The issue has to do with the same information used by deniers to show that we’re entering an ice age: ice core data. That shows that global warming is part of a natural cooling and warming cycle that takes place every 100,000 years or so, with surface temperature anomaly and CO2 ppm tracking each other.
The problem is that during this cycle CO2 ppm never exceeded 300, and it’s now 400. Can that have an effect on climate?
The NAS, which is considered the “gold standard” of science review in the US, studied dozens of reports, and concluded that CO2 acts as both a forcing and feedback factor, and the former might be now kicking in.
Deniers wanted and funded an independent study, BEST, and came up with the same conclusion. So did dozens of science organizations worldwide.
Also important is the idea of a forcing factor. That implies that CO2 is not the main cause of warming. Rather, it affects other causes of warming. Because, as deniers even admit, the science is complex, then there is a possibility that an increase in CO2 that’s above what happens in natural cycles may lead to positive feedback loops kicking in which increase warming. So far, over 50 of them have been detected the last two decades.
Finally, what’s also worrisome are negative feedback, or those that absorb warming. That may be good news unless one realizes the cost of such. In this case, we might be underestimating the problem because we’ve been looking at surface temperature. What is the cost of heat being absorbed by oceans?