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A stylized study of the climate response to longwave and shortwave forcing at the altitude of aviation-induced cirrus

Tresa Mary Thomas, Lei Duan, Govindasamy Bala, and Ken Caldeira · Earth's Future 13 · 2025

Key finding. In simulations imposing stylized forcing at aviation-cirrus altitude, longwave absorbers produce their largest instantaneous radiative forcing in the subtropics but their largest global temperature response in the polar regions, implying that contrail-induced warming could be reduced most effectively by avoiding aviation-induced cirrus at night, and at high latitudes during their winters.

Six global maps in two columns, longwave forcing on the left and shortwave on the right. The top row is instantaneous radiative forcing, confined to a narrow northern mid-latitude band, red for longwave and blue for shortwave. The middle row is effective radiative forcing, still concentrated in that band but with weaker signals spreading elsewhere. The bottom row is equilibrium surface temperature change, which is spread across the whole globe and strongest at high latitudes, far from the band where the forcing was applied.
Forcing goes in along one narrow band; temperature comes out everywhere, and most strongly near the poles. That mismatch between where the energy is added and where the climate responds is the whole result — it is why the latitude at which contrails form matters more than the forcing they produce there. Figure 4 from Thomas et al. (2025), Earth's Future 13, e2025EF006201. Reproduced under CC BY 4.0. Extracted from the published PDF and resized for web display.

What question did this research address?

Aviation-induced cirrus clouds warm the planet, but they act through two competing channels at once. They trap outgoing longwave radiation, which warms, and they scatter incoming shortwave radiation, which cools. The net climate effect of that combination is not well understood.

This paper asked how the climate responds to longwave and to shortwave forcing imposed at the altitude where aviation-induced cirrus form, and whether the response depends on the latitude at which that forcing occurs.

What did we find?

Using the Community Earth System Model CESM1.2.2, the study imposed stylized longwave and shortwave forcing agents in different latitude bands, isolating each radiative channel rather than modelling contrails directly.

For longwave absorbers, the latitude of largest radiative forcing and the latitude of largest temperature response are not the same. At equal concentration, subtropical absorbers produce the largest instantaneous radiative forcing, while polar absorbers produce the largest effect on global mean temperature.

Shortwave scatterers behave differently. Their instantaneous and effective radiative forcing is largest at low latitudes, but the resulting global temperature response is not strongly sensitive to the latitude at which the forcing is applied.

Because the longwave effect operates day and night while the shortwave effect operates only in daylight, the results imply that the warming attributable to aviation-induced cirrus is concentrated in night-time cirrus and in high-latitude winter cirrus.

Why does it matter?

The finding separates two quantities that are often treated as interchangeable. Radiative forcing is a useful summary of how strongly something perturbs the energy budget, but here the place where forcing is largest is not the place where warming is largest, so forcing alone is a poor guide to which contrails matter most.

It also points at an unusually tractable mitigation lever. If the warming is concentrated in cirrus formed at night and at high latitudes in winter, then rerouting or rescheduling a modest subset of flights would address a disproportionate share of the effect.

Citation

Tresa Mary Thomas, Lei Duan, Govindasamy Bala, and Ken Caldeira (2025). A stylized study of the climate response to longwave and shortwave forcing at the altitude of aviation-induced cirrus. Earth's Future 13.

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