ModelE2.1-CC2 is the successor to the carbon-cycle configuration that NASA-GISS contributed to the sixth Climate Model Intercomparison Project (CMIP6), known as GISS-E2.1-G-CC. The physical climate core is unchanged from that predecessor, but the new version carries substantially upgraded ocean and land carbon cycle components, longer and more carefully equilibrated simulations, and a battery of conservation diagnostics that track every gram of carbon, every joule of heat and every unit of water as it shuttles between reservoirs. The result, the authors report, is a model with smaller drifts in all of its carbon pools than the previous release — a quiet but crucial achievement, because drifting carbon reservoirs can masquerade as climate responses that are actually numerical artifacts.

The technical architecture is ambitious. The atmosphere runs at 2 by 2.5 degrees of latitude and longitude with 40 vertical layers reaching into the lower mesosphere, coupled to an ocean at 1 by 1.25 degrees that is non-Boussinesq, meaning it explicitly represents density variations in its momentum and continuity equations. The ocean conserves potential enthalpy, freshwater and salt, and uses the quadratic upstream advection scheme, which tracks subgrid-scale moments of each tracer and thereby sharpens the effective resolution of the simulated biogeochemistry. Mesoscale eddy transport is handled through a quasi-isopycnal formulation with surface-intensified diffusivity that allows eddies to restratify the Southern Ocean realistically, and vertical mixing combines the K-profile parameterization with tidally driven dissipation — a detail that matters because the model’s Atlantic overturning circulation proves sensitive to it.

The ocean carbon cycle is built on the NASA Ocean Biogeochemistry Model, expanded into a version called NOBMg. It simulates four phytoplankton functional groups — diatoms, chlorophytes, cyanobacteria and coccolithophores — competing for four nutrients: nitrate, silicate, ammonia and iron. Dissolved organic and inorganic carbon are carried prognostically, carbonate chemistry is solved with the MOCSY routines, and gas exchange with the atmosphere follows the CMIP6 protocol. The new version adds prognostic oxygen and alkalinity, an updated iron cycle with regionally varying organic ligands, and a revised treatment of nitrogen fixation in which cyanobacteria draw on an effectively unlimited reservoir of atmospheric N2 but are inhibited when nitrate and ammonium are abundant, mirroring laboratory findings on the marine diazotroph Trichodesmium.

Getting such a model to sit still — to hold atmospheric CO2 steady in a preindustrial control climate — demanded an elaborate spin-up and tuning campaign. The physical model was run to radiative equilibrium over roughly 3,000 model years; the land carbon cycle was equilibrated by iterating 9 years of simulated meteorology and litterfall 750 times through the soil carbon pools, equivalent to 6,750 simulated years and completed in under 24 hours on 88 processor cores; and the ocean carbon cycle was spun up for about 300 years atop a preindustrial ocean equilibrated over 7,000 years. Tuning experiments adjusted phytoplankton growth rates, remineralization rates and iron scavenging until the air-sea CO2 flux approached zero and ocean net primary production rose from 15 to about 30 petagrams of carbon per year. Notably, the tuned iron scavenging rate came out two orders of magnitude higher than in the CMIP6-era model, but consistent with observed scavenging rates for trace metals that behave like iron.

On land, the Ent Terrestrial Biosphere Model simulates 12 plant functional types with a coupled photosynthesis-stomatal conductance scheme based on the Farquhar and Ball-Berry formulations, canopy radiative transfer that resolves sunlit and shaded leaf layers, and soil biogeochemistry adapted from the Carnegie-Stanford-Ames model. In its current configuration, Ent runs in biophysics-only mode: vegetation structure and leaf area index are prescribed from MODIS satellite observations, while the plant labile carbon pool and nine soil carbon pools evolve prognostically. Irrigation is a prognostic field drawing first from rivers and lakes, then from groundwater, contributing about 0.2 millimeters per year of sea-level-equivalent freshwater in 2010 — an imbalance the team attributes to underestimated surface water storage and the absence of reservoirs in the model.

How does it perform against reality? In historical simulations spanning 1850 to 2021, driven by CO2 emissions rather than prescribed concentrations, the model tracks observed atmospheric CO2 closely, with the largest discrepancies between 1900 and 1970. Surface temperature anomalies follow the GISTEMP record well, particularly after the 1950s, though the model overestimates the cooling after the Pinatubo eruption and shows stronger El Niño-driven variability. Ocean heat uptake agrees well with Argo float estimates and the ECCO-V4r4 ocean state estimate. Soil carbon storage is a striking success: the simulated global total of 1,693 petagrams of carbon is virtually identical to the 1,700 petagrams from inventory compilations cited by the Global Carbon Budget.

The model is candid about its weaknesses. Ocean net primary productivity, at roughly 30 petagrams of carbon per year, sits well below the 50 to 60 petagrams suggested by observations, and the team traced part of the shortfall to a genuine coding error in the underwater radiative transfer routines — layer depth was used where layer thickness belonged, starving subsurface waters of light. The absence of photoadaptation, which would let phytoplankton raise their chlorophyll-to-carbon ratios in dim waters, compounds the problem. On land, gross primary production of about 127 petagrams per year falls at the low end of the 110 to 170 petagram range, and aboveground biomass is grossly overestimated at 533 petagrams of carbon — nearly double satellite estimates — because the prescribed forest heights derive from early ICESat/GLAS lidar data now known to be about six meters too tall. The model also omits deforestation, wood harvest and fire, missing roughly 1 petagram of carbon per year in land-use emissions, which likely explains its slightly underpredicted atmospheric CO2.

Despite these caveats, the model has already proven itself in the wider community. In a 13-model intercomparison of remaining carbon budgets for 1.5 and 2 degrees of warming, its responses fell consistently within the ensemble spread, and in experiments probing positive, zero and negative CO2 emissions it behaved like its peers except for a stronger late-phase response to carbon dioxide removal driven by a weakened Atlantic overturning circulation. Its estimates of the fraction of CO2 escaping to the atmosphere after seafloor trawling disturbances agreed closely with a data-assimilated ocean model. These are exactly the kinds of questions — carbon budgets, carbon dioxide removal, geoengineering feasibility — that Earth system models exist to answer.

The roadmap ahead is substantial. The ocean team plans to add diel vertical migration, denitrification and a closed nitrogen cycle, particulate inorganic carbon for the calcium carbonate pump, and interactive dust-iron deposition. On land, a new geometric-optical canopy radiative transfer scheme will ingest vertical foliage profiles from NASA’s GEDI and ICESat-2 lidar missions, updated tree allometry from the Tallo database will replace the biased height prescriptions, and a fire module coupled to land carbon and atmospheric chemistry is under development, paving the way for explicit deforestation dynamics. Future versions will be benchmarked against pioneering NASA observations from the PACE and SWOT missions and the EMIT hyperspectral imager. In an era when the remaining carbon budget is the number that governs global climate policy, models like ModelE2.1-CC2 — transparent about their errors, conserving their quantities to machine precision, and grounded in satellite observation — are the instruments by which that number will be refined.

Subject of Research: Development and observational evaluation of the NASA-GISS ModelE2.1-CC2 Earth system model with interactive ocean and land carbon cycles

Article Title: The NASA-GISS ModelE2.1-CC2 ESM: development and evaluation

Article References: The NASA-GISS ModelE2.1-CC2 ESM: development and evaluation. (n.d.). https://doi.org/10.5194/gmd-19-9131-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9131-2026

Keywords: Earth system model, NASA GISS, carbon cycle, climate modeling, ocean biogeochemistry, terrestrial biosphere, CMIP6, CO2 emissions, phytoplankton, soil carbon, iron cycle, model evaluation

The logic behind using iron as an oxygen proxy rests on straightforward chemistry. Throughout most of Earth’s early history, the deep ocean was rich in dissolved iron, delivered by hydrothermal vents and the chemical breakdown of rocks on land. In an oxygen-free ocean, that dissolved iron reacted with sulfur to form pyrite, the glittering mineral better known as fool’s gold. When early marine microbes began releasing oxygen as a byproduct of photosynthesis, the oxygen combined with dissolved iron instead, precipitating it out as iron oxides such as hematite. The spectacular banded iron formations of the Lake Superior region in Michigan and Minnesota, layered rocks that hold much of the world’s iron ore, are the most famous monuments to this process. Geologists have long read the transition from pyrite to hematite in sedimentary rocks as a marker of when the ocean, and later the atmosphere, accumulated oxygen.

That interpretive framework has served science well, but it rests on an assumption: that the amount of iron reaching the sea floor was relatively steady, so that changes in the mineral record must reflect changes in seawater chemistry. Hardisty and colleagues from the University of Hamburg in Germany and ETH Zurich set out to test that assumption across a vast stretch of geologic time. Drawing on and augmenting data from the Sedimentary Geochemistry and Paleoenvironments Project database, an international collaborative effort compiling geochemical measurements from sedimentary rocks worldwide, the team reconstructed the iron record for the last 1.2 billion years of Earth history, a span that covers the oxygenation of the ocean, the colonization of land by plants, and repeated episodes of mountain building.

The analysis produced two insights that reshape how the record should be read. First, the continents played a far larger and previously underappreciated role in supplying iron to the ocean than standard models acknowledged. Second, pyrite formed from that continental supply of iron was itself an important player in Earth’s oxygenation, because burying pyrite in sediments removes reduced sulfur and iron from the ocean-atmosphere system and influences the long-term oxygen budget. In other words, the iron record is not a passive gauge of ocean oxygen; it is an integrated signal of oxygen chemistry and the flux of iron from land.

For the oldest portion of the study window, the long-standing theory holds up well. The iron record in ocean sediments from that early interval was controlled primarily by low-oxygen conditions in the water column, exactly as the classical interpretation predicts. But as the researchers moved forward in time, the delivery of iron to the ocean took center stage. Peaks in the iron record over the past 500 million years coincide with major mountain building events, periods when tectonic collisions raised vast chains of peaks and exposed fresh, iron-rich rock to the elements. The Variscan mountain building event, the great Paleozoic collision that helped assemble the supercontinent Pangea, correlates with a prominent peak in the iron cycle, and other major orogenic episodes leave comparable signatures.

The mechanism linking mountains to iron makes physical sense. When elevation rises, so does the steepness of slopes and the exposure of fresh bedrock, and elevated atmospheric oxygen accelerates the oxidative weathering of iron-bearing minerals. Rain, snowmelt and glacial grind strip the weakened rock apart, and rivers carry the liberated iron toward the sea. Hardisty explained that the combination of changed elevation and atmospheric oxygen intensified the weathering of iron-rich rocks at Earth’s surface, amplifying the continental flux precisely during the intervals when mountain belts were being uplifted. The iron record, in this light, becomes a proxy for the tempo of tectonics and erosion as much as for ocean chemistry.

Land plants added their own powerful lever to the system. Hardisty noted that plant roots release chemicals that help break down rocks and sediments, freeing the iron they contain, a process known as chelation in which organic acids bind metal ions and pry them loose from mineral lattices. Roots also bind streambed sediments in place, slowing the rush of water and giving dissolved iron more time to react with oxygen and form iron-oxide minerals that can eventually be transported to the ocean. The spread of rooted vegetation from the Devonian period onward would therefore have progressively transformed how iron moved from continent to sea, layering a biological signal on top of the tectonic one within the same sedimentary archive.

None of this tears down the old tool, the researchers emphasize; it broadens it. The iron that scientists were tracing was tracking more than changes in ocean oxygen, which is how the records were interpreted in the past, Hardisty said. We didn’t tear down the tool, we added another component to it to broaden the application and added new insight. That added component carries real consequences for how paleoceanographers interpret their data. A spike in iron oxides in a 300-million-year-old rock, once read automatically as evidence of rising ocean oxygen, may instead reflect an erosion pulse from a rising mountain belt or an intensification of plant-driven weathering on land. Disentangling these signals requires pairing iron measurements with independent indicators of tectonic activity, vegetation and paleogeography, a more nuanced but ultimately more faithful reading of deep time.

The implications extend beyond reconstructing the past. Understanding what shaped the iron cycle in previous eras could help scientists anticipate how Earth responds to environmental change now underway. Weathering rates, sediment delivery and the chemistry of rivers are all sensitive to climate, and a warming world is already altering erosion patterns and the flux of nutrients and metals from land to sea. Iron remains an important way to understand the past, Hardisty said, and our findings will help researchers by expanding their tools to continue to study how climate change will affect the planet. The study was supported by the German Research Foundation, and its synthesis of a billion years of sedimentary chemistry shows that even the most familiar geochemical archives, when reexamined at scale, still have new things to tell us about the restless, interconnected machinery of the Earth’s surface.

Subject of Research: The evolution of Earth's surface iron cycle and its links to oxygenation, mountain building and land plants

Article Title: Iron helped scientists trace Earth’s oxygen. Now it’s telling a bigger story

Article References: Iron helped scientists trace Earth’s oxygen. Now it’s telling a bigger story. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: iron cycle, oxygenation, banded iron formations, pyrite, hematite, mountain building, land plants, weathering, sedimentary geochemistry, Pangea, paleoenvironments, biogeochemistry