Student Group Version
Global climate change is becoming a threat to our current way of life on Earth. One consequence of climate change is the melting of ice caps, glaciers, and sea ice, including polar ice in Greenland and Antarctica. Substantial melt of these massive glaciers will cause a rise in sea level along coastlines throughout the globe. This activity will explore how melting ice impacts sea level.
Water is an unusual liquid because it expands when it freezes. In general, liquids do not expand upon freezing, but rather contract and become denser as temperature drops. Like other liquids, as water begins to cool, it becomes more and more dense. But, because of the physical structure of the water molecule, it continues to become denser until just before freezing, when it expands. This expansion occurs at the point that freezing begins (around 4°C). At this temperature water molecules arrange themselves into a crystal lattice structure that is significantly less dense than the liquid form. Because of this decrease in density at the point of freezing, ice always floats on water.
When objects are totally submerged in water, they displace an amount of water equal to their volume. However, because ice floats on water and is not completely submerged, ice does not displace an amount of water equal to its volume. Instead, it displaces less than its total volume of water. The water that floating ice displaces is equal to the volume that the ice would take up if it melted and became water again. In other words, floating ice displaces water equal to the mass of the ice. When ice melts, the mass of the ice is conserved, but the crystal lattice structure of ice disappears and the volume decreases and becomes equal to the volume of water it displaced in its ice form.
Therefore, when floating ice melts, the melted water is equal only to the volume of the ice that was submerged. This means that when floating ice melts, it contributes no additional volume to the body of water. We see this phenomenon when we let ice melt in a glass of water. The water does not overflow because the ice has already displaced water equal to the volume it will take up upon melting.
Ice already in the oceans does not contribute to sea level rise, but ice covering land will contribute to sea level rise upon melting. Greenland, for example, is covered by vast quantities of continental ice. The melting of this ice will contribute to sea level rise. The sea ice in the area of the North Pole is floating in water and thus the melting of this ice will not contribute to sea level rise.
Local relevance: Although many of the icy areas that could melt with climate change are far away from the San Francisco Bay area, this issue is extremely relevant locally. According to maps created by the Bay Conservation and Development Commission for the San Francisco Chronicle, a one-meter rise in sea level would submerge parts of Corte Madera, San Rafael, Hayward, Newark and much of the Silicon Valley shoreline. In San Francisco, Mission Bay housing and office developments, Caltrain tracks, Candlestick Point redevelopment, Heron's Head Park, parts of Treasure Island, and the San Francisco and Oakland airports would all be under water.
In this activity, students will learn which masses of ice pose the biggest threat for rising global sea level and why.
Water is a wonderful solvent. This means many substances dissolve in water, adding molecules to the liquid. As molecules are added to a particular volume of water, the density increases. Thus, seawater is noticeably denser than fresh water and fresh water will float on the surface of seawater.
In the North Atlantic, a phenomenon based on this concept drives a process known as thermohaline circulation or the “great ocean conveyor belt.” In this area, surface water moving north from lower latitudes becomes saltier (due to evaporation) and colder as it moves northward. This causes the density of the water to increase, and the water eventually sinks as it enters the North Atlantic. When the water sinks, it drives a current that plays a significant role in global ocean circulation. The sunken water (it’s colder and more dense) slowly flows along the bottom of the ocean back toward the lower latitudes where it eventually rises, like a conveyor belt, to the surface and starts the journey north again. Thermohaline circulation is extremely important in maintaining hospitable climates around the globe because it contributes to the overall circulation of warm water from near the equator towards the poles.
Glacial melting in Greenland has caused some concern because of the potential for significant increase of fresh water in the North Atlantic. If melting rates continue to increase with global warming, a layer of freshwater could theoretically form in the North Atlantic. This fresh water could mix with the salty, dense water of the North Atlantic and stop the sinking of North Atlantic water, thereby altering the driving force behind the great ocean conveyor belt current. Although projections are speculative, scientists suggest that a disruption in this circulation could lead to a cold climate shift in Europe as well as unpredictable changes in other parts of the globe.
In this exercise, students will be able to visualize differences in water density and relate this to potential consequences of increased glacial melting.