4 ms·

Only slightly educated guess forthcoming. Air pollution was pretty bad in the United States from the 1950s through the late 1980s, and we know that https://en.

Only slightly educated guess forthcoming. Air pollution was pretty bad in the United States from the 1950s through the late 1980s, and we know that https://en.wikipedia.org/wiki/Global_dimming https://en.wikipedia.org/wiki/Global_dimming has kept temperatures down a bit. So basically from the early 1970s through the 1990s in the United States, the amount of 'global dimming' type air pollution decreased tremendously, because of a number of factors, including all sorts of clean air mandates and regulation.

kossTKR 5y ago
The dimming factor seems a bit frightening. I don't know how sound the theory is, but i remember listening to the climate podcast Radio Ecoshock where a scientist talked about aerosols in general and that there could be a negative feedback loop that would disappear if we stop polluting heavily, ironically then raising temperatures significantly - this could mean that an extremely agressive decrease towards 2030 could increase temperature weirdly enough. The effect was seen over the US after 9/11 as far as i remember. A damned if you do and don't situation.
- pedrocr 5y ago
  > The effect was seen over the US after 9/11 as far as i remember. There was a study that claimed that but there's a second study that claims general cloud cover was a confounding factor not controlled for that explains the actual result: https://en.wikipedia.org/wiki/Global_dimming#Contrails_and_clouds https://en.wikipedia.org/wiki/Global_dimming#Contrails_and_c... Even with a full set of airplanes in the air contrails are a tiny minority of the cloud cover. Seems pretty hard for them to have any visible impact. Aerosols in general are another matter all together. Those are all over the atmosphere and it wouldn't be surprising if they had a large effect as they accumulate.
  - kossTKR 5y ago
    Thanks for the clarification. Always seemed a bit too quick and neat a causation. But as you say the bigger picture is still worrying - and i am not sure that disregarding cloud formation there isn't still certain aerosols hanging around changing things for the worse after heavy air traffic - it's hard to gauge from the Abstracts of papers i just skimmed, but something seems to be happening : https://link.springer.com/article/10.1023/A:1006600107117 https://link.springer.com/article/10.1023/A:1006600107117 https://www.researchgate.net/publication/279155481_Impact_of_Coupled_NOxAerosol_Aircraft_Emissions_on_Ozone_Photochemistry_and_Radiative_Forcing https://www.researchgate.net/publication/279155481_Impact_of...
  - kobieyc 5y ago
    Related to cloud cover: one of the biggest uncertainties in climate models is how to cater for the albedo effect also known as cloud feedback. It goes something like this: 1) The most potent greenhouse gas is water vapor (not CO2) 2) Increased temperatures result in more water vapor (simple evaporation) 3) Water vapor forms clouds 4) Clouds are white and reflect incoming solar radiation A lot of models have to make big assumptions about the strength of this negative feedback loop, and the assumptions have a big impact on the output of the models. https://en.wikipedia.org/wiki/Cloud_feedback https://en.wikipedia.org/wiki/Cloud_feedback
    - Retric 5y ago
      Conversely, further increased evaporation > even more water vapor > increased precipitation > fewer clouds. Also, clouds reflect sunlight > less warming, but clouds reflect IR > less cooling. It’s a really complex interaction that doesn’t have a clear global maximum as each region has a slightly different impact and they all interact. Atmosphere dust is one of the biggest drivers here and dust alone involves a lot of non linear feedback loops.
      - mcswell 5y ago
        "further increased evaporation > even more water vapor > increased precipitation > fewer clouds." No. You can't get fewer clouds from more water vapor, that's not how equilibria work.
        Retric 5y ago
        Clouds are dependent upon the particulate matter in the air, which is reduced by precipitation. You still get cloud formation without that, but the air ends up very supersaturated. https://en.wikipedia.org/wiki/Cloud_condensation_nuclei https://en.wikipedia.org/wiki/Cloud_condensation_nuclei
- ip26 5y ago
  Well, we can always spray sulphur aerosols in the stratosphere without all the CO2 and acid rain.

kossTKR 5y ago

The dimming factor seems a bit frightening. I don't know how sound the theory is, but i remember listening to the climate podcast Radio Ecoshock where a scientist talked about aerosols in general and that there could be a negative feedback loop that would disappear if we stop polluting heavily, ironically then raising temperatures significantly - this could mean that an extremely agressive decrease towards 2030 could increase temperature weirdly enough. The effect was seen over the US after 9/11 as far as i remember. A damned if you do and don't situation.

pedrocr 5y ago
> The effect was seen over the US after 9/11 as far as i remember. There was a study that claimed that but there's a second study that claims general cloud cover was a confounding factor not controlled for that explains the actual result: https://en.wikipedia.org/wiki/Global_dimming#Contrails_and_clouds https://en.wikipedia.org/wiki/Global_dimming#Contrails_and_c... Even with a full set of airplanes in the air contrails are a tiny minority of the cloud cover. Seems pretty hard for them to have any visible impact. Aerosols in general are another matter all together. Those are all over the atmosphere and it wouldn't be surprising if they had a large effect as they accumulate.
- kossTKR 5y ago
  Thanks for the clarification. Always seemed a bit too quick and neat a causation. But as you say the bigger picture is still worrying - and i am not sure that disregarding cloud formation there isn't still certain aerosols hanging around changing things for the worse after heavy air traffic - it's hard to gauge from the Abstracts of papers i just skimmed, but something seems to be happening : https://link.springer.com/article/10.1023/A:1006600107117 https://link.springer.com/article/10.1023/A:1006600107117 https://www.researchgate.net/publication/279155481_Impact_of_Coupled_NOxAerosol_Aircraft_Emissions_on_Ozone_Photochemistry_and_Radiative_Forcing https://www.researchgate.net/publication/279155481_Impact_of...
- kobieyc 5y ago
  Related to cloud cover: one of the biggest uncertainties in climate models is how to cater for the albedo effect also known as cloud feedback. It goes something like this: 1) The most potent greenhouse gas is water vapor (not CO2) 2) Increased temperatures result in more water vapor (simple evaporation) 3) Water vapor forms clouds 4) Clouds are white and reflect incoming solar radiation A lot of models have to make big assumptions about the strength of this negative feedback loop, and the assumptions have a big impact on the output of the models. https://en.wikipedia.org/wiki/Cloud_feedback https://en.wikipedia.org/wiki/Cloud_feedback
  - Retric 5y ago
    Conversely, further increased evaporation > even more water vapor > increased precipitation > fewer clouds. Also, clouds reflect sunlight > less warming, but clouds reflect IR > less cooling. It’s a really complex interaction that doesn’t have a clear global maximum as each region has a slightly different impact and they all interact. Atmosphere dust is one of the biggest drivers here and dust alone involves a lot of non linear feedback loops.
    - mcswell 5y ago
      "further increased evaporation > even more water vapor > increased precipitation > fewer clouds." No. You can't get fewer clouds from more water vapor, that's not how equilibria work.
      - Retric 5y ago
        Clouds are dependent upon the particulate matter in the air, which is reduced by precipitation. You still get cloud formation without that, but the air ends up very supersaturated. https://en.wikipedia.org/wiki/Cloud_condensation_nuclei https://en.wikipedia.org/wiki/Cloud_condensation_nuclei
ip26 5y ago
Well, we can always spray sulphur aerosols in the stratosphere without all the CO2 and acid rain.