Day 1 Question 1: Feedbacks that complicate climate change prediction

Question 1:

List and describe some of the demographic, atmospheric, oceanic, and land surface feedbacks that complicate attempts to predict future climate change (associated with rising carbon dioxide levels). Explain how scale constrains our ability to detect these feedbacks, model them, and integrate them into predictive scenarios. Devise a Stommel diagram(s) to illustrate the scales at which these feedbacks play out.

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Interactions between various parts of the climate system create complex feedbacks and add to the overall complexity of the entire climate system (Rial et al. 2004). Due to these complexities, and the vast number of feedbacks and variables in the climate system, it is nearly impossible to model the future climate scenario with great certainty. Feedback mechanisms can be organized into several categories, including socio-economic, ecological, climatic, and atmospheric.

Among the socio-economic feedbacks, some of which are the most uncertain, are changes in energy use technologies, changes in energy production technologies, and land-use changes. Among ecological feedbacks are the acclimation of plants, soil microorganisms, and algae and plankton to warmer temperatures and high CO2 concentrations, and land cover and vegetation changes with changes in temperature and precipitation. Climatic feedbacks include the air temperature response to higher CO2 concentrations, albedo feedbacks with the response of glaciers, ice caps, and sea ice to climate change, sea level response to climate change, the release of greenhouse gasses from the thawing of frozen peat bogs, and the transfer of heat and CO2 into the ocean. Within the atmosphere, several feedbacks exist, including the mixing of greenhouse gasses, and changes in dust and aerosol concentrations in the atmosphere. Selected feedback mechanisms are illustrated in a time-space diagram below.

Each system has its own set of complex feedbacks and responses, as well. For example, as temperatures rise, and more melt occurs on the surface of an ice sheet, meltwater from the surface can percolate down through crevasses and moulins in the ice to reach the surface. As the water pressure at the base of a glacier increases, the glacier may actually float on the water and slide along the now lubricated base. This enhances the flow and dynamics of the glacier, but cannot easily be predicted.

As glaciers melt and contribute more freshwater to the oceans, other changes can occur. Higher sea levels allow water to seep further beneath tidewater glaciers, also allowing these to slip and surge. These feedbacks are extremely important, since when glaciers melt, they leave behind a material which is always less reflective than the snow or ice surface itself. Consequently, warmer conditions tend to lead to melting of snow and ice, which in turn leads to more radiation being absorbed by the surface, which ultimately leads to more snow and ice melt. This is an example of a positive feedback, often referred to as the albedo feedback.

Furthermore, glaciers contribute freshwater to the oceans, particularly in the North Atlantic, where temperature and salinity drive the thermohaline circulation, or THC, the major conveyor of heat and water in the world’s oceans, and therefore a major driver of global climate. As freshwater enters this system in the North Atlantic, the water becomes more buoyant and sinks more slowly. It is that sinking that drives the entire conveyor. Therefore, a freshening of the North Atlantic is expected to cause the THC to slow down. This would then drive less warm water northward along the Gulf Stream current, shifting Europe into a much cooler climate. This cooler climate could, potentially, allow for the flourishing of alpine glaciers, adding to the planet’s reflectivity, and ultimately dampening the effects of fossil fuel consumption.

On the other hand, the slowdown of the THC also leads to less upwelling of nutrients in the ocean. This ultimately leads to less benthic plant life, which is an enormous carbon sink. Such complexities and non-linearities in the climate system preclude the certain prediction of future climates.

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Works Cited:
 
Rial, J. A., et al. (2004), Nonlinearities, Feedbacks and Critical Thresholds within the Earth's Climate System, Climatic Change, 65, 11-38.