Slides from a presentation about modeling past and future climate as part of the "School of Ice" workshop for educators at Oregon State University on Aug. 2, 2021.
4. Hierarchy of Climate Models
• 0D Energy Balance Model (EBM)
• 1D EBM
• 1D Radiative-Convective Models
• Intermediate Complexity Models (e.g. 2D EBMs)
• 3D General Circulation Models (GCMs)
Increasing
Complexity
5. Climate Models Equations based
on conservation of
• mass
• momentum
• energy
• water
• carbon
• ...
Equations are solved in each of the grid boxes of the model.
Typically global climate models have
about 20-30 layers in the atmosphere
and about the same amount in the
ocean. The horizontal grid box size
varies from about 5 degrees (500 km) to
about 1 degree (100 km) or less.
Models with smaller grid box sizes
r e s o l v e m o r e d e t a i l s ( h i g h e r
resolution), but the y are also more
computationally expensive to run.
6. Interactive Components Included
• Atmosphere
• Ocean
• Sea Ice
• Land Surface (sometimes with interactive vegetation)
• Ocean Biology and Chemistry (e.g. Carbon, Nutrient,
and Oxygen Cycles)
• (Ice Sheets, usually prescribed)
Models that include biogeochemistry are also often
called Earth System Models
7. Parameterizations
• Processes that cannot be resolved (smaller than
grid box size need to be expressed in terms of
resolved quantities; often empirical formulas)
• Examples are clouds, convection (atmosphere),
and mixing (ocean)
8. Forcings
• Models are driven by boundary conditions, e.g. incident
solar radiation at the top-of-the-atmosphere, surface
properties (albedo), changes in atmospheric CO2
• Interior is solved without the use of observations
11. What are climate models used for?
• Paleoclimate Studies
How did climate change in the past and why?
• Detection and Attribution
Is climate changing signi
fi
cantly and if so why?
• Projections
How may climate change in the future?
13. IPCC AR5 Scenarios
Uncertainty #1: Emissions
We don’t know the future economic
development. Will the real world follow more
closely scenario RCP2.6, 4.5, 6, or 8.5?
Di
ff
erent scenarios result in di
ff
erent
emissions of substances that a
ff
ect climate
(CO2, CH4, SO2)
}AR4
}AR5
High emissions
Strong emission reductions
IPCC 2013
RCP = Representative Concentration Pathways (GHG concentrations prescribed)
SRES = Special Report on Emissions Scenarios
SSP = Shared Socioeconomic Pathways will be used in AR6
14. IPCC 2013
Strong emission reductions
Uncertainty #2: Carbon Cycle
For a given CO2 emission scenario, we don’t know exactly how much carbon is
taken up by the ocean and land and how much remains in the atmosphere a
ff
ecting
climate. In AR5 concentrations (e.g. of CO2) are given (inset). Each concentration
scenario corresponds to a range of emissions due to carbon cycle uncertainty.
Spread only due to
uncertainty in
carbon cycle
15. Spread only due to
uncertainty in
climate sensitivity
Uncertainty #3: Climate Sensitivity
For given CO2 concentration scenario, we don’t know exactly how much climate will warm.
# of models used
16. IPCC 2013
Projections of
Surface Warming
More warming over land than over the ocean.
Why?
More warming in Arctic.
Why?
Note that we saw this pattern (larger
changes over land than over the oceans)
also in the temperature reconstructions from
the Last Glacial Maximum (textbook chapter
3, Fig. 10; lecture 2.2 slide 13) and in the
modern observations (textbook chapter 2,
Fig. 2)
Stippling: signi
fi
cant changes (model mean > 2 std of int. var.)
18. • Total temperature e
ff
ect (45
ppm) much larger than
previously thought due to
enhanced disequilibrium
• Iron e
ff
ect (26-35 ppm) also
ampli
fi
ed by disequilibrium
• Together T and Fe account for
3/4 (67-87 ppm) of atmospheric
CO2 change
• Sea ice and circulation not
important due to compensating
e
ff
ects of di
ff
erent C
components
Khatiwala et al., 2020
19. Muglia and Schmittner, 2021
• LGM AMOC depth well
constrained by sediment data
• LGM AMOC strength NOT well
constrained by sediment data
20. Schmittner and Lund, 2015
• Did AMOC collapse during
Heinrich Stadial 1 cause initial
rise of CO2?
