I'm struggling a little with what seems to be a mismatch between analyses that correlate aerosol emissions (particularly in North America and Western Europe) with large-scale cooling of oceans in the 1980s (particularly the North Atlantic), and other analyses that talk of fairly limited flying times for (sulphate) aerosols.
Linked to that is the question of what concentrations are needed to have a cooling effect.
If I look at earth.nullschool.net (the SO2 and SO4 visualisations), the concentrations reduce dramatically the moment you get more than 100 km away from point sources, with occasionally a plume stretching up to about 500 km from the point source (sometimes volcanoes, but also heavy industry, powerplants or densely populated cities).
If I look at SST anomaly patterns within about 500 km from heavily polluting areas (for example the East Coast of the US, or the Chinese coastline, or the Indian subcontinent) and correlate these with country-specific emission data, I cannot find a pattern. Even when I compare areas that are upstream along ocean currents (for example around Florida in the case of the US East Coast) with downstream areas nearest to the polluted areas, or when I compare areas that are outside strong currents, I cannot find a pattern that matches.
Can anyone enlighten me?
Kind regards,
Timmo Gaasbeek
Small correction
It is officially called life-time when googling for it
There are two aspects
1. Aerosol is produced over land
2. Weather is transported in the sense that the cooling persists over a larger area downwind because local temperatures, for instance in the Netherlands, are governed by where the air is coming from not so much the local surface temperature:
Winter: winds from the east = cold; from the west, like this year, = warm
Summer: winds from the south: hot
An informative publication is that of Geert Jan van Oldenborgh een wereldexpert op gebied van klimaatverandering
https://www.knmi.nl/kennis-en-datacentrum/publicatie/western-europe-is-warming-much-faster-than-expected
en samenvatting van zijn Nature artikel:
Europe witnessed a strong surface warming during the last decades that reached more than twice the global warming rate. The brightening due to a decrease of aerosol burden and cloudiness in the lower troposphere is suspected to have contributed to this additional warming, but this link has not been quantified so far. From a statistical analysis of a large multi-decadal set of surface horizontal visibility data, we show that phenomena leading to low visibility such as fog, mist or haze strongly declined during the last three decades. Using an original statistical methodology linking local visibility changes with temperature changes, we show that this decline could have contributed on average to about 20% of recent daytime warming, and to more than 50% in parts of Eastern Europe.
Harry ten Brink, thanks for your response. I understand that you'd expect downwind cooling effects from high-emission areas. However, how far does the cooling effect of wind go? And is there a difference between land temperatures and SST responses to cold/warm air brought in from elsewhere? An interesting example then would be to look at the Indian Ocean on both sides of India, where the monsoon winds blow the Asian Brown Cloud alternately eastward and westward offshore, and where you might see changes in interseasonal SST gradients.
For the US East Coast, the strange thing is that the SST trend near the shore does not follow the US emission trends at all. Is it possible that the winds carry cool air away from the shore without cooling the near-shore sea areas?
Interesting that you mention fog, mist and haze declining. I'm based in Rwanda, and here both people's anecdotal observations and the CRU cloud fraction data seem to indicate a similar reduction, but here it is seen by most people whom i speak to as caused by warming, not the other way around. It may actually be more complicated than that. We've had an 1.4 degree warming between 1970 and 2006 (Kigali airport meteo station), but since then there has been a cooling trend. The warming accellerated after the 1994 genocide, when large areas of forest east (upwind) of Kigali were cleared for settlement and agriculture. This change in vegetation may also contributed to changes in fogginess. There was not much urban sprawl upwind of the airport, so the urban heat island theory does not explain the warming. There have been substantial efforts at reforestation in the country, and one hypothesis is that the growing trees contribute to cooling. Interesting puzzle to investigate.
Kind regards,
Timmo
Timmo Gaasbeek
Calculations of how the aerosol effect works in day-by-day forecasts or retrospective is not done because it is too detailed. The effects of local "forcings", aerosol but also tropospheric ozone are only accounted for in highly simplified climate models in which the formation and removal of aerosol and ozone is calculated with a chemical model and their effect is calculated with a simplified radiation model. The results are then used to make an estimate of the regional effect and actually for the effect on the global temperature as an average number which is the most reliable number for the change at the moment
I like to mention that aerosol is only effective when there are no clouds because then the radiation of the sun is already reflected by the clouds. This means that the effect is thus dependent on time (there is no aerosol-effect at night)
However there is an other indirect effect via clouds. Aerosol particles serve as the nucleus on which cloud droplets form and in this way they are also cooling, but only during daytime when clouds reflect solar radiation. This aerosol-cloud interaction effect is the most uncertain of the "forcings"
Fig. SPM5 Summary for PolicyMakers IPCC
https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_SPM_FINAL.pdf
Row: Cloud adjustments due to (manmade; sic) aerosols
Full chapter:
https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter07_FINAL-1.pdf
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