Chris Jones and Sonia Seneviratne

1. Chris Jones
IPCC Lead Author, WGI Chapter 4
IPCC Synthesis Report Core Writing Team
Met Office Hadley Centre, Exeter, UK
UNFCCC COP27, Science Pavilion, 8th November 2022
OVERSHOOT – WHAT DOES IT MEAN
TO EXCEED AND RETURN TO 1.5°C:
EARTH SYSTEM CLIMATE RESPONSE

2. • Amount of global warming is
proportional to the total
(“cumulative”) CO2 emissions
• Limiting global warming requires
limiting total emissions
• There is a finite amount
(“budget”) we can emit
before we reach a warming
level
• We have 500 GtCO2 left of our
1.5°C carbon budget
• from the start of 2020
• for a 50% chance
• Annual emissions currently
approx. 40 GtCO2 per year
WGI Figure SPM.10
Every tonne of CO2 emissions adds to global warming

3. The more we emit…
…the more we warm
To reverse global warming, we must reverse the emissions

4. … we must reverse
the emissions
to reverse
the warming…
• Globally “net negative”
• CDR must be greater than
any residual emissions
To reverse global warming, we must reverse the emissions

5. • For the SSP5-3.4-overshoot
scenario global temperature
reverses quite linearly when
emissions are net-negative
• There may be some small
hysteresis/asymmetry
• WGI Chapter 5
• Some feedbacks not in these
models may make reversing
harder
• E.g. thawing permafrost
(WGII SPM B.6.2)
ssp534-over
(shown by Jan)
Earth system models
show reversibility of
global temperature:
Koven et al., 2022
Global temperature is approximately reversible with net-negative emissions

6. • As CO2 decreases and global
temperature reverses, so does
global land precipitation and
Arctic sea-ice area…
Some, but not all, climate components also reverse
WGI Figure 4.34

7. • …but sea-level rise* does not
reverse
• any sea-level rise from ice-sheet
collapse would be irreversible
• but reduced emissions (and in
this case overshoot) prevent
further acceleration of sea-level
rise
* here just showing thermosteric sea-level rise
(expansion from warming) WGI Figure 4.34
Some degree of sea-level rise is unavoidable

8. • Global temperature can be
reversed
• but only if we reverse (net
negative) emissions
• The more we exceed our
carbon budget the greater
amount of net-negative we
require to reverse temperature
• Both higher levels of overshoot,
and longer duration increase the
amount of unavoidable changes,
and risks of irreversible or abrupt
changes
Climate system summary of overshooting 1.5°C

9. Overshoot pathways: Regional climate and extremes
Sonia I. Seneviratne, ETH Zurich
Coordinating lead author, IPCC AR6 “Physical climate basis” Chapter 11
sonia.seneviratne@ethz.ch
@SISeneviratne
#ClimateReport #IPCC
Overshoot – what does it mean to exceed 1.5°C?
UNFCCC COP27, Science Pavilion, 8th November 2022

10. Overshoot: Regional climate and extremes
Implications of global warming levels before vs after an overshoot are not the same for
regional climate, extremes, and impacts:
• Some changes in the climate system are irreversible on long time scales (e.g. glacier loss,
sea level rise associated with ice melting, tipping points)
• Some climate variables show a hysteresis, i.e. delayed recovery
• Some associated impacts are irreversible (death of people, biodiversity loss)
The implications of overshooting are large for risks to natural and human systems,
especially if the temperature at peak warming is high, because some risks may be long
lasting and irreversible, such as the loss of some ecosystems
(IPCC SR15, Chapter 3, Cross-chapter Box 8: 1.5°C Warmer Worlds

11. Europe, 2022 summer
24’000 fatalities in Europe,
of which 18’000 in West-
Central Europe
Up to 6 meter glacier melt
Burnt forest area in EU
“worst since records’ start
in 2006”
Europe, 2022 summer: Extreme drought & heat
“likely driest summer in 500 years”

12. Overshoot: Regional climate and extremes
Duration
(years)
Magnitude (°C)
Peak
temperature
1.5°C
The peak temperature
determines many of the
impacts of an overshoot
pathway

13. Overshoot: Regional climate and extremes
Asymmetric response of droughts as function of global warming (hysteresis)
Frequency and increase in intensity of agroecological drought events that occurred once every 10 years in a
climate without human influence
"CO2 up phase" "CO2 down phase" Difference (down - up)
Frequency
per
10
years
Intensity
increase
(Liu et al., in preparation; ETH Zurich)

14. Overshoot: Regional climate and extremes
Conclusions:
“1.5°C with overshoot” ≠ “1.5°C”
Every 10th of a degree matters: Overshoots of more than 0.1°C matter
A true ”1.5°C-compatible pathway” can only be with “no or limited overshoot”