Confidential manuscript submitted to Geophysical Research Letters1

Supplementary Information

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Spurious Late Historical-Era Warming in CESM2 Driven by Prescribed Biomass Burning

3Emissions

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J. T. Fasullo*1,2, Jean-Francois Lamarque1, Cecile Hannay1, Nan Rosenbloom1, Simone Tilmes1,

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Patricia DeRepentigny2, Alexandra Jahn2, and Clara Deser1

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Contents of this file Figures S1 to S2 and descriptive text.

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The spatial structure of anomalous warming of CESM2 during the GFED era relative to that in8

CESM1, estimated by the inter-model differences in the changes between 20-yr intervals, 1995-92014 and 1970-1989, is characterized by disproportionately strong warming at northern mid and

10high latitudes. The regions of disproportionate warming (Figure S1a) are located over, and to the11east of, eastern continental boundaries in Eurasia and North America. Contrasting changes in 850

12hPa relative humidity (RH; Figure S1b), where low clouds are common at high latitudes, are13

more spatially complex than for temperature and likely reflect both dynamical responses to BB

14emissions and contrasts in model physics. Yet systematically stronger decreases are generally

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evident in regions of disproportionate warming (shown below). Strongly coherent with the16

BOAS and BONA regions are disproportionate reductions in cloud drop number in CESM2

17(Figure S1c), reductions that are much larger than model differences elsewhere on the globe.

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Relative cloud liquid water path (LWP) changes (Figure S1d) are also negative in CESM2, both

19in regions coherent with cloud drop number changes and to their east, suggesting a potential role

20for advection of cloud anomalies downstream from the BOAS and BONA regions. Spatially21

correlated to reductions in LWP are reductions in low cloud amount (Figure S1e) and increases

Confidential manuscript submitted to Geophysical Research Letters

22in SWsfc (Figure S1f). Considered in tandem, these diagnostics provide a mechanistic perspective

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on the drivers of disproportionate high latitude warming in CESM2 during the GFED era

24whereby the increased variability in emissions drive net reductions in RH and cloud drop number

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that decrease low cloud amount and thickness, and increase SWsfc and warming, both locally and26

downstream from the BOAS and BONA emission regions.

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Figure S1. Differences in spatial structure of late historical-era changes between CESM2 and29

CESM1. Shown are CESM2 minus CESM1 differences in changes (1995-2014 minus30

1970-1989) in (a) temperature and (b) relative humidity at 850 hPa, vertically integrated

Confidential manuscript submitted to Geophysical Research Letters

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cloud droplet number (c) and liquid water path (d), low cloud amount (e), and net surface

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shortwave flux (f). Regions where the difference in changes is less than twice the33

ensemble standard error are stippled. Note that scaling is applied to some fields (see34panel titles).

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Figure S2. Evidence for analogous sensitivity to BB in other CMIP6 simulations. Ensemble
mean historical-era evolution of anomalous downwelling solar radiation from 40oN-70oN

Confidential manuscript submitted to Geophysical Research Letters

38for (a) all simulation members submitted to the CMIP3 (blue), CMIP5 (red), and CMIP639(black) archives, and (b) by model submitted simulations to CMIP6 for which at least 440

historical-era members are available. The standard deviation range across CMIP641members is shown (grey) and the average from 1980-1989 was used to baseline the42timeseries. The ensemble-mean anomalies for 1997-2014 are shown with model names

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and number of ensemble members (parenthesis). In (a), CMIP3 and CMIP5 members

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have been extended through 2014 with SRES-a1b and RCP85, respectively.

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