When did humans begin to change the climate?

Answer 1

Local climate was affected by agricultural practices as early as ten thousand years ago. Goats, for example, at all the saplings in certain areas of the Middle East and around the Mediterranean, resulting in the gradual disappearance of forests. This loss altered earth's albedo, resulting in local climate affects.

Humans did not begin liberating fossil CO2 in earnest until the 1700s. Before the industrial revolution, CO2 changed naturally about 5 ppm per thousand years. However, from 1700 to 1900 it rose about 5 ppm per century, 10 times faster than any natural global variation. From 1900 to 1950 it rose another 10 ppm, or 5 ppm per quarter century. That is 40 times faster than the natural rate of change.

From 1950 to 1962 it rose another 10 ppm, accelerating the rate of change about 80 times faster than the natural rate. From 1996 to 2001 it rose from 360 ppm to 370 ppm. Just 5 years to equal what normally would occur in two thousand. And we have kept this rate up (whereas the natural change was a gradual rise or fall of about 10 ppm), decade after decade. The measured temperature increases have been associated with our consumption of fossil fuels by half a dozen robust, independent climate models.

Throughout human existence CO2 never exceeded 300 ppm. Most of the last 800,000 years it has varied only from 250 to 290 ppm. Today it is near 400 ppm, and before 2050 we will exceed 500 ppm. The climate effect of that activity is expected to persist for centuries to come.