Some denialists claim that the IPCC is assuming CO₂ levels in the atmosphere will rise exponentially, whereas observations show a straight line.

• Any smooth-ish curve will look like a straight line if you zoom in on a sufficiently short part of it.  A ship may appear to be steaming in a dead straight line, but it has to follow the curve of the Earth's surface.  The curve is clear over 50 years or more.
  
• The data support the view that the level of CO₂ is accelerating.  In the sixties it was growing at under 1ppmv annually; now it's at 2 to 3ppmv annually.
  
• The IPCC has not assumed atmospheric concentrations will increase exponentially.  They do predict acceleration (faster-than-linear growth), but the "exponential" tag comes from the denialists' interpretation.
  
• Our emissions change according to the economic cycle and technology, so such predictions need to be based on forecasts of those.  Long term, economies have grown exponentially.  Although it's physically impossible for that to go on forever, it could continue for some decades yet if we don't wake up to the need for zero growth.  Exponentially growing emissions do not imply exponentially increasing concentrations in the atmosphere, but they do suggest something faster than linear.
• It is also important to predict how fast the oceans will absorb extra CO₂, and this certainly appears to  be slowing.  As they warm, it is expected to go into reverse.
  

 It has even been claimed that the CO2 concentration has not risen in 160 years!  This seems to be from a misunderstanding of a finding that the fraction of emissions that stays in the atmosphere has not changed in that time.