How much will the planet warm if we double atmospheric carbon dioxide levels?

Nature · Published 3 October 2026 Swipe more like this →

In short: Climate sensitivity is the core metric for predicting our future. It measures the global temperature rise following a doubling of atmospheric CO2. While the physics of a 1°C increase without feedback is clear, the complex, real-world interactions of clouds and ice make the final temperature outcome a subject of intense scientific debate.

Climate sensitivity is defined by how the Earth's surface temperature responds to a doubling of atmospheric carbon dioxide compared to pre-industrial levels, which were approximately 260 parts per million. Because we cannot measure this directly, scientists rely on three primary lines of evidence: historical instrumental records since the mid-19th century, complex mathematical climate models, and paleoclimate data from ice cores. Each method involves assumptions and limitations, such as the inability of models to perfectly simulate small-scale cloud processes or the difficulty of using ancient climate data to predict centennial-scale changes.

Scientists distinguish between two main measures of this sensitivity. The Transient Climate Response (TCR) estimates the temperature increase at the exact moment CO2 concentrations double, providing a gauge for expected warming over the current century. In contrast, Equilibrium Climate Sensitivity (ECS) accounts for the long-term adjustment of the entire climate system, including the slow-responding oceans, which can take hundreds or thousands of years to stabilize. Because of these long timescales, ECS is often considered less practical for immediate policy decisions, though it remains a vital theoretical benchmark.

The uncertainty in these estimates stems largely from feedback effects—processes that can either accelerate or dampen warming. Without any feedbacks, a doubling of CO2 would result in roughly 1°C of warming. However, positive feedbacks, such as increased water vapor in a warmer atmosphere or the loss of reflective sea ice, significantly amplify this effect. The IPCC’s fifth assessment report suggests a 'likely' ECS range of 1.5°C to 4.5°C, while more recent estimates place it between 2°C and 5°C. This range is critical: if sensitivity is on the higher end, meeting the Paris Agreement goal of limiting warming to below 2°C becomes significantly more difficult, potentially requiring drastic reductions in emissions that may be impossible to achieve in time.

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