Posted by: Michael Ritter PhD | September 17, 2009

Future Geographies: Feedbacks Driving Global Warming

There are two types of feedbacks, positive feedbacks that drive system change and negative feedbacks that seek to keep systems in a state of equilibrium. Geoscientists like physical geographers are recognizing that positive feedback mechanisms may drive the Earth system past thresholds and towards a new state of equilibrium. In so doing, a new physical geography of the Earth system will appear. The
distribution of Earth’s regional climate’s and ecosystems may be irreversibly altered.

Examples of Feedbacks Driving Global Warming

Rising temperatures are expected to cause increased evaporation of water into
the atmosphere, most of which will originate from oceans. The additional water
vapor boosts the absorption of infrared radiation emitted by the earth
resulting in more warming (a positive feedback). The increased warmth promotes
more evaporation yielding an enhanced greenhouse effect. However, the addition
of water may cause an increase in cloud cover resulting in a higher
atmospheric albedo and reflection of incoming solar radiation. If this were to
occur, the reduction in insolation would lead to cooling. Such contradictory
consequences makes it difficult to determine what actually will occur in the future.

Figure 1. Tropical forest – climate change feedback Courtesy NASA
(Source )

Throughout history, humans have cut forests to build structures, warm their
homes, and cook their meals, and clear the land for agriculture. Removing
forests removes a powerful sink for carbon dioxide.  Leaving more CO2 in
the atmosphere enhances global warming and thus an increase in temperatures.
As a result, temperature conditions that may be too warm to support healthy
forest ecosystems. With less vegetation present, more carbon dioxide is left
in the atmosphere causing more warming, another positive feedback driving the
earth system toward ever warmer conditions. As temperatures increase,
evaporation increases causing drier conditions and the threat of wildfires and
forest destruction.

Figure 2. Permafrost – Climate change feedback. Image Courtesy USGS

Geoscientists agree that the Arctic has been and will continue to be
significantly impacted by global warming. Much of the land surface in the
Arctic is underlain by permanently frozen ground called “permafrost”.
They uppermost “active layer” experiences seasonal thawing. Recent studies
indicate that climatic warming my result in in a 12 to 15% reduction in the area covered by permafrost and a 15 to 30% increases in the thickness of the active layer.  As temperature rises permafrost melts, releasing stored carbon, but just as importantly, methane.  Increased warming results in more permafrost melting pushing the earth system ever forward into a future enhanced greenhouse environment.

Changes in Arctic ecosystems has already occurred as a result of global warming. Figures 3 a & b shows two photographs from the same location in Alaska, showing the transition from tundra to wetlands over the last twenty years. When permafrost melts, water collects in small  ponds on the surface increasing the heat gain nearly ten-fold. The additional heat continues to melt the underlying permafrost causing it to collapse and increasing the size of the pond. This positive feedback further degrades the permafrost.

Tundra

Figure 3a Tundra

Courtesy: Torre Jorgenson/NOAA (Source)

Wetlands

Figure 3b Wetland

Courtesy: Torre Jorgenson/NOAA (Source)

Carbon dioxide makes up a greater proportion of  the atmosphere by volume, but methane absorbs energy much more efficiently. Increased warming at high latitudes may cause an increase in the
release of methane from bogs or peatlands. Methane release from organic decomposition in wetlands coupled with carbon dioxide from melting permafrost will drive greenhouse gas levels higher, creating warmer temperatures.

Figure 4. Sea Ice – climate change feedback
sea ice feedbacks Image Courtesy USGS
Changes to the reflectivity of the surface (called the albedo) affects the amount of solar radiation absorb by the Earth. As Arctic sea ice melts it exposes open water which is less reflective (albedo decreases). The reduction in albedo allows more light to be absorbed by the ocean. As the ocean water warms, more heat is added to the air creating a positive feedback and driving Arctic temperatures ever higher. The reduction in sea ice is having a significant impact on arctic ecosystems.

Tipping Points

Positive feedbacks drive the physical environment towards new physical states. In June of 2008, twenty years after his landmark testimony about global warming, Dr. James Hansen reiterated his warnings before the U.S. Congress. He cited several examples of earth systems reaching or nearing a tipping point. A tipping level (point) is a level at which “no additional  forcing is required for large climate change and impacts.” (Hansen, 2008).  According to Hansen, a  “point of no return” is reached when unstoppable and irreversible (on a practical time scale) occurs. The disintegration of the Greenland ice cap is an example.

Time is also an important factor in assessing whether a tipping point has been reached or a point of no return. Some, like Josefino Comiso of the NASA Goddard Space Flight Center,  feel that the tipping point for perennial Arctic sea ice has already passed (National Geographic, 2007). David Barber, of the University of Manitoba is projecting that the North Pole will be ice free for the first time in history. For example, sea ice may completely disappear from the Arctic Ocean during the summer in a few years. This would represent a new state for the Arctic ocean. But temperature conditions could change in the relatively near future to permit sea ice to reform during the summer.


[Adapted from “Future Geographies: Feedbacks Driving Global Warming“. The Physical Environment: an Introduction to Physical Geography.]


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