How would a world power create an artificial winter?

Question

With the setting and criteria provided below, how would a country make an artificial winter that targets the North American Continent?

Setting:

The story that will use this artificial winter is set in the following setting:

• Alternate universe that mirrors our own.
• The year is 2025, and science, technology and world politics continue on the same track they are currently on.
• Besides the different year, the only differences between our universe and this one are that a powerful nation has been planning this artificial winter for quite a while now and has put their plan into effect.

Technological setting:

This whole artificial winter thing has been in the works for quite a while now, so you can assume that a lot of R&D has gone into making this happen. As such, you can incorporate some advances in climate control that are not currently possible today. However, these must meet the following criteria:

• Your proposed advance in technology must not provide a complete black box solution to this problem. IE The Snowy-Blowy-0-Matic Artificial Winter Creator 4000™ hasn't been invented quite yet.
• Should be based in our current understanding of the laws of physics and/or chemistry.

Criteria for the artificial winter:

• It must last at least 6 months,
• Cause enough snowfall to be disruptive to become a national crisis
• It does not need to cover the whole of the North America Continent, but must cover enough to be majorly disruptive to the US and Canada on a national level.
• The plan to make this winter cannot have potentially civilization-ending (or resetting) consequences.
• The artificial winter itself will be the main disruption to North America, not the side effects of making the winter happen. (I'm looking at you Tsar Bomba;)
• The winter should mostly be confined to North America.

A good answer will:

• Be scientifically based (no magic)
• Be plausible
• Explain the side effects of your plan to make the artificial winter happen
  • Note: this does not mean that you also have to explain the effects of a 6 month long winter, but I would love to hear those if you want to go into detail.
• Take no more than 50 years of setup time (IE: cannot have been started before the year 1975)

Edit:

The criteria for the artificial winter conditions have been changed slightly.

Answer 1

Year Without a Summer, sulfur aerosols are your friend, you can even largely restrict the damage to the northern hemisphere too. The Year Without a Summer was caused by a large volcanic eruption in Indonesia that released huge quantities of sulfur into the upper atmosphere. The sulfur mixes with water vapour and forms minute droplets of sulfuric acid aerosol that stay in the atmosphere for a year or two and absorb and/or reflect a noticeable percentage of the incoming insolation sharply lowering temperatures.

Because of the patterns of global air circulation sulfur released into the atmosphere of the northern hemisphere tends to stay there and vice versa so effecting North America won't cause much havoc south of the equator. A north/south split is about the best you can do though so Europe and Asia will also be in the firing line, but at least the country causing the disruption knows what to expect. You might be able to intensify the effect in North America by creating a high localised concentration of shorter stay large droplet aerosols but there will be some spillover. Sulfur aerosols are rather shortlived with even very high concentrations "raining out" of the atmosphere over the course of a couple of years so the fimbulwinter thus created has a definitely limited lifespan once the sulfur stops being supplied to the stratosphere.

To achieve the desired effect you need a lot of sulfur, a lot of heat, oxygen, and a way to deliver the sulfur oxide compounds produced to the upper atmosphere. The second and third parts are really easy and the fourth provision isn't much harder, there are two ways I can see of delivering the sulfur compounds to the upper atmosphere:

• the first is to create them at altitude, use high altitude bombers dumping thousands of specially designed shells that create clouds of sulfuric acid vapour as they detonate in the stratosphere.

• the second is to use enhanced airflow to pump sulfur from the ground into the stratosphere, this would ultimately involve large furnaces fueled by high sulfur coal, with added sulfur, and pumping the resulting exhaust gas into something like a hyperboloid cooling tower. The tower would need to be redesigned for maximum vertical laminar acceleration and minimal gas diffusion to boost the hot, sulfur rich gases into the upper atmosphere.

Personally I favour the bombing-run method, since the equipment can be built away from prying eyes, kept away from prying eyes, and some of it used for other purposes like spy planes until Operation Fimbulwinter is ready to roll.

Finally to address sourcing millions of tonnes of sulfur for the job, you could deliberately mine or buy in cheap high sulfur coal and use it for electricity generation while capturing the sulfur in the waste gas, you even look good internationally since your coal plants are so clean and environmentally friendly. Or you can mine the sulfur from the sea; sure your country doesn't need to use desalination plants and you really don't need to go to the added expense of stripping all that sulfate out of the brine to make the plant work but you can pass it off. Desalination means you can preserve your country's natural watersheds without needing dams for irrigation and drinking water. And the brine is less polluting when it's sulfur free right? Actually if you're going to go to the trouble of breaking down the brine chemically you should go the whole way and extract the rarer commercial metals while you're at it, lithium, manganese, gallium and the like.

Answer 2

What you want kinda already exists... But you can't target just North America.

This is the device:

It's called a Tsar Bomba, and it's the most potent nuke ever. Seriously. Just look at its blast radius:

And this is how it works:

Nuclear winter is the severe and prolonged global climatic cooling effect hypothesized to occur after widespread firestorms following a nuclear war. The hypothesis is based on the fact that such fires can inject soot into the stratosphere, where it can block some direct sunlight from reaching the surface of the Earth.

How to use: blow a lot of these worldwide, especially around the equator. Remember to always have a Geiger counter with you at all times after that.

Oh, and about this:

The plan to make this winter cannot have potentially civilization-ending consequences.

I am sure civilization would thrive. Our ancestors managed to live without electricity and clean water before, we sure can too.

Answer 3

Block the sun with satellites with big panels

You can put a network of satellites orbiting Earth covering all the planet (more or less like the GPS satellites). Of course you will need more satellites to ensure a proper sun blocking. And put big movable panels in the satellites, so you can make them block the sunlight at will. With that, you can have the winter where you need it and for the amount of time you want it.
This image is how the GPS satellite network is represented:

Answer 4

In the comments, you mention:

"It must last at least 6 months". Presumably spring and summer? – RonJohn 37 mins ago

@RonJohn actually there I’m not too picky, any contiguous 6 months of the year should be enough. – Jack 24 mins ago

So, an extended (starts in October, ends in May) and harsh winter would work just as well? – RonJohn 14 mins ago

@RonJohn correct – Jack 2 mins ago

The blocking high pressure region over the North Pacific needs to be larger and come earlier, so as to push the Polar Jet Stream further north over Alaska earlier in the year and push the Variable Pacific Jet Stream further south.

Warm air from the Gulf of Mexico needs to be prevented from coming north.

This allows the Polar Jet Stream to get much colder and then plunge deeper into the continent.