Global Warming and Climate Change
Evidence for a Warming Planet and a Changing Atmosphere
Jun 7, 2009
What is the compelling evidence that human activities are changing the temperature of the planet and the composition of the atmosphere?
Global Temperatures Rising
The temperature of the troposphere – the thick layer of air nearest the surface – and land surfaces are warming while the stratosphere – the layer of thin air above the troposphere – is cooling. This has been interpreted as a sign that greenhouses gases are trapping energy and keeping that energy close to the surface of the earth.
The temperature of the planet has fluctuated many times in the past, but it got warmer or cooler over millions of years. Humankind is now seeing rates of increase in warming that exceed past natural rates by a factor of 100.
Polar Ice Disappearing
Both poles are getting warmer. In Greenland and Antarctica the surface of the ice is dropping and there is less mass when measured from space. The ice sheets have come and gone in the past so how is this different? In a June 2009 Discover magazine article entitled “The Big Heat” Robin Bell explains that the worrisome reason is that in the past the ice sheets didn’t move together — one would lead and then the other would follow. However, now both the north and south are spewing ice into the global ocean, accelerating in decline at the same time.
Sea Levels Rising
Like global temperature, sea levels have fluctuated in the past. Some 35,000 years ago sea levels were as high as they are now. A few thousand years later, sea levels began to drop with the onset of a cold glacial period and by about 15,000 years ago, the oceans had declined by almost 500 feet, the water locked up as ice in advancing glaciers. Then sea levels started to rise and they have been going up ever since.
Current sea level rise is due partly to human-induced global warming, which will increase sea levels over the coming century and longer periods. Increasing temperatures result in sea level rise by the thermal expansion of water and through the addition of water to the oceans from the melting of continental ice sheets.
Thermal expansion, which is well-quantified, is currently the primary contributor to sea level rise and is expected to be the primary contributor over the course of the next century. Melting glacial contributions to sea-level rise are less important,and are more difficult to predict and quantify.
Greenhouse Gases Increasing
Bubbles of ancient air trapped in deep ice cores reveal a disturbing increase in the atmosphere since the mid-20th century of the so-called greenhouse gases, predominantly carbon dioxide but also methane, nitrous oxides, and hydrofluorocarbons. Increases in these gases are believed to lead to warming of the surface of the planet as well as the lower atmosphere by increasing the greenhouse effect.
Ocean Chemistry Changing
When carbon dioxide dissolves in sea water, it forms carbonic acid which in high enough concentrations, is corrosive to the shells and exoskeletons of many marine organisms. Some scientists postulate that if current trends in burning fossil fuels and generating carbon dioxide continue, the ocean could become more acidic than any time in the past 65 million years.
Global temperatures during the Paleocene-Eocene Thermal Maximum (PETM) 55–65 million years ago were much warmer than both those presently and those predicted for the next century. At the height of the PETM, sea surface temperatures in the oceans rose 9 degrees F (5 degrees C) in the tropics and 11 degrees F (6 degrees C) in the Arctic. The oceans became more acidic, possibly resulting in 30 to 50 percent of sea floor life becoming extinct at that time.
In the June 2009 Discover magazine article “The Big Heat” global ecologist Ken Caldeira sums it up by stating, “It is the basic view of the vast majority of working scientists that human-induced climate change is real. However, there is a real diversity of informed opinion on how important climate change is going to be to the various things that affect humans, and there is a diversity of opinion on how to address this problem, but the debate over human-induced climate change is over.”
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