A thermodynamic approach is proposed to measure the physical value of the world's renewable water resources and Antarctica and Greenland ice sheets. This involves the Exergy Replacement Cost, defined as the energy required by the best available technologies to return a resource to the same conditions as it was delivered by the ecosystem(s). The replacement cost of the world's renewable water resources varies between a minimum of 3,592 Mtoe/year and a reasonable value of 53,304 Mtoe/year. Thus, each year we would need between 0.4 to 6.4 times more fossil energy than consumed in 1997, to replace only part of the functions of the hydrological cycle. In the case of ice sheets, its minimum physical replacement cost is 3.840E+08 Mtoe, being the actual exergy replacement cost near of 20 times higher, that is 7.210E+09 Mtoe. If all existing ice sheets melted, the required exergy for recover them in the same conditions that are now in nature would be around 9,000 times greater than the total amount of fossil fuels reserves in the Earth. Accordingly, Earth's ice sheets correspond to our most important global exergy reserve.