I've read scientific papers that hypothesize that the earth's average temperature ranged from ~10C to ~ 25C over the past 600 million years. Temperature minima occurred within the past 2 million years, about 150 million years ago, 300 million years ago and 450 million years ago. Is this wrong? If not, why does anyone believe that human activity is causing the earth to warm since it warmed and cooled over wide temperature extremes when no humans existed.
You'll simply need to do quite a bit more reading on the topic.You could start here, on the way to further exploration of the data:https://grist.org/series/skeptics/
The real problem is not average temperature change itself, it is the rate at which the average temperature changes. Historically it has taken 800 - 8000 years for the average global temp (AGT) to change by ~1C. Since the dawn of the Industrial Revolution only 200 years ago, the AGT has risen by nearly 1.5C. So you see, it is not really a matter of human activity causing climate change so much as it is accelerating it. When climate change occurs over several hundreds or thousands of years, species' have time to adapt to it. When the climate changes as rapidly as we're seeing now though, there's not enough time for natural evolutionary processes to compensate and adapt, which tends to lead to mass extinction.
Also of note: a list of economists who support I-732: http://www.env-econ.net/2016/10/dozens-of-economists-around-the-state-who-endorse-the-i-732-carbon-tax.html