"Additional heat source" refers to an extra object or device that produces heat in addition to the existing heat source. It provides more warmth or heat to a specific area or room. Full definition
In other words, the team needs to find an additional heat source. (scientificamerican.com)
The oceanic heat store should be regarded as an additional heat source that adds or subtracts the effect of earlier solar irradiance (or lack of it) to or from the present day effect of current solar irradiance. (newclimatemodel.com)
The oceanic heat store should be regarded as an additional heat source that adds or subtracts the effect of earlier solar irradiance (or lack of it) to or from the present day effect of current solar irradiance over variable periods of time. (newclimatemodel.com)