The concentration of greenhouse gases in the atmosphere.

Are there other complications?

Yes. A whole series of other feedbacks will influence the concentration of greenhouse gases in the atmosphere. Not all the CO2 that we put into the atmosphere stays there. Some is absorbed by vegetation on land, mostly forests, while more is taken up by the oceans. 

Change these uptake rates and the rate of buildup of the gas in the atmosphere from human activities will also change--speeding up or slowing down global warming. One way this uptake might be reduced is if we chop down all the tropical forests. Another could be the impact of warming on ocean currents, particularly the global "conveyor belt" that begins in the North Atlantic. 

Here the formation of ice leaves the remaining water increasingly saline, and so more dense. As a result it descends to the ocean floor, where it begins a long journey through the oceans that lasts an estimated thousand years. This water carries dissolved CO2 with it on its long journey. 

Some oceanographers believe that as warming takes hold and ice formation is reduced, these currents could slow down or carry less water, which could mean less CO2 is removed from the atmosphere. If they are right, global warming could happen faster than predicted.