In the 1970's scientists were becoming convinced that the Earth was indeed cooling and we were headed of another ice age. Various schemes were proposed, such as spreading carbon soot or ash on the polar ice caps to decrease albedo and increase the amount of solar energy absorbed buy the Earth. Now that it has been confirmed that solar activity is sliding downward towards a Maunder Minimum in the 2030's and the possibility of a "Little Ice Age" is looming, I'm sure that some of these sorts of schemes will surface again.
There are two issues to address here.
Firstly, climactic change is a natural cycle and part of the long term evolution of the Earth. Vikings were croft farming in Greenland in the 1100's (we know this because some of their farms are being exposed by retreating glaciers), and George Washington could take advantage of the last Little Ice Age and pull artillery across frozen rivers to surprise the British during the Revolutionary War. (Union and Rebel commanders fighting over the same ground a century later had no such advantage in the Civil War). So this is natural, and people were able to adapt with far lower technology and fewer resources than we have.
Second, the "cure" is worse than the disease. Climate is a non-linear, adaptive system where inputs do not have linear outputs. We simply have no way of knowing or predicting the outcomes of geoengineering projects on the scales proposed (either now or in your scenario), it is quite possible that trends that we consider negative would be amplified rather than dampened, or the timescales would be wrong and the inputs of today could show up centuries down the line as oceanic currents changed (for example).
Perhaps strangely, the ideal solution in either case would be to reduce the interdependence of human systems, since they are also nonlinear and adaptive, and individuals changing behaviours in response to increasing/decreasing temperatures could adversely affect tightly coupled systems separated physically and temporally from the events. More loosely coupled systems rather than more centralized systems will be far less brittle and inflexible, and allow people to make appropriate changes based on their own local circumstances.