When did Global warming come into effect?

Answer 1

The Vostok Ice Core data would indicate that events of global warming began over 400,000 years ago. It is most likely that global warming events occurred millions of years before and followed by global cooling that led to the formation of ice.

Data indicates there have been over 75 major global temperature swings in just the last 5,000 years.

Answer:

The last ice age began about 110,000 years ago, and ended 12 to 15 thousand years ago. The maximum ice extent occurred about 22,000 years ago. Therefore, the previous ice age warming began roughly 22,000 years ago. Although the age had lasted a long time, like previous ice ages once it began to thaw a feedback mechanism kicked in making the temperature rise quickly.

So for natural global warming the answer would be 22,000 years ago. That ice age, like previous ages, was caused by orbital variations. Currently earth is closest to the sun in January, making northern hemisphere winters (where most of the land mass resides) slightly warmer. When the dead of winter occurs at aphelion, winters tend to be colder. This is one cycle. There are other, longer natural climate cycles.

The current Milankovitch Cycle (as these orbital cycles are known) indicates earth should have entered a slight gradual cooling phase some centuries ago, culminating with the onset of another ice age in another 50,000 years. Instead, earth's temperature has been climbing rather dramatically, in response to significantly increased greenhouse gas emissions from human activity.

The accompanying temperature chart shows the most significant part of the recent upswing took effect beginning in the 1970s (see links).