What was done
The authors examined the potential for vegetation feedbacks on climate via a "new, fully coupled, Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) - Integrated Biosphere Simulator (IBIS) climate-vegetation model with boundary conditions appropriate for 21,000 years before present."

What was learned
Under the colder and drier conditions of the Last Glacial Maximum compared to the present, grasslands and tundra largely replace present-day forests in temperate and boreal latitudes.  In addition, the physiological effects of the lower atmospheric CO2 concentration at the time of the Last Glacial Maximum - a concentration of approximately 200 ppm - "cause a reduction in tropical and subtropical forest cover (compared to present) in favor of C4 grasslands."

These vegetation changes produce two opposing climate feedbacks in different parts of the planet.  In middle and high latitudes, the reduced tree cover leads to further cooling, as reduced tree cover exposes more snow and raises the surface albedo.  In the tropics and subtropics, however, the sparser vegetation cover weakens the hydrological cycle and leads to warmer and drier conditions.

What it means
If we reverse the direction of change explored in this study and move from the colder and lower atmospheric CO2 concentrations of the Last Glacial Maximum to the warmer and higher CO2 concentrations of the present and beyond, what do we see?  In all parts of the planet, including high, middle and low latitudes, we find a proliferation of trees [see also Trees (Range Expansions) in our Subject Index].  In the high and middle latitudes, this phenomenon has a warming effect on climate; while in the lower latitudes it has a cooling effect.  Together, these two trends tend to reduce the latitudinal temperature gradient, which tends to weaken the potential for stormy weather and the many weather extremes that many people worry will increase as a result of rising temperatures and CO2 concentrations.  The results of this study thus suggest that most of that worry is wildly misplaced.