Thread by @crentsch

I went through the technical guide for a modern climate model and counted the user-specified parameters:

1,737 (and I know I missed some).

If you've ever wondered what dials must be set to run a GCM, here they are...🧵

I have been told 'these just physics simulators' suggesting that after entering π, σ, 𝑔, etc and some earth-specific information, the rest is simply the consequence of laws of nature. I would observe this is not entirely the case.

First, the model needs 207 plant optical properties specifying the IR/vis reflectance and absorbance of leaves and stems

Dry and saturated soil/snow albedo
207 + 96 = 303

Beam weights for six spectral bands
303 + 10 = 313

Albedos for various types of dirty ice
313 + 50 = 363

More dirty ice optical parameters
363 + 50 = 413

Last set of dirty ice optical parameters
413 + 50 = 463

Growth/shrink rate of individual grains of ice
463 + 1 = 464

Orbital parameters
464 + 3 = 467

Glacier, soil, and snow emissivity
467 + 3 = 470

Reynold's number calculation for small turbulent eddies in/across snow
470 + 2 = 472

Plant aerodynamic properties 💨🌱
472 + 72 = 544

Eighth-order polynomial fits providing water vapor pressure and its derivative
544 + 36 = 580

Bedrock thermal conductivity
580 + 1 = 581

Weight of rain or snow that a canopy leaf can support
581 + 2 = 583

Estimation of microtopography
583 + 2 = 585

Hydraulic conductivity of ice and dirt
585 + 16 = 601

Melting snow self-cleaning & compaction
601 + 12 = 613

Plant stomatal resistance
613 + 22 = 635

Photosynthesis rate dependence on temperature
635 + 18 = 653

Plant functional type parameters
653 + 44 = 697

Photosynthesis photon conversion efficiency
697 + 4 = 701

Plant root distribution parameters
701 + 24 = 725

Urban construction materials
725 + 54 = 779

Leaf molecular ratio and respiration model parameters
779 + 36 = 815

Nitrogen fixation rate
815 + 4 = 819

Plant carbon/nitrogen allocation parameters
819 + 184 = 1003

Decomposition rates
1003 + 34 = 1037

Turnover rates for century model
1037 + 23 = 1060

Nitrogen fixation rate
1060 + 3 = 1063

Plant mortality
(interesting that it is called out as a 'gross oversimplification of an important process')
1063 + 1 = 1064

Lightning strikes and rate of fires started per person per month
1064 + 5 = 1069

Combustibility, fire economic influence, seasonality of fires, acreage burned each year
1069 + 17 = 1086

Combustion completeness by biomass type
1086 + 165 = 1251

Methane model parameters
1251 + 21 = 1272

Diffusion rates for methane and oxygen
1272 + 10 = 1282

Crop plant functional type
1282 + 63 = 1345

Crop phenology and morphology, planting date range
1345 + 136 = 1481

Crop allocation by functional type
1481 + 136 = 1617

Irrigation parameters
1617 + 6 = 1623

Biomass density, aspect ratio
1623 + 10 = 1633

Water stress impact on respiration rates
1633 + 11 +10 = 1654

Canopy trimming
1654 + 3 = 1657

Background plant mortality rate and carbon starvation
1657 + 2 = 1659

Biomass fuel burn time and moisture content
1659 + 8 = 1667

Windspeed impact on fire spread
1667 + 16 = 1683

Fire intensity and duration
1683 + 15 = 1698

Dust compensatory factor
1698 + 1 = 1699

Dust particle size, transport between atmospheric layers
1699 + 15 + 17 = 1731

Plant carbon isotope discrimination
1731 + 5 = 1737

Solar NIR/vis ratio. Interesting that it is "assumed to 0.5" but Wikipedia has it as 0.52-0.55 (en.wikipedia.org/wiki/Sunlight)
1736 + 1 = 1737

My goal is to locate the sensitivity analysis for GCMs.

In a proper sensitivity analysis, each independent parameter is increased or decreased X% and the impact on 50 yr planet temperature change displayed, with the highest-impact variable at the top.

So far, I cannot locate a GCM sensitivity analysis. Please share if you know of one.