Storms of My Grandchildren

[eagle12304]

Hi all. I am new here and just got done reading Hansen's Storm of My Grandchildren.

I wanted to see if there were any people knowledgeable about the claim in his book that if humans burned all the coal or had methane release we could trigger a runaway greenhouse like on Venus.

I was wondering if there other reading material on this in the scientific literature?

I was also wondering why we don't have a runaway now, given the alleged importance of positive feedbacks like ice albedo.

Wouldn't they tend to cause a infinite positive feedback loop?

Thanks.

[Snow_Miser]

Hello eagle,

Net feedback in the climate system is not positive, but strongly negative. Multiple analyses by many climate scientists have confirmed this.

[TerryM]

Hello eagle,

Net feedback in the climate system is not positive, but strongly negative. Multiple analyses by many climate scientists have confirmed this.

Can you provide a link or two? My understanding is that positive feedback is overwhelming.

Terry

[Snow_Miser]

Can you provide a link or two? My understanding is that positive feedback is overwhelming.

Terry

Sure,

http://www.gewex.org...eb2010GNews.pdf

Paper by Graeme Stephens

Radiative feedbacks involving low level clouds are a primary cause of uncertainty in global climate model projections. The feedback in models is

not only uncertain in magnitude

,

but even its sign varies across climate models

(e.g., Bony and Dufresne, 2005). These low cloud feedbacks have been hypothesized in terms of the effects of two primary cloud variables—low cloud amount and cloud optical depth. The basis of these feedbacks relies on the connection between these variables and the solar radiation leaving the planet exemplified in the following simple expressions (Stephens, 2005). ...an increase in optical depth with an increase in temperature results in an increase in cloud albedo, suggesting a

negative feedback

.

...

The net consequence of these biases is that the optical depth of low clouds in GCMs (General Circulation Models) is more than a factor of two greater than observed, resulting in albedos of clouds that are too high. This model low-cloud albedo bias is not a new finding and is not a feature of just these two models. The study of Allan et al. (2007), for example, also noted how the reflection by low-level clouds in the unified model of the UK Meteorological Office is significantly larger than matched satellite observations of albedo, suggesting that this bias also exists in that model. The mean LWP (cloud liquid water path) of model clouds that contributed to this in the most recent Intergovernmental Panel on Climate Change assessment is close to 200 g/m2, which is also

nearly a factor of two larger than observed.

The implication of this optical depth bias that owes its source to biases in both the LWP and particle sizes is that

the solar radiation reflected by low clouds is significantly enhanced in models compared to real clouds. This reflected sunlight bias has significant implications for the cloud-climate feedback problem. The consequence is that this bias artificially suppresses the low cloud optical depth feedback in models by almost a

factor of four

and thus its potential role as a negative feedback.

This bias explains why the optical depth feedback is practically negligible in most global models (e.g., Colman et al., 2003) and why it has received scant attention in low cloud feedback discussion. These results are also relevant to the model biases in absorbed solar radiation discussed recently by Trenberth and Fasullo (2010) and as explored in more detail in Stephens et al. (2010).

A bunch of Lindzen, Spencer, Pielke and Douglass papers also show this, mainly that observations show a much less sensitive climate than do models.

[Vergent]

Sure,

http://www.gewex.org...eb2010GNews.pdf

Paper by Graeme Stephens

Radiative feedbacks involving low level clouds are a primary cause of uncertainty in global climate model projections. The feedback in models is

not only uncertain in magnitude

,

but even its sign varies across climate models

(e.g., Bony and Dufresne, 2005). These low cloud feedbacks have been hypothesized in terms of the effects of two primary cloud variables—low cloud amount and cloud optical depth. The basis of these feedbacks relies on the connection between these variables and the solar radiation leaving the planet exemplified in the following simple expressions (Stephens, 2005). ...an increase in optical depth with an increase in temperature results in an increase in cloud albedo, suggesting a

negative feedback

.

...

The net consequence of these biases is that the optical depth of low clouds in GCMs (General Circulation Models) is more than a factor of two greater than observed, resulting in albedos of clouds that are too high. This model low-cloud albedo bias is not a new finding and is not a feature of just these two models. The study of Allan et al. (2007), for example, also noted how the reflection by low-level clouds in the unified model of the UK Meteorological Office is significantly larger than matched satellite observations of albedo, suggesting that this bias also exists in that model. The mean LWP (cloud liquid water path) of model clouds that contributed to this in the most recent Intergovernmental Panel on Climate Change assessment is close to 200 g/m2, which is also

nearly a factor of two larger than observed.

The implication of this optical depth bias that owes its source to biases in both the LWP and particle sizes is that

the solar radiation reflected by low clouds is significantly enhanced in models compared to real clouds. This reflected sunlight bias has significant implications for the cloud-climate feedback problem. The consequence is that this bias artificially suppresses the low cloud optical depth feedback in models by almost a

factor of four

and thus its potential role as a negative feedback.

This bias explains why the optical depth feedback is practically negligible in most global models (e.g., Colman et al., 2003) and why it has received scant attention in low cloud feedback discussion. These results are also relevant to the model biases in absorbed solar radiation discussed recently by Trenberth and Fasullo (2010) and as explored in more detail in Stephens et al. (2010).

A bunch of Lindzen, Spencer, Pielke and Douglass papers also show this, mainly that observations show a much less sensitive climate than do models.

The reality of the feedback is greater than any of the models. Or, maybe its the hot air from the right that is melting all the ice.

[Snow_Miser]

The reality of the feedback is greater than any of the models. Or, maybe its the hot air from the right that is melting all the ice.

How do you know that this is simply not a product of natural variability being underestimated in the models?

[cmc0605]

Eagle,

Good questions. I will answer them in reverse order...

1) Regarding why positive feedbacks don't lead to runaway warming in the present climate, it's because the outgoing radiation of the planet still increases with temperature. This is a fundamental consequence of Planck's radiation law, which relates emission of an object to its temperature (to the power of four).

Consider two planets, one without a water vapor feedback, and the second one with a water vapor feedback (ice-albedo is actually pretty small potatoes unless you're transitioning into something like a snowball Earth, particularly in the present climate when rather little area is covered in ice to begin with. The feedback also obviously stops when ice is gone, and there's no ice as you approach the runaway greenhouse limit).

The first planet will radiate to space at a rate in proportion to the planets temperature to the fourth power. Any tendency for the planet to warm (say by adding CO2, or turning up the sun) will raise the temperature, but the planet will then radiate more energy to space in order to come to a new equilibrium at that higher temperature. Consider the second planet now. The same planet begins to warm, radiating like T^4 in order to come back to balance; but it also has a water vapor feedback, so that infrared opacity increases with temperature too. This results in a competition between outgoing radiation increasing with T, and outgoing radiation decreasing with water vapor.

The result is that the planet loses energy at a slower rate than T^4. The basic physics is illustrated in the first figure of this post. This diagram can be a bit tricky to follow, but think of our warming mechanism as increasing the amount of sunlight the planet receives. The amount of sunlight the planet initially receives is shown by the red horizontal line, and the final increased amount is the green horizontal line. The black and blue curves show how each planet would radiate...the black curve with no water vapor feedback, the blue curve with a water vapor feedback, which cancels out some of the curvature you get from T^4. The blue squares and red circles are equilibrium points. This is where the outgoing radiation equals the absorbed incoming sunlight. The existence of a greenhouse gas itself shifts the equilibrium points to higher temperatures (that's just the greenhouse effect), but the greenhouse feedback reduces the slope of the line, which increases the distance between the two red circles relative to the two blue circles. Notice that for the water vapor case, since the emission to space is more "sluggish," you need to raise the temperature by a further amount in order to reach the same equilibrium point.

Note: the comment above focuses on cloud responses, which is quite uncertain and important for future projections, but Earth's stability is not dependent on the details of cloud feedback and so is not really pertinent to your question. Cloud feedback could very well be positive and not lead to a runaway. In that case, it could change the curvature of the outgoing radiation in the same way as water vapor (or change the albedo, thus impacting the position of the horizontal lines). This all matters for climate sensitivity, but the stability of Earth's climate is not dependent on cloud feedback being negative.

2) Jim Hansen has a remarkable track record but I really don't think he understands the runaway greenhouse. The runaway greenhouse is actually rather independent of the CO2 concentration, and the physics can be inferred again from the figure above. Notice that at very high temperatures, in excess of 300 K, the outgoing radiation begins to flatten out. There is in fact an upper limit to the amount of radiation a planet with a water vapor feedback can emit to space. Whether Earth is prone to a runaway or not is intimately dependent on whether the absorbed radiation (the horizontal lines) are above or below that threshold. In the modern Earth, we receive about ~240 W/m2 of solar energy, well below the limiting OLR of the planet. What happens in a runaway is that the absorbed radiation is above that limit, so it's receiving more energy than it can emit back to space. This continues to increase the temperature but not the outgoing radiation (since water vapor opacity is increasing rapidly). The only way to come back to equilibrium is to lower the amount of sunlight or boil or turn off the water vapor feedback, which isn't usually until the entire ocean is in the atmosphere.

Hope that helps and serves as a useful starting point...

[WeatherRusty]

cmc0605 approached the answer for your question as to why there can be no 'runaway' greenhouse on current day Earth from a radiative physics point of view. I completely agree, but lets look at it in terms of chemistry.

The major reason, as stated by cmc0605, is that Earth holds a large body of liquid water at it surface. This liquid water is largely responsible for the long term carbon cycle taking place on Earth. Venus lacks such oceans of water at it's surface, and any Earth like carbon cycle long ago ceased to function.

On Venus, any out gassed CO2 from within it's crust and mantle remain trapped in the atmosphere. There is no mechanism available to scrub CO2 from from the atmosphere. On Earth, the oceans aid in the process of chemical weathering and the deposition of Carbon on the sea floor in the formation of carbon based sedimentary rocks such as limestone.

You can read about how this works in more detail HERE.

This is a relatively slow process, working on geologic time scales down to centuries and milennia. The warmer it becomes, the more rapid the chemical weathering which draws down the atmospheric CO2 preventing a runaway condition to a vastly hotter equilibrium point.

[Jonger]

The reality of the feedback is greater than any of the models. Or, maybe its the hot air from the right that is melting all the ice.

More proof that much of this entire science is political.

[TerryM]

Jonger

I don't see any of the science as politicized. I do however see opposition to science as having become politicized over a relatively short period.

cmc0506 & Rusty

I haven't though much about us mimicking fate of Venus, as I've assumed that the last oil shale would probably be knapped into some tribally significant symbol, possibly a gorget or ornamental atlatl point, by our not too distant antecedents.

As far as positive/negative feed backs, I assume that as long as complex civilization is possible, some idiot will be burning fossil fuel, and the positive will overwhelm the negative. I don't however see a long time horizon for complex civilization.

Terry

[cmc0605]

Jonger

I don't see any of the science as politicized. I do however see opposition to science as having become politicized over a relatively short period.

cmc0506 & Rusty

I haven't though much about us mimicking fate of Venus, as I've assumed that the last oil shale would probably be knapped into some tribally significant symbol, possibly a gorget or ornamental atlatl point, by our not too distant antecedents.

As far as positive/negative feed backs, I assume that as long as complex civilization is possible, some idiot will be burning fossil fuel, and the positive will overwhelm the negative. I don't however see a long time horizon for complex civilization.

Terry

One thing to keep in mind regarding feedbacks is how climate scientists define "feedback."

In climate science we always define a "reference system" which serves as an anchor for evaluating other feedbacks. In other words, the chosen reference system is a baseline, or a "no-feedback" scenario; any other climate system response that is separate from the reference system then becomes a feedback.

The most popular reference system is the one I described above, the so-called Planck response, which is the increased emission to space as the planet warms (or decreased emission as the planet cools). All other feedbacks (water vapor, lapse rate, clouds, surface albedo) modify the efficiency of that reference system. The sum of those other feedbacks can be net positive, but their magnitude is still smaller than the strongly stabilizing Planck restoring system, and so the total system response is net negative on Earth. What "positive feedbacks" really mean is that this restoring tendency is "less negative." There are the possibility of bifurcations (like a snowball earth or ruanway greenhouse) in which other feedbacks can overwhelm the Planck response, but this is not considered to be relevant for modern global warming.

The magnitude of the reference system feedback is generally found by perturbing the troposphere temperature uniformly (say, by 1 K) and seeing how the outgoing radiation changes in response to that perturbation. The increase in outgoing radiation is around 3.5 W/m2 per degree warming. Put another way, in this chosen reference system, doubling CO2 would raise the temperature by about 1 K. If you add a positive (say longwave) feedback, then the radiative restoring efficiency would go down...in other words, the outgoing radiation might go up by only 2 W/m2 in response to a 1 K warming; equivalently, it takes more warming to achieve radiative equilibrium.

The choice of the reference system is arbitrary. We typically use the Planck response, because it is very well understood, but one could (for example) define a fixed relative humidity reference system (instead of fixed specific humidity). This would increase the "no-feedback" reference sensitivity, but then also decrease the magnitude of the feedback, since you'd no loner be counting most of the water vapor response as a feedback. There may be advantages to changing how you frame the problem, but in the end the system response is of course independent of how you set it up.

Carbon cycle feedbacks (like methane release, CO2 outgassing from oceans, etc) are a separate class of feedbacks, because they do respond to climate change, but often we think of them as modifying the forcing (e.g., if you ask what is the system response to a doubling of CO2, then you have already included any carbon cycle response automatically...they only modify how easy it is to get to a doubling).

[TerryM]

Right

The feedback I fear most is the change from latent to sensible heat as polar ice volume diminishes. At some point most of the energy presently masked by the change of state from ice to water is going to present itself as sensible heat, and that's a lot of warming.

It takes 141 BTU to change a pound of 32F ice into a pound of water at 32F, that same energy will raise the 32F water to 173F, less evaporative losses. Albedo concerns are real of course, although this year's vast ice extent (= high albedo), has shown that rising surface and oceanic temperatures will still melt the ice out in record breaking fashion.

I'm not convinced that a world without polar caps (plural), is hospitable for mankind. We may never boil away the oceans, but peak humanity may closely follow peak oil.

Terry

[The_Global_Warmer]

Right

The feedback I fear most is the change from latent to sensible heat as polar ice volume diminishes. At some point most of the energy presently masked by the change of state from ice to water is going to present itself as sensible heat, and that's a lot of warming.

It takes 141 BTU to change a pound of 32F ice into a pound of water at 32F, that same energy will raise the 32F water to 173F, less evaporative losses. Albedo concerns are real of course, although this year's vast ice extent (= high albedo), has shown that rising surface and oceanic temperatures will still melt the ice out in record breaking fashion.

I'm not convinced that a world without polar caps (plural), is hospitable for mankind. We may never boil away the oceans, but peak humanity may closely follow peak oil.

Terry

In a decade Greenland Ice Mass went from relatively stable to rapidly melting out of control.

In just over a decade Greenland is now losing ice at a constantly worsening rate.

If Albedo's over Greenland continue to lower at the rate we have already seen for the 0-1500M range for the 1500M-3000M+.

We can see that a continued drop in albedo would yield a huge change in how much melting take place.

This year's results will be interesting.

[TerryM]

In a decade Greenland Ice Mass went from relatively stable to rapidly melting out of control.

In just over a decade Greenland is now losing ice at a constantly worsening rate.

If Albedo's over Greenland continue to lower at the rate we have already seen for the 0-1500M range for the 1500M-3000M+.

We can see that a continued drop in albedo would yield a huge change in how much melting take place.

This year's results will be interesting.

Yea

I'm inclined to think that the time table for Greenland's melt is due for a major revision. Thus far everyone's predictions regarding the rapidity of Arctic melt have been far too conservative. Why would Greenland be the one they got right.

Terry

BTW Anyone want to invest in bottled water from "Natural Glacial Waters", after seeing the photos?

[eagle12304]

Thanks all. Not sure I understand everything but a good start.

[WxUSAF]

I attended a conference last week where Dr. Hansen was present (as were a number of other distinguished climate scientists). The conclusion is that Earth will NOT runaway to a Venus-like state, even if we burn all our CO2. We'd still be screwed, but Earth wouldn't turn into Venus.

[Mallow]

Snowlover, you claim that:

Hello eagle,

Net feedback in the climate system is not positive, but strongly negative. Multiple analyses by many climate scientists have confirmed this.

You cite as proof a paper which concludes that negative cloud-albedo feedbacks may be underestimated.

However, it does not logically follow that if the paper's conclusion holds, your statement is true. Net feedback in the climate system is driven by several factors. Cloud feedbacks have the most uncertainty in IPCC reports, but even with uncertainties taken into account, the net feedback is expected to be positive:

http://cdn.greenopti...007_radforc.jpg

[cmc0605]

I attended a conference last week where Dr. Hansen was present (as were a number of other distinguished climate scientists). The conclusion is that Earth will NOT runaway to a Venus-like state, even if we burn all our CO2. We'd still be screwed, but Earth wouldn't turn into Venus.

Thanks. It's good to know he no longer thinks this. (You mentioned Ray Pierrehumbert was at the conference in the other thread, and I know he would be pretty vocal against someone who thinks CO2 could trigger a runaway at current Earth-like conditions). It's important to note though that this point has been reasonably understood for a couple decades.