P.S. Reelya: Some of the countries in Europe, post WWI, did not repay their debts to the USA, as I recall.
Yeah.Loud Whispers: Only if they couldn't borrow the money from another nation.
Anywho, to answer the OP:
Over the years leading up to 2060, climate change has been accelerating, along with severe weather events, droughts, and so forth. War, famine, and civil war have all been rising over the previous decades.
In 2026, US President [REDACTED] announces the launch a solar shade. He declares that this will alleviate the heating from global warming.
By 2028, the planet is dramatically cooler thanks to the solar shade, but also considerably darker. The mass droughts which have been plaguing much of the world for the past two decades were continuing, and the decrease in sunlight exacerbates the situation, further cutting crop growth rates, with the heat decrease also contributing.
In 2033, becoming somewhat panicked about the dwindling arable farmland and water sources, US President [REDACTED BUT DIFFERENT] attempted an invasion of Canada, after most of Europe essentially self-destructed. The occupation was successful, temporarily, until the Brits launched a sort of stealth nuke which was undetected by the US's anti-missile defenses, and which exploded in the upper atmosphere above the US, setting off an EMP which knocked out the entire US infrastructure and advanced military, drone fleet, and electronic firing guns (which had largely replaced firing pin weapons in the US military, but not among private citizens or the Canadian military). The Canadians then pulled their actual military tanks and equipment and so forth out of hidden faraday-caged bunkers to wipe out the US military in their country. (They also had earlier mandated that all factories be shielded from EMP, preventing them from being knocked out, etc)
In 2034, the solar shade is de-activated by a robotic spacecraft belonging to a private space corporation (the only one remaining after the space corp mergers in 2016-2017, which coincided with the beginning of the 2016-2019 global nation-state default crisis and financial collapse (which is to say that it started with Japan defaulting and restructuring its debt in January, followed by a domino effect of 16 full or partial defaults of nations over the next few months, then resulting in Rand Paul's election as President of the USA (despite that the US had not defaulted)).
After trying to re-invade Canada and failing again, the US essentially collapsed and a civil war over the remains of the country broke out, and eventually petered out inconclusively. The United States, as of 2040, is a lawless wasteland where one is likely to be shot by steampunk defense bots for wandering into the few farms that remain, or near water or oil/gas/oil shale/oil sands 'wells'. There isn't much arable land left, and people start dying off fairly quickly as a result of the US's massive size and total lack of any way of restoring its capabilities (or allies, at this point - it has alienated everyone).
Elsewhere... By 2050, Africa has been united as a result of constant warfare since 2029, but still has only a small amount of arable land and water. (In 2045, there were only three empires remaining in Africa)
Russia broke out into a 10 year civil war after Vladimir Putin's (yes, that wasn't redacted) death in 2041 at the age of 81. China invaded Russia in 2051, and by 2055, had conquered Siberia and Eastern Russia (planning for the changing climate, Siberia may eventually be ideal territory) and installed a puppet leader in the rest of Russia.
The Middle Eastern Alliance is formed in 2055, as the countries in the region, including Pakistan and India, agree to look past their differences (a massive step for Israel, Palestine, and Iran) and ally to work together in the changing world to preserve and protect each other and ensure the survival of all members to the greatest extent possible. Politics has been almost completely rewritten across much of the world over the past few decades as a result of the clear existential danger that the climate crisis posed to all humankind. (Although much of humanity devolved into warfare, some did rise above. Paradoxically, it was those who were least expected to.)
In 2060, orbital solar power is provided as a pilot program to all the countries in the middle east, who are suffering considerably from climate change and being forced to acquire water by drilling (with estimates that they will run out within a few years). This effectively free, carbon neutral, power source (rapidly increasing in power gathered and transmitted as time passes) provides a means for desalination and removal of carbon from ocean water, allowing the construction of new desalination plants on the coast, which begin to provide water to the entire middle east. Attempts are made to sequester the resulting carbon, with moderate success.
For comparison, China is getting most of its power from nuclear plants, Russia from burning various fossil fuels without bothering to deal with the GHG emissions, and Canada with earth-based clean energy and natural gas.
I omitted Europe (among other things). Haven't decided quite what should happen to them. Perhaps some kind of animal-based contagion which can infect humans but is only infectious from that specific (mainland, one hopes) europe-based animal species to humans and not from human to human, limiting its spread to europe. I'd need to find an animal which would be appropriately range-restricted to Europe (and probably not Russia or Britain) in the years in question (2020-2030), and I am definitely unfamiliar with what wildlife is in Europe and what ranges it has.