Perhaps for a reason similar to the notion that a .01% change in CO2 concentrations can lead to a 20' change in sea level.
The bold is either repeating blatant misinformation or not knowing math. CO2 content in the atmosphere has increased from under 300 ppm in 1960 (at which point a significant quantity was already anthropogenic) to 399 ppm as of last month. That's 33%, not 0.01%. The rate of increase has increased by 217% since 1950, and all of the increase is verifiably anthropogenic.
We know it's anthropogenic because our primary activities that produce it (that is, burning fossil fuels) produce a different and identifiable distribution of carbon isotopes than natural processes (including respiration and burning non-fossil fuels like wood). It's not just a matter of "There's more carbon, where could it have come from?" it's a matter of, "There's more carbon, let's see what kind it is so we know where it came from."
I want that to bear emphasis: The research is in agreement. 97.5% of climate scientists agree that global warming is anthropogenic. And what truthers omit is that of the other 2.5% (which is already statistically insignificant), the majority agree that global warming is caused by
anthropogenic carbon, and are playing a semantic game that technically the heat itself is not anthropogenic. The actual remaining dissent, primarily among retired and discredited ex-climatologists (there are a total of three known climatologists in the english speaking who will actually put their name on dissenting opinions and not try to distance themselves from it afterwards. Or rather, there were three - one of them turned out to have a fake degree to begin with), is a rounding error.
That almost noexistent disagreement is literally linguistic pedantry. This is you walking into a room of climatologists and saying, "Wow, it's so hot out there I'm literally sweating my eyes out." 97 of them say, "Oh I know, it's awful," and the other three, "Really? LITERALLY? I think you mean FIGURATIVELY."
The "thousands of scientists" who signed that petition that truthers liked so much? Only a few of them were climatologists, and when contacted all denied having signed it. After trimming off Mickey Mouse and the cast of M*A*S*H*, the few hundred other actual scientists who signed it were a mix of aging physicists, and aging physicists making bold proclamations and believing they can jump into other sciences and outsmart the professionals without new education is... Well, it's something of an
old joke in science. Of course, many of them denied signing it to begin with. Over 90% of
all scientists agree with the 97.5% of climate scientists (most don't appreciate the pedantry of the next 2%), but outside of their field their opinions matter for nothing, as do the other 10% (and I'm not even going to try to find figures on how many of them are also pedants). The cross-training in college stops early in the undergrad level, meaning a physics doctorate has no more expertise of the field of climatology than he does in neurosurgery.
Meanwhile, the CO2 in the surface of the ocean (a bigger deal in the short term than atmospheric) has more than doubled in the same amount of time, and we can look at the isotopes to know where that came from, too. CO2 in water is a nasty thing for several reasons, the biggest of which is that it binds into CaCO3, which reduces pH. pH changes are bad for the majority of aquatic organisms.