Positive Feedback Loops Create Problems for Climate Models and Possibly Wicked Outcomes

The global warming effects from positive feedback loops that are in some cases included by some climate models are poorly understood by climate scientists because the extreme complexity of their interactions cause huge uncertainty. The IPCC's Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.". The authors of AR4 went on to say, "that scientific understanding of carbon cycle feedbacks was poor".

Until recently climate models looked at a limited set of factors and often measured changes in the ocean and on land separately. One new approach, developed at the Hadley Centre for Climate Prediction and Research in Bracknell, accounts for as many influences as possible, including some of the feedback loops both negative and positive. Their attempts, and those of other modelers, to integrate feedback mechanisms have often produced results far outside, both above and below, the 'consensus' numbers by a factor of 10.

One of the most confounding positive feedbacks is the loss of carbon from some types of ecosystems due to increased evaporation as the climate warms and thereby increases the rate of desertification. Desert soils contain little humus, and support little vegetation. Also there's the increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere and much of the Amazon Rainforest. While models disagree on the strength of any terrestrial carbon cycle feedback, they each suggest any such feedback would accelerate global warming, a known unknown.

Another positive feedback that's been overlooked until recently is the release of dissolved organic carbon (DOC) from peat bogs into water courses from where it would in turn enter the atmosphere. Western Siberia, for instance, has a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release of large quantities of methane.

That issue, Arctic methane release - the release of methane from seas and soils in permafrost regions of the Arctic - is the most accepted positive feedback mechanism and the most feared because of its potential to create non-linear catastrophic change.

Large quantities of methane are stored in the Arctic in natural gas deposits, permafrost, and as submarine clathrates. Permafrost and clathrates degrade on warming, in addition organic matter stored in permafrost generates heat as it decomposes in response to the permafrost melting, thus large releases of methane from these sources may arise as a result of this positive feedback loop.

Staying in the Arctic, there's the ice-albedo feedback loop. When ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Sea ice, and the cold conditions it sustains, serves to stabilize methane deposits on and near the shoreline,  preventing the clathrate breaking down and outgassing methane into the atmosphere. Recent observations in the Siberian arctic show increased rates of methane release from the Arctic seabed as well as the land.

There is about 50 times more carbon in the oceans than there is in the atmosphere and cooler water can absorb more CO2 than warmer water. As ocean temperatures rise the oceans will absorb less CO2 resulting in more warming. In addition to the water itself the ecosystems of the oceans also sequester carbon. Their ability to do so is also expected to decline as the oceans warm because less carbon will be available to be incorporated into the shells and bones throughout every part of the food chain all of which eventually falls to ocean floor and, in the long term, becomes sequestered as limestone.

Then there's water vapor feedback. As the atmosphere is warmed, the saturation vapor pressure increases, and the amount of water vapor in the atmosphere will increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor. A recent study on global warming concluded, “We have high confidence that the most extreme rainfalls will become even more intense."

Of course there's more, far more, feedbacks than can be touched on in one blog post. The point here is that climate models have so many interdependent parameters that it's no wonder they produce such a wide range of results and even less wonder that proponents and opponents alike easily find reasons to disagree with parts, both inputs and outputs, that don't fit their worldview and agree with those that do.

Tomorrow we'll take a look at why climate science, like all of science, uses probability to express its confidence in the most common range of results and why this is a good strategy for directing the focus of further research, but a terrible way to direct public policy and a very dangerous way forward. Dangerous because if a low probability, non-linear, black swan type outcome occurs on the low side [lower sensitivity, less warming] society will simply be over prepared. But if it happens on the high side, as many experts are now warning, there will be no second chance to exercise precaution.