Dangerous tipping points loom ahead

Some dangerous tipping points may have already been passed, many more lie just ahead.
The problem of forecasting the future climate -- even for so close a date as 2050 -- is made more difficult because of the existence of "tipping points" in our climate system. More important, the transitions that occur when tipping points are reached make our future extremely dangerous. It is important to know more about them. The way a tipping point functions can best be understood using the concept of feedback loops.

Feedback loops are classified as "positive" and "negative." A positive feedback loop is one that amplifies a small change in the system, making it have a larger effect than it would otherwise. A little positive feedback can be good, or your stereo amplifier wouldn't function. But too much can be bad, as when your speakers feed back the output signal back into the input signal -- a microphone -- and you get this loud, unpleasant squeal. In terms of global warming, positive feedback is nearly always bad. What would be a small change in global warming can be amplified into a bigger change as a result of positive feedback.

Negative feedback, on the other hand, reduces the effect of any change in the system. It provides system stability. In terms of global warming, "negative" feedback is almost always "good."

Presently, the most important example of a tipping element involves permafrost melting. Permafrost is a layer of soil that remains frozen year round, even during summer when local air temperatures are well above freezing. If the permafrost begins to melt under anaerobic conditions, say at the bottom of a lake or the ocean, bacterial action produces methane. The methane bubbles to the surface and into the atmosphere. Methane is a very potent greenhouse gas, eight times more potent molecule for molecule than carbon dioxide. If the methane is produced in large enough quantities to affect the global climate, permafrost melting can become part of a runaway greenhouse effect. We would then say that the tipping point for permafrost melting would have occurred. This possibility seems increasingly likely and very dangerous, but so far has received little public attention.

The diagram to the left shows in a schematic way how a tipping point transition occurs. The system starts out, in the front panel, symbolized as the red ball sitting in a bowl to represent stability of the initial climate. The shape of the blue surface is set by the parameters of the system. If one of the parameters, the global temperature for example, begins to increase, one can imagine that the blue surface moves upward. The bowl becomes more shallow, to indicate that the climate has become less stable. Eventually, the initial climate state becomes unstable, and the ball crosses into a new state as the tipping point is crossed. The new climate state, while stable, may have completely different characteristics from the initial climate state.

A table is presented below which summarizes our current knowledge of tipping elements in the Earth climate system. The tipping elements in the table are listed in the order of the global temperature at which the tipping temperature is reached. The top four elements listed are those of most immediate concern, those with tipping transitions estimated to be within zero and a 2 degree increase in the present global temperature.

In this table, the columns in order list: the tipping element, the applicable direction of change (e.g., -rainfall means less precipitation), the global temperature at which the transition will occur (relative to the average 1980-99 value) , the approximate time in years that it should take to complete the transition, the overall impact of the transition, and references and notes listed in the "details" section below. Note that all of the transitions have potential ecological impacts, so these are not noted separately.

Note in the table starting at the top line: The permafrost is already melting and may have already reached its tipping temperature. It will take a while to melt, as even at a constant temperature it takes some time to melt ice.

There is some debate over whether the loss of the Arctic summer sea ice is a true tipping point, as it may be completely reversible (2, 3, 5). This is because the date at which the Arctic freezes over in the next winter does not appear to depend on how early it unfroze the last year. Whether a true tipping point or not, it is included in the table because the loss of ice in summer has already begun and this loss is having, and will have, a large effect on Arctic countries and on the northern ecosystem.

The loss of the ice cap in Greenland has had a lot of attention in the press. While its tipping temperature will probably be reached soon, note that the transition time is estimated to be more than 300 years. This means that the 7 m rise in sea level associated with this transition will occur over many centuries, allowing considerable time for adaptation.

This may not be true for impending loss of the northern ice and snow cover. The absence of ice and snow year-round on the ground appears to constitute a true tipping point. Ice and snow, when present, reflect 90% of the energy from the sun right back into space. Therefore, ice and snow cover tends to stabilize the planet when it is in a ice age.

If the ice cover starts to melt for any reason, however, the bare ground (or ocean) absorbs about 80% of the energy from the sun during the day. At night, the ground will re-radiate this energy as heat. Whether this heat escapes the earth or not depends on the greenhouse gas concentration in the atmosphere, among other things. If the greenhouse gas is at a high enough concentration, not all the gained heat is lost at night, and the bare ground warms and acts to accelerate the warming. A tipping point has been reached, and now the planet will warm until it becomes stable in an ice-free mode. In this case, the tipping point is actually set by the CO2 concentration itself.

Note that all four of the top tipping points involve the Arctic. This is because the average temperature rise in the Arctic is about twice that at the equator. Therefore, the effect of global warming impacts this area first and hardest. The effect on the ice sheet covering west Antarctica is delayed somewhat beyond the northern effects for several reasons, e.g., location and depth of the sheet.

The next dangerous tipping point is already underway: Melting permafrost.

DETAILS

Notes and references for the table above:

1) A. K. Walter (2009) "Methane: A menace surfaces," Scientific American, 301 (6) 68-75.
This article notes that 1/3 to 1/2 of the permafrost is within 1 and 1.5 degrees Centigrade of melting. The author predicts an additional rise of 0.32 degrees Centigrade in global warming by 2100 due to methane release from melting permafrost.

2) T. M. Lenton, et al. (2008) "Tipping elements in the Earth's climate system," Proc. Nat. Acad. Sci. (PNAS) 105, 1786-1793. First definition of a tipping element; summarizes tipping points derived from formal presentations, discussions and surveys at an international meeting of climate scientists. Especially see Table 1 in this article for further details.

3) D. Notz (2009) "The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss," PNAS, 106, 20590-20595. Suggests that the loss of summer sea ice is reversible, but probably not the year-round loss of sea ice or complete loss of ice and snow cover on the ground.

4) The rate of failure of the Indian monsoon is thought to increase because of warming but the probability cannot be directly related to the global temperature change (2).

5) M. M. Holland, C. M Bitz, B. Tremblay (2006) "Future abrupt reductions in the summer Arctic sea ice," Geophysical research letters, 33, L23503

Other tipping points and recent information can also be found in a special tipping point section in PNAS (2009), edited and with an introduction by H. J. Schellnhuber.