@s@d -
Last time the average world temperature was -5C lower than it is now was during the last ice age. The local climate in Chicago, which was covered by vast amounts of ice at that time, was ... rather different in the -5C world than it is now---more like the interior of Greenland buried under a kilometer of ice if you want a modern equivalent. Certainly not fit for gardening (permaculture or not) or running major trading hub/city; or solvable by buying a better snowsuit, cranking the thermostat up a bit or weatherstripping the windows
The point here is that a shift of a few degrees is a much bigger deal that most laymen apparently appreciate.
The ocean currents were definitely shifted. Huge polar caps stretching down to 45-55deg latitude would be somewhat of a show stopper. Several places that used to be land are now covered, e.g. Doggerland mentioned above. A map of the world would look rather strange/unfamiliar in certain places.
You may recognize the outline of what would become Britain over the next few thousand years as oceans rose. Continental Europe is a bit harder to make out.
Almost all post-neolithic civilization (7+ billion people, the location of major human centers, and the limited/handful of food crops humans rely on (wheat, potato, squash, etc.) developed on a world map that looks very close to what we see today. The Holocene climate has been very stable in the past 10000 years until things started changing/accelerating about ~100 years ago.
Maybe the reason civilized humans have such a hard time appreciating that "climate change" really means "changing climates" and not just reprogramming the HVAC thermostat by a few degrees over the next few hundred years is because it's NEVER happened to civilized humans before. There's zero cultural memory when it comes to climate change. We're only culturally aware or familiar with the one we've always been in.
By "civilized" I mean cultures which grow crops and have permanent cities.
As for the aerosol thing ... we're mainly talking sulfates and nitrates. This is well-understood by now and fully included in the complex/integrated models. Now, we already inject these particles into the atmosphere via car exhaust, coal burning, etc. near the surface and they reflect incoming sunlight which provides a cooling effect. If you ever noticed a white haze over a major city, that's what we're talking about. To you it might be better known as smog ... but as far as the climate is concerned, it keep things a bit cooler than they would otherwise be. The combined cooling effect from these emissions corresponds to removing about 25 years of CO2 emissions ... so kinda like taking a loan in the bank. Unlike CO2 which stays up for thousands of years, aerosols wash out/break down quickly. Therefore, if we all transitioned to electric cars, PV, ... it would be the radiative forcing equivalent of adding in an additional 25 years worth of CO2 emissions in short order.
Regardless, the geoengineering idea is to increase sulfate emissions but put them in the stratosphere instead where they stay up for about 10 years (instead of 1-2). This will have a cooling effect, w/o the associated smog problems down on the ground (e.g. acid rain, asthma), so that's the proposed plan.
Note, that all these fixes to solve the radiative forcing problem (including giant mirrors in space) does nothing to solve the chemistry problem, i.e. ocean acidity.
