Boulders on Hills
Posted by Jeff Id on May 25, 2009
Another guest post by Dr. Weinstein, I found this one to be interesting as well. He makes a good point about the possibility of strong positive feedbacks. Before we get into his post, there is a subtlety of positive feedbacks which is often missed on blogs. Positive feedback does not guarantee instability. You can have some positive feedback in a stable complex system as long as the net positive is less than 1. If the net feedback is greater than 1 then the system becomes unstable and jumps to extremes. As you approach 1 the system becomes gradually less stable and would exhibit stronger variations in response to everyday weather disturbances.
Personally I think it is unlikely that strong feedbacks of the nature the IPCC requires could exist, Dr. Weinstein points out some of the logic problems with a strong positive feedback climate below. I don’t believe his post precludes a weaker yet positive response which would not cross the threshold to create an unstable climate. IMHO, I’m still putting my $4 on a negative response, it explains why our temps are as stable as they are.
I read a quote from someone on AGW who’s name I can’t remember right now. It was in reference to the stability of our current climate and I think it is appropriate.
You don’t find round boulders perched on the sides of steep hills.
Debunking the CO2 Positive Feedback Myth
Leonard Weinstein, ScD
May 11, 2009
The climate models used in the IPCC reports have made predictions of increases in global temperature of 2C to 6C over present levels by 2100. These predictions were based on two assumptions:
- The atmospheric CO2 level would go from 290ppm in about 1850 to at least 580ppm in 2100.
- The increase in CO2 would have both a direct greenhouse gas effect, and trigger a positive feedback effect. The result would be a small direct increase in temperature from the CO2, but the increased temperature would also result in an increase in water evaporation. The water vapor is the major greenhouse gas in the atmosphere, and its increase would further increase temperature, resulting in a positive feedback until the process self-limited at a significantly higher level.
The predicted direct CO2 effect is estimated to be about 1C for the doubling of CO2 level. The present level of about 388ppm would have already caused about half of the direct rise, since the effect is nonlinear. The global temperature has apparently increased by about 0.7C in the last 150 years, which is slightly more than the estimated direct CO2 effect, but far short of the expected feedback imposed value. In addition, the temperature level up to about 1850 was significantly lower than typical levels during the last several thousand years. Much of the period between about1200 to 1850 has in fact been called the “Little Ice Age”. The abnormal low temperature starting point for the change makes the distinction between natural temperature rise due to a recovery from the abnormally low temperature to the present difficult to separate from CO2 and positive feedback induced increases. The CO2 increase was small until about 1940, so the positive contribution from the CO2 is based on an even smaller maximum temperature increase (about 0.3C) and a shorter time. All of these facts indicate that calculations of any CO2 effects and positive feedback additions would have badly missed the actual present temperature if we did not already know it.
The proposed solution to the discrepancy by the IPCC is that sulfate gas and particulate pollution from burning fossil fuels, have caused atmospheric “Global Dimming”, which greatly inhibited the correct level of warming. While this cannot be totally refuted, it is not specifically supportable either. Since the need for a strong positive feedback is needed to support the projections for the temperature rise to 2100, the mechanism for such a rise is examined.
Proposed mechanism for glacial to interglacial temperature increases:
It is very likely that over the last several hundred thousand years, axial tilt and precession in the Earth’s orbital motion (Milankovitch cycles). have triggered the transitions from glacial conditions (lasting about 100,000 years) to the significantly warmer inter-glacial periods (lasting 10,000 to 20,000 years) During the glacial cycles massive spreads of glaciers and ocean ice formed over large regions at higher latitudes, and the average Earth temperature was significantly lower than for the inter-glacial periods. The increase of average Solar insolation that occurred due to these orbital variations is not nearly sufficient to directly explain the rapid increase in global temperature, and the rapid melting of much of that ice. The variation has to have been triggered more by the change in distribution of the Solar insolation on the surface. However, once the transition was triggered, it has been hypothesized that some forms of positive feedback amplified the increase in temperature.
The most likely form of the initial trigger was due to local increases in ocean temperature in higher latitudes causing some of the marginal ocean ice to melt, and the increased absorption from the increased ocean area resulting in additional ocean absorption of Solar energy. This positive feedback may have been limited due to the small level of direct increased warming. However, the increased area of ocean and slight increased temperature also caused the water vapor pressure to slightly increase. Since water vapor is a strong greenhouse gas, this led to further temperature increases, resulting in some positive feedback. The increasing ocean temperature eventually resulted in large amounts of CO2 to be released, since the solubility of the water to CO2 is lower at higher temperature, and the oceans hold the vast majority of ocean+air CO2. It appears that the large increase in CO2 lagged the overall increasing surface temperature by about 800 years or so. It is then proposed by some (but it is not likely) that the increased greenhouse effect, due to the increasing CO2 level, supercharged the positive feedback by resulting in more heating. This additional heating then released more water vapor, and the positive feedback then took off until some mechanism stopped the process (possibly cloud formation).
Present conditions and additional heating:
The present total greenhouse effect from water vapor, CO2, Methane, and other greenhouse gases is estimated to make the surface 33C warmer than a surface without greenhouse gases. The direct contribution from the CO2 is estimated to be about 2C. All greenhouse gases other than water vapor are estimated to be about 3C. Why would a small amount of CO2, or all other gases than water vapor cause more of a positive feedback for heating that that due to the water vapor itself? If there is positive feedback from just the initial forcing, it would not require CO2 to do something strange, the water vapor would do it (i.e., heating causes more water vapor, which causes more heating, etc.).
Also, since water vapor is the main greenhouse gas, positive feedback would make an unstable system unless there was some mechanism that halted runaway conditions. It is likely that increased cloud formation, or even a haze condition caused by the water vapor, would decrease the effective Solar insolation so that a stable temperature is reached. Any increase in any greenhouse gas with changes small compared to the existing total would be self-limited by the water vapor limiting properties.
There is one way the small direct temperature increase from the CO2 could trigger a positive feedback (of limited scope). That would be if the small direct increase were world wide, and significantly decreased total global ice cover on the oceans. The total yearly average of the extent of ocean ice has only been measured for a few decades, and even though it has recently decreased somewhat, the present net effect is that less than 0.3 percent of the Earth’s surface has been exposed to a lower albedo (and thus higher absorption of energy), and this is at locations of very low Solar insolation. Most of even this ice area change is probably due to the natural variation, but the net change of absorbed energy is not sufficient to make a significant difference even if most of it was due to the CO2 increase. In addition, the melted area is now decreasing as we go into a multi-year cooling period. Historical records indicate this slight variation is not unusual over several decade long cycles.
Since water vapor is by far the largest greenhouse gas on the Earth, and since the Earth is mostly water covered, it is easy to see why the response of water and ice to perturbations in the level and distribution of Solar insolation would be most important in the shift from glacial ages to interglacial periods. Reasonable arguments can be made for some positive feedback of water vapor to explain the initial rapid temperature increase during the transition. It is clear that such water vapor feedback would have to be self-limiting, since the increase stops. The claim that the much smaller CO2 contribution can then cause an even stronger positive feedback, using water vapor as the main additional feedback mechanism, defies logic.
A recent significant increase of CO2, possibly with a large anthropogenic input, still only contains a total of less than 7% of the atmospheric greenhouse gas content. Only about 1/3 of that is larger than the claimed “natural” levels. It is posited that this can somehow override the water vapor self-limiting mechanism by triggering a small increase in temperature to thus release more water vapor and supercharge a temperature rise to several times the direct effect of the CO2 itself. Since the self-limiting mechanism for the water vapor is present, this does not follow logically. If just a small temperature change from any cause would cause a large feedback, then changes from day to night, or winter to summer, which are orders of magnitude larger than the small direct CO2 induced level change, would cause large temperature overshoot to new levels. They don’t (the change can be determined by the change in insolation), and thus there is no significant positive feedback at the present levels. Actual temperature changes have many drivers, but CO2 does not appear to be a significant driver beyond it’s direct contribution, at the levels or variations in levels present, and certainly can’t have the amplifying effect claimed. In fact the direct contribution is likely somewhat reduced by the water vapor self-limiting mechanism.