Fixed anvil temperature hypothesis

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Anvil cloud over the Tiwi Islands, Australia

Fixed anvil temperature hypothesis is a

anvil clouds is constrained by radiative processes and thus does not change in response to surface warming. Since the amount of radiation emitted by clouds is a function of their temperature, it implies that it does not increase with surface warming and thus a warmer surface does not increase radiation emissions (and thus cooling) by cloud tops. The mechanism has been identified both in climate models and observations of cloud behaviour, it affects how much the world heats up for each extra tonne of greenhouse gas in the atmosphere
. However, some evidence suggests that it may be more correctly formulated as decreased anvil warming rather than no anvil warming.

Background and hypothesis

In the

anvil clouds – the topmost convective clouds – concentrate.[1]

The "fixed anvil temperature hypothesis" stipulates that owing to energetic and thermodynamic constraints imposed by the Clausius-Clapeyron relationship, the temperature and thus radiative cooling of anvil clouds does not change much with surface temperature.[1] Specifically, cooling decreases below −73 °C (200 K) as the ineffective radiative cooling by CO
2 becomes dominant below that temperature.[2] Instead, the elevation of high clouds rises with surface temperatures.[3]

A related hypothesis is that tropopause temperatures are insensitive to surface warming; however it appears to have distinct mechanisms from the fixed anvil temperature process.[4] They have been related to each other in several studies,[5] which sometimes find a fixed tropopause temperature a more reasonable theory than fixed anvil temperature.[6]

Evidence

The fixed anvil temperature hypothesis has been widely accepted and even extended to the non-tropical atmosphere. Its strength relies in part on its reliance on simple physical arguments.[7]

Models

The fixed anvil temperature hypothesis was initially formulated by Hartmann and Larson 2002 in the context of the NCAR/PSU MM5

relative humidity[13] or that the fixed anvil temperature is more properly expressed as anvil temperature changing more slowly than surface temperature.[14] Climate models also simulate an increase in cloud top height[15] and some radiative-convective models apply it to the outflow of tropical cyclones.[16]

The fixed anvil temperature hypothesis has also been obtained in simulations of

runaway greenhouse however, other physical effects pertaining to cloud opacity may take over and dominate the fixed anvil temperature as surface temperatures reach extreme levels.[18]

Observations

The fixed anvil temperature hypothesis has been backed by observational studies

Brewer-Dobson circulation.[13] Xu et al. 2007 found that cloud temperatures are more stable for clouds with sizes exceeding 150 kilometres (93 mi).[21] The ascent of cloud top height with warming is also supported by observations.[15]

Implications

Clouds are the second biggest uncertainty in future climate change after human actions, as their effects are complicated and not properly understood.[22] The fixed anvil temperature hypothesis has effects on global climate sensitivity, since anvil clouds are the most important source of outgoing radiation linked to tropical convection[23] and their temperature being stable would render the outgoing radiation non-responsive to surface temperature changes.[24] This creates a positive feedback component of cloud feedback.[25] The fixed anvil temperature hypothesis has also been used to argue that climate modelling should use temperature rather than pressure to model the height of high clouds.[26]

Alternative views

A hypothesis which would have the opposite effect on climate is the

atmospheric stability and appears to reflect actual climate conditions more closely.[26] Finally, there is a view that cloud top temperatures could actually decrease with surface warming[29] as convection height rises. This may constitute a non-equilibrium response.[30]

Research

As of 2020[update] further research is needed to properly understand the physics of some cloud feedbacks,[31] as they differ between models,[32] and progress on properly modelling clouds globally is very slow.[22]

References

  1. ^ a b Hartmann & Larson 2002, p. 1.
  2. ^ Hartmann & Larson 2002, p. 3.
  3. S2CID 182771431
    .
  4. S2CID 118967908
    .
  5. ISSN 2169-8996
    .
  6. ^ Seeley, Jeevanjee & Romps 2019, p. 1849.
  7. ^ Seeley, Jeevanjee & Romps 2019, p. 1842.
  8. ^ Hartmann & Larson 2002, p. 2.
  9. ^ Del Genio 2016, p. 107.
  10. ^ Igel, Drager & van den Heever 2014, p. 10516.
  11. S2CID 146704580
    .
  12. ^ Noda et al. 2016, p. 2313.
  13. ^
    ISSN 0022-4928
    .
  14. ^ Seeley, Jeevanjee & Romps 2019, p. 1848.
  15. ^
    S2CID 198420050
    .
  16. doi:10.1002/2014MS000342
    .
  17. S2CID 146053272
    .
  18. PMID 25061956
    .
  19. S2CID 131478611
    .
  20. ^ a b Noda et al. 2016, p. 2307.
  21. ^ Noda et al. 2016, p. 2312.
  22. ^ a b Irfan, Umair (2021-05-19). "Scientists aren't sure what will happen to clouds as the planet warms". Vox. Retrieved 2021-07-05.
  23. ^ Hartmann & Larson 2002, pp. 1–2.
  24. ^ Hartmann & Larson 2002, p. 4.
  25. ^ Del Genio 2016, p. 116.
  26. ^
    S2CID 135038760
    .
  27. S2CID 134486980
    .
  28. doi:10.1029/2011JD016459
    .
  29. ^ Igel, Drager & van den Heever 2014, p. 10530.
  30. ^ Igel, Drager & van den Heever 2014, p. 10531.
  31. ^ "Cooling effect of clouds 'underestimated' by climate models, says new study". World Economic Forum. 10 June 2021. Retrieved 2021-07-05.
  32. ISSN 0894-8755
    .

Sources

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