The Feedbacks

Once total warming reaches about 2 degrees various feedbacks become important, and most of them will cause additional rapid warming. Computer models and historical records show that once these take off the warming will accelerate out of our control. Several major extinction events in history appear to be associated with this type of warming

  • Water Vapour Effect
    • As the atmosphere warms it can hold more water vapour, which is itself an important greenhouse gas, which in turn speeds up the warming. This can cause more clouds to form, which can have either a small negative (cooling) feedback effect or a larger positive effect depending on the altitude of the clouds.
  • Albedo Effect
    • Ice reflects more than 80% of the sunlight that falls on it, but as the ice melts to dark water only 5-10% is reflected, which causes increasingly rapid warming near the poles, and so more melting ice.
  • Ocean acidification and saturation
    • The oceans have been taking up a quarter of the CO2 we produce and this increases their acidity. Many key species such as plankton build shells from calcium, which becomes more difficult as the acidity increases. A study highlighted in the latest The Copenhagen Synthesis Report states that when CO2 reaches 450 ppm, large areas of the polar oceans are likely to become corrosive to their shells and loss of shell weight in Antarctic plankton has already been observed. These species are important in removing CO2 via photosynthesis and the rate at which they remove it will slow or stop as acidity grows.
    • The rate of change in acidity is much faster than previous ocean acidification-driven extinctions in Earth’s history, from which it took hundreds of thousands of years for ecosystems to recover.
    • Separately the oceans will become saturated and the rate of take-up will slow. There is some evidence that this is already happening.
  • Tropical Forests start to emit rather than store CO2 while pests, bushfires and CO2 destroy forests
    • 20-40% of the Amazon will likely die from a 2 degree warming rise, while double that rise will probably destroy most of it. The Amazon (and others) are not adapted to fire, and are very vulnerable to increasing drought. As these forests currently lock away a large amount of our emissions this would accelerate warming drastically.
    • Northern forests are being devastated by pests who are no longer killed off in winter.
    • Bushfires in California, the Mediterranean, Australia and elsewhere are destroying forests at increasing rates as the temperature rises.
    • Studies show that increasing CO2 reduces growth in tropical forests, contrary to previous belief.
  • Permafrost Methane Release
  • Wetlands release methane and CO2 as they Dry Out or are Drained
    • A further large amount of greenhouse gas is stored in peatlands, swamps and bogs. Many of these are being deliberately drained, and others are drying out due to drought releasing methane and CO2.
  • Ocean Methane Hydrate Release
    • A huge amount of methane is trapped on the continental shelves of the oceans as a frozen methane hydrate. As the oceans warm these may become destabilised. The release of even a small percentage would be disastrous, and this may have happened 55 Million years ago during the Paleocene-Eocene Thermal Maximum (PETM).
  • Microbial Activity Increases with Temperature and Desertification releases soil carbon

As you can see these feedbacks are complex and interrelated, making it very difficult to predict how quickly they will become important. However it is very worrying that we are already seeing evidence of several of these feedbacks although we are still well below the 2 degrees of total warming that was considered safe.