might experience

under climate change. They can be expressed in terms of the

extreme hot, cold, wet or dry seasons which are associated

with the heatwaves, cold spells, floods, and droughts that

impact society.

Using these simulations, the natural variability of future

climates can be compared by selecting those with global

average temperature rises of 2 °C or 4 °C by 2100. In the test

region of England and Wales, changes in average seasonal

temperature still follow the global patterns. However, the

inclusion of year-to-year variability shows that although

the chances of cold winters or wet summers reduce as the

world warms under climate change, they are still possible in

individual years, even with 4 °C of warming. An increasing

chance of dry summers with only a modest reduction in the

chances of very wet summers is expected. Hence, despite the

chance of cold winters and wet summers becoming less with

climate change, they do not vanish entirely; hence we still

have to be prepared for them.

By including the uncertainties derived from a wide range

of natural processes, these future projections can be used

to estimate the range of extreme weather events that could

be experienced. The average maximum daily temperatures

experienced in London during a 1-in-50 year event increase

from 35.7 °C over 1961-90 to 38.8 °C by the 2050s. And the

average autumnal 5-day rainfall event totals increase from

78.4 mm to 87.0 mm.





The chances of a colder than average winter

(according to 1961-90 long-term averages) are

about 20% by 2020, but they drop to 4% by 2100.

The chance of the very cold winter temperatures

seen in 2009/10 was about 6%, but by 2100 the

chance drops to less than 1%.

No strong change in winter average rainfall

amounts was found. However significant increases

in extreme rainfall events in autumn are expected.

The chances of a summer considered very hot

historically (happening once every 20 years) rises to

occurring 9 times out of 10 by 2100 (i.e. happening

much more often than not).

For the next 20 years there is still a 35-40% chance

of getting a wetter than average summer. The

chance drops to about 20% by 2100.

The chances of a very wet summer (defined as 20%

more rain than the 1961-90 average) are expected to

fall from 18% in 2020 to 10% by 2100.

TABLE 1: Changes to England and Wales summer and winter temperature and rainfall during the 21st century under a medium-high emissions scenario.


Winter temperature

Summer temperature



-4 -2 0 2 4 6 8 10

Temperature anomaly (°C)

Winter precipitation

-4 -2 0 2 4 6 8 10 12

Temperature anomaly (°C)

Summer precipitation



-100 -50 0 50 100 150 200

Precipitation anomaly (%)

-100 -50 0 50 100 150 200 250

Precipitation anomaly (%)

2 °C of global average temperature rise 4 °C of global average temperature rise

FIGURE 10: Distributions of summer and winter temperature and rainfall under global average warming of 2 °C and 4 °C for England

and Wales. A clear shift to warmer temperatures is seen for both cases, but with more extreme summer and winter warm periods

more likely under 4°C. There is little change in average winter rainfall, but drier summers are more likely in a warmer world.


Climate change and food insecurity

A warming climate increases vulnerability to food insecurity across the globe,

but mitigation can limit this trend

Among the most significant impacts of climate change is

the potential increase of food insecurity and malnutrition.

Understanding the specific impacts of climate change on

food security is challenging because the relationship

between climate and the dimensions of food

security is complex. Food security itself

is more than a simple measure of

production; it also includes not

only availability of food, but

also access, utilisation and

stability. Climate change

will affect all aspects

of food security,

and threatens to

exacerbate existing


Here we show

a measure of

vulnerability to

food insecurity,


with climaterelated

events, in

developing and


countries. This is

a measure of how

vulnerable to disruption

a country’s food security

system is as a result of flood

and drought events. The presentday

measure of vulnerability to food

insecurity is shown in Figure

11. Figures 12-15 show future

projections under two scenarios

of greenhouse gas emissions,

RCP2.6 and RCP8.5, for the 2050s

and 2080s.


Vulnerability to food insecurity


The index shows vulnerability to food insecurity increasing

by the 2050s under both RCP scenarios. However there are

marked differences between the two by the 2080s. The

scenario with rapid and sustained reductions in future

global emissions (RCP2.6) changes little from

the 2050s to the 2080s. However, the

high global emissions scenario

(RCP8.5) shows further

substantial increases in

vulnerability by the 2080s,

as climate change

continues unabated in

this scenario.


FIGURE 11: Vulnerability is comprised of an index that measures

the exposure of national food systems to climate-related hazards,

their sensitivity to impacts of the climate hazards, and their

ability to respond in the face of these impacts. The index has

been designed so that it is best suited to measuring vulnerability

in developing and least developed countries.

These projections

of vulnerability to

food insecurity

show that in the

next few decades,


warming as a

result of the inertia

in the climate

system will threaten

food security in

developing and least

developed countries,

regardless of emissions

over that timeframe.

However, beyond the 2050s

the benefits of mitigation are

clear. Under RCP8.5 the levels of

vulnerability to food insecurity continue

to rise, but under RCP2.6 they

remain stable at approximately

the same levels as in the 2050s.

RCP2.6 – 2050s

RCP2.6 – 2080s

Vulnerability to food insecurity



FIGURE 12: Vulnerability to food insecurity under RCP2.6 in the 2050s.

Vulnerability to food insecurity is projected to increase from the present

day. Even with rapid and sustained reductions in greenhouse gas

emissions, the climate will continue to change in the next few decades,

with negative consequences for food security.

FIGURE 13: Vulnerability to food insecurity under RCP2.6s in the 2080s.

Vulnerability to food insecurity is projected to remain steady after

the 2050s. With rapid and sustained reductions in greenhouse gas

emissions, the climate stabilises over time and, although the situation is

worse than the present day, it does not continue to deteriorate.

RCP8.5 – 2050s

RCP8.5 – 2080s

Vulnerability to food insecurity


FIGURE 14: Vulnerability to food insecurity under RCP8.5 in the 2050s:

Vulnerability to food insecurity is projected to increase from the present

day, as a result of the impacts of climate change.


FIGURE 15: Vulnerability to food insecurity under RCP8.5 in the 2080s.

Levels of vulnerability to food insecurity across the developing world are

projected to increase further still, as a result of climate change.

For further information on these results and to explore how climate change compares against scenarios of adaptation

please visit



The focus of the work by the Met Office Hadley Centre is driven by the Hadley Centre Climate Programme (HCCP).

Related work from other projects benefits from underpinning science from the HCCP. Here we highlight an example of a

project we are leading.


The Met Office Hadley Centre is working in partnership with

the Grantham Institute, Tyndall Centre and Walker Institute

on the AVOID2 programme, which is providing answers

to questions around the issue of “preventing dangerous

anthropogenic interference with the climate system”.

The top-level, policy-relevant result from the programme is

that keeping warming to less than 2 °C is still within reach.

However, this would involve global emissions peaking

before 2030 or sooner, and a significant and sustained global

decarbonisation with removal of atmospheric CO 2

later in

the century.

Recent results describe some cost-optimal pathways to

achieving less than 2 °C with the following characteristics:

electricity highly decarbonised by 2050; sectors such as

industry, transport and buildings shift to electricity and other

low-carbon fuels; and, a substantial deployment of carboncapture

and storage (CCS). The sooner global mitigation

action begins and emissions peak, and the sooner CCS is

scaled-up, the lower costs will be. For example, meeting less

than 2 °C but waiting until 2030 for global mitigation action

to start would cost two-thirds more in GDP compared with

global action starting in 2020. Similarly, the necessary rates of

decarbonisation to meet less than 2 °C would be two to three

times greater when starting in 2030 compared with action

starting in 2020.

If reductions in CO 2

emissions occur later in the century,

then to keep within the 2 °C limit there will be a greater

requirement for BECCS (BioEnergy with CCS), which may hit

planetary limitations. Changing land-use to bio energy crops

can give rise to extra CO 2

emissions (e.g. from deforestation),

which need to be included when assessing the net effect of

BECCS on future global CO 2

emissions. Therefore CO 2


is not a substitute for near-term emissions reductions.

Investigation of the Intended Nationally Determined

Contributions (INDCs) show a long term global average

surface temperature change of likely less than 2.7 °C by 2100,

compared with over 5 °C with no mitigation. The proportion

of impacts avoided by following the INDC pledge pathway

compared to a no mitigation scenario is of the order of 75%

(e.g. for heatwave exposure) to 50% reduction of people

affected by flooding per year, to 25% reduction in area of

cropland experiencing a decline in suitability for crop growth.

The avoided impacts depend strongly on what happens to

emissions after 2030.

No mitigation




Avoided impacts:


millions of people

exposed to heatwaves/year






Emissions capped at INDC level

Strong further action to meet 2°C target


Cropland decline

thousand km 2








millions of people

affected/ year







FIGURE 16: Global impacts of mitigation at INDC and higher levels

compared to no action.

Water stress

millions of people exposed

to increased water stress










Accumulated Cyclone Energy – a measure used to quantify

the intensity of individual tropical cyclones and also entire

seasons. For the seasonal measure it accounts for the number,

duration and intensity of all the storms that occurred during

that season.

Adaptation – Actions taken to adapt to a future, changing

climate to cope with, for example, rising sea-levels, changes

in rainfall amounts or intensity and increases in heatwave

duration and severity.

Earth-system – Used to describe situations where all physical,

chemical and biological processes occurring on, in and

around the Earth are considered.

Mitigation – Actions taken to prevent given levels of climate

change occurring. This is usually achieved by decreasing

the amount of greenhouse gases present in the atmosphere

through reducing emission rates or enhancing processes

which remove these gases.

HadCRUT4 – Met Office and the Climate Research Unit,

University of East Anglia (


NOAA-GlobalTemp – NOAA-National Centers for

Environmental Information Merged Land Ocean Surface

Temperature (


NASA-GISS – NASA Goddard Institute for Space Studies


GHCNDEX – Global Historical Climate Network – Daily –

Extremes (

NSIDC – National Snow and Ice Data Center


University of Colorado – Sea Level Research Group


Projections – Climate model experiments run using plausible

estimates for future greenhouse gas emissions and other

processes that affect the climate to provide best possible

information for future conditions. Different scenarios can

be created by using different emission models. See RCP.

RCP – Representative Concentration Pathway. These indicate

possible trajectories of greenhouse gas (and aerosol)

concentrations over the 21st Century. They do not prescribe

the emissions that lead to these concentrations. Four standard

RCPs are used, and are identified by the equivalent increase

in radiative heating that the greenhouse gas concentrations

result in by 2100. RCP2.6 has concentrations of greenhouse

gases peaking early in the 21st century, before falling

rapidly by 2100, whereas RCP8.5 has continued increases

in atmospheric greenhouse gas concentrations throughout

the 21st century. A further two, RCP4.5 and RCP6.0, are

intermediate pathways.


Contributing authors:

Robert Dunn

Simon Brown

Joanne Camp

Fiona Carroll

Nikolaos Christidis

Glen Harris

Chris Jones

Richard Jones

John Kennedy

Kirsty Lewis

Felicity Liggins

Jason Lowe

Rachel McCarthy

Colin Morice

Katy Richardson

David Sexton

Peter Stott

Paul van der Linden

Kate Willett

Dan Williams

Andy Wiltshire

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