1.The density of water is a function of temperature and pressure. Water ice has at least 10 different solid phases in addition to liquid, super critical and vapor phases. At a molecular level, there is not an abrupt border between phases. The difference between liquid water at 0 C and ice at 0 C is the persistence of long range order. In the liquid, molecules form small clusters with the same ordering as ice, but the clusters break apart and reform continually. A small cluster of molecules has a large surface to volume ratio and hence a large surface energy. In the solid phase (ice) the clusters are large enough to be stable against thermal fluctuations of order k(sub)B T where k(sub)B is the Boltzman constant and T is the temperature in Kelvin. Water freezes by a nucleation and growth process in which the larger, more stable clusters grow at the expense of the smaller clusters. The effect of applying pressure is to slightly reduce the intermolecular distances which favors cluster formation. The interesting bit for the present question is that the density depends on the cluster size. The density of water is a population average over the distribution of clusters at any given temperature. An interesting side note is that molecules tend to order beside a surface. That means that the water in your cells has a slightly different density than the water in say a glass at the same temperature. Oceans are large and contain water at many different temperatures. The volume of the ocean will depend upon the fraction of the total at each temperature. From 0 to 4 C the density increases and above 4 C the density of water decreases. One water is above 4 C, any increase in temperature will increase the volume of the oceans even if the mass remains the same.
The comments above are mine. You may also want to consult.
http://www.lsbu.ac.uk/water/phase.html
The authors have spent more time than I explaining the details.
2. Water from ice melting over land flows into the oceans and increases the total mass, thereby raising the sea level. Although the mass of ice in the Greenland and Antarctic ice sheets is huge, the enthalpy of ice is also huge. By my calculation, it would take more than 10,000 years for these ice sheets to warm up to 0 C and then melt. Most of the melt would occur in the second half of the process. Sea levels are rising due to global warming, but the process is slow (3 mm/yr) and people will have plenty of time to adapt. In my humble opinion, the most serious effect of global warming from a human perspective is the effect on food crops.
