[DoveBrown]

From your link: "These increases in greenhouse gas concentrations and their marked rate of change are largely attributable to human activities since the Industrial Revolution (1800)." and "Data for the past 2000 years show that the atmospheric concentrations of CO2, CH4, and N2O – three important long-lived greenhouse gases – have increased substantially since about 1750."

I don't think your link supports the statement "the weaking started well before the industrial revolution."

[jcoffland]

The link clearly shows that significant rise in CO2 did not occur until after 1900 so it could not have caused weakening ocean currents that began, at the latest estimate in 1850. I do not understand where anyone got the idea that a constantly changing climate is abnormal.

[DoveBrown]

I'm not a climatologist but as I understand it when looking at the climate over the long scale there are cycles as the climate changes that are predictable by natural processes (including effects of the Earths orbit and co2 changes). Then from the 19th century the climate has been moving off the known path as understood by these cycles. The difference has been attributed to humans. The worry is that when you put the cyclic natural of climate change together with the human caused climate change then what you may end up with is a climate which is not conducive to food productive for the human population.