What about CO2?
In the late 1990s, this was the evidence suggesting that carbon emissions caused global warming:
Carbon dioxide is a greenhouse gas, proved in a laboratory a century ago.
Global warming has been occurring for a century and concentrations of atmospheric carbon have been rising for a century. Correlation is not causation, but in a rough sense it looked like a fit.
Ice core data, starting with the first cores from Vostok in 1985, allowed us to measure temperature and atmospheric carbon going back hundreds of thousands of years, through several dramatic global warming and cooling events. To the temporal resolution then available (data points more than a thousand years apart), atmospheric carbon and temperature moved in lockstep: they rose and fell together. Talk about a smoking gun!
There were no other credible causes of global warming.
But starting in about 2000, the last three of the four pieces of evidence above fell away. Using the same point numbers as above:
Better data shows that from 1940 to 1975 the earth cooled while atmospheric carbon increased. That 35 year non-correlation might eventually be explained by global dimming, only discovered in about 2003.
The temporal resolution of the ice core data improved. By 2004 we knew that in past warming events, the temperature increases generally started about 800 years before the rises in atmospheric carbon. Causality does not run in the direction we had assumed in 1999 — it runs the opposite way!