When are we committed to crossing critical (1.5 or 2 °C) temperature thresholds? Cristian Proistosescu 1 Kyle Armour 1 Gerard Roe 1 Peter Huybers 2 1 University of Washington 2 Harvard University AGU Fall Meeting 2017 Courtesy of NASA’s Earth Observatory
Two questions 1. When will we cross 1.5 or 2 °C global warming thresholds (e.g., following high or low emission scenarios) – subject to constraints from the observed global energy budget? 2. When will we be geophysically committed to crossing 1.5 or 2 °C global warming thresholds?
What do CMIP5 models say? CMIP5 projections (IPCC AR5) 2 °C
What do CMIP5 models say? § Models may not agree with observed CMIP5 projections (IPCC AR5) global warming and energy budget constraints § Models may not span full range of plausible future warming § Computationally expensive to run different emissions scenarios, so can’t ask questions like, when are we geophysically committed to 2 °C? 2 °C § Not clear which physical factors are contributing to uncertainty in projected warming
Our approach § Use a 2-layer ocean model (Held et al. 2010; radiative radiative + F + = � T u + response Armour 2017) that includes the essential physics forcing governing global-mean surface warming: Ocean heat uptake efficacy dT u upper ocean T u dt = � T u + F + "� ( T d � T u ) ( c u deep ocean T d dT d = � ( T u � T d ) ( dt = � ( T u � T d ) ( c d
Our approach § Use a 2-layer ocean model (Held et al. 2010; Global surface temperature response Armour 2017) that includes the essential physics to abrupt CO 2 quadrupling governing global-mean surface warming: Temperature change T (°C) 6 3 dT u dt = � T u + F + "� ( T d � T u ) ( c u 2 4 Slow warming on timescale of the dT d deep ocean dt = � ( T u � T d ) 1 ( c d 2 Fast warming on timescale of the surface 0 0 0 50 100 150 0 50 100 150 Year after CO 2 quadrupling
Our approach § Use a 2-layer ocean model (Held et al. 2010; Global surface temperature response Armour 2017) that includes the essential physics to abrupt CO 2 quadrupling governing global-mean surface warming: Temperature change T (°C) 6 3 dT u dt = � T u + F + "� ( T d � T u ) ( c u 2 4 dT d dt = � ( T u � T d ) 1 ( c d 2 0 0 0 50 100 150 0 50 100 150 Year after CO 2 quadrupling
Our approach § Use a 2-layer ocean model (Held et al. 2010; � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T Armour 2017) that includes the essential physics of 2-layer model to CMIP5 model response to governing global-mean surface warming: CO 2 forcing (Geoffroy et al. 2013) dT u dt = � T u + F + "� ( T d � T u ) ( c u dT d dt = � ( T u � T d ) ( c d
Our approach § Use a 2-layer ocean model (Held et al. 2010; � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T Armour 2017) that includes the essential physics of 2-layer model to CMIP5 model response to governing global-mean surface warming: CO 2 forcing (Geoffroy et al. 2013) Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 dT u dt = � T u + F + "� ( T d � T u ) ( c u dT d dt = � ( T u � T d ) ( c d
Our approach § Use a 2-layer ocean model (Held et al. 2010; � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T Armour 2017) that includes the essential physics of 2-layer model to CMIP5 model response to governing global-mean surface warming: CO 2 forcing (Geoffroy et al. 2013) Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 dT u dt = � T u + F + "� ( T d � T u ) ( c u Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on dT d observed warming and energy budget (see dt = � ( T u � T d ) ( c d also: Forest et al. 2002, 2006; Stott & Forest 2007) ⇥ T obs = 0.75 ± 0.2 °C (Otto et al. 2013; = 0.65 ± 0.27 Wm -2 − Q obs 2000-2009 relative to ⇥ 1860-1879) = F obs = 2.3 ± 1 Wm -2
Our approach � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T of 2-layer model to CMIP5 model response to 2-layer model historical warming CO 2 forcing (Geoffroy et al. 2013) Temperature change [°C] Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on observed warming and energy budget (see also: Forest et al. 2002, 2006; Stott & Forest 2007) ⇥ T obs = 0.75 ± 0.2 °C (Otto et al. 2013; = 0.65 ± 0.27 Wm -2 − Q obs 2000-2009 relative to ⇥ 1860-1879) = Year F obs = 2.3 ± 1 Wm -2
Our approach � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T of 2-layer model to CMIP5 model response to 2-layer model historical warming CO 2 forcing (Geoffroy et al. 2013) Temperature change [°C] Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on observed warming and energy budget (see also: Forest et al. 2002, 2006; Stott & Forest 2007) Step 4: Use the observationally-constrained parameter/forcing estimates to project warming, and committed warming, following RCP2.6 and RCP8.5 emissions scenarios Year
Our approach � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T of 2-layer model to CMIP5 model response to 2-layer model projections CO 2 forcing (Geoffroy et al. 2013) Temperature change [°C] Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on observed warming and energy budget (see also: Forest et al. 2002, 2006; Stott & Forest 2007) Step 4: Use the observationally-constrained parameter/forcing estimates to project warming, and committed warming, following RCP2.6 and RCP8.5 emissions scenarios Year
Our approach � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T of 2-layer model to CMIP5 model response to 2-layer model projections CO 2 forcing (Geoffroy et al. 2013) Temperature change [°C] Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on observed warming and energy budget (see also: Forest et al. 2002, 2006; Stott & Forest 2007) Step 4: Use the observationally-constrained parameter/forcing estimates to project warming, and committed warming, following RCP2.6 and RCP8.5 emissions scenarios Year
Our approach � ( Step 1: Draw priors of , , , and from fits + "� c u c d � T of 2-layer model to CMIP5 model response to 2-layer model projections CO 2 forcing (Geoffroy et al. 2013) Temperature change [°C] Step 2: Drive model with timeseries of historical radiative forcing (Meinshausen et al. 2011), with priors drawn from forcing range in IPCC AR5 Step 3: Use Bayesian inference to estimate posterior parameters/forcings based on observed warming and energy budget (see also: Forest et al. 2002, 2006; Stott & Forest 2007) Step 4: Use the observationally-constrained parameter/forcing estimates to project warming, and committed warming, following RCP2.6 and RCP8.5 emissions scenarios Year
Our approach 2-layer model projections CMIP5 projections (IPCC AR5) Temperature change [°C] Year
When are we going to cross 1.5 or 2 °C thresholds? 2-layer model projections Year at which 2 °C is crossed Temperature change [°C] Probability density [1/yr] Year Year
When are we going to cross 1.5 or 2 °C thresholds? 2-layer model projections Year at which 2 °C is crossed Temperature change [°C] Probability density [1/yr] Year Year Probability of crossing 2.0 °C along RCP 2.6 = 0.13
When are we going to cross 1.5 or 2 °C thresholds? 2-layer model projections Year at which 1.5 °C is crossed Temperature change [°C] Probability density [1/yr] Year Year Probability of crossing 1.5 °C along RCP 2.6 = 0.41
Zero-emissions climate commitment § How much more warming will occur given no further human influence on climate? § Constant atmospheric composition requires continued emissions; the climate commitment is better defined with respect to past emissions only 1.5 Constant composition IPCC AR4 models Global temperature change ( °C) This study 1.0 Zero emissions BERN2.5CC model 0.5 HadCM3LC model (Matthews and Weaver 2010) 0.0 1800 1900 2000 2100 2200 2300 Year
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