Development and Evaluation of an Earth-System Model: HadGEM2
Observation and Context
Standard climate models typically focus only on physical interactions between the atmosphere and the ocean. However, real-world climate change is heavily influenced by biological and chemical processes. Natural systems—such as growing forests, ocean plankton, and atmospheric gases—can create feedback loops that either speed up or slow down warming. Previous models often relied on artificial data corrections or fixed values to keep climate simulations stable, missing out on how these living systems dynamically react to rising greenhouse gases.
Hypothesis
If scientists integrate interactive land ecosystems, ocean biology, and tropospheric chemistry into a physical climate model, the model will maintain a stable climate simulation without artificial corrections while accurately capturing complex biogeochemical feedbacks and present-day environmental conditions.
Experiment and Methodology
Researchers developed the HadGEM2-ES (Hadley Centre Global Environmental Model version 2 Earth System):
- They integrated the TRIFFID dynamic vegetation model and the RothC soil model to calculate land carbon storage and plant growth.
- They added the Diat-HadOCC marine biology model to simulate ocean carbon uptake and plankton growth (including iron fertilization via airborne dust).
- They implemented the UKCA chemistry scheme to interactively simulate methane, tropospheric ozone, and sulfur-based aerosols.
- To prevent runaway errors, the team carefully stabilized (spun up) the model under pre-industrial (1860) conditions before running historical simulations through the 20th century to compare simulated data against satellite and ground-based observations.
Results and Data
- Stability: Under 1860 control conditions, the carbon cycle achieved near-perfect balance; atmospheric carbon dioxide experienced negligible drift (about 0.01 ppm per year), and the air-to-sea carbon flux hovered near zero.
- Vegetation and Carbon Stores: Simulated present-day soil carbon totaled 1107 GtC and vegetation carbon reached 478 GtC, aligning well with observed global estimates. Terrestrial Net Primary Productivity (NPP) increased by roughly 22% from pre-industrial levels to 70.5 GtC/yr, fitting within the expected multi-model range.
- Atmosphere and Ocean: The model accurately reproduced seasonal carbon dioxide swings measured at Mauna Loa and Pt. Barrow, fixed previous timing errors, and reliably simulated vertical ozone profiles.
- Deficiencies: Overly dry bare soil simulated in regions like Australia led to localized overestimates of mineral dust emissions.
Conclusion and Climate Impact
The researchers confirmed their hypothesis: HadGEM2-ES successfully operates as a stable, coupled Earth system model without requiring artificial flux corrections. Adding interactive biological and chemical systems did not distort the physical climate. This model significantly enhances climate science by simulating real-world biogeochemical feedbacks—such as how shifting forests or warming oceans alter greenhouse gas absorption—providing more realistic projections of long-term future warming for international assessments like CMIP5.
Full Citation
Collins, W. J., Bellouin, N., Doutriaux-Boucher, M., Gedney, N., Halloran, P., Hinton, T., Hughes, J., Jones, C. D., Joshi, M., Liddicoat, S., Martin, G., O’Connor, F., Rae, J., Senior, C., Sitch, S., Totterdell, I., Wiltshire, A., & Woodward, S. (2011). Development and evaluation of an Earth-System model – HadGEM2. Geoscientific Model Development, 4(4), 1051–1075. https://doi.org/10.5194/gmd-4-1051-2011