Carbon Brief - Zeke HausfatherThe Intergovernmental Panel on Climate Change’s (IPCC)
sixth assessment report
(AR6) is the first major IPCC review to specifically focus on when the
world might pass the 1.5C and 2C warming levels.
AR6 uses a combination of historical observations, climate models and an
updated estimate
of climate sensitivity to provide a best-estimate that the world will pass
– or temporarily “reach” – 1.5C somewhere between 2030 and 2035,
depending on the future emissions scenario.
Even in the most stringent mitigation scenario examined in the report
– SSP1-1.9 – the world exceeds 1.5C in most models during the
middle of the 21st century, before falling back down below 1.5C by 2100
due to the large-scale deployment of
negative emissions technologies
(for more details on the Shared Socioeconomic Pathways (SSPs) used in the
AR6, read
Carbon Brief’s explainer).
The date that the world is expected to pass 1.5C is in “the early part of
the range” suggested in Chapter 1 of the IPCC’s 2018
special report on 1.5C
(SR15), due to a combination of
revisions to historical temperature records
and higher near-term warming projections.
However, a separate, more
directly comparable estimate found in Chapter 2 of the SR15 report is
nearly identical to the new AR6 estimates.
The world is expected to pass 2C in
emissions scenarios
that do not feature strong near-term mitigation with a best-estimate of
between the early 2040s and the early 2050s. These – and the 1.5C
exceedance dates – are quite similar to those given in an
earlier Carbon Brief analysis
published in late 2020.
AR6 has also updated the remaining “
carbon budget” that can be emitted before the world is committed to 1.5C or 2C of
warming.
The carbon budget for a 50% chance to limit warming to 1.5C is quite
similar to that
given in the SR15 report. It shows that the world can emit around 460bn tonnes of CO2 (GtCO2) –
or just 11.5 years of current (2020) emissions – after 1 January 2021
before being committed to 1.5C.
The carbon budgets for limiting warming to 1.5C with a 66% chance has been
slightly increased due to the
narrower range
of
climate sensitivity
in the AR6. The remaining carbon budget to limit warming to 2C is
similarly affected.
In addition, the report introduces new carbon budgets for limiting warming
to 1.7C, as well as for limiting to each temperature with higher (83%) or
lower (17%) probability.
The
narrower range
of future projected warming in the AR6 compared to past IPCC reports –
coupled with more historical observations over the past few years –
provides a better sense of when these important warming levels may be
exceeded.
When will the world pass 1.5C?
AR6 focuses in some depth on when the world might pass 1.5C and 2C warming
levels relative to the pre-industrial period. The authors suggest that the
world is likely to pass 1.5C in the early 2030s, in the absence of rapid
emissions mitigation. This is in the earliest part of the 2030-52 range
reported in the
SR15
summary for policymakers
(SPM).
The 1.5C and 2C warming levels are noteworthy in part because they reflect
the targets set in the
Paris Agreement.
However, Paris targets specifically refer to end-of-century warming
outcomes, so scenarios such as
SSP1-1.9
– which overshoot 1.5C mid-century before reducing temperatures back down
through a large-scale deployment of negative emissions – are nominally
consistent with Paris goals, even if they may exceed 1.5C during the 21st
century.
(See
Carbon Brief’s Q&A on the IPCC report for an introduction to the scenarios used
by the IPCC, or
Carbon Brief’s explainer for more details.)
Calculating when the world exceeds a particular temperature threshold is
not a straightforward exercise. Global average surface temperatures in any
given year are driven by a combination of long-term warming and short-term
natural variability.
The latter – driven by
El Niño and La Niña events
or
volcanic eruptions
– can result in a year being up to 0.2C warmer or cooler than the
trajectory of long-term
human-caused warming.
This means it is quite possible for humans to have only warmed the
world by 1.3C – only slightly above today’s level – and see a single year
that exceeds 1.5C. In fact, the World Meteorological Organization
estimated
last year that there is a one-in-four chance that the world will exceed
1.5C for at least one year by 2025.
The international community of researchers and policymakers is more
concerned with the effects of long-term human-caused warming than
short-term natural variability. Because of this, passing the 1.5C and 2C
limits has
generally been defined
based on a multi-year average rather than a single year, though there is
no clear agreed-upon approach.
To avoid the problem of over-interpreting short-term variability, the AR6
authors calculate the 20-year periods where the average temperature
reaches 1.5C (or 2C) warming above
pre-industrial levels.
For example, in the SSP2-4.5 scenario, they find that 2021-40 is the first
20-year period when the average is most likely to exceed 1.5C.
This
formulation is a bit confusing – in order to simplify it (and compare it
to other approaches) Carbon Brief uses the mid-point of the 20-year period
(for example, 2030 in the case of a 2021-40) as the likely point at which
the longer-term average temperature exceeds 1.5C, following the approach
used in Chapter 4 of the AR6.
This generally provides a good estimate of
the likely exceedance year, though may be slightly off in cases where the
rate of warming changes significantly over the course of the 20-year
period (for example, in very high emissions scenarios).
Both the
SR15 report
and a
2020 analysis
by Carbon Brief used a somewhat different approach; rather than looking at
20-year averages, these methods smoothed the data to remove short-term
temperature variability before calculating the likely exceedance year.
Carbon Brief used the new
CMIP6 models, while the SR15 Chapter 1 assessment simply extrapolated historical
warming trends and uncertainties into the future. AR6, on the other hand,
uses its own
assessed warming range
based on CMIP6 models constrained by both observations and climate
sensitivity estimates.
The SR15 report actually included two separate assessments of when the
world might pass 1.5C.
Chapter 1
of the report suggested that the world would pass 1.5C somewhere between
2030 and 2052, adding that there was stronger evidence to support the
early part of this range.
The Chapter 1 text did not give a central estimate, but did include a
figure, showing the world passing 1.5C around 2040. This was based on an
assumed continuation of the (at the time) historical warming trend of 0.2C
per decade.
However, this value underestimates both recent observed warming trends (in
part due to
corrections to observational temperature records
in the years since the publication of the SR15) and is lower than the
near-term warming projections in most of scenarios evaluated in the SR15
report (and those in the new AR6).
In addition, deep in the
supplementary materials of Chapter 2
of the SR15 report was an estimate (in Table 2. SM.12) that a 1.5C low
overshoot scenario (similar to the SSP1-1.9 scenarios) would pass 1.5C
around 2035.
It suggested that well-below 2C scenarios (similar to
SSP2-2.6) would pass 1.5C around 2033. These values are nearly identical
to the best estimates in the new IPCC report, though because they were not
highlighted in the main text, this was not widely understood at the time.
The figure below shows the main
SR15 projection
of 1.5C exceedance times (grey bar), along with the new AR6 likely
exceedance year (and uncertainty ranges) for each of the new
SSP
scenarios. In addition,
Carbon Brief’s estimates of when the world might pass 1.5C are shown alongside the new IPCC
projections for comparison purposes.
Range of possible 1.5C exceedance years from
Chapter 1
and
Chapter 2
of the IPCC
SR15 report
(grey bar and dots, respectively), as well as AR6 and
Carbon Brief
exceedance year estimates for each of the SSPs. Dots represent central
estimates (when available), while bars represent the very likely (5–95%)
range. 20-year average periods in the AR6 are converted to expected
exceedance years by taking the midpoint of the range. Note that bars
extending to the top of the graph represent cases where the uncertainty
range encompases outcomes that never exceed 1.5C. Chart by Carbon Brief
using
Highcharts.
The new AR6 values are notably in the earliest part of the range given by
SR15 Chapter 1. The AR6 estimates that the world will pass 1.5C around
2030 under a moderate emissions scenario (SSP2-4.5), whereas SR15 gave a
range of 2030-52.
However, as mentioned above, the AR6 numbers are in line with those in the
SR15 Chapter 2 supplementary materials.
More broadly, the AR6 finds a
best-estimate exceedance year of 2027-35 across all of the models
examined, albeit with a wide uncertainty range for each. In scenarios
without rapid mitigation, the world is very likely to exceed 1.5C in the
late 2030s (SSP5-8.5) or mid-2040s (SSP2-4.5) at the latest.
The new AR6 central estimates of when the world will likely pass 1.5C are
generally quite similar to Carbon Brief’s
2020 analysis, though are, on balance, a few years earlier. More importantly, the
uncertainty range in the AR6 is considerably larger, suggesting that the
long-term average temperature may pass 1.5C as early as 2020 or 2021.
However, passing 1.5C this early is a very unlikely outcome given observed
temperatures today. These early potential exceedance dates are likely
something of an artifact of model-observation mismatches after the
1995-2014 normalisation periods – where models and observations are
matched in the AR6.
Carbon Brief’s
earlier analysis, by contrast, used model projections from 2020 onwards based on a
best-estimate of the
human contribution to warming.
Crossing 2C
In addition to 1.5C exceedance years, the AR6 report provides estimates of
when the world may pass 2C warming relative to pre-industrial levels.
These are shown for each SSP scenario expected to pass 2C in the figure
below, alongside Carbon Brief’s own estimates. Note that SSP1-1.9 and
SSP2-2.6 are excluded as temperatures are unlikely to exceed 2C in most
models under these scenarios.
Range of possible 2C exceedance years in the AR6 and Carbon Brief’s 2020
analysis. Dots represent central estimates (when available), while bars
represent the very likely (5–95%) range. 20-year average periods in the AR6
are converted to expected exceedance years by taking the midpoint of the
range. Chart by Carbon Brief using
Highcharts.
AR6 finds that in the modest-mitigation SSP2-4.5 scenario the world is
likely to exceed 2C around 2052, with a range of 2037 to 2084. For the
high emissions SSP3-7.0 scenario, the world is likely to pass 2C around
2046 (with a range of 2035-2062), while in the very high emissions
SSP5-8.5 scenario it is 2041 (with a range of 2032 to 2053).
These values are effectively identical to Carbon Brief’s earlier
estimates, except for a wider range of possible later exceedance dates in
SSP2-4.5 and SSP3-7.0. This may reflect differences between the unweighted
CMIP6 models used by Carbon Brief and the assessed warming range in the
AR6.
The results of the new AR6 report are clear: the best estimate is that the
world will pass 1.5C in the 2030s, even under the rapid mitigation
scenarios of SSP1-1.9 and SSP1-2.6.
However, there is still a chance in
these scenarios that the world will not pass 1.5C, particularly under the
SSP1-1.9 scenario and if sensitivity of the climate to CO2 and other
greenhouse gas emissions is on the low end of the range assessed in the
AR6 report.
In the SSP1-1.9 scenario, the best estimate is that the world will
overshoot 1.5C in the middle of the 21st century – warming by 1.6C
relative to pre-industrial levels – before temperatures fall back down to
below 1.5C by 2100 through the widespread use of negative emissions
technologies.
The world is unlikely to warm more than 2C in either of the deep
mitigation scenarios. In the other three scenarios examined, however, the
best estimate is that the world will pass 2C somewhere between the early
2040s and early 2050s.
Updates to the remaining carbon budget
The year in which the world will exceed 1.5C and 2C warming levels is a
related, but somewhat separate, question from the remaining carbon
budget.
Carbon budgets
are a simplified way to measure the maximum emissions that can still enter
the atmosphere if the world wishes to limit global warming to levels such
as 1.5C. They are based on the fact that the amount of warming that will
occur can be
approximated
by total – that is, cumulative – CO2 emissions.
In practice, though, carbon budgets mask a lot of complexity. Because the
world is already most of the way to 1.5C of warming, the remaining budget
is relatively small and, therefore, quite sensitive to the approach used.
The 2018 SR15 report significantly expanded the carbon budget, relative to
what was reported in the IPCC fifth assessment report (AR5) in 2013-14.
This was primarily due to the use of
observations
rather than climate model projections to estimate historical warming; for
more technical details see this
Carbon Brief analysis.
AR6 used a broadly similar approach to that of the SR15 (
Chapter 2) to calculate the remaining budget and generally obtained similar
results. The figure below shows the remaining carbon budgets for both a
50% chance and a 66% chance of avoiding more than 1.5C warming above
pre-industrial levels from the SR15 and the AR6, as of 1 January
2021.
These are calculated by subtracting observed global emissions (as reported
by the
Global Carbon Project) over the 2018-20 period from the SR15 budget (which gives the remaining
allowable budget starting in 2018) and by subtracting observed 2020
emissions from the AR6 budget (which provides a budget starting in 2020).
Remaining carbon budgets for a 50% and 66% chance of avoiding more than 1.5C
warming as of January 1st 2021. Published budgets have been adjusted using
observed CO2 emissions from the
Global Carbon Project. Chart by Carbon Brief using
Highcharts.
Both the SR15 and AR6 suggest that the world has around 460GtCO2 remaining
in the 1.5C budget for a 50% avoidance chance. This means that the
remaining carbon budget would be fully exhausted in just 11.5 years of
current (2020) emissions – and less than that if global emissions rebound
(as expected) in 2021 and do not decrease over the next decade.
For
example, the carbon budget would be used up in 10.7 years if emissions
continued at 2019 levels. This would result in a world committed to a 1.5C
temperature increase around 2031 – quite similar to the best estimate
exceedance dates found in the AR6 discussed above.
For a 66% chance of limiting warming to 1.5C, AR6 reports that the world
has a remaining carbon budget of 360GtCO2 – or nine years of current
emissions.
This is a notable increase from the SR15 carbon budget of 295GtCO2. This
increased carbon budget for a 66% avoidance chance reflects a narrower
transient climate response to cumulative carbon emissions (
TCRE) value calculated in the AR6 report (1C to 2.3C per 1000 gigatons of
carbon – GtC) than in the SR15 (0.8C to 2.5C per 1,000GtC), resulting from
the narrower estimate of climate sensitivity in the AR6.
This TCRE
revision does not affect the 50% avoidance chance as the best-estimate of
TCRE is unchanged at 1.65C per 1,000GtC.
Methodological changes
A number of other important elements were updated in the AR6 carbon budget
calculations.
First,
updated temperature records
were used that moved the world a bit closer to 1.5C than was assumed in
the SR15 report, which, all things being equal, would result in a lower
carbon budget. However, in the SR15 report the global surface air
temperature (GSAT) was assumed to be warming faster than the global mean
surface temperature (GMST), resulting in a smaller carbon budget.
(GMST is the standard metric used when discussing historical,
observation-based recordings. It is based on a combination of land-surface
air temperatures from weather stations and the sea surface temperature,
measured using buoys and ships.
It is subtly different from GSAT, which is
generally used by climate models. It is also based on land surface air
temperatures, but this is combined with temperatures of the air above
seawater, rather than of the seawater itself.)
AR6 – based on more recent studies – found more limited evidence
for this assumption, and did not assume any difference between GSAT and
GMST. This change largely counterbalanced the reduction in the carbon
budget associated with updated temperature records, resulting in a 50%
avoidance budget nearly identical to that of the SR15.
The SR15 report suggested that unrepresented Earth system feedbacks – such
as
thawing permafrost
– could result in a reduction of remaining carbon budgets of up to
100GtCO2 over the course of this century – and these were not
included in the remaining carbon budgets numbers.
The AR6 lowers this
estimate of Earth system feedbacks to 26GtCO2 per degree C warming (albeit
with an uncertainty of ±97GtCO2) and includes these feedbacks in its
carbon budget numbers – while acknowledging that there is still
low confidence in the exact magnitude of these estimates.
Carbon budget for 2C
In addition to remaining carbon budgets for 1.5C, both the SR15 and AR6
provide carbon budgets for 2C. These are shown – calculated as
starting in 2021 – in the figure below.
Remaining carbon budgets for a 50% and 66% chance of avoiding more than 2C
warming as of January 1st 2021. Published budgets have been adjusted using
observed CO2 emissions from the
Global Carbon Project. Chart by Carbon Brief using
Highcharts.
AR6 slightly reduces the remaining carbon budget for a 50% chance of
avoiding 2C, relative to that in the SR15 – from 1,375GtCO2 (or 34
years of current emissions) to 1,310GtCO2 (or 33 years of current
emissions).
This suggests that the world will exhaust its remaining 2C
carbon budget around 2053 if current emissions continue – quite
similar to the 2052 exceedance year in the modest mitigation SSP2-4.5
scenario discussed earlier.
For a 66% chance of avoiding 2C warming – which is how the Paris
Agreement goal of limiting warming to well-below 2C is commonly
interpreted – AR6 has slightly increased the remaining carbon budget
from 1,045GtCO2 (26 years of current emissions) to 1,110GtCO2 (28 years of
current emissions). This reflects the updated TCRE values in the AR6
compared to the SR15 report.
Overall, both the exceedance years and carbon budgets reported in the AR6
are similar to values reported in the SR15 report – despite notable
updates to the models used for calculating exceedance years and the
approach used for calculating carbon budgets.
The AR6 exceedance estimate
of around 2030 is in the earliest part of the range reported by Chapter 1
of SR15 but is very similar to the values reported in the supplementary
materials of the SR15.
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