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D4.3.3—Boreal forest tipping point

Boreal forests face tipping-point risk when warming, drought, fire, pests, and decomposition shift them from carbon sinks to carbon sources.

Syllabus
First assessment 2025
Objective
D4.3.3
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceNovember 2018
Most common paperPaper2
Typical marks2

Common command terms

  • Explain

Recent exam appearances

November 2018Paper2 ["SL"] · TZ03(c)[ 2 ]D4.3.3—Boreal forest tipping point

Coverage 2018–2018 · Updated 16 Jul 2026

A Boreal Forest Is a Carbon Sink Only while Uptake Exceeds Release

Carbon gain Carbon loss
photosynthesis and tree growth plant and microbial respiration
carbon stored in wood and litter decomposition of litter and thawed soil carbon
accumulation in cold or waterlogged soils combustion during peat and forest fires

net sink: uptake > release → ecosystem carbon store grows
net source: release > uptake → ecosystem carbon store shrinks and atmospheric carbon rises

A forest can remain visibly wooded while switching from sink to source. The classification depends on the net carbon balance, not tree presence alone.

Warming Can Push Boreal Forests into a Self-Reinforcing Carbon-Loss State

  1. Warmer winters and reduced snowfall alter soil insulation and water supply.
  2. Drought stress and heat reduce growth and cause forest browning.
  3. Insects, tree mortality and fire increase.
  4. Combustion and decomposition release biomass and soil carbon.
  5. Permafrost thaw exposes additional carbon to decomposition.
  6. Extra CO₂ and methane reinforce warming, drought and fire risk.

The tipping point is crossed when repeated carbon loss and impaired recovery maintain a net-source state instead of returning to net accumulation.

The relevant evidence must combine carbon fluxes, tree growth, mortality, fire and soil thaw through time; one photograph of a burned forest cannot establish the budget shift.

Climate Effects Follow Forcing, Feedback, Exposure and Biological Response

human greenhouse-gas source → enhanced infrared retention → climate variable changes → feedback may amplify the change → habitat or physiological mechanism alters survival, production or distribution → population and ecosystem consequences

Impact Mechanism that must appear
boreal sink becomes source carbon release exceeds uptake through drought, mortality, fire and thaw
ice-dependent species decline required resting or breeding platform is lost at the critical time
marine productivity falls stratification reduces nutrient upwelling to phytoplankton
range contracts or shifts climate space moves but dispersal and habitat constrain tracking
reef degrades bleaching removes symbiont energy and acidification suppresses calcification

Do not jump directly from “warming” to “species decline.” The biological mechanism and the relevant time or place make the explanation causal.

Boreal forest tipping point

[ 2 ]

An increase in global temperatures poses a critical threat to boreal forests. Explain the consequences of climate change to this northern ecosystem.

Concept essentials

  • Boreal warming can increase drought, fires, pests, disease, and tree mortality.
  • Faster decomposition and permafrost thaw can release carbon stored in cold soils.
  • A tipping point occurs when feedbacks shift the forest into a different, less stable state.