Frame challenge: France wins the Seven Years' War.
No change to climate and geography, but border changes instead.
The Seven Years' War opposed France (with Spain, Austria and Russia) to Britain (with Prussia and Portugal). It concluded in 1763 with France ceding its American territories to England and Spain, and was effectively the end of New France.
A mere decade later, in 1775, the Thirteen Colonies rebel, starting the Revolutionary War, with notable help from the Kingdom of France, in which Britain would have to cede much of the territories it had just gained. From there, the US expanded to the west to form the country we know today.
Instead, the Seven Years' War still ends in 1763, with a French victory that allows Britain to keep its Thirteen Colonies. The rest of British America is split between France and Spain.
Still having reason to rebel, and with indirect support of the Kingdom of France still having reasons to spite the Brits, the United States formally gains independent in 1783.
The three colonies of New France - Louisiana, Acadia and Canada - remain under French control until some time during the First French Revolution, where New France is granted independence. The new country takes the name of the largest colony, Canada, with its capital in Québec.
Over the 19th and early 20th century, Mexico and Canada expand through the rest of the North American continent. Ultimately, the Mexico-Canada border follows a rough line from California to Louisiana, while the United States spans from Maine to Florida.
Due to its sheer side and access to a vast amount of resources, Canada becomes the powerhouse of North America, and forms excellent trade relations with the United States to access it vast Atlantic coastline.
Other suggested changes may include:
Alaska is a Russian or Canadian territory. Hawaii is an independent territory. Various First Nations independent enclaves are peppered throughout the North American continent.
Slavery is abolished in France and its colonies, including Canada, by the French Revolution. The United States follows suits under the economic influence of Canada.
Canada's only official language is French, though still includes sizeable communities of Spanish and English speakers. The United States speak a version of English influenced by Canadian French.