Its the sort of sloppy journalism the denialists love to dispute so they can discredit the people the journalists are writing about.

The journalist said

"the trend is going up and at faster and faster rates."

which is not true. The trend is level. What he meant is the levels are going up, not the trend. Mr Butler said

"The 400 is a reminder that our emissions are not only continuing, but they're accelerating; that's a scary thing," Butler said Saturday. "We're stuck. We're going to keep going up."

"Accelerating" is a misleading term. The emissions are accelerating, that is, the CO2 concentration increases every year, and overwhelming the earth's ability to remove it, which the chart clearly shows. The RATE at which they are increasing seems to have been level for a few years, as you point out.

Put in mathematical terms, the function is increasing, the first derivative of the function is positive, and the second derivative is zero.

Think of it like distance, velocity, acceleration. Distance is the height of the curve above the x axis. velocity is the slope of the curve (distance/time). Acceleration is the change in slope (velocity/time).

In this case, the distance is increasing, the velocity is constant, and the acceleration is zero. The fact you're not speeding up doesn't
necessarily mean you're not going too fast.