System Feedback and Tipping Points

 "Feedback" is a term to describe myriad processes in both the biological and physical components of our ecosystems.  Feedback is not at all a new discovery, or one that is unique to global warming/climate change.  While in the past feedback processes have been cited for contributing to the stability of ecosystems, feedback processes are also critical threats to the stability of global systems in response to global warming.  In this context, feedback processes can be considered a response to an external addition of energy to a system that magnifies, or increases, that energy input.  Some examples may help.

Mankind burns fossil fuels for energy which releases Carbon Dioxide (CO₂) into the atmosphere.  CO₂ is a greenhouse gas, absorbing and re-radiating energy back to the surface of Earth.  One of the often cited, and dramatic, consequences of such man-induced global warming is the melting of ice in the Arctic regions, a direct impact of increased atmospheric CO_2.  But:

• Ice is far more reflective than water, thus as the ice melts the exposed water absorbs more energy and warms.
• In turn, the warm water causes more ice to melt, which warms and melts more ice.

Thus we have a feedback that reinforces and magnifies the original process.  Technically, any process that adds energy to a system is often referred to as a "forcing process".  In this example, the original forcing process is magnified, by the feedback process resulting in a much greater change in system stability than either process alone.  However as in this example, the situation is far more complicated as:

• As the water warms, it releases CO₂ since cold water holds more CO₂ than warm water (envision the bubbles in a warming glass of any carbonated beverage).
• As CO₂ is released from the warming ocean it increases the concentration of CO₂ in the atmosphere and additional warming occurs which melts more ice. 

and,

• The melting Arctic ice adds fresh water to the saline ocean.
• As both fresh water and warm water are less dense than salt water and cold water, the melt water tends to form a layer of fresh water "floating" on the ocean surface.
• As discussed in Currents, Energy and Climate the major ocean currents are driven by the temperature and density of the ocean waters.  Thus lower density, warmer water resulting from ice melt may alter the pattern of ocean currents, resulting in severe climate change. 

and, as the ice melts over land regions of the Arctic,

• A dark soil surface is exposed, which rapidly warms causing additional melt.
• As the soil warms and the permafrost melts, additional greenhouse gases, primarily methane, are released into the atmosphere.  Methane is a much more potent greenhouse gas than CO₂, but fortunately does have a somewhat shorter residence time in the atmosphere.  Nonetheless, it does enter the atmosphere and cause additional warming. 

and, as the climate of northern regions has warmed biological processes have also been altered such as,

• The pine beetle (Dendroctonus ponderosae) has spread through western Canada, killing pines over more than a 50,000 square mile region.  This massive deforestation is resulting in the release of large amounts of CO₂ from the decaying vegetation.  The Canadian Forestry Service has estimated that 270 megatons of CO₂ will be released by 2020, which exceeds the total amount of CO₂ that Canada is committed to reduce under the Koyto Protocol. 

While this simple example identifies several feedback processes in response to the initial forcing process of man-caused CO₂ release into the atmosphere it far over-simplifies the complex interactions that constitute climate change. It should also be apparent that the sum of the feedback processes at some point will be sufficiently large to be self-sustaining – that is they will continue even if the original forcing factor (combustion of fossil fuels) were to be completely removed.  That point is the so-called "Tipping Point" and it represents the point at which we can no longer have any control over global warming.

We have no idea what the actual tipping point will be, nor when we will reach it.  In fact, we really will not have any way of knowing when we reach it, or are even past it.  We do know that the rate of global warming is increasing far more rapidly than was predicted only a few years ago, and that we are thus more rapidly approaching the tipping point than we had previously thought.  And, the rate of man-induced increase in atmospheric CO₂ is also increasing more rapidly than previously predicted.  Most scientists agree that we are rapidly approaching the tipping point, and will pass it within a few decades if not sooner.  In fact, many scientists believe that when the planets climates stabilize in response to the current levels of atmospheric CO₂ (387ppm) the Earth’s will not support human life as we know it today.