Okay... the Fourth of July is generally considered one of the hottest days of the year in New York City (probably because everyone is out and about, perception is reality, so the legend gets imprinted over the fact).
Thus, I got to wondering one day, what if ice was denser than water? That is, what if ice SANK when if froze, not float?
(And we'll ignore the other FOURTEEN versions of crystalline water, and concentrate on Ice Ih, your typical crystalline ice found all over the planet.)
So, first, a few facts.
Ice has a density of 0.917 g/cm³. As the temperature drops, the density increases. Liquid water has a density of about 0.997 g/cm³ at room temperature. These numbers are for your average everyday existence. Pressure can affect melting and freezing points.
So, as we learn at an early age, ice floats. By floating, it insulates lakes and other bodies of water, and thus allows life to thrive. (Since water is most dense at 4°C, this cold water sinks to the bottom, leaving warmer water above, covered by the much colder frozen water.) If ice sinks, then lakes, ponds, and rivers would "silt up", freezing everything within the water. While this would have a very negative effect upon life, it could be argued that perhaps the organisms within this frozen ecosystem would adapt, just as frogs adapt to dry seasons and the Pompeii Worm adapt to extreme temperatures near hydrothermal vents.
As for oceans, there's a different system involved. Most scuba divers know that the weight of water creates water pressure. The deeper one dives, the greater the mass of water above the diver, which creates a greater pressure. Divers must carefully regulate their bodies to the change in pressure (much like an airline passenger), and the current deep sea diving is 1,010 feet. Regelation, using pressure to change the melting point, might not affect the ice, as this "heavy ice" I postulated might not expand upon freezing. If it does, than each atmosphere of pressure lowers the melting point 0.0072 °C. The deepest depth visited by humans has a pressure about one thousand times that of sea level, so the freezing point would be about -7°C. Of course, the salinity of the water would also lower the freezing point (which is why we salt sidewalks in the winter time). So the ice would sink, then hit the regelation zone, and become liquid. So, there might be a thick layer of frozen ice at the poles, then a zone beneath that ice where the water remains liquid (but under high pressure and low temperature). What type of ecosystem would exist here? Would the boundary of ice pack and open water differ than it does now?
Given the increased density of ice, how much more of Antarctica would be sunk beneath sea level?
A small density increase would be negligible during the winter... electrical wires, tree branches, and roofs would still collapse under the weight, but perhaps a little bit more. (Engineering could mitigate this in man-made structures.)
If glaciers are denser, they would move more quickly via basal sliding. (The pressure of the glacier melts the ice at the bottom, allowing the glacier to slide.)
Okay... enough trespassing for today. Hope this cooled you off!
(If you need more cooling, or just an excuse to stay inside, visit the Official Snowflake Bentley Web Site. Wilson A. "Snowflake" Bentley photographed some 5,000 snowflakes via microscope, and all snowflake images on this post are from that website.)