Your challenge, if you chose to except it, is to make it so that the roles of the North and the South are reversed. The North would have to be pro-slavery, while the South are Anti-slavery. How could this have come about?
Assuming you mean north and south of the US...
At the start of hostilities during the American Revolution, Loyalists in the northern colonies flee to the Southern colonies, away from the radicalism of Boston. The British, realizing their newfound support in the southern colonies, focus the war plan there.
The eventual peace settlement frees New England, NY, NJ, PA, DE, MD and VA from the British, in exchange for the colonies of NC, SC and GA willingly remaining in the British Empire as the unified colony "Confederation of British Atlantic America" (CBAA). The unifying ceremony took place at Charlotte, NC, forever to be nicknamed the Birthplace of Confederation. The British Empire outlawed slavery in 1833, over the objections of the CBAA. The British threatened a complete war if they did not agree to abolition. Faced with a massive military threat, the south decided abolition was the safest route. They simply established a sharecropping and peonage system in it's place, but were technically against slavery. (In the 1980's, rock and roll groups would hold concerts to raise money for the fight against Aparthood in the Atlantic America Confederation. Aparthood would be abolished with the moral leadership of Bishop Daniel Berrigan in 1990.)
Meanwhile, slavery had been in decline in the northern colonies for decades, and dwindled to a few scattered slaves here and there. The Constitutional Convention developed a plan of gradual abolition in which all persons currently in slavery would remain slaves and those born to current slaves would remain slaves until the age of majority. Virginia was not happy at being outvoted, but what could they do?
In 1787, the midnight before the day the US Constitution went into effect, a baby boy (Jubilee Johnson) was born in Dover, DE. He would live to be 97 years old, a slave until 1884. He had a son at the age of 96, the result of a young bride sharing his war pension despite his advanced age. Their daughter (Evangeline Johnson) was born in 1884, and legally remained a slave until her age of majority in 1902. She was the last slave in the US.
Religious groups such as Quakers, Universalists and Unitarians became increasingly concerned with the welfare of slaves in the US in the early and mid 1800s. As the slaves aged, their masters were less and less willing to provide them with adequate food, shelter, clothing and healthcare.
Harriet Beecher Stowe, for instance, wrote a popular novel, "Uncle Tom's Cabin", in which the cruel slave owner, Simon Legree, banished from the plantation any slave too old or infirm to work. The freed slaves hobbled through the countryside until kindly old Uncle Tom, a freed slave himself, took them in and allowed them to live in his cabin. The community in the novel supported itself by selling traditional handicrafts. The novel swayed public opinion in the north in favor of requiring slaveowners to care for their slaves from cradle to grave, with minimum levels of expenditure on care being set by the federal government, regardless of a slave's ability to work. With popular opinion swung strongly against forced emancipation, freeing slaves early was made illegal in 1850.
In 1862 President Lincoln, faced with a labor shortage during the War of Western Rebellion, signed the Confiscation Proclamation which confiscated, as eminent domain for the war effort, the very few, and mostly elderly, slaves remaining in the US. They became federal property and generally were left to live lives as freedmen after the war, though still technically slaves and government property. With help from liberal charities and federal war pensions, retired slaves were cared for in small communities. Most elderly slaves simply lived with their now grown and emancipated children. The children of these families were educated at what are now some of the US's most prestigious historically black colleges, like Wilberforce, Lincoln, Tufts and Bates.
Those fertile plains would still be cold though because most of the warm waters from the Atlantic would still be absorbed in the south and would not reach the north.