We test hypotheses about the unknown sink for carbon by analyzing time series for the unknown carbon sink, carbon emissions, atmospheric concentrations, and surface temperature between 1860 and 1990. During this period, the time series for the unknown carbon sink is determined by annual changes in carbon emissions and summer land surface temperature in the northern hemisphere. The first difference of carbon emissions indicates that an increase in carbon emissions may generate a short run increase in oceanic uptake that is not simulated correctly by models. The temperature effect may have its greatest impact on terrestrial vegetation in the midlatitudes and high latitudes of Eastern North America and Europe. We also test and reject hypotheses that increases in the atmospheric concentration of CO2, nitrogen deposition associated with fossil fuel combustion, or uncertainty about the rate at which forests are cut and/or regrow are responsible for changes in the unknown carbon sink over time.