You talk like +10C is nbd. Are you aware of how much landmass was covered by water during that time?
I'm no expert but that would cause human civilization to take a dramatically different form if not outright extinction.
Also, even a number they chose is plain trolling. Of all numbers, 420 ppm is what they "calculated".
There are regionally used strong currencies like the Euro. Us Euros mostly do our internal trade in, well, Euros, without being bombed or even sanctioned. That is quite a lot of trade.
Aside from the Euro, there isn't much of a competition in the international arena. China wants to control the renminbi tightly, which precludes its wider adoption as a reserve currency. UK is a shadow of its former imperial self, and so is the pound. The Japanese economy is too weak for the yen to be a serious competitor.
And with currencies like the rupee, you will find that few people outside India are willing to trust them. Russians now sell a lot of oil to India for rupees (and note that there does not seem to be any initiative to bomb India for engaging in this trade), but then are basically forced to use those same rupees to buy stuff from India again, and Indian exports aren't a great match for Russian economic needs.
That is how the world economy used to work, before the petrodollar.
Now, Saudis or Qatar or pick your favorite Middle eastern monarch sell oil to US and everyone else in exchange for $USD. Since oil is used in nearly every other product line, either for transport or actual product, now world trade depends on how much $USD you have to buy said oil.
Euro is irrelevant outside of Europe.