This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the National Science Foundation under Cooperative Agreement No. 1852977.

Water Isotope Modeling with CESM

Jiang ZhuPaleo & Polar Climate, NCAR

B. Otto-Bliesner, E. Brady, & The iCESM TEAM

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What are water isotopes (isotopologues)?

H216O

m=18

8p

8n 8p

10n

H218O

m=20 Heavier water has slightly

o higher ρ, Tmelting, Tboiling, ν, …o lower chemical reaction rates

Fractionations occur during physical, chemical, & biologicalprocesses

Other water isotopologues include H217O, HD17O, D217O, …

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The delta notation

o Natural abundance H218O

H216O ≈ !

"##

o Challenge to measure absolute value

𝛿!$O =

18O16O %&'()*18O16O %+&,-&.-

− 1 ×1000‰

o Standards• Water: VSMOW (Vienna Standard Mean Ocean Water)

• Carbonate: VPDP (Vienna Pee Dee Belemnite)https://open.oregonstate.education

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Why water isotopes? – “Paleo-Thermometer”

Carbonate-water thermometer• 𝑇 = −4.8 (𝛿!$O/&.01,&+* − 𝜹𝟏𝟖𝐎4&+*.) + 16.5

• T-dependent reaction ratesCaC16O+ 3H18

2 O ⌦ CaC18O+ 3H162 O

Pearson, 2012, The Paleontological Society Papers

Ice-core thermometer • 𝛿!$O = 𝜶 𝑇 + 𝛽

• Rayleigh fractionation (heavy water condenses more easily)

–50 –40 –20 –10–30

–40

–20

–30

Temperature (°C)

δ18 O

(‰)

Jouzel et al., 1997, JGR-Oceans

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Why water isotopes? – “Paleo-Rain gauge” (the amount effect)

Tropical precipitation amount (mm / year)

δ18 O

in p

reci

pita

tion

(‰; V

SMO

W)

Rozanski et al., 1993, Geophysical Monograph Series

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δ18 O

(‰)

Warminterglacial

Coldglacial

Stronger

MONSOON

Weaker

Deep ocean

Greenland

Chinese Cave

Lisiecki & Stern, 2016, Paleoceanography

Why water isotopes? – infer past changes (largely qualitatively)

𝑇 = −4.8 (𝛿!"O#$%&'($)* − 𝜹𝟏𝟖𝐎-$)*%) + 16.5

𝛿!"O = 𝜶 𝑇 + 𝛽

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An isotope-enabled Community Earth System Model (iCESM)

o Physically consistent simulation of water isotopes in the Earth system

• Better interpret the isotope records

• Direct synthesis of model-data information

o https://github.com/NCAR/iCESM1.2 (Not yet available for CESM2)

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How does iCESM work?

H218O water cycle = H216O water cycle + fractionations

Brady et al., 2019, JAMES

CESMiCESM

A complete suite of physical processeso Ocean & atmosphere circulationo Clouds & convectiono Turbulence & mixingo Soil & vegetationo Snow & iceo Rivero Flux exchangeso …

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LGM SST proxies (956)

18O (594)62%!

Mg/Ca (150) UK'37 (192) TEX86 (20)

An application: how cold was the Last Glacial Maximum (LGM)?

LGM (20,000 years ago)

Tierney, Zhu, King, Malevich, Hakim, & Poulsen, 2020, Nature

–30‰

𝑇 = −4.8 (𝛿!"O#$%&'($)* − 𝜹𝟏𝟖𝐎𝐰𝐚𝐭𝐞𝐫) + 16.5

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A

iCESM EnsemblesB

LGM SSTC

-10

-5

0°C

LGM SATD

-15

-10

-5

0°C

LGM PrecipE

-4-2024

mm/day

+ =

LGM SST proxies (956)

18O (594) Mg/Ca (150) UK'37 (192) TEX86 (20)

iCESM enables direct assimilation of δ18O records

LGM is 5.7–6.5°C colder

cf. 3–8°C in IPCC (2013)

Tierney et al. 2020a,b, Nature; Science

𝑇 = −4.8 (𝛿!"O#$%&'($)* − 𝜹𝟏𝟖𝐎2$)*%) + 16.5

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LGM informs equilibrium climate sensitivity (ECS)

-14-12-10- -6-4-24G5 global Kooling (3)

0

1

2

3

4

5

6

-CS (3)

Sherwood2020

0PCC2013

TPerney2020

CS51CCS53

CCS54

C-S51

C-S52r = -0.97

Zhu et al., 2021a,b, GRL; JAMES (in preparation)

o ECS = global warming due to 2×CO2o One of the most important metrics in

climate science

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Summary

o Water isotopes are paleo-thermometer & rain gauge but imperfect

o We need a dynamical model, iCESM, to • provide a physically consistent simulation of water isotopes in the Earth system

• better interpret the isotope record

• directly synthesize information from model & data for a mechanism understanding of past and future climate change

o iCESM can also be used to study present-day climate: e.g. Bailey, A. (2020). A New Lens for Evaluating Dynamic Controls on Shallow Convection. Journal of Advances in Modeling Earth Systems

Thank you!jiangzhu@ucar.edu