this is definitely not one of my strong points so feel free to correct me as necessary.
however the way i understand it is ...
in 1860
the northern per capita GDP was only a couple percentage points behind the south. however there were almost 3 times as many whites in the north than the south and so the northern total production far outweighed that of the south.
cotton represented about 80% of US exports but if that represented the GDP then the north built it's war machine with less than the remaining 20 %.
the GDP represents the value of ALL goods and services, whether exported or not.
the south exported almost all of it's GDP. the north had a much larger domestic market, including southern states, and did not export to the extent that the south did.
the south preferred to export to britain rather than fuel northern industry which they saw as a threat to their political power. if they had sold the bulk of their cotton to northern industry that 80% figure would be much lower.
the northern GDP remained largely at home and so to equate exports alone with the GDP in invalid.
the bottom line is that the north out produced the south by about 10 to 1 and the per capita GDP of the north continued to rise after the war while the south declined.
does this make sense ?
No, cotton represented 80% of U.S. exports, but the 80% of those cotton exports equated to 5-6% of the entire U.S. GDP. The south held about 5-6% of the GDP from cotton and maybe 1-2% more from sugar and rice, whereas the north's economy comprised of the rest of the GDP, like 90%. Therefore, the north's GDP was about 90% greater than the south's 8-10% that combined made up the entire U.S. GDP. Total GDP = 100% + 90% from the north + 10% from the south = 100% GDP, so the northern GDP was way ahead of the southern GDP, not behind it. Consequently, the north built its war machine with 90% of the GDP, which only 20% of the GDP would have never done the trick.
You are confusing export percentages with GDP percentages, the 80% export percentages is what made up the south's 6% GDP. 80% cotton export = 5-6% GDP. The U.S. had pretty much a closed economy, which exports accounted for a small percentage of the GDP, and that's why the threat of tariffs on imports always loomed.
Cotton went through boom and bust cycles, so for about 30-40 years prior to the Civil War it went through a boom phase, were it expanded its cotton production and simultaneously expanded slavery. But the increase of cotton production did not increase its GDP because exports were not that significant compared to the Industrial Revolution in the north. The plantation owner made more money but the south's overall contribution to the U.S. GDP was always basically small, perhaps it fluctuated a few point to 6% during the boom decades. Check out the graph below.
Subsequently, the war expanded the north's economy probably 100 fold, whereas it bankrupted the south's economy. Yes, the north's economy increased every decade after the Civil War, conversely the south's economy remained depressed until Roosevelt gave them an astronomical amount of New Deal subsdies.
For the purpose of this thread, all anyone needs to know is that cotton was not remotely indispensable to the U.S. GDP or tax revenue and pretty much expendable. The north could have easily built a nation without cotton exports, and it did prior, during and following the Civil War.