Valuing Climate Damages Updating Estimation of the Social Cost of Carbon Dioxide (2017) / Chapter Skim
Currently Skimming:
4 Climate Module
4 Climate Module
Pages 85-128
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 85... ...
Those other variables may include regional temperature, regional precipitation, statistics of weather extremes, global and regional sea level, and ocean pH. In so doing, it must accurately represent within a probabilistic framework the current understanding of the climate and carbon cycle systems and associated uncertainties.
|
From page 86... ...
See Box 4-1 for definitions of TCR, TCRE, IPT, and ECS. using planetary energy balance to estimate global mean surface temperature changes, to Earth system models of intermediate complexity and full complexity Earth system models that project coupled changes in the atmosphere, oceans, and land surface.2 At each stage of the development of Earth system models, more comprehensive representations of feedbacks and response characteristics have been added (Flato et al., 2013)
|
From page 87... ...
CHARACTERISTICS OF AN ADEQUATE CLIMATE MODULE The committee's Phase 1 report (National Academies of Sciences, Engineering, and Medicine, 2016) suggested several criteria that could be used to evaluate whether any simple Earth model considered for use in SC-CO2 estimation reflects current scientific understanding of the relationships between CO2, other greenhouse gases, emissions, concentrations, forcing, and global mean surface temperature change, as well as their uncertainty and profiles over time.
|
From page 88... ...
. In the context of the climate module, this means: • Scientific basis and uncertainty characterization: The mod ule's behavior should be consistent with the current, peer reviewed scientific understanding of the relationships over time between CO2 emissions, atmospheric CO2 concentra tions, and CO2-induced global mean surface temperature change, including their uncertainty.
|
From page 89... ...
Performing well against these diagnostics is not a guarantee that a climate module is appropriate for all applications, so conclusions can also, where possible, be checked against direct calculations carried out with more comprehensive models.3 As defined in Box 4-1, four key metrics can describe the configuration of a simple Earth system model: equilibrium climate sensitivity (ECS) , transient climate response (TCR)
|
From page 90... ...
BENCHMARK EXPERIMENTS FOR CALIBRATING AND EVALUATING SIMPLE EARTH SYSTEM MODELS Temperature Response to Idealized Concentration Changes The simplest benchmark experiments involve changing atmospheric CO2 concentrations in a (simple or complex) climate model and com
|
From page 91... ...
Simple climate models,5 including those used in estimating the SC-CO2, have 5The committee refers to "simple climate models" here rather than "simple Earth system models" to encompass models that do not have a fully interactive carbon cycle (i.e., calculating, rather than prescribing, the distribution and fluxes of carbon within the climate model.)
|
From page 92... ...
assessed range for TCR (red vertical bar) and the response of a simple Earth model system (blue plume)
|
From page 93... ...
of the CMIP5 ensemble of comprehensive climate models (black lines) and of the FAIR model (see text)
|
From page 94... ...
and (b) show "likely"6 ranges of uncertainty for the transient climate response and the equilibrium climate sensitivity as assessed by IPCC AR5.
|
From page 95... ...
Relationship between Emissions and Concentrations Since the mid-2000s, many Earth system models have incorporated interactive carbon cycles, and these idealized experiments have been extended to diagnose the emissions required to increase CO2 concentrations at a prescribed rate, in addition to the uptake of CO2 by land and ocean and the residual "airborne fraction." Panel (c) in Figure 4-2 shows atmospheric concentration of CO2 in the idealized experiments shown in panel (a)
|
From page 96... ...
The coupled climate carbon cycle response to emissions can be summarized in a plot of global mean surface temperature change against diagnosed cumulative CO2 emissions from the comprehensive Earth system models included in CMIP5 under the 1 percent per year increasing-CO2 scenario (Figure 4-2, Panel a)
|
From page 97... ...
. Response to a Pulse Injection of CO2 Since the SC-CO2 is defined in terms of the impact of a pulse injection of CO2 into the atmosphere, one highly relevant test of the performance of a simple Earth model system is to compare its response to a pulse injection with that of more comprehensive models.
|
From page 98... ...
against a background scenario of approximately constant CO2 concentrations from 2010. NOTES: The figure includes a range of full-complexity Earth system models, Earth system models of intermediate complexity, and simple Earth system models (black thin lines)
|
From page 99... ...
Response to Historical Forcings and Future Scenarios Another test of a simple Earth model system is to compare its behavior with that of more comprehensive models when driven with observed emissions and radiative forcing over the historical period followed by a range of future forcing scenarios, such as the RCPs (Van Vuuren et al., 2011)
|
From page 100... ...
Year (b) FIGURE 4-4 Fossil fuel CO2 emissions, concentrations, and temperature response to the representative concentration pathway (RCP)
|
From page 101... ...
The AR5 provided formally assessed uncertainty ranges for ECS, TCR, and TCRE, although it does not specify either distributional forms or joint distributions. The AR5 also does not provide formally assessed ranges for other climate metrics that are relevant to the SC-CO2 estimates, including IPT, the TCR/ECS ratio (also known as the realized warming fraction, [RWF]
|
From page 102... ...
For TCRE, AR5 estimates a likely global warming of 0.8-2.5 °C per 1000 GtC cumulative emission for cumulative emissions less than 2000 GtC; subsequent studies (Tokarska et al., 2016) suggest the linearity may extend to a higher range, while others have found that it may not (Herrington and Zickfeld, 2014)
|
From page 103... ...
shows temperature response to a pulse emission of 100 GtC released in 2020 into a background RCP 2.6 (blue) and RCP 8.5 (red/ pink)
|
From page 104... ...
While their global average climate forcing can be crudely approximated as proportional to total emissions, different spatial patterns of emissions give rise to significantly different spatial patterns of temperature change. These spatial patterns cannot be directly modeled in a simple Earth system model (see discussion of disaggregation below)
|
From page 105... ...
FAIR is extended with a state-dependent carbon uptake to incorporate feedbacks between the climate and the carbon cycle and thus reproduce the CO2 behavior of more complex models, in particular the changing airborne fraction with rising temperature and cumulative emissions, which is shown in Figure 4-2c (above)
|
From page 106... ...
FAIR assumes that iIRF100 is a simple linear function of accumulated perturbation carbon stock in the land and ocean, which is the difference between cumulative emissions to date ("reference" emissions plus pulse) and the excess carbon in the atmosphere (i.e.
|
From page 107... ...
, in that it is almost straight and slightly concave at high values. A simple climate model that omits carbon cycle uncertainty represented in the state-dependent iIRF100 would necessarily display a very different shape, strongly concave over the full range.
|
From page 108... ...
, using a representative distribution of parameters: note how both comprehensive Earth system models and the simple Earth system models show a rapid initial adjustment (short IPT) to a pulse emission in 2020.
|
From page 109... ...
and (b) incorporating responses to forcing agents other than CO2 requires: • two timescales, one subdecadal, the other centennial, of the surface temperature and ocean heat content response to radiative forcing; • at least three distinct timescales of the atmospheric CO2 response to emissions, corresponding to atmospheric exchanges with the land and surface ocean, the deep ocean, and geological reservoirs; and • a state-dependent carbon cycle in which the fraction of emitted CO2 that remains in the atmosphere increases in response to higher temperatures and accumulation of car bon in the land and ocean.
|
From page 110... ...
dt ⎣ ⎦ dϕ = −ϕ / ρ 2 , (8) dt where T is global mean temperature, Te is the global mean temperature with which sea level is in quasi-equilibrium, is a multi-millennial scale contribution to sea level rise from Earth's long-term climate cycles, b is a
|
From page 111... ...
Advances in semi-empirical models of sea level rise are qualitatively different from most new publications addressing metrics for energy balance models, such as ECS and TCR, that appear between IPCC assessments because they are incorporating physical processes that have not previously been taken into account. FIGURE 4-6 Projections of GMSL rise under three RCPs using the semi-empirical model of Kopp et al.
|
From page 112... ...
RECOMMENDATION 4-3 In the near term, the Interagency Working Group should adopt or develop a sea level rise com ponent in the climate module that (1) accounts for uncertainty in the translation of global mean temperature to global mean sea level rise and (2)
|
From page 113... ...
Median scale factor κ(x) for the relationship between climatically driven local sea level change and global mean sea level change, panel (b)
|
From page 114... ...
Globally averaged D surface ocean pCO2 lags behind globally averaged atmospheric CO2 by approximately 1 year, and so the trend in pH can be readily derived from the trend of atmospheric CO2 in simple Earth system models. Another approach for deriving pH in simple models is as a quadratic function of concentration of dissolved inorganic carbon (DIC)
|
From page 116... ...
In turn, acidification alters the relative abundance of carbonate species in surface waters and slows the ocean uptake of anthropogenic CO2. This feedback is captured implicitly in simple earth system models whose pH projections are consistent with those in earth system models where ocean carbonate chemistry and biology is included explicitly.
|
From page 117... ...
(b) Year FIGURE 4-9 Projected mean change in acidification of the surface ocean from 1990 under RCP 2.5 and RCP 8.5 scenarios.
|
From page 118... ...
is consistent with carbon uptake in the climate module, (2) accounts for uncertainty in the transla tion of global mean surface temperature and carbon uptake to surface ocean pH, and (3)
|
From page 119... ...
. With the climate module providing global mean temperature T(t)
|
From page 120... ...
. Temperature maps in units of °C regional change per °C global mean temperature change; precipitation maps in units of percent regional precipitation change per °C global mean temperature change.
|
From page 121... ...
In the long run, it may be useful to use more comprehensive climate models, or statistical emulators of them (e.g., Castruccio et al., 2014) , to directly estimate the joint probability distribution of global mean temperature change and regional climate changes.
|
From page 122... ...
CONCLUSION 4-3 In the near term, linear pattern scaling, although subject to numerous limitations, provides an accept able approach to estimating some regionally disaggregated vari ables from global mean temperature and global mean sea level. If necessary, projections based on pattern scaling can be aug mented with high-frequency variability estimated from obser vational data or from model projections.
|
From page 123... ...
From the ensemble of model outputs, additional outputs will need to be extracted for the other components of the climate module (sea level rise, pH, and disaggregated variables)
|
From page 124... ...
Earth system models may capture some of these additional feedbacks, but simple Earth system models often do not, and they are generally held constant when diagnosing ECS in general circulation models and those of intermediate complexity. The experiments to assess ECS and TCR prescribe CO2 concentrations, so carbon cycle feedbacks are also excluded.
|
From page 125... ...
For example, current research suggests it is more likely than not that the warming response to an increase in forcing increases in a warmer global mean climate. Likewise, TCRE may decrease with warming less quickly than indicated by many climate models of intermediate complexity.
|
From page 126... ...
CONCLUSION 4-5 Research focused on improving the repre sentation of the Earth system in the context of coupled climate economic analyses would improve the reliability of estimates of the SC-CO2. In the near term, research in six areas could yield benefits for SC-CO2 estimation: • coordinated research to reduce uncertainty in estimates of the capacity of the land and ocean to absorb and store car bon, especially in the first century after a pulse release, applied to a range of scenarios of future atmospheric com position and temperature; • coordinated Earth system model experiments injecting identical pulses of CO2 and other greenhouse gases in a range of scenarios of future atmospheric composition and temperature; • development of simple, probabilistic sea level rise models that incorporate the emerging science on ice sheet stability and that can be linked to simple Earth system models; • systematic assessments of the dependence of patterns of regional climate change on spatial patterns of forcing, the relationship between regional climate extremes and global mean temperature, the temporal evolution of patterns under conditions of stable or decreasing forcing, and nonlineari ties in the relationship between global means and regional variables; • systematic assessments of nonlinear responses to forcing in Earth system models and investigations into evidence for such responses in the geological record; and • the development of simple Earth system models that incor porate nonlinear responses to forcing and assessments of the effects of such nonlinear responses on SC-CO2 estimation.
|
From page 127... ...
CLIMATE MODULE 127 Earth system models, including the use of comprehensive models to represent low-probability, high-consequence states of the world, as well as the use of decision support science approaches to identify and evaluate key decision-relevant uncertainties in Earth system models.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.