I started reading up on environmental issues 25 years ago when I was in science at university, and have had a passing interest in the topic ever since then.
In particular, the global warming topic was visible even back then.
Eventually, I came across the work of geologist Don Easterbrook, who points out, using both the (unassailable) geologic record and recent (decades) of world temperature data (somewhat assailable, I suppose), that global warming is not happening.
Instead, he points to the PDO (Pacific decadal oscillator), which is now on a 30-year cooling cycle, and argues that global cooling is now in effect. I found his work to be most convincing. In particular, he shows a stunning graph of planet temperature built from the geologic record (ice core samples, ocean floor oxygen isotopes), visible at the link below.
After seeing that graph, with the current world temperature on a vanishingly small, slightly downward tilting trajectory that is scarcely thicker than the width of the chart line itself, it was impossible for me to believe in global warming at all (if I ever did).
I could go on about the spectroscopy science that shows that methane (CH4, cow flatulence, garbage dumps, decomposition) and water vapor (H20) are both vastly stronger greenhouse gases than CO2 (and yet are ignored by GW proponents), and that world temperature actually declined over the 30 years from 1945 to 1975, where man-made use of petroleum fuels accelerated the most in history, but Don Easterbrook makes a much better case than me.
Look him up here: www.globalresearch.ca/global-cooling-is-here/10783 and here: http://myweb.wwu.edu/dbunny/