Partially True: Some Earth System Models Do Blur Photosynthesis and Wood Growth — But It's More Complicated Than That
“Many Earth system models equate photosynthesis rates with wood growth”
The argument in brief
The claim that many Earth system models equate photosynthesis rates with wood growth contains a real kernel of truth but overstates the problem. Most models don't literally set them equal — instead, they use oversimplified rules that cause wood growth to track photosynthesis far too closely. In reality, wood captures only about 20–40% of what a forest photosynthesizes, a ratio many older models get badly wrong.
Data: Pugh et al. 2019, Nature Geoscience; Collalti & Prentice 2019, New Phytologist
Why it spread
Climate model limitations are a legitimate area of scientific debate, and researchers themselves publish papers criticizing these shortcuts. That gives the claim credibility with technically minded readers. It also fits neatly into a broader narrative that climate models are unreliable, making it easy to share as a gotcha — even though the actual issue is a known, documented bias that modelers are actively fixing, not a fundamental flaw in climate science.
The claim is that many Earth system models (ESMs) — the large computer simulations used to project climate futures — treat photosynthesis and wood growth as essentially the same thing. The verdict is: partially true. No serious model literally writes an equation saying photosynthesis equals wood growth, but a significant number of models, especially older ones, use such crude shortcuts that the practical effect is similar.
Here's the biology the models need to capture. A tree photosynthesizes a certain amount of carbon — that's gross primary production, or GPP. But a large chunk of that gets burned off in the tree's own respiration, some feeds roots and soil microbes, and some gets stored as sugars rather than built into wood. According to Pugh et al. (2019) in Nature Geoscience, wood growth in real forests captures only about 20–40% of GPP. Tropical forests sit around 25%, temperate forests around 32%, and boreal forests around 35%.
Many ESMs, particularly those used in earlier climate model comparisons, don't reproduce this. Fatichi et al. (2019) in Nature Plants found that numerous land surface models skip explicit carbon allocation entirely, effectively using total photosynthesis as a stand-in for biomass growth. De Kauwe et al. (2014) in Global Change Biology confirmed that some models lack separate wood carbon pools altogether. Trugman et al. (2018) showed that fixed, oversimplified allocation rules mean wood growth ends up scaling too tightly with photosynthesis — with some models implicitly allocating over 50% of GPP to wood, nearly double the observed rate.
To be fair to the models, this is an area of active improvement. Collalti and Prentice (2019) in New Phytologist laid out the theoretical case for why NPP and GPP shouldn't be assumed to scale together, and newer model generations like CLM5 and JULES have added more realistic allocation modules. The problem is real, but it's a known bias being worked on — not a hidden flaw or evidence that climate projections are fundamentally broken.
This claim spreads because it appeals to two audiences at once: scientists who genuinely know ESMs have carbon allocation weaknesses, and broader audiences already skeptical of climate models. The real issue — oversimplified allocation fractions in some older models — gets compressed into a sharper, more damning-sounding accusation. The honest version is less dramatic but still worth knowing: some models overestimate how much carbon forests lock into wood, which matters for projecting how much CO₂ trees will absorb as the climate changes.
Sources
- Fatichi et al. (2019) - Nature Plants
Many land surface models and Earth system models do not explicitly represent carbon allocation to different plant tissues, effectively treating net primary production as a proxy for biomass accumulation without distinguishing wood growth from total photosynthesis or NPP.
- De Kauwe et al. (2014) - Global Change Biology
A multi-model analysis found that many ESMs poorly represent carbon allocation, with some models lacking explicit wood carbon pools or conflating gross/net photosynthesis with biomass increment, leading to overestimates of woody biomass growth.
- Trugman et al. (2018) - Global Change Biology
ESMs often use fixed or overly simplified carbon allocation schemes that do not account for the fraction of photosynthate used for respiration, root exudation, and non-structural carbohydrates, meaning wood growth is not properly separated from photosynthesis.
- Collalti & Prentice (2019) - New Phytologist
The authors explicitly critique the assumption in some models that net primary production (NPP) scales directly with gross primary production (GPP), noting that wood growth represents only a fraction of GPP after autotrophic respiration and allocation to other tissues.
- Friedlingstein et al. (2006) - Journal of Climate
Early coupled carbon-climate ESMs used in CMIP exercises had highly simplified vegetation carbon cycle representations, with limited distinction between photosynthesis and wood biomass accumulation, contributing to spread in carbon cycle projections.
- Pugh et al. (2019) - Nature Geoscience
Observational data show that wood growth captures only about 20-40% of GPP in forests, a ratio that many ESMs fail to reproduce accurately, confirming that equating photosynthesis with wood growth is an oversimplification present in some but not all models.