Climate sensitivity

CS diagram

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Schematic climate sensitivity

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Fictional Snowball Earth 1 Neethis

Hansen & Sato, Climate Sensitivity Estimated From Earth's Climate History Figure 7

ECS assessments IPCC

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Frequency distribution of climate sensitivity, based on model simulations (NASA)

Climate sensitivity refers to the temperature change in response to a change in the radiative forcing of the Earth's atmosphere. It is a key concept in climatology and Earth system science, providing a quantitative way to predict how the Earth's climate will respond to various greenhouse gas emissions scenarios. Climate sensitivity is typically measured in degrees Celsius (°C) per doubling of atmospheric carbon dioxide (CO2) concentration, though it can also apply to other forcings.

Definition

Climate sensitivity is often divided into two main types: Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR).

• Equilibrium Climate Sensitivity (ECS) refers to the equilibrium change in global mean surface temperature following a doubling of atmospheric CO2 concentration, once the climate system has reached a new equilibrium state, typically over a timescale of several centuries.
• Transient Climate Response (TCR) measures the change in global mean surface temperature, at the time of CO2 doubling, under a scenario where CO2 concentrations increase by 1% per year.

Determining Factors

The sensitivity of the climate system is influenced by various feedback mechanisms, including but not limited to:

• Water vapor feedback: Water vapor is a potent greenhouse gas, and warming leads to increased water vapor in the atmosphere, further amplifying the warming.
• Albedo feedback: Changes in surface albedo, due to melting of ice and snow, expose darker surfaces underneath, which absorb more solar radiation and increase warming.
• Cloud feedback: Clouds can both cool the Earth by reflecting sunlight and warm it by trapping outgoing infrared radiation. The net effect of cloud feedbacks on climate sensitivity is complex and remains a significant area of research.

Implications

Understanding climate sensitivity is crucial for predicting future climate change and informing climate policy and mitigation strategies. Higher climate sensitivity would imply more significant changes in climate for a given increase in greenhouse gas concentrations, necessitating more urgent and substantial emission reductions to limit global warming to targets such as those set by the Paris Agreement.

Challenges in Estimation

Estimating climate sensitivity is challenging due to uncertainties in feedback processes, the historical temperature record, and the representation of physical processes in climate models. Studies use a combination of paleoclimate data, observational data, and model simulations to estimate ECS and TCR, leading to a range of estimates. The Intergovernmental Panel on Climate Change (IPCC) reports provide assessments of current understanding and estimates of climate sensitivity, which are updated as new evidence becomes available.

Recent Findings

Recent assessments suggest that ECS is likely between 1.5°C and 4.5°C, with a best estimate of about 3°C. However, some studies suggest the possibility of higher values, which would imply greater risks associated with greenhouse gas emissions.

Conclusion

Climate sensitivity is a central concept in understanding future climate change. Despite the challenges in precisely determining its value, the concept is crucial for developing effective climate policies and strategies for mitigation and adaptation. Ongoing research aims to narrow the uncertainties surrounding climate sensitivity to better predict and prepare for future climate changes.