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Sunday June 24th 2018

Ocean-Front Property in Arizona

Saint Michael's Mount. Penzance, Cornwall, England. August 2015.

A hotly debated topic today is Global Climate Change. Some scientists predict that, over the coming decades and centuries, the Earth will get warmer. As a result of warming, the Earth’s polar icecaps and glaciers will melt, raising sea level by many meters, flooding low-lying regions of the planet such as Florida, the low countries in Europe and many Pacific islands.

How serious is the problem of rising sea level? Can anything be done about it?

We are going to address this important subject in a series of articles. In this article we’ll have a general look at water on earth. In a subsequent article, we’ll look at the timescales that are of concern to human life on this planet. Then we’ll look at how much sea level has varied in the the past. After that, we’ll consider various courses of action that can be  taken to avoid or mitigate the problem

1 Water on Earth
Earth is a water world. Liquid water has made life possible on the blue and white marble we call Planet Earth. When observed from space, the blue part is liquid water in the vast oceans and seas. The white part is water clouds in the atmosphere. The clouds are a collection of tiny droplets of liquid water and ice crystals that float in the air. Much of what is visible from space is water! [1]

The Blue Marble. Earth from space. NASA Composite image from Terra satellite, 2002.

Water (H2O) serves as a solvent and to transport the organic chemicals that make life possible. There would be no life on Earth as we know it without an abundant amount of liquid water.

The surface area of Earth is about 196.9 million square miles (510.1×106 km2). About 71% of Earth’s surface area is covered with water, mostly oceans, seas and bays, but also icecaps, glaciers, lakes, rivers, swamps, groundwater, permafrost and water in living things.

The average depth of the ocean is about 12,100 feet (3,388 meters) and the deepest part is about 36,200 feet (11,034 meters).

The total volume of water on Earth is about 332.5 million cubic miles. (1,386 x 109 km3). As shown in the figure, all the water on Earth would fit in an 860-mile diameter sphere. [2] The large sphere shows all the water; the smaller sphere shows all the liquid fresh water. 99% of that fresh water is groundwater. The tiny sphere shows the fresh water in lakes and rivers that is available for use by humans and animals.

Global Water Volume Sphere.

Key quantities are listed in Table 1.

Table 1. Global Water
Quantity Value Percent
Mass of Earth 5.972 x x1024 kg
Radius of Earth 6,371 km
Surface Area of Earth 510.1 x 106 km2
Surface Area of Water 362.1 x 106 km2 71%
Average Ocean Depth 3,388 meters
Max Ocean Depth 11,034 meters
Total Volume of Water 1.386 x 109 km3 100%
Ocean, Seas and Bays 1.338 x 109 km3 96.54%
Ice Caps, Glaciers, Permanent Snow 2.406 x 107 km3 1.68%
Groundwater 2.340 x 107 km3 1.75%
Fresh Water 1.063 x 107 km3 0.77%
Lakes 1.764 x 105 km3 0.013%
Atmosphere 1.290 x 104 km3 0.00093%
Rivers 2.120 x 103 km3 0.00015%
Biological Water 1.120 x 103 km3 0.000081%

Only 1.68% of Earth’s water is frozen in ice caps and glaciers. Even if it all melted it would not add much to the total volume of liquid water. However water density varies with temperature, i.e. warmer water is less dense than cooler water. Warmer ocean water will take up a greater volume. How much does it vary? Table 2 shows the density and Specific Volume of Water vs. temperature. [3]

Table 2. Water Density at Standard Sea-Level Atmospheric Temperature
Temp
(° F)
Temp
(° C)
Density
(grams / cm3)
1000 x Specific Volume
(cm3 / gram)
32 0.0 0.9987 1.0013
39.2 4.0 1.000 1.000
40 4.4 0.99999 1.00001
50 10.0 0.99975 1.00025
60 15.6 0.99907 1.00093
70 21.0 0.99802 1.00198
80 26.7 0.99669 1.00332
90 32.2 0.99510 1.00492
100 37.8 0.99318 1.00687
120 48.9 0.98870 1.01143
140 60.0 0.98338 1.01690
160 71.1 0.97729 1.02324
180 82.2 0.97056 1.03033
200 93.3 0.96333 1.03807
212 100 0.95865 1.04313

The average temperature of ocean surface water is about 17 ° C (62.6 ° F). If the temperature were to increase from 60 ° F to 70 ° F, the volume of water would expand by 0.8%.

Quick estimate: the combined effect of melting and density decrease due to a 10 ° F temperature increase would add about 2.68% to the volume of liquid water. At an average ocean depth of 3,388 meters, increased volume could cause sea level to rise by as much +90 meters. This assumes none of the increase goes into a greater area, only increased height. More accurate estimates puts the maximum possible sea level rise at +70 meters.

An extreme scenario is for sea level to rise by about +70 meters.

The biggest question is how quickly can sea level change, i.e. how many millimeters or meters per year or century.

Next: Timescales
With this general understanding of water in the planet, next it will be useful to look at various timescales in order to put bounds on the problems that we need to solve. For example, if we were to know that all the water on Earth will evaporate into space a billion years from now, but humans will only be around for a million years, that would not be an urgent problem for us to worry about.

The topic of the next article will be the timescales that are of interest to human life.

———————————————-
References:

[1] Debunked: “Blue Marble” Photos show a Changing Earth

[2] How much water is on Earth?

[3] USGS - Water Density

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