Where did the 97% of scientists figure come from re: climate change?

I don't know enough about climate change to take a personal position on whether we have global warming or not. People keep telling me "97% of scientists say there is global warming that's caused by humans." Where did this 97% figure come from? I was recently told that the past two years have been exceptionally cold.

via International Skeptics Forum https://ift.tt/2FIqVfG