Research assistant in the SPECIAL group, Jierong Zhao, and SPECIAL group PI, Sandy Harrison, have recently released a pre-print that investigates the importance of CO₂ and climate in influencing changes in GPP over the Quaternary. Here we summarise of the key highlights but you can access the full pre-print below for all of the details of this new and exciting study!

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Eco-evolutionary Modelling of Global Vegetation Dynamics and the Impact of CO2 during the late Quaternary: Insights from Contrasting Periods

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Why Study Global Greening?

Predicting how terrestrial vegetation will respond to future climate scenarios is crucial for understanding global greening. While climate is a key driver of vegetation change, carbon dioxide (CO₂) levels also play a fundam ental role by influencing plant productivity and the balance between C3 and C4 plants.

However, disentangling the relative importance of climate and CO₂ is challenging, especially since modern climate change is largely CO₂-driven. Studying past climate shifts—where climate was altered by natural orbital cycles rather than rising CO₂—offers a unique opportunity to isolate these effects.

To do this, researchers use a counterfactual experiment, where climate and CO₂ are manipulated separately in models to see which factor has a bigger impact and where those effects are strongest. This approach allows us to understand how vegetation responded to past changes and, in turn, improve predictions for the future.

The Key Takeaways

To model the effects of climate and CO₂ on gross primary production (GPP) (a measure of plant productivity), the study used:

  • The P model, which simulates light-use efficiency and GPP.
  • A leaf area index (LAI) model, which estimates seasonal vegetation cycles.
  • Climate simulations from MPI-ESM1.2-LR (a low-resolution version of the Max Planck Institute’s Earth System Model) which was included in PMIP4 (Paleoclimate Modelling Intercomparison Project).

For full details of the modelling approach and counterfactual experiments, check out the full pre-print here.

Findings at the LGM and MH:

  • LGM (Last Glacial Maximum, ~21,000 years ago)
    • GPP was significantly lower than in the pre-industrial period.
    • The LGM also saw a major increase in C4 plants, which became responsible for 56% of global GPP—almost double their pre-industrial abundance.

Figure 1: Simulated change in total annual gross primary production (GPP) between the pre-industrial (PI) and (a) the Last Glacial Maximum (LGM) and (b) the mid-Holocene (MH).

  • Mid-Holocene (~6,000 years ago)
    • GPP was slightly higher than in the pre-industrial period.
    • However, lower CO₂ levels limited plant growth, preventing a larger increase.
  • Comparing Transitions
    • GPP increased by ~32% from the LGM to the MH (in the common area of plant coverage between all periods of investigation).
    • However, the transition from the MH to pre-industrial conditions resulted in only a small decrease in global GPP.

Climate vs. CO₂: What Had the Bigger Impact?

By running factorial experiments (modelling the effects of each variable separately), the study found:

  • At the LGM, both lower CO₂ and colder/drier climate had a negative effect on GPP.
  • However, climate had a slightly stronger negative impact on plant productivity than CO₂ alone.
  • At the MH, climate had a positive effect on GPP, but this was offset by low CO₂ levels, limiting plant growth.

“The impact of lowered CO₂ on GPP is only slightly smaller than the changes caused by climate at the LGM, reinforcing the overall reduction of GPP. The impact of lowered CO₂ in the MH is larger than the impact of climate, offsetting the positive impacts of climate change in the MH experiment.”

Uncertainties and Challenges

Estimating GPP at the LGM remains uncertain, as different models produce widely varying values.

  • CMIP6/PMIP4 models estimate LGM GPP at 61-109 PgC, with this study’s estimate (84 PgC) falling in the middle of this range.
  • Ice core records confirm this uncertainty, as factors like ocean productivity and respiration rates are difficult to estimate.
  • Despite this, pollen evidence strongly supports the idea that tree cover was significantly reduced at the LGM, aligning with model results.

Why Does This Matter?

Understanding how vegetation responded to past climate and CO₂ shifts helps improve future climate projections. This study shows that while climate is a dominant factor in plant productivity, CO₂ availability plays a major role in limiting or enhancing plant growth—a key consideration in predicting future ecosystem changes.

As CO₂ levels continue to rise, it’s essential to factor both climate and CO₂ effects into vegetation models to create more accurate forecasts for global greening in a warming world.

 

Zhao, J., Zhou, B., Harrison, S. P., and Prentice, I. C.: Eco-evolutionary Modelling of Global Vegetation Dynamics and the Impact of CO2 during the late Quaternary: Insights from Contrasting Periods, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3897, 2025.