2._CRD Materials_NN2 and_CJC Methods_NN2 
2.1_CRD ._SENT Sample_NN1 Collection_NN1 and_CJC Preparation_NN1 
We_PNP collected_VVD tree-ring_AJ0 samples_NN2 from_PRP living_VVG P._NP0 engelmannii_NP0 trees_NN2 at_PRP four_CRD sites_NN2 (_PUL FLS_NP0 ,_PUN RPS_NP0 ,_PUN MWS_NP0 ,_PUN and_CJC SLS_NP0 )_PUR across_PRP the_AT0 GYE_NP0 in_PRP northwest_NN1 Wyoming_NP0 ,_PUN US_NP0 (_PUL 44.4_CRD –_PUN 45.0_CRD °_NN0 N_ZZ0 ,_PUN 109.5_CRD –_PUN 110.5_CRD °_NN0 W_ZZ0 )_PUR ,_PUN two_CRD located_VVN in_PRP Yellowstone_NP0 Na-_AJ0 tional_AJ0 Park_NN1 and_CJC two_CRD on_PRP the_AT0 Shoshone_NN1 National_AJ0 Forest_NN1 (_PUL Figure_NN1 1a_NN1 and_CJC Table_NN1 S1_CRD )_PUR ._SENT At_PRP each_DT0 site_NN1 ,_PUN we_PNP collected_VVD in-_AJ0 crement_NN1 cores_NN2 from_PRP living_AJ0 trees_NN2 (_PUL 2_CRD per_PRP tree_NN1 )_PUR (_PUL Stokes_NP0 &_NP0 Smiley_NP0 ,_PUN 1968_CRD )_PUR ,_PUN and_CJC at_PRP the_AT0 FLS_NP0 site_NN1 ,_PUN we_PNP used_VVD a_AT0 chainsaw_NN1 to_TO0 remove_VVI cross_NN1 sections_NN2 from_PRP downed_AJ0 remnant_NN1 ,_PUN subfossil_NN1 P._NP0 engelmannii_NP0 logs_NN2 ._SENT All_DT0 cores_NN2 and_CJC cross_NN1 sections_NN2 were_VBD processed_VVN using_VVG standard_AJ0 dendrochronological_AJ0 techniques_NN2 (_PUL Stokes_NP0 &_NP0 Smiley_NP0 ,_PUN 1968_CRD )_PUR ._SENT Because_CJS samples_NN2 did_VDD not_XX0 exhibit_VVI visible_AJ0 color_NN1 changes_NN2 between_PRP heartwood_NN1 and_CJC sapwood_NN1 or_CJC between_PRP living_AJ0 and_CJC subfossil_AJ0 sam-_AJ0 ples_NN2 ,_PUN no_AT0 chemical_AJ0 processing_NN1 was_VBD required_VVN prior_AJ0 to_PRP collecting_VVG LWB_NP0 measurements_NN2 ._SENT 
2.2_CRD ._SENT Chronology_NN1 Development_NN1 
We_PNP obtained_VVD LWB_NP0 measurements_NN2 for_PRP all_DT0 samples_NN2 using_VVG an_AT0 Epson_NP0 Expression_NN1 XL_CRD 12000_CRD commercial_AJ0 flatbed_AJ0 scanner_NN1 in_PRP combination_NN1 with_PRP the_AT0 software_NN1 CooRecorder_NN1 (_PUL Larsson_NP0 ,_PUN 2014_CRD )_PUR ,_PUN the_AT0 absolute_AJ0 dates_NN2 of_PRF which_DTQ were_VBD then_AV0 validated_VVN statistically_AV0 with_PRP the_AT0 software_NN1 COFECHA_NN1 (_PUL Holmes_NP0 ,_PUN 1983_CRD )_PUR ._SENT We_PNP inverted_VVD the_AT0 raw_AJ0 LWB_NP0 values_NN2 to_TO0 allow_VVI for_PRP detrending_NN1 by_PRP methods_NN2 similar_AJ0 to_PRP those_DT0 used_VVN for_PRP MXD_AJ0 data_NN0 (_PUL Rydval_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD )_PUR ._SENT We_PNP removed_VVD age-related_AJ0 trends_NN2 by_PRP detrending_VVG all_DT0 LWB_NP0 series_NN0 using_VVG the_AT0 age-dependent_AJ0 spline_NN1 using_VVG the_AT0 power_NN1 trans-_NN1 formed_VVN residuals_NN2 in_PRP the_AT0 SignalFree_AJ0 framework_NN1 (_PUL Melvin_NP0 et_NP0 al._NP0 ,_PUN 2007_CRD )_PUR ._SENT 
2.3_CRD ._SENT Climate_NN1 Response_NN1 and_CJC Reconstruction_NN1 Model_NN1 
We_PNP compare_VVB each_DT0 site_NN1 LWB_NP0 chronology_NN1 with_PRP monthly_AJ0 CRU_NN1 TS_ZZ0 4.03_CRD (_PUL land_NN1 )_PUR 0.5Tmax_NN1 and_CJC Tmean_AJ0 data_NN0 (_PUL Harris_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD )_PUR averaged_VVD over_PRP the_AT0 GYE_NP0 study_NN1 region_NN1 (_PUL 44.00_CRD –_PUN 44.50_CRD °_NN0 N_ZZ0 ,_PUN −109.00_CRD to_PRP 109.50_CRD °_NN0 W_ZZ0 )_PUR for_PRP the_AT0 period_NN1 1905_CRD –_PUN 2015._CRD We_PNP use_VVB Pearson_NP0 correlation_NN1 analysis_NN1 to_TO0 test_VVI temporal_AJ0 (_PUL e.g._AV0 ,_PUN target_NN1 reconstruction_NN1 season_NN1 )_PUR and_CJC spatial_AJ0 relationships_NN2 between_PRP temperature_NN1 and_CJC LWB_NP0 data_NN0 using_VVG KNMI_AJ0 Climate_NN1 Explorer_NN1 (_PUL Trouet_NP0 &_NP0 Van_NP0 Old-_NP0 enborgh_NP0 ,_PUN 2013_CRD )_PUR ._SENT We_PNP then_AV0 employ_VVB a_AT0 nested_AJ0 principal_AJ0 components_NN2 regression_NN1 (_PUL PCR_NP0 )_PUR model_NN1 ,_PUN which_DTQ accounts_VVZ for_PRP the_AT0 decrease_NN1 in_PRP number_NN1 of_PRF predictor_NN1 chronologies_NN2 back_AVP through_PRP time_NN1 (_PUL E_ZZ0 ._SENT R_ZZ0 ._SENT Cook_NP0 et_NP0 al._NP0 ,_PUN 1999_CRD )_PUR ._SENT LWB_NP0 chronologies_NN2 are_VBB retained_VVN for_PRP modeling_NN1 if_CJS they_PNP are_VBB positively_AV0 and_CJC significantly_AV0 (_PUL p_ZZ0 <_NN1 0.01_CRD )_PUR correlated_VVN to_PRP the_AT0 target_NN1 instrumental_AJ0 data_NN0 during_PRP the_AT0 common_AJ0 period_NN1 ._SENT To_TO0 validate_VVI /_PUN cross-validate_VVB the_AT0 reconstruction_NN1 model_NN1 ,_PUN we_PNP split_VVD the_AT0 common_AJ0 period_NN1 into_PRP two_CRD equal_AJ0 periods_NN2 for_PRP calibration_NN1 on_PRP the_AT0 early_AJ0 period_NN1 and_CJC verification_NN1 on_PRP the_AT0 late_AJ0 period_NN1 ._SENT To_TO0 assess_VVI model_NN1 uncertainty_NN1 ,_PUN we_PNP use_VVB the_AT0 maximum_AJ0 entropy_NN1 bootstrapping_AJ0 method_NN1 (_PUL MEBoot_NP0 )_PUR (_PUL Vinod_NP0 &_NP0 López-de_NP0 Lacalle_NP0 ,_PUN 2009_CRD )_PUR to_TO0 produce_VVI 300_CRD reconstruction_NN1 replicates_VVZ ._SENT We_PNP conduct_VVB a_AT0 severity_NN1 –_PUN duration_NN1 analysis_NN1 (_PUL González_NP0 &_NP0 Valdés_NP0 ,_PUN 2003_CRD )_PUR on_PRP reconstructed_AJ0 values_NN2 of_PRF at_PRP least_AV0 2+_CRD years_NN2 to_TO0 determine_VVI the_AT0 magnitude_NN1 (_PUL average_AJ0 cumulative_AJ0 departure_NN1 from_PRP the_AT0 long-term_AJ0 mean_NN1 )_PUR ,_PUN duration_NN1 (_PUL time_NN1 span_NN1 of_PRF each_DT0 event_NN1 )_PUR ,_PUN and_CJC intensity_NN1 (_PUL duration_NN1 divided_VVN by_PRP the_AT0 magnitude_NN1 )_PUR of_PRF warm_AJ0 and_CJC cool_AJ0 events_NN2 ._SENT The_AT0 top_AJ0 10_CRD warm_AJ0 and_CJC cool_AJ0 events_NN2 are_VBB based_VVN on_PRP the_AT0 overall_AJ0 rank_NN1 (_PUL duration_NN1 rank_NN1 +_NN1 magnitude_NN1 rank_NN1 )_PUR ._SENT 
3._CRD Results_NN2 and_CJC Discussion_NN1 
3.1_CRD ._SENT Chronology_NN1 Development_NN1 
Of_PRF the_AT0 four_CRD GYE_NP0 LWB_NP0 chronologies_NN2 ,_PUN FLS_NP0 is_VBZ the_AT0 only_AJ0 record_NN1 that_CJT spans_VVZ >_NN1 1_CRD ,_PUN 000_CRD years_NN2 (_PUL 737_CRD –_PUN 2016_CRD CE_NP0 )_PUR due_AJ0 to_PRP the_AT0 successful_AJ0 inclusion_NN1 of_PRF remnant_NN1 material_NN1 into_PRP the_AT0 chronology_NN1 (_PUL Table_NN1 S1_CRD )_PUR ._SENT RBAR_AJ0 values_NN2 indicate_VVB that_CJT all_DT0 LWB_NP0 chronologies_NN2 show_VVB adequate_AJ0 signal_NN1 coherence_NN1 ,_PUN ranging_VVG from_PRP 0.38_CRD (_PUL FLS_NP0 )_PUR to_PRP 0.29_CRD (_PUL MWS_NP0 )_PUR over_PRP the_AT0 period_NN1 during_PRP which_DTQ EPS_NP0 >_NP0 0.80_CRD ._SENT RBAR_AJ0 values_NN2 for_PRP these_DT0 chronologies_NN2 are_VBB similar_AJ0 to_PRP those_DT0 found_VVN in_PRP other_AJ0 BI_AJ0 studies_NN2 (_PUL Heeter_NP0 et_NP0 al._NP0 ,_PUN 2020_CRD ;_PUN Wilson_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD ,_PUN 2019_CRD )_PUR ._SENT Correlations_NN2 between_PRP each_DT0 site_NN1 LWB_NP0 chronology_NN1 indicate_VVB a_AT0 high_AJ0 level_NN1 of_PRF similarity_NN1 (_PUL Figure_NN1 S1_CRD )_PUR ,_PUN and_CJC thus_AV0 all_DT0 four_CRD LWB_NP0 chronologies_NN2 advanced_VVN to_PRP the_AT0 candidate_NN1 pool_NN1 of_PRF predictors_NN2 for_PRP the_AT0 PCR_NP0 model_NN1 ._SENT 
3.2_CRD ._SENT Climate_NN1 Response_NN1 and_CJC Seasonality_NN1 
We_PNP found_VVD the_AT0 strongest_AJS significant_AJ0 and_CJC positive_AJ0 relationships_NN2 between_PRP all_DT0 four_CRD LWB_NP0 chronologies_NN2 and_CJC cur-_AJ0 rent-year_AJ0 August_NP0 Tmax_NP0 (_PUL r_ZZ0 =_NP0 0.43_CRD –_PUN 0.76_CRD ,_PUN p_ZZ0 <_NN1 0.001_CRD )_PUR ._SENT This_DT0 is_VBZ consistent_AJ0 with_PRP other_AJ0 studies_NN2 ,_PUN which_DTQ suggest_VVB that_CJT LWB_NP0 typically_AV0 shows_VVZ stronger_AJC relationships_NN2 with_PRP Tmax_NP0 than_CJS with_PRP Tmean_NP0 (_PUL e.g._AV0 ,_PUN Heeter_NP0 et_NP0 al._NP0 ,_PUN 2020_CRD ;_PUN Wilson_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD ,_PUN 2019_CRD )_PUR ._SENT Because_CJS the_AT0 LWB_NP0 /_PUN instrumental_AJ0 data_NN0 relationships_NN2 were_VBD consistently_AV0 strong_AJ0 after_PRP ca._NN0 1920_CRD ,_PUN we_PNP use_VVB 1920_CRD to_TO0 be_VBI the_AT0 starting_NN1 year_NN1 for_PRP our_DPS subsequent_AJ0 analysis_NN1 ._SENT We_PNP find_VVB the_AT0 climate_NN1 response_NN1 of_PRF all_DT0 four_CRD sites_NN2 to_TO0 be_VBI representative_AJ0 of_PRF Tmax_NN1 variability_NN1 across_PRP the_AT0 GYE_NP0 and_CJC broader_AJC western_AJ0 US_NP0 (_PUL Figure_NN1 S2_CRD )_PUR ._SENT Across_PRP all_DT0 four_CRD sites_NN2 ,_PUN we_PNP note_VVB a_AT0 positive_AJ0 relationship_NN1 between_PRP site_NN1 elevation_NN1 and_CJC temperature_NN1 signal_NN1 strength_NN1 ;_PUN the_AT0 highest_AJS elevation_NN1 sites_NN2 yielded_VVD the_AT0 strongest_AJS spatial_AJ0 and_CJC temporal_AJ0 temperature_NN1 responses_NN2 (_PUL Table_NN1 S1_CRD )_PUR ._SENT Wil-_AJ0 son_NN1 et_NP0 al._NP0 (_PUL 2019_CRD )_PUR and_CJC Salzer_NP0 ,_PUN Larson_NP0 ,_PUN et_NP0 al._NP0 (_PUL 2014_CRD )_PUR show_VVB a_AT0 similar_AJ0 relationship_NN1 between_PRP site_NN1 elevation_NN1 and_CJC the_AT0 signal_NN1 strength_NN1 of_PRF LWB_NP0 chronologies_NN2 across_PRP the_AT0 Yukon_NP0 ,_PUN Canada_NP0 ,_PUN and_CJC TRW_NP0 chronologies_NN2 in_PRP the_AT0 White_AJ0 Mountains_NN2 of_PRF California_NP0 ,_PUN USA_NP0 ,_PUN respectively_AV0 ._SENT We_PNP limit_VVB our_DPS reconstruction_NN1 target_NN1 to_PRP August_NP0 Tmax_NP0 due_AJ0 to_PRP a_AT0 combination_NN1 of_PRF several_DT0 factors_NN2 ._SENT First_ORD ,_PUN recent_AJ0 studies_NN2 have_VHB shown_VVN the_AT0 growing_AJ0 season_NN1 for_PRP high-elevation_NN1 P._NP0 engelmannii_NP0 ,_PUN particularly_AV0 at_PRP the_AT0 FLS_NP0 site_NN1 ,_PUN is_VBZ restricted_VVN to_PRP late_AJ0 July_NP0 through_PRP early_AJ0 September_NP0 ,_PUN such_DT0 that_DT0 late-_AJ0 wood_NN1 tracheids_NN2 are_VBB most_AV0 likely_AJ0 differentiating_VVG and_CJC maturing_VVG during_PRP the_AT0 month_NN1 of_PRF August_NP0 (_PUL Rochner_NP0 ,_PUN 2019_CRD )_PUR ._SENT Second_ORD ,_PUN the_AT0 LWB_NP0 metric_AJ0 is_VBZ shown_VVN to_TO0 be_VBI most_AV0 strongly_AV0 representative_AJ0 of_PRF summer_NN1 temperature_NN1 variability_NN1 (_PUL Rydval_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD ;_PUN Wilson_NP0 et_NP0 al._NP0 ,_PUN 2014_CRD ;_PUN Wilson_NP0 ,_PUN D'Arrigo_NP0 ,_PUN et_NP0 al._NP0 ,_PUN 2017_CRD )_PUR ,_PUN and_CJC at_PRP the_AT0 4_CRD GYE_NP0 sites_NN2 ,_PUN August_NP0 temperatures_NN2 appear_VVB to_TO0 be_VBI the_AT0 most_AV0 limiting_AJ0 factor_NN1 on_PRP densiometric_AJ0 growth_NN1 ._SENT Heeter_NP0 et_NP0 al._NP0 (_PUL 2020_CRD )_PUR document_VVB similar_AJ0 seasonal_AJ0 reconstructions_NN2 (_PUL e.g._AV0 ,_PUN August_NP0 –_PUN September_NP0 Tmax_NP0 )_PUR due_AJ0 to_PRP shortened_AJ0 growing_AJ0 seasons_NN2 at_PRP high_AJ0 elevations_NN2 in_PRP the_AT0 Southwest_NN1 US_NP0 ._SENT 
3.3_CRD ._SENT GYE_NP0 Tmax_NP0 Variability_NN1 770_CRD –_PUN 2019_CRD CE_NP0 
We_PNP provide_VVB a_AT0 1_CRD ,_PUN 250-year_AJ0 reconstruction_NN1 of_PRF August_NP0 Tmax_NP0 that_CJT accounts_VVZ for_PRP 60_CRD %_NN0 of_PRF the_AT0 instrumental_AJ0 period_NN1 variance_NN1 (_PUL Figure_NN1 1_CRD )_PUR ._SENT Beta_NN1 values_NN2 for_PRP the_AT0 four_CRD LWB_NP0 chronologies_NN2 (_PUL Table_NN1 S1_CRD )_PUR indicate_VVB that_CJT all_DT0 chronologies_NN2 are_VBB retained_VVN for_PRP the_AT0 reconstruction_NN1 model_NN1 ._SENT Over_PRP the_AT0 common_AJ0 period_NN1 (_PUL 1822_CRD –_PUN 2015_CRD )_PUR ,_PUN our_DPS model_NN1 strength_NN1 (_PUL r2_CRD =_NP0 0.60_CRD )_PUR is_VBZ comparable_AJ0 to_PRP other_AJ0 temperature_NN1 reconstructions_NN2 for_PRP the_AT0 western_AJ0 US_NP0 (_PUL e.g._AV0 ,_PUN MXD-derived_NP0 Briffa_NP0 et_NP0 al._NP0 ,_PUN 1992_CRD r2_CRD =_NP0 0.40_CRD at_PRP grid_NN1 point_NN1 #14_AJ0 and_CJC TRW-derived_AJ0 Salzer_NN1 ,_PUN Bunn_NP0 ,_PUN et_NP0 al._NP0 ,_PUN 2014_CRD r2_CRD =_NP0 0.46_CRD )_PUR (_PUL Figure_NN1 S3_CRD )_PUR ._SENT Validation_NN1 /_PUN cross-validation_NN1 statistics_NN0 remain_VVB robust_AJ0 for_PRP each_DT0 forward_AV0 (_PUL n_ZZ0 =_NN1 2_CRD )_PUR and_CJC backward_AJ0 (_PUL n_ZZ0 =_NN1 3_CRD )_PUR nest_NN1 spanning_VVG the_AT0 period_NN1 770_CRD –_PUN 2019_CRD and_CJC ensure_VVB that_CJT the_AT0 reconstruction_NN1 is_VBZ time-stable_AJ0 with_PRP high_AJ0 explanatory_AJ0 power_NN1 (_PUL Table_NN1 S2_CRD )_PUR ._SENT Within_PRP the_AT0 long-term_AJ0 context_NN1 ,_PUN the_AT0 reconstruction_NN1 exhibits_VVZ multidecadal_AJ0 to_PRP centennial-scale_AJ0 temperature_NN1 variability_NN1 from_PRP 770_CRD to_PRP the_AT0 middle_AJ0 twelfth_ORD century_NN1 ,_PUN after_PRP which_DTQ cool_AJ0 temperatures_NN2 dominate_VVB until_PRP the_AT0 middle_AJ0 fifteenth_ORD century_NN1 (_PUL Figure_NN1 2a_NN1 )_PUR ._SENT Prolonged_AJ0 warm_AJ0 temperatures_NN2 start_VVB ca._NN0 1450_CRD and_CJC persist_VVB through_PRP the_AT0 middle_AJ0 seventeenth_ORD century_NN1 ._SENT Temperature_NN1 conditions_NN2 over_PRP next_ORD ca._NN0 250_CRD years_NN2 are_VBB characterized_VVN by_PRP low_AJ0 variability_NN1 until_PRP the_AT0 turn_NN1 of_PRF the_AT0 twentieth_ORD century_NN1 ._SENT 
3.3.1_CRD ._SENT Notable_AJ0 Warm_AJ0 and_CJC Cool_AJ0 Events_NN2 
The_AT0 intense_AJ0 warming_NN1 trend_NN1 since_PRP ca._NN0 2000_CRD CE_NN1 far_AV0 exceeds_VVZ conditions_NN2 during_PRP any_DT0 other_AJ0 period_NN1 over_PRP the_AT0 past_AJ0 1_CRD ,_PUN 250_CRD years_NN2 ,_PUN particularly_AV0 during_PRP the_AT0 MCA_NP0 ._SENT The_AT0 severity_NN1 –_PUN duration_NN1 analysis_NN1 indicates_VVZ that_CJT 2016_CRD was_VBD the_AT0 warmest_AJS single-year_AJ0 anomaly_NN1 since_PRP 770_CRD ,_PUN followed_VVN by_PRP the_AT0 year_NN1 1056_CRD ,_PUN which_DTQ was_VBD centered_VVN during_PRP the_AT0 MCA_NP0 (_PUL Figures_NN2 2b_AJ0 ,_PUN 2c_AJ0 ,_PUN and_CJC 3a_NN1 ;_PUN Table_NN1 S3_CRD )_PUR ._SENT Yet_AV0 across_PRP the_AT0 GYE_NP0 ,_PUN the_AT0 MCA_NP0 is_VBZ characterized_VVN by_PRP substantial_AJ0 multidec-_AJ0 adal_NN1 summer_NN1 temperature_NN1 variability_NN1 rather_AV0 than_CJS one_CRD prolonged_AJ0 homogeneous_AJ0 warming_NN1 event_NN1 (_PUL Figures_NN2 2a_NP0 and_CJC 4a_NP0 )_PUR ._SENT Within_PRP the_AT0 time_NN1 frame_NN1 of_PRF the_AT0 MCA_NP0 ,_PUN however_AV0 ,_PUN is_VBZ the_AT0 top-ranked_AJ0 warm_AJ0 event_NN1 (_PUL based_VVN on_PRP intensity_NN1 ,_PUN duration_NN1 ,_PUN and_CJC magnitude_NN1 )_PUR lasting_VVG ca._NN0 20_CRD years_NN2 from_PRP 1050_CRD to_PRP 1070._CRD Although_CJS the_AT0 period_NN1 2015_CRD –_PUN 2019_CRD is_VBZ the_AT0 tenth-ranked_AJ0 event_NN1 ,_PUN the_AT0 warming_NN1 over_PRP the_AT0 past_AJ0 5_CRD years_NN2 is_VBZ twice_AV0 as_AV0 intense_AJ0 as_CJS warming_VVG during_PRP the_AT0 MCA_NP0 ;_PUN hence_AV0 ,_PUN the_AT0 overall_AJ0 ranking_NN1 of_PRF the_AT0 2015_CRD –_PUN 2019_CRD event_NN1 is_VBZ lower_AV0 due_AJ0 to_PRP the_AT0 short_AJ0 duration_NN1 ._SENT As_PRP 2020_CRD rivaled_NP0 2016_CRD for_PRP the_AT0 highest_AJS global_AJ0 surface_NN1 air_NN1 temperatures_NN2 on_PRP record_NN1 (_PUL Voosen_NP0 ,_PUN 2021_CRD )_PUR ,_PUN current_AJ0 and_CJC impending_AJ0 projections_NN2 of_PRF increasing_AJ0 anthropogenic_AJ0 warming_NN1 (_PUL IPCC_NP0 ,_PUN 2013_CRD )_PUR ,_PUN especially_AV0 for_PRP the_AT0 midlatitudes_NN2 (_PUL Li_NP0 et_NP0 al._NP0 ,_PUN 2012_CRD )_PUR ,_PUN suggest_VVB that_CJT the_AT0 modern_AJ0 warming_NN1 will_VM0 likely_AV0 soon_AV0 surpass_VVI the_AT0 MCA_NP0 (_PUL as_CJS defined_VVN by_PRP the_AT0 severity_NN1 –_PUN duration_NN1 analysis_NN1 )_PUR ._SENT 
The_AT0 twentieth_ORD century_NN1 contains_VVZ two_CRD noteworthy_AJ0 warming_NN1 anomalies_NN2 :_PUN the_AT0 year_NN1 1988_CRD and_CJC the_AT0 1931_CRD –_PUN 1940_CRD pe-_AJ0 riod_NN1 (_PUL Figure_NN1 3a_NN1 and_CJC Table_NN1 S3_CRD )_PUR ._SENT The_AT0 fourth-highest-ranking_AJ0 warm_AJ0 year_NN1 since_PRP 770_CRD CE_NP0 ,_PUN 1988_CRD ,_PUN coincides_VVZ with_PRP the_AT0 timing_NN1 of_PRF the_AT0 1988_CRD Greater_NP0 Yellowstone_NP0 Area_NN1 fires_NN2 ,_PUN which_DTQ collectively_AV0 formed_VVD the_AT0 largest_AJS complex_AJ0 fire_NN1 recorded_VVN for_PRP the_AT0 GYE_NP0 (_PUL Christensen_NP0 et_NP0 al._NP0 ,_PUN 1989_CRD ;_PUN Romme_NP0 &_NP0 Despain_NP0 ,_PUN 1989_CRD )_PUR ._SENT Multiple_AJ0 studies_NN2 suggest_VVB that_DT0 climate_NN1 ,_PUN rather_AV0 than_CJS fuels_NN2 ,_PUN was_VBD the_AT0 primary_AJ0 driver_NN1 of_PRF the_AT0 1988_CRD Complex_NN1 (_PUL Romme_NP0 &_NP0 Turner_NP0 ,_PUN 2004_CRD ;_PUN Turner_NP0 et_NP0 al._NP0 ,_PUN 2003_CRD )_PUR and_CJC that_DT0 continued_AJ0 warming_NN1 could_VM0 push_VVI GYE_NP0 fire_NN1 regimes_NN2 past_PRP a_AT0 tipping_NN1 point_NN1 ,_PUN to_PRP where_AVQ the_AT0 system_NN1 is_VBZ qualitatively_AV0 altered_VVN ,_PUN by_PRP the_AT0 middle_AJ0 21st_ORD century_NN1 (_PUL Westerling_NP0 et_NP0 al._NP0 ,_PUN 2011_CRD )_PUR ._SENT The_AT0 GYE_NP0 summer_NN1 tem-_NN1 perature_NN1 reconstruction_NN1 characterizes_VVZ 1988_CRD as_PRP highly_AV0 abnormal_AJ0 ,_PUN with_PRP a_AT0 return_NN1 interval_NN1 in_PRP excess_NN1 of_PRF 500_CRD years_NN2 (_PUL Figure_NN1 2c_NN1 )_PUR ._SENT Our_DPS reconstruction_NN1 provides_VVZ long-term_AJ0 contextualization_NN1 of_PRF extreme_AJ0 warm_AJ0 year_NN1 abnormalities_NN2 like_PRP 1988_CRD ,_PUN which_DTQ alludes_VVZ to_PRP the_AT0 increased_AJ0 role_NN1 of_PRF temperature_NN1 as_PRP a_AT0 driver_NN1 of_PRF fire_NN1 activity_NN1 in_PRP recent_AJ0 decades_NN2 ._SENT As_CJS the_AT0 influence_NN1 of_PRF warming_VVG temperatures_NN2 on_PRP aridification_NN1 and_CJC increased_AJ0 fire_NN1 activity_NN1 across_PRP many_DT0 parts_NN2 of_PRF the_AT0 western_AJ0 US_NP0 continues_VVZ to_TO0 be_VBI explored_VVN (_PUL Abatzoglou_NP0 &_NP0 Williams_NP0 ,_PUN 2016_CRD ;_PUN Westerling_NP0 et_NP0 al._NP0 ,_PUN 2011_CRD )_PUR ,_PUN historical_AJ0 con-_AJ0 textualization_NN1 of_PRF modern_AJ0 climatic_AJ0 trends_NN2 (_PUL e.g._AV0 ,_PUN identifying_VVG historical_AJ0 analogs_NN2 )_PUR is_VBZ important_AJ0 for_PRP developing_VVG a_AT0 more_AV0 comprehensive_AJ0 understanding_NN1 of_PRF consequences_NN2 of_PRF future_AJ0 climatic_AJ0 change_NN1 for_PRP the_AT0 GYE_NP0 ._SENT The_AT0 1931_CRD –_PUN 1940_CRD period_NN1 ranks_NN2 as_PRP the_AT0 fifth-warmest_AJS event_NN1 and_CJC coincides_VVZ with_PRP the_AT0 Dust_NN1 Bowl_NN1 ,_PUN a_AT0 well-documented_AJ0 period_NN1 of_PRF prolonged_AJ0 drought_NN1 across_PRP much_DT0 of_PRF the_AT0 western_AJ0 US_NP0 (_PUL B_ZZ0 ._SENT I_PNP ._SENT Cook_NP0 et_NP0 al._NP0 ,_PUN 2008_CRD ,_PUN 2009_CRD )_PUR ._SENT The_AT0 spatial_AJ0 imprint_NN1 of_PRF warming_NN1 during_PRP the_AT0 1930s_CRD is_VBZ extensive_AJ0 ,_PUN as_CJS this_DT0 event_NN1 can_VM0 be_VBI seen_VVN in_PRP both_DT0 regional_AJ0 and_CJC broader_AJC temperature_NN1 reconstructions_NN2 for_PRP the_AT0 NH_NP0 (_PUL Figure_VVB 4d_NN0 )_PUR ._SENT 
As_CJS we_PNP found_VVD with_PRP the_AT0 MCA_NP0 ,_PUN our_DPS reconstruction_NN1 provides_VVZ further_AJC evidence_NN1 that_CJT the_AT0 Little_AJ0 Ice_NN1 Age_NN1 (_PUL LIA_NP0 ;_PUN ca._NN0 1300_CRD –_PUN 1850_CRD )_PUR was_VBD characterized_VVN by_PRP multidecadal_AJ0 variability_NN1 ,_PUN rather_AV0 than_CJS by_PRP a_AT0 solitary_AJ0 ,_PUN sustained_AJ0 cool_AJ0 period_NN1 (_PUL Briffa_NP0 et_NP0 al._NP0 ,_PUN 1992_CRD ;_PUN Salzer_NP0 &_NP0 Kipfmueller_NP0 ,_PUN 2005_CRD ;_PUN Wise_NP0 ,_PUN 2014_CRD )_PUR (_PUL Figure_NN1 2a_NN1 )_PUR ._SENT The_AT0 Maunder_NP0 Minimum_AJ0 (_PUL MM_NN0 ;_PUN ca._NN0 1645_CRD –_PUN 1715_CRD ;_PUN Eddy_NP0 ,_PUN 1976_CRD )_PUR is_VBZ particularly_AV0 pronounced_VVN ,_PUN with_PRP the_AT0 second_ORD highest-ranked_AJS cool_AJ0 event_NN1 occurring_VVG from_PRP 1697_CRD to_PRP 1709_CRD (_PUL Figure_NN1 2b_NN1 and_CJC Table_NN1 S3_CRD )_PUR ._SENT This_DT0 cool_AJ0 event_NN1 is_VBZ strongly_AV0 and_CJC negatively_AV0 correlated_VVN (_PUL p_ZZ0 <_NN1 0.01_CRD )_PUR with_PRP the_AT0 highest_AJS period_NN1 of_PRF snowpack_NN1 anomalies_NN2 (_PUL ca._NN0 1695_CRD –_PUN 1735_CRD CE_NP0 )_PUR for_PRP the_AT0 GYE_NP0 presented_VVD by_PRP Pederson_NP0 et_NP0 al._NP0 (_PUL 2011_CRD )_PUR (_PUL Figure_NN1 4c_CRD )_PUR ._SENT We_PNP also_AV0 show_VVB evidence_NN1 of_PRF the_AT0 Dalton_NP0 Minimum_AJ0 (_PUL DM_NN0 ;_PUN 1790_CRD –_PUN 1830_CRD ,_PUN Wagner_NP0 &_NP0 Zori-_NN1 ta_ITJ ,_PUN 2005_CRD )_PUR ,_PUN characterized_VVN by_PRP more_DT0 moderately_AV0 cool_AJ0 ,_PUN late-season_AJ0 Tmax_NN1 and_CJC high_AJ0 cool-season_AJ0 snowpack_NN1 ._SENT 
3.3.2_CRD ._SENT Broad-Scale_AJ0 Forcings_NN2 on_PRP GYE_NP0 Temperature_NN1 Variability_NN1 
The_AT0 period_NN1 ca._NN0 1500_CRD –_PUN 1640_CRD is_VBZ the_AT0 longest-sustained_AJ0 ,_PUN multidecadal-scale_AJ0 warming_NN1 in_PRP the_AT0 reconstruction_NN1 ,_PUN and_CJC it_PNP aligns_VVZ with_PRP the_AT0 widely_AV0 documented_AJ0 sixteenth_ORD century_NN1 Megadrought_NN1 over_PRP much_DT0 of_PRF North_NP0 America_NP0 (_PUL E_ZZ0 ._SENT R_ZZ0 ._SENT Cook_NP0 et_NP0 al._NP0 ,_PUN 2007_CRD ;_PUN Stahle_NP0 et_NP0 al._NP0 ,_PUN 2000_CRD )_PUR ,_PUN reduced_AJ0 precipitation_NN1 within_PRP the_AT0 Upper_AJ0 Missouri_NP0 ,_PUN Columbia_NP0 ,_PUN and_CJC Colorado_NP0 River_NP0 basins_NN2 (_PUL Gray_NP0 et_NP0 al._NP0 ,_PUN 2007_CRD )_PUR ,_PUN anomalously_AV0 frequent_AJ0 wildfire_NN1 activity_NN1 across_PRP the_AT0 GYE_NP0 (_PUL Whit-_NP0 lock_NN1 et_NP0 al._NP0 ,_PUN 2008_CRD )_PUR ,_PUN and_CJC a_AT0 ridge_NN1 pressure_NN1 pattern_NN1 over_PRP the_AT0 region_NN1 ,_PUN as_CJS demonstrated_VVN in_PRP the_AT0 geopotential_AJ0 height_NN1 reconstructions_NN2 by_PRP Wise_NP0 &_NP0 Dannenberg_NP0 (_PUL 2014_CRD )_PUR ._SENT As_CJS we_PNP note_VVB a_AT0 strong_AJ0 ,_PUN positive_AJ0 relationship_NN1 between_PRP Tmax_NP0 and_CJC upper_AJ0 air_NN1 pressure_NN1 (_PUL Figure_NN1 S4_CRD )_PUR ,_PUN we_PNP posit_VVB that_CJT this_DT0 ca._NN0 140-year_AJ0 warm_AJ0 event_NN1 ,_PUN initially_AV0 brought_VVN about_AVP by_PRP increased_AJ0 upper_AJ0 air_NN1 pressure_NN1 ,_PUN caused_VVD prolonged_AJ0 drought_NN1 and_CJC increased_AJ0 fire_NN1 across_PRP the_AT0 GYE_NP0 ._SENT 
While_CJS various_AJ0 decadal_NN1 to_PRP multidecadal_AJ0 cooling_AJ0 events_NN2 align_VVB with_PRP well-documented_AJ0 periods_NN2 of_PRF reduced_AJ0 solar_AJ0 activity_NN1 (_PUL e.g._AV0 ,_PUN Spörer_NP0 Minimum_AJ0 ;_PUN Figures_NN2 4b_CRD and_CJC S5_CRD )_PUR ,_PUN volcanic_AJ0 forcing_NN1 likely_AV0 also_AV0 enhanced_VVN the_AT0 expression_NN1 of_PRF prolonged_AJ0 cool_AJ0 periods_NN2 such_DT0 as_PRP the_AT0 MM_NN0 and_CJC DM_NN0 ._SENT We_PNP find_VVB strong_AJ0 and_CJC significant_AJ0 (_PUL p_ZZ0 <_NP0 0.01_CRD ;_PUN n_ZZ0 =_NN1 16_CRD )_PUR depar-_AJ0 tures_NN2 in_PRP GYE_NP0 summer_NN1 temperatures_NN2 following_VVG many_DT0 of_PRF the_AT0 largest_AJS volcanic_AJ0 eruptions_NN2 since_PRP 1100_CRD CE_NP0 ._SENT The_AT0 SEA_NN1 shows_VVZ significant_AJ0 reductions_NN2 in_PRP August_NP0 Tmax_NP0 1_CRD (_PUL p_ZZ0 <_NN1 0.01_CRD )_PUR to_PRP 2_CRD (_PUL p_ZZ0 <_NN1 0.05_CRD )_PUR years_NN2 following_VVG major_AJ0 volcanic_AJ0 eruptions_NN2 (_PUL Crowley_NP0 &_NP0 Unterman_NP0 ,_PUN 2013_CRD ;_PUN Sigl_NP0 et_NP0 al._NP0 ,_PUN 2015_CRD )_PUR (_PUL Figure_NN1 4b_CRD )_PUR ._SENT Based_VVN on_PRP the_AT0 lag_NN1 value_NN1 denoted_VVN by_PRP SEA_NN1 ,_PUN the_AT0 ninth-ranking_AJ0 coldest_AJS single-year_AJ0 anomaly_NN1 ,_PUN 1993_CRD (_PUL Table_NN1 S3_CRD )_PUR ,_PUN can_VM0 be_VBI partially_AV0 explained_VVN by_PRP the_AT0 ef-_AJ0 fects_NN2 of_PRF the_AT0 June_NP0 1991_CRD eruption_NN1 of_PRF Mount_NP0 Pinatubo_NP0 ,_PUN Philippines_NP0 (_PUL Figure_NN1 4a_CRD )_PUR ._SENT Although_CJS both_DT0 periods_NN2 show_VVB pronounced_AJ0 reductions_NN2 in_PRP solar_AJ0 irradiance_NN1 ,_PUN most_DT0 reconstructions_NN2 show_VVB solar_AJ0 activity_NN1 to_TO0 be_VBI lower_AJC during_PRP the_AT0 MM_NN0 compared_VVN to_PRP the_AT0 DM_NN0 (_PUL Hoyt_NP0 &_NP0 Schatten_NP0 ,_PUN 1993_CRD ;_PUN Lean_NP0 et_NP0 al._NP0 ,_PUN 1995_CRD )_PUR ._SENT The_AT0 MM_NN0 coincides_VVZ with_PRP the_AT0 1641_CRD Parker_NP0 and_CJC 1696_CRD Serua_NP0 eruptions_NN2 ,_PUN thereby_AV0 demonstrating_VVG the_AT0 influence_NN1 of_PRF both_DT0 solar_AJ0 and_CJC volcanic_AJ0 forcing_NN1 ._SENT 
