True. The clincher is that the earth is not a uniform system, and not so simple as merely ice and heat. In a closed system, sure. As heat energy is applied to the test tube, the ice inside will melt until the heat energy within the test tube is at equilibrium with the outside. Not so with our planet, otherwise we would all be ice pops. We have a planet leaning on it's axis, rotating, revolving, convection currents in the air and in the seas, and then man adds to the mix with green house gases, flouro-carbons and all manner of environmental contaminants, which alter the delicate balance between these systems. WadadliEmpress raised a good point as far as temperature change. 0 C and 0.1 C is the difference between ice and water.