Many climate scientists say they see a "fingerprint" in recent temperature increases that they claim is distinctive and makes current temperature increases different from past "natural" temperature increases.
So, to see if we are all as smart as the climate scientists, here are two 51-year periods from the 20th century global temperature record as provided by the Hadley CRUT3. Both are scaled the same (each line on the y-axis is 0.2C, each x-axis division is 5 years) — in fact, both are clips from the exact same image. So, which is the anthropogenic warming and which is the natural?
One clip is from 1895 to 1946 (the"natural" period) and one is from 1957 to present (the supposedly anthropogenic period).
If you have stared at these charts long enough, the el Nino year of 1998 has a distinctive shape that I recognize, but otherwise these graphs look surprisingly similar. If you are still not sure, you can find out which is which here.