The choice of 2°C of global warming is not arbitrary, nor is it a political slogan. It represents a boundary identified by decades of climate science as the point beyond which the risk of large-scale, irreversible change rises sharply.
Global average temperature is a proxy. What ultimately matters are the physical consequences that temperature enables: ice loss, sea-level rise, ecosystem collapse, extreme weather, and disruption to carbon cycles. As warming increases, these impacts accelerate nonlinearly.
At a planetary scale, fractions of a degree are not marginal. The difference between 1.5°C and 2°C of warming corresponds to:
These impacts are not evenly distributed. They disproportionately affect already vulnerable regions, amplifying inequality, displacement, and geopolitical instability.
Source: https://www.climatecouncil.org.au/resources/impacts-degrees-warming/
Earth’s climate system contains multiple tipping elements: components that can shift abruptly once a threshold is crossed. Examples include:
Once triggered, many of these processes cannot be reversed on human timescales, even if temperatures later stabilise or decline. This means that overshooting 2°C is not simply a temporary excursion; it risks permanently altering the trajectory of the planet.
The 2°C threshold is therefore less a target than a guardrail - a point beyond which the probability of runaway or self-reinforcing change increases materially.
Climate models show a near-linear relationship between cumulative CO₂ emissions and global temperature rise. This relationship allows scientists to translate temperature thresholds into carbon budgets, creating a direct link between emissions today and warming tomorrow.
Importantly, this also means that:
At higher temperatures, uncertainty becomes more dangerous. Small errors in emissions or removals can push systems past thresholds unintentionally.
Staying below 2°C remains technically possible, but only within a narrowing window. It requires not only rapid emissions reductions, but also:
As the margin for error shrinks, measurement uncertainty itself becomes a climate risk.
Understanding why 2°C matters leads directly to the question of how much carbon we can still afford to emit, and how confidently we can account for what is removed.
That question is addressed next.