# Projected rapid response of stratospheric temperature to stringent climate mitigation

## TL;DR

This study shows that stratospheric cooling trends could provide early evidence that climate mitigation efforts are working, within 5-10 years of rapid CO2 reductions.

## Contribution

The study identifies stratospheric temperature as an early indicator of climate mitigation effectiveness, distinct from surface temperature.

## Key findings

- Stratospheric cooling trends weaken within 5 years under rapid emission cuts (SSP1–1.9).
- Stratospheric temperature trends have higher signal-to-noise ratios than surface temperature for detecting mitigation effects.
- Stratospheric temperature could serve as an early indicator for policymakers and the public.

## Abstract

Deep, rapid and sustained reductions in greenhouse gas emissions are required to meet the 2015 Paris Agreement climate target. If the world strengthens efforts toward near-term decarbonisation and undertakes major societal transformation, this will be met with requests from policymakers and the public for evidence that our actions are working and there are demonstrable effects on the climate system. Global surface temperature exhibits large internal variability on interannual to decadal timescales, meaning a reduction in the magnitude of surface warming would not be robustly attributable to climate mitigation for some time. In contrast, global stratospheric temperature trends have much higher signal-to-noise ratios and could offer an early indication of the effects of climate mitigation. Here we examine projected near-term global temperature trends at the surface and in the stratosphere using large ensemble climate models following three future emission scenarios. Under rapid, deep emission cuts following SSP1–1.9, modelled middle and upper stratospheric cooling trends show a detectable weakening within 5 years compared to a scenario approximately representing current climate commitments (SSP2–4.5). Therefore, stratospheric temperature trends could serve as an early indicator to policymakers and the public that climate mitigation is taking effect.

Where will the first signals of climate mitigation emerge? This study finds stratospheric cooling trends, a hallmark of climate change, would weaken within 5-10 years of rapid CO2 reductions, offering early signs the climate system is changing course.