Risks of unavoidable impacts on forests at 1.5 °C with and without overshoot
- Hakan Sener
- Jun 18
- 2 min read
Updated: Jul 2
A 2025 study warns that overshooting 1.5°C—even briefly—risks centuries-long Amazon dieback and irreversible shifts in boreal forests.
A 2025 study explores a critical question for the climate era: How much damage do forests suffer if we overshoot 1.5°C of warming before returning to it later? The answer, according to Gregory Munday and co-authors, is deeply concerning.
Using a high-resolution climate–ecosystem model ensemble (PRIME), the study evaluates the resilience of Amazon and Siberian forests under three policy-relevant emissions pathways—ranging from minimal overshoot to prolonged exceedance of 1.5°C. The findings suggest that even brief overshoots can lock in long-term or irreversible damage to vulnerable forest ecosystems.
Key Findings: Amazon and Boreal Forests Respond Differently—But Both Face Risks
Amazon Forests Face Long-Term Dieback Risks
Temporary overshoot increases long-term risk of Amazon forest dieback, with up to 146,000 km² lost by 2300.
Even under strong mitigation scenarios, tree cover continues to decline for centuries if overshoot is involved.
In the most severe scenarios, up to 55% of simulations show Amazon dieback even when global temperature returns below 1.5°C.
Boreal Forests May Expand, But That Brings Its Own Problems
Siberian forests are projected to expand due to warming and increased net primary productivity (NPP).
This woody encroachment can lead to major changes in ecosystem structure, biodiversity, and water cycles—shifting these regions into new ecological states.
These changes may offset some carbon loss, but also introduce irreversible ecosystem transformations.
Stabilizing at 1.5°C Is Better Than Returning to It
The least damaging outcomes occur in scenarios that stay near 1.5°C without overshooting, confirming that temperature trajectory—not just end-state—matters.
In contrast, overshoot scenarios—even if emissions are aggressively reversed later—cause long-lasting reductions in forest health and productivity.
Tipping Points, Tail Risks, and Deep Uncertainty
The Amazon, in particular, shows tipping point behavior: when regional warming exceeds 2°C, the risk of dieback increases sharply.
Simulations also reveal “tail risks”—low-probability but high-impact outcomes—where even modest overshoot leads to major ecosystem collapse.
Precipitation decline and extreme temperature spikes combine to push forests past safe operating zones, sometimes long after global temperatures stabilize.
Implications: The Case Against Complacency in Overshoot Strategies
The study delivers a strong warning for policymakers: not all 1.5°C pathways are created equal. Plans that allow global temperatures to exceed 1.5°C before coming back down are not equivalent in terms of ecosystem impact. Overshoot might seem temporary on a human timescale—but forests remember.
Protecting forest ecosystems, and their carbon storage and biodiversity functions, requires not just net-zero targets, but urgency in avoiding temperature overshoot altogether. Otherwise, we risk committing the world’s forests to centuries of degradation, even if we meet the Paris Agreement’s end goals.
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