Cumberland Times-News

Bob Doyle - Astronomy

March 22, 2014

Earth’s climate keeps changing, but why?

Earth’s climate has been subject to change, long before humans walked the Earth. Why should the climate change?

The Earth is a dynamic planet, subject to the shifting of the crustal plates (which can lead to increased volcanic eruptions), the advance and retreat of glaciers and changes in the Earth’s motion about the sun (Earth’s axial tilt and the varying ovalness of the Earth’s orbit).

On our neighbor planets, Venus and Mars there is little or no climate change.

Venus’ surface conditions are dominated by its heavy heat trapping atmosphere which elevate its surface temperature to about 870 degrees F, day and night.

Direct sunlight never reaches Venus surface owing to the planet’s thick sulfurous clouds, averaging 10 times higher than our clouds. Venus has no seasons, as its poles are vertical to the plane of its nearly circular orbit.

Mars has seasons due to its oval shaped orbit and tilt of its axis. But even when Mars is closest to the sun, noon ground temperatures barely reach above freezing; but through the night, the daytime heat leaks into space, bringing the surface temperature before dawn to -100 F.

Mars is half as big as the Earth so most of its internal heat has leaked away. There were volcanoes in Mars’ distant past, emitting enough steam to form oceans and lakes.

But Mars’ low gravity has permitted most of its atmosphere to leak into space. The low Martian air pressure (about 1 percent of Earth’s) causes water to boil away. (The hydrogen gas in the early Earth’s atmosphere has similarly escaped.)

Through scientific analysis of ice cores and sediment cores from the ocean floor, we have an approximate record of the Earth’s climate over the past billion years.

We are now in a warmer interglacial period, emerging from a time when glaciers covered St. Louis about 15,000 years ago. But during this warming, our global temperatures have fluctuated. From AD 200 to 600, the average global temperature was 1 degree F warmer than today.

The Dark Ages (600 to 900 AD) had temperatures 2 degrees F colder than today. The Medieval Warm Period was 1 degree F warmer than today. During the Little Ice Age (1300 to 1850) the temperatures were 2 to 3 degrees F colder than today. (Currently, our average surface temperature is 59 degrees F.)    

Extensive glacier coverage such as the recent episode that Earth just emerged from are rather rare in the Earth’s past billion years. The three others were: Southern Hemisphere (260 to 340 million years ago), North Africa (450 million years ago) and the most severe (550 to 850 million years ago) when most of the Earth’s surface was frozen (called Snowball Earth).

The locations of the second and third glacial coverage may seem strange, but crustal plate motion had moved these areas near the poles at the time when they were covered by glaciers.

During the interglacial periods, the concentration of carbon dioxide in the atmosphere rose with the average global temperature. As the global temperatures dropped, the ocean could absorb more carbon dioxide and its concentration in the atmosphere fell as well.

What about today, when humanity is burning fossil fuels and driving our atmosphere’s carbon dioxide concentration to levels not seen in hundreds of thousands of years?

Past measurements of both the average global temperatures and the carbon dioxide levels show a good correlation between the two numbers.

Since the interglacial warming began, sea level has risen about 400 feet, owing to the melting of glaciers, whose waters flowed into the rivers and then into the sea.

More than half of the 300 largest port cities in the ancient world, built from 3000 BC to the fall of the Roman Empire are now submerged. It is clear that climate change occurred long before human history.

Now what part of the current change in climate is due to humans? What can be done to arrest any further warming? When the Earth warmed in the distant past, some animals were able to migrate northward and continue their way of life.

But if the temperatures rise markedly in the midwest U.S., it will be hard to move farms 300 miles northward.

Humanity is in for some hard adjustments. The people who will suffer the most include some Pacific Islanders (likely to be submerged), southern Bangladesh and southern China where about 200 million people will no longer be able to farm (as their ground water will be contaminated as the ocean encroaches).

Reference: Chapter 13 of “Earth: A Tenant’s Guide” by Frank Rhodes, a Ph.D. geologist and former President of Cornell.

SKY SIGHTS AHEAD: The moon is now seen only in the a.m. sky. On March 27 the brilliant planet Venus will appear just to the right of the lower tip of the crescent moon in the 6:30 a.m. eastern dawn.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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