A Colorful look at Climate Sensitivity

Stevens, Björn; Kluft, Lukas

doi:10.5194/egusphere-2022-1460

Cited by 11 publications

(19 citation statements)

References 19 publications

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“…The reference response is the anvil cloud-masked clear-sky 408 feedback. By virtue of a fixed temperature and area fraction, 409 anvils contribute no additional emission to space with surface 410 warming, so they destabilize the climate relative to the clear-411 sky response, λcs (37,58). Assuming λcs = −2 Wm −2 K −1 412 (36,37) and f h = 0.17 (Table 1) implies λ0 ≈ −1.7 Wm thereby influencing climate sensitivity in divergent ways (20).…”

Section: R a F Tmentioning

confidence: 99%

Physical and observational constraints on the anvil cloud area feedback

McKim¹,

Bony²,

Dufresne³

2023

Preprint

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Changes in anvil cloud area with warming are one of the largest sources of uncertainty in Earth's climate sensitivity (Sherwood et al 2020). Here, we develop a simple theory of cloud radiative effects and derive an equation for the tropical anvil cloud area feedback. Our theory shows that the feedback is given by the product of the present day (and thus measurable) cloud radiative effect and the fractional change in anvil area with warming. Satellite observations suggest a cloud radiative effect ≈ −1 Wm −2 and a sensitivity of anvil clouds to surface temperature ≈ −11% K −1 at the interrannual time scale (Saint-Lu et al, 2020), leading to a tropical anvil cloud area feedback of 0.08 ± 0.05 Wm −2 K −1 . This feedback is thus weaker and better constrained than previously thought. We then use our theory to derive an equation for the proportionally higher anvil temperature feedback, which depends on the change in anvil temperature with warming. Satellite observations suggest this change is ≈ 0.44 K K −1 . Combining the resultant temperature feedback with the area feedback leads to a total anvil cloud feedback of −0.01 ± 0.09 Wm −2 K −1 . Changes in anvil clouds with warming appear to have little effect on climate sensitivity. anvil clouds | cloud feedbacks | climate sensitivity | climate change A nvil clouds are the cirrus cover formed by detrainment from deep convection. Anvils blanket the tropics and modify Earth's energy balance by reflecting sunlight and trapping infrared radiation. Their reduction in area with warming (1, 2) is a leading source of uncertainty in estimating climate sensitivity (3), and given new constraints on low cloud feedbacks (4, 5), the anvil cloud area feedback is now the most uncertain cloud feedback of all. But can it be better constrained? Changes in anvil clouds with warming Ramanathan and Collins (6) first explored the idea that changing anvil cloud cover could alter Earth's climate sensitivity by regulating ocean surface temperature like a thermostat. However, the drop off in frequency of both deep convection and surface temperature above a certain threshold temperature are no longer considered evidence of a tropical thermostat (7-10). Ten years later, Lindzen et al (11), through a different chain of logic, reasoned that reduced anvil cloud cover with warming could act like a shrinking iris and significantly dampen further warming. Critiques of this work's methodology soon followed (12-14), but they did not preclude the existence of a strong area feedback. Twenty years later, a recent comprehensive assessment of Earth's climate sensitivity (3) by the World Climate Research Program (WCRP), relying primarily on the observational study by Williams and Pierrehumbert (15), estimated the anvil area feedback to be −0.2 ± 0.2 Wm −2 K −1 -a range wide enough to encompass the possibility that on one end the anvil cloud area feedback is zero, and on the other end is −0.4 29 Wm −2 K −1 , a value large enough to make the overall cloud 30 feedback zero, given that low cloud feedbacks are less posit...

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Section: R a F Tmentioning

confidence: 99%

Physical and observational constraints on the anvil cloud area feedback

McKim¹,

Bony²,

Dufresne³

2023

Preprint

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“…Charney et al. (1979) had estimated the ECS to be within 1.5 and 4.5°C (consistent with recent theoretical assessments, see Stevens & Kluft, 2023). In the following decades, this estimate has not changed and the uncertainty not narrowed (Collins et al., 2013).…”

Section: Introductionmentioning

confidence: 58%

“…

The equilibrium climate sensitivity (ECS) is defined as the equilibrium warming to be expected following a doubling of atmospheric CO 2 concentrations. Charney et al (1979) had estimated the ECS to be within 1.5 and 4.5°C (consistent with recent theoretical assessments, see Stevens & Kluft, 2023). In the following decades, this estimate has not changed and the uncertainty not narrowed (Collins et al, 2013).

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mentioning

confidence: 76%

Climate Change Response of Tropical Atlantic Clouds in a Kilometer‐Resolution Model

Heim,

Schär

2023

JGR Atmospheres

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Estimates of the tropical cloud feedback of global climate models (GCMs) show a large inter‐model spread due to the small‐scale nature of convective cloud processes. Estimates from large‐eddy simulations (LES) are more consistent among themselves, but difficult to scale to the climate system. Here we consider a compromise between GCMs and LES, and study how tropical clouds over the Atlantic respond to a climate perturbation in a regional kilometer‐resolution model. We perform two 4‐year‐long simulations at 3.3 km horizontal grid spacing with the limited‐area model COSMO on a 9,000 × 7,000 km2 domain covering the tropical Atlantic: a control simulation and a climate change simulation using the pseudo‐global warming approach. In a previous publication we have demonstrated that this approach yields a credible representation of the tropical climate without the double‐ITCZ bias commonly seen in GCMs. Here we address the cloud feedback and find a reduction of ITCZ high‐cloud cover resulting in a negative longwave cloud feedback similar to the CMIP6 ensemble mean. We find a reduction of stratocumulus clouds, which we argue is primarily a thermodynamic response. More surprisingly, the shallow cumulus cloud cover over the West Atlantic increases, which we argue is due to increased stability resulting in weaker entrainment and a more humid boundary layer. The mean shortwave cloud feedback over the tropical Atlantic is positive, comparable to the CMIP6 ensemble mean. The emerging estimate of the total cloud‐radiative feedback over the Atlantic is slightly negative, consistent with CMIP6 GCMs, but it is substantially stronger than that of the driving GCM.

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“…The preceding analysis shows that the value of CO 2 radiative forcing can be understood in terms of the quantum properties of the CO 2 molecule, with the influence of Fermi resonance on the band structure coefficient w particularly critical. To tie this analysis back to Equation (2) in the Introduction (Section 1) and link global warming to radiative forcing, the last thing we need is an expression for the climate feedback parameter λ. Analytic approaches to this problem have been discussed in detail in other recent works (Ingram 2010;Jeevanjee et al 2021a;Stevens & Kluft 2022;Jeevanjee 2023;Koll et al 2023), so we only provide a very brief summary here, in the interests of completeness.…”

Section: Climate Sensitivitymentioning

confidence: 99%

Fermi Resonance and the Quantum Mechanical Basis of Global Warming

Wordsworth,

Seeley,

Shine

2024

Planet. Sci. J.

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Although the scientific principles of anthropogenic climate change are well-established, existing calculations of the warming effect of carbon dioxide rely on spectral absorption databases, which obscures the physical foundations of the climate problem. Here, we show how CO2 radiative forcing can be expressed via a first-principles description of the molecule’s key vibrational-rotational transitions. Our analysis elucidates the dependence of carbon dioxide’s effectiveness as a greenhouse gas on the Fermi resonance between the symmetric stretch mode ν 1 and bending mode ν 2. It is remarkable that an apparently accidental quantum resonance in an otherwise ordinary three-atom molecule has had such a large impact on our planet’s climate over geologic time, and will also help determine its future warming due to human activity. In addition to providing a simple explanation of CO2 radiative forcing on Earth, our results may have implications for understanding radiation and climate on other planets.

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A Colorful look at Climate Sensitivity

Cited by 11 publications

References 19 publications

Physical and observational constraints on the anvil cloud area feedback

Physical and observational constraints on the anvil cloud area feedback

Climate Change Response of Tropical Atlantic Clouds in a Kilometer‐Resolution Model

Fermi Resonance and the Quantum Mechanical Basis of Global Warming

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