Frequently Asked Questions

FAQ

Coordinating Editors:

Sophie Berger (France/Belgium), Sarah L. Connors (France/United Kingdom)

Drafting Authors:

Richard P. Allan (United Kingdom), Paola A. Arias (Colombia), Kyle Armour (United States of America),

Terje Berntsen (Norway), Lisa Bock (Germany), Ruth Cerezo-Mota (Mexico), Kim Cobb (United States

of America), Alejandro Di Luca (Australia, Canada/Argentina), Paul Edwards (United States of America),

Tamsin L. Edwards (United Kingdom), Seita Emori (Japan), François Engelbrecht (South Africa), Veronika

Eyring (Germany), Piers Forster (United Kingdom), Baylor Fox- Kemper (United States of America),

Sandro Fuzzi (Italy), John C. Fyfe (Canada), Nathan P. Gillett (Canada), Nicholas R. Golledge (New

Zealand/United Kingdom), Melissa I. Gomis (France/Switzerland), William J. Gutowski (United States

of America), Raﬁq Hamdi (Belgium), Mathias Hauser (Switzerland), Ed Hawkins (United Kingdom),

Nigel Hawtin (United Kingdom), Darrell S. Kaufman (United States of America), Megan Kirchmeier-

Young (Canada/ United States of America), Charles Koven (United States of America), June-Yi Lee

(Republic of Korea), Sophie Lewis (Australia), Jochem Marotzke (Germany), Valérie Masson-Delmotte

(France), Thorsten Mauritsen (Sweden/Denmark), Thomas K. Maycock (United States of America),

Shayne McGregor (Australia), Sebastian Milinski (Germany), Olaf Morgenstern (New Zealand/

Germany), Swapna Panickal (India), Joeri Rogelj (United Kingdom/Belgium), Maisa Rojas (Chile), Alex

C. Ruane (United States of America), Bjørn H. Samset (Norway), Trude Storelvmo (Norway), Sophie

Szopa (France), Jessica Tierney (United States of America), Russell S. Vose (United States of America),

Masahiro Watanabe (Japan), Sönke Zaehle (Germany), Xuebin Zhang (Canada), Kirsten Zickfeld

(Canada/Germany)

These Frequently Asked Questions have been extracted from the chapters of the underlying report and are compiled

here. When referencing speciﬁc FAQs, please reference the corresponding chapter in the report from where the FAQ

originated (e.g., FAQ 3.1 is part of Chapter 3).

Frequently Asked Questions

FAQ

Frequently Asked Questions

FAQ 5.1 | Is the Natural Removal of Carbon From the Atmosphere Weakening?

For decades, about half of the carbon dioxide (CO

) that human activities have emitted to the atmosphere

has been taken up by natural carbon sinks in vegetation, soils and oceans. These natural sinks of CO

have

thus roughly halved the rate at which atmospheric CO

concentrations have increased, and therefore slowed

down global warming. However, observations show that the processes underlying this uptake are beginning to

respond to increasing CO

in the atmosphere and climate change in away that will weaken nature’s capacity

totake up CO

in the future. Understanding of the magnitude of this change is essential for projecting how the

climate system will respond to future emissions and emissions reduction efforts.

Direct observations of CO

concentrations in

theatmosphere, which began in 1958, show that the

atmosphere has only retained roughly half of the CO

emitted by human activities, due to the combustion of

fossil fuels and land-use change such as deforestation

(FAQ 5.1, Figure1). Natural carbon cycle processes on

land and in the oceans have taken up the remainder

of these emissions. These land and ocean removals or

‘sinks’ have grown largely in proportion to the increase

in CO

emissions, taking up 31% (land) and 23%

(ocean) of the emissions in 2010–2019, respectively

(FAQ 5.1, Figure1). Therefore, the average proportion

of yearly CO

emissions staying in the atmosphere

has remained roughly stable at 44%over the last six

decades, despite continuously increasing CO

emissions

from human activities.

On land, it is mainly the vegetation that captures CO

from the atmosphere through plant photosynthesis,

which ultimately accumulates both in vegetation and

soils. As more CO

accumulates in the atmosphere,

plant carbon capture increases through the CO

fertilization effect in regions where plant growth is not

limited by, for instance, nutrient availability. Climate

change affects the processes responsible for the uptake

and release of CO

on land in multiple ways. Land CO

uptake is generally increased by longer growing seasons

due to global warming in cold regions and by nitrogen

deposition in nitrogen-limited regions. Respiration by

plants and soil organisms, natural disturbances such

as ﬁres, and human activities such as deforestation all

release CO

back into the atmosphere. The combined

effect of climate change on these processes is to weaken

the future land sink. In particular, extreme temperatures

and droughts as well as permafrost thaw (see FAQ 5.2)

tend to reduce the land sink regionally. Inthe ocean,

FAQ 5.1, Figure1 | Atmospheric carbon dioxide (CO

) and natural

carbon sinks. (Top) Global emissions of CO

from human activities and

the growth rate of CO

in the atmosphere; (middle) the net land and

ocean CO

removal (natural sinks); and (bottom) the fraction of CO

emitted by human activities remaining in atmosphere from 1960 to 2019.

Lines are the ﬁve years running mean, error bars denote the uncertainty

of the mean estimate. SeeTable5.SM.6 for more information on the data

underlying this ﬁgure.

FAQ 5.1: Is natural removal of carbon

from the atmosphere weakening?

No, natural carbon sinks have taken up a near constant

fraction of our carbon dioxide (CO

) emissions over the

last six decades. However, this fraction is expected to

decline in the future if CO

emissions continue to increase.

Atmosphere

Natural sinks

remaining in the atmosphere

1960 1980 2000 2020

100%

1960 1980

Years

2000 2020

Billion tonnes of

per year

Billion tonnes of

per year

Proportion of emitted CO

remaining in the atmosphere

Human

caused

emissions

Atmospheric

growth rate

Ocean

Land

Frequently Asked Questions

FAQ

several factors control how much CO

is captured: the difference in CO

partial pressure between the atmosphere

and the surface ocean; wind speeds at the ocean surface; the chemical composition of seawater (that is, its

buffering capacity), which affects how much CO

can be taken up; and the use of CO

in photosynthesis by

seawater microalgae. The CO

-enriched surface ocean water is transported to the deep ocean in speciﬁc zones

around the globe (such as the Northern Atlantic and the Southern Ocean), effectively storing the CO

away from

the atmosphere for many decades to centuries. The combined effect of warmer surface ocean temperatures on

these processes is to weaken the future ocean CO

sink.

The ocean carbon sink is better quantiﬁed than the land sink, thanks to direct ocean and atmospheric carbon

observations. The land carbon sink is more challenging to monitor globally, because it varies widely, even

regionally. There is currently no direct evidence that the natural sinks are slowing down, because observable

changes in the fraction of human emissions stored on land or in oceans are small compared to year-to-year

and decadal variations of these sinks. Nevertheless, it is becoming more obvious that atmospheric and climate

changes are affecting the processes controlling the land and ocean sinks.

Since the land and ocean sinks respond to the rise in atmospheric CO

and to human-induced global warming,

the absolute amount of CO

taken up by land and ocean will be affected by future CO

emissions. This also implies

that, if countries manage to strongly reduce global CO

emissions, or even remove CO

from the atmosphere,

these sinks will take up less CO

because of the reduced human perturbation of the carbon cycle. Under future

high-warming scenarios, it is expected that the global ocean and land sinks will stop growing in the second

half of the century as climate change increasingly affects them. Thus, the total amount of CO

emitted to the

atmosphere and the responses of the natural CO

sinks will both determine what efforts are required to limit

global warming to acertain level (see FAQ 5.4), underscoring how important it is to understand the evolution of

these natural CO

sinks.

FAQ 5.1 (continued)

Frequently Asked Questions

FAQ

Frequently Asked Questions

FAQ 5.2 | Can Thawing Permafrost Substantially Increase Global Warming?

In the Arctic, large amounts of organic carbon are stored in permafrost–ground that remains frozen throughout

the year. If signiﬁcant areas of permafrost thaw as the climate warms, some of that carbon may be released

into the atmosphere in the form of carbon dioxide or methane, resulting in additional warming. Projections

from models of permafrost ecosystems suggest that future permafrost thaw will lead to some additional

warming–enough to be important, but not enough to lead to a‘runaway warming’ situation, where permafrost

thaw leads to adramatic, self-reinforcing acceleration of global warming.

The Arctic is the biggest climate-sensitive carbon pool on Earth, storing twice as much carbon in its frozen soils,

or permafrost, than is currently stored in the atmosphere. As the Arctic region warms faster than anywhere else

on Earth, there are concerns that this warming could release greenhouse gases to the atmosphere and therefore

signiﬁcantly amplify climate change.

The carbon in the permafrost has built up over thousands of years, as dead plants have been buried and

accumulated within layers of frozen soil, where the cold prevents the organic material from decomposing. As

the Arctic warms and soils thaw, the organic matter in these soils begins to decompose rapidly and return to the

atmosphere as either carbon dioxide or methane, which are both important greenhouse gases. Permafrost can

also thaw abruptly in agiven place, due to melting ice in the ground reshaping Arctic landscapes, lakes growing

and draining, and ﬁres burning away insulating surface soil layers. Thawing of permafrost carbon has already

been observed in the Arctic, and climate models project that much of the shallow permafrost (<3 m depth)

throughout the Arctic would thaw under moderate to high amounts of global warming (2°C–4°C).

While permafrost processes are complex, they are beginning to be included in models that represent the

interactions between the climate and the carbon cycle. The projections from these permafrost carbon models show

awide range in the estimated strength of acarbon–climate vicious circle, from both carbon dioxide and methane,

equivalent to 14–175 billion tonnes of carbon dioxide released per 1°C of global warming. By comparison, in

2019, human activities have released about 40 billion tonnes of carbon dioxide into the atmosphere. This has two

implications. First, the extra warming caused by permafrost thawing is strong enough that it must be considered

when estimating the total amount of remaining emissions permitted to stabilize the climate at agiven level of

global warming (i.e.,the remaining carbon budget, see FAQ 5.4). Second, the models do not identify any one

amount of warming at which permafrost thaw becomes a‘tipping point’ or threshold in the climate system

that would lead to arunaway global warming. However, models do project that emissions would continuously

increase with warming, and that this trend could last for hundreds of years.

Permafrost can also be found in other cold places (e.g.,mountain ranges), but those places contain much less

carbon than in the Arctic. For instance, the Tibetan plateau contains about 3% as much carbon as is stored in

the Arctic. There is also concern about carbon frozen in shallow ocean sediments. These deposits are known as

methane hydrates or clathrates, which are methane molecules locked within acage of ice molecules. Theyformed

as frozen soils that were ﬂooded when sea levels rose after the last ice age. If these hydrates thaw, they may

release methane that can bubble up to the surface. The total amount of carbon in permafrost-associated

methane hydrates is much less than the carbon in permafrost soils. Global warming takes millennia to penetrate

into the sediments beneath the ocean, which is why these hydrates are still responding to the last deglaciation.

As aresult, only asmall fraction of the existing hydrates could be destabilised during the coming century. Even

when methane is released from hydrates, most of it is expected to be consumed and oxidised into carbon dioxide

in the ocean before reaching the atmosphere. The most complete modelling of these processes to date suggests

arelease to the atmosphere at arate of less than 2% of current human-induced methane emissions.

Overall, thawing permafrost in the Arctic appears to be an important additional source of heat-trapping gases

to the atmosphere, more so than undersea hydrates. Climate and carbon cycle models are beginning to consider

permafrost processes. While these models disagree on the exact amount of the heat-trapping gases that willbe

released into the atmosphere, they agree that: (i) the amount of such gases released from permafrost will increase

with the amount of global warming; and (ii) the warming effect of thawing permafrost is signiﬁcant enough to

be considered in estimates of the remaining carbon budgets for limiting future warming.

Frequently Asked Questions

FAQ

FAQ 5.2, Figure1 | The Arctic permafrost is abig pool of carbon that is sensitive to climate change. (Left) Quantity of carbon stored in the

permafrost, to 3m depth (NCSCDv2 dataset) and (right) area of permafrost vulnerable to abrupt thaw (Circumpolar Thermokarst Landscapes dataset).

FAQ5.2:

Can thawing permafrost substantially increase global temperatures?

Carbon stored in the Arctic permafrost

Permafrost vulnerable to abrupt thaw

North Pole

Arctic circle

North Pole

Arctic circle

The thawing of frozen ground in the Arcrtic will release carbon that will amplify global warming but this will not lead

to runaway warming.

0 1000 2000 km

0 20 40 60 80+

Kg of organic carbon per m

FAQ 5.2 (continued)

Frequently Asked Questions

FAQ

Frequently Asked Questions

FAQ 5.3 | Could Climate Change Be Reversed By Removing Carbon Dioxide From the Atmosphere?

Deliberate removal of carbon dioxide (CO

) from the

atmosphere could reverse (i.e., change the direction

of) some aspects of climate change. However, this

will only happen if it results in anet reduction in the

total amount of CO

in the atmosphere, that is, if

deliberate removals are larger than emissions. Some

climate change trends, such as the increase in global

surface temperature, would start to reverse within

a few years. Other aspects of climate change would

take decades (e.g., permafrost thawing) or centuries

(e.g.,acidiﬁcation of the deep ocean) to reverse, and

some, such as sea level rise, would take centuries to

millennia to change direction.

The term negative carbon dioxide (CO

) emissions

refers to the removal of CO

from the atmosphere by

deliberate human activities, in addition to removals

that occur naturally, and is often used as synonymous

with carbon dioxide removal. Negative CO

emissions

can compensate for the release of CO

into the

atmosphere by human activities. They could be

achieved by strengthening natural CO

sequestration

processes on land (e.g.,by planting trees or through

agricultural practices that increase the carbon content

of soils) and/or in the ocean (e.g., by restoration of

coastal ecosystems) or by removing CO

directly from

the atmosphere. If CO

removals are greater than

human-caused CO

emissions globally, emissions are

said to be net negative. It should be noted that CO

removal technologies are unable, or not yet ready, to

achieve the scale of removal that would be required to

compensate for current levels of emissions, and most

have undesired side effects.

In the absence of deliberate CO

removal, the CO

concentration in the atmosphere (a measure of

the amount of CO

in the atmosphere) results from

a balance between human-caused CO

release and

the removal of CO

by natural processes on land and

in the ocean (natural ‘carbon sinks’; see FAQ 5.1).

If CO

release exceeds removal by carbon sinks, the

concentration in the atmosphere would increase;

FAQ 5.3: Could climate change be reversed

by removing CO

from the atmosphere?

Removing more carbon dioxide (CO

) from the atmosphere

than is emitted into it could reverse some aspects of climate

change, but some changes would continue in their current

direction for decades to millennia.

Atmospheric CO

(ppm)

peak

600

550

500

450

400

350

300

250

1900 2000 2100 2200 2300

YES, BUT YEARS

Global surface

temperature

change (°C)

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

1900 2000 2100 2200 2300

YES, BUT DECADES

Permafrost

area change

(millions of km

)

Ocean thermal

expansion (m)

-1

-2

-3

-4

-5

-6

-7

-8

1900 2000 2100 2200 2300

NO, CENTURIES, MILLENNIA

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1900 2000 2100

Year

2200 2300

FAQ 5.3, Figure 1 | Changes in aspects of climate change

in response to a peak and decline in the atmospheric CO

concentration (top panel). The vertical grey dashed line indicates the

time of peak CO

concentration in all panels. It shows that the reversal

of global surface warming lags the decrease in the atmospheric CO

concentration by a few years, the reversal of permafrost area decline

lags the decrease in atmospheric CO

by decades, and ocean thermal

expansion continues for several centuries. The quantitative information in

the ﬁgure (i.e.,numbers on vertical axes) is not to be emphasized as it

results from simulations with just one model and will be different for other

models. The qualitative behaviour, however, can be expected to be largely

model independent.

Frequently Asked Questions

FAQ

if CO

release equals removal, the atmospheric CO

concentration would stabilize; and if CO

removal exceeds

release, the CO

concentration would decline. This applies in the same way to net CO

emissions–that is, the sum

of human-caused releases and deliberate removals.

If the CO

concentration in the atmosphere starts to go down, the Earth’s climate would respond to this change

(FAQ 5.3, Figure1). Some parts of the climate system take time to react to achange in CO

concentration, so

a decline in atmospheric CO

as a result of net negative emissions would not lead to immediate reversal of

all climate change trends. Recent studies have shown that global surface temperature starts to decline within

afew years following adecline in atmospheric CO

, although the decline would not be detectable for decades

due to natural climate variability (see FAQ 4.2). Other consequences of human-induced climate change, such as

reduction in permafrost area, would take decades; yet others, such as warming, acidiﬁcation and oxygen loss

of the deep ocean, would take centuries to reverse following adecline in the atmospheric CO

concentration.

Sea level would continue to rise for many centuries to millennia, even if large deliberate CO

removals were

successfully implemented.

‘Overshoot’ scenarios are a class of future scenarios that are receiving increasing attention, particularly in

the context of ambitious climate goals, such as the global warming limits of 1.5°C or 2°C included in the Paris

Agreement. In these scenarios, aslow rate of reduction in emissions in the near term is compensated by net

negative CO

emissions in the later part of this century, which results in atemporary breach or ‘overshoot’ of

agiven warming level. Due to the delayed reaction of several climate system components, it follows that the

temporary overshoot would result in additional climate changes compared to ascenario that reaches the goal

without overshoot. These changes would take decades to many centuries to reverse, with the reversal taking

longer for scenarios with larger overshoot.

Removing more CO

from the atmosphere than is emitted into it would indeed begin to reverse some aspects

of climate change, but some changes would still continue in their current direction for decades to millennia.

Approaches capable of large-scale removal of CO

are still in the state of research and development or unproven

at the scales of deployment necessary to achieve a net reduction in atmospheric CO

levels. CO

removal

approaches, particularly those deployed on land, can have undesired side effects on water, food production

and biodiversity.

FAQ 5.3 (continued)

Frequently Asked Questions

FAQ

Frequently Asked Questions

FAQ 5.4 | What Are Carbon Budgets?

There are several types of carbon budgets. Most often, the term refers to the total net amount of carbon dioxide

(CO

) that can still be emitted by human activities while limiting global warming to aspeciﬁed level (e.g.,1.5°C

or 2°C above pre-industrial levels). This is referred to as the ‘remaining carbon budget’. Several choices and value

judgements have to be made before it can be unambiguously estimated. When the remaining carbon budget is

combined with all past CO

emissions to date, a‘total carbon budget’ compatible with aspeciﬁc global warming

limit can also be deﬁned. Athird type of carbon budget is the ‘historical carbon budget’, which is ascientiﬁc way

to describe all past and present sources and sinks of CO

The term remaining carbon budget is used to describe the total net amount of CO

that human activities can

still release into the atmosphere while keeping global warming to aspeciﬁed level, like 1.5°C or 2°C relative

to pre-industrial temperatures. Emissions of CO

from human activities are the main cause of global warming.

Aremaining carbon budget can be deﬁned because of the speciﬁc way CO

behaves in the Earth system. That

is, global warming is roughly linearly proportional to the total net amount of CO

emissions that are released

into the atmosphere by human activities –also referred to as cumulative anthropogenic CO

emissions. Other

greenhouse gases behave differently and have to be accounted for separately.

The concept of a remaining carbon budget implies that, to stabilize global warming at any particular level,

global emissions of CO

need to be reduced to net zero levels at some point. ‘Net zero CO

emissions’ describes

a situation where all the anthropogenic emissions of CO

are counterbalanced by deliberate anthropogenic

removals so that, on average, no CO

is added or removed from the atmosphere by human activities. Atmospheric

concentrations in such asituation would gradually decline to along-term stable level as excess CO

in the

atmosphere is taken up by ocean and land sinks (see FAQ 5.1). The concept of aremaining carbon budget also

means that, if CO

emissions reductions are delayed, deeper and faster reductions are needed later to stay within

the same budget. If the remaining carbon budget is exceeded, this will result in either higher global warming

or aneed to actively remove CO

from the atmosphere to reduce global temperatures back down to the desired

level (see FAQ 5.3).

Estimating the size of remaining carbon budgets depends on aset of choices. These choices include: (1)theglobal

warming level that is chosen as alimit (for example, 1.5°C or 2°C relative to pre-industrial levels); (2) the probability

with which we want to ensure that warming is held below that limit (for example, aone-in-two, two-in-three,

or higher chance), and (3) how successful we are in limiting emissions of other greenhouse gases that affect the

climate, such as methane or nitrous oxide. These choices can be informed by science, but ultimately represent

subjective choices. Once these choices have been made, to estimate the remaining carbon budget for agiven

temperature goal, we can combine knowledge about: how much our planet has warmed already; the amount

of warming per cumulative tonne of CO

; and the amount of warming that is still expected once global net

emissions are brought down to zero. For example, to limit global warming to 1.5°C above pre-industrial

levels with either a one-in-two (50%) or two-in-three (67%) chance, the remaining carbon budgets amount

to 500and 400 billion tonnes of CO

respectively, from 1January 2020 onward (FAQ 5.4, Figure1). Currently,

human activities are emitting around 40 billion tonnes of CO

into the atmosphere in asingle year.

The remaining carbon budget depends on how much the world has already warmed to date. This past warming

is caused by historical emissions, which are estimated by looking at the historical carbon budget –a scientiﬁc way

to describe all past and present sources and sinks of CO

. It describes how the CO

emissions from human activities

have redistributed across the various CO

reservoirs of the Earth system. These reservoirs are the ocean, the land

vegetation, and the atmosphere (into which CO

was emitted). The share of CO

that is not taken up by the

ocean or the land, and that thus increases the concentration of CO

in the atmosphere, causes global warming.

The historical carbon budget tells us that, of the about 2560 billion tonnes of CO

that were released into the

atmosphere by human activities between the years 1750 and 2019, about aquarter were absorbed by the ocean

(causing ocean acidiﬁcation) and about athird by the land vegetation. About 45% of these emissions remain in

the atmosphere (see FAQ 5.1). Adding these historical CO

emissions to estimates of remaining carbon budgets

allows an estimate of the total carbon budget consistent with aspeciﬁc global warming level.

Frequently Asked Questions

FAQ

In summary, determining aremaining carbon budget –that is, how much CO

can be released into the atmosphere

while stabilizing global temperature below achosen level –is well understood but relies on aset of choices.

However, it is clear that, for limiting warming below 1.5°C or 2°C, the remaining carbon budget from 2020

onwards is much smaller than the total CO

emissions released to date.

FAQ 5.4, Figure1 | Various types of carbon budgets. Historical cumulative carbon dioxide (CO

) emissions determine to alarge degree how much

the world has warmed to date, while the remaining carbon budget indicates how much CO

could still be emitted while keeping warming below speciﬁc

temperature thresholds. Several factors limit the precision with which the remaining carbon budget can be estimated. Therefore, estimates need to specify the

probability with which they aim at limiting warming to the intended target level (e.g.,limiting warming to 1.5°C with a67% probability).

This remaining carbon budget

can increase or decrease

depending on how deeply

we reduce greenhouse gases

other than CO

FAQ 5.4: What are Carbon Budgets?

The term carbon budget is used in several ways. Most often the term refers to the total net amount of carbon dioxide

(CO

) that can still be emitted by human activities while limiting global warming to a specified level.

Historical budget

GtCO

already emitted between 1750–2019

Remaining carbon budget

GtCO

in line with keeping

global warming to 1.5°C or 2°C

1.5°C

2°C

2560

GtCO

+/-220

(500) GtCO

50%Probability67%

(1350) GtCO

1150

(Gt = billion tonnes)

400

FAQ 5.4 (continued)