Ice age terminations

Marshall and Clark:
“…Our simulations suggest that a substantial fraction (60% to 80%) of the ice sheet was frozen to the bed for the first 75 kyr of the glacial cycle, thus strongly limiting basal flow. Subsequent doubling of the area of warm-based ice in response to ice sheet thickening and expansion and to the reduction in downward advection of cold ice may have enabled broad increases in geologically- and hydrologically-mediated fast ice flow during the last deglaciation.
Increased dynamical activity of the ice sheet would lead to net thinning of the ice sheet interior and the transport of large amounts of ice into regions of intense ablation both south of the ice sheet and at the marine margins (via calving). This has the potential to provide a strong positive feedback on deglaciation.
The timescale of basal temperature evolution is of the same order as the 100-kyr glacial cycle, suggesting that the establishment of warm-based ice over a large enough area of the ice sheet bed may have influenced the timing of deglaciation. Our results thus reinforce the notion that at a mature point in their life cycle, 100-kyr ice sheets become independent of orbital forcing and affect their own demise through internal feedbacks.”
– Science of Doom
I like to think of the Antarctic ice sheet not as a result, but as having a purpose helping to cause a survivable climate.

 

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White Earth Climate

Looking at the idea of a climate equilibrium, I came across this and wish I had paid more attention to Gleick:

“Their natural bias is to make models with a strong tendency to return to the equilibrium we measure every day on the real planet. Then, to explain large changes in climate, they look for external causes—changes in the earth’s orbit around the sun, for example.” – James Gleick

Hansen & Lacis took this approach in 1984 I think. It has been said that CO2 is only thing that makes the GCMs work. I think there is a bias towards looking for external causes.

“Climatologists who use global computer models to simulate the long-term behavior of the earth’s atmosphere and oceans have known for several years that their models allow at least one dramatically different equilibrium. During the entire geological past, this alternative climate has never existed…”

The White Earth equilibrium. Lots of snow and sea ice.

“Then, for no reason whatsoever, it shifts into a different sort of behavior, still fluctuating but producing a different average. The people who design computer models are aware of Lorenz’s discovery, but they try at all costs to avoid almost-intransitivity. It is too unpredictable.”

It’s unclear to me if he’s still talking about GCMs or models in general? We may agree there is a bias towards one equilibrium or the current equilibrium. In some ways that’s preferable to an unpredicted state.

“Computer models have such a strong tendency to fall into the White Earth equilibrium that climatologists find themselves wondering why it has never come about. It may simply be a matter of chance.”

It’s my understanding that the White Earth is colder than the recent glacial periods. If it’s true that they try at all costs to avoid this, why do the models go there?

“Yet it takes no great imagination for a climatologist to see that almost-intransitivity might well explain why the earth’s climate has drifted in and out of long Ice Ages at mysterious, irregular intervals. If so, no physical cause need be found for the timing. The Ice Ages may simply be a byproduct of chaos.”

And I would take the long time scales of ice ages and shorten them all the way down to decades and shorter time spans. I like the idea of understanding the glacial interglacial cycle to find the same processes that governs the time spans we worry about.

“The system is not in equilibrium, never was and will never be. It is even its defining feature. It can’t “return” to some equilibrium it has never been in to begin with. I thought at least this basic truth would be known.” – Tomas Milanovic

“…components in the system are inherently chaotic; there are feedbacks that could potentially switch sign, and there are central processes that affect the system in a complicated, non-linear manner. These complex, chaotic, non-linear dynamics are an inherent aspect of the climate system.” – IPCC 14.2.2

I finding it difficult to accept the idea of a climate equilibrium. I do agree that that point of view will often work (the shorter the time span the better), and is more tractable than multiple climate states. Equilibrium states solve all kinds of engineering problems, are practical and economical. But perhaps at the tails of a probability distribution function, we find other states.

More complete quote:

“Climatologists who use global computer models to simulate the long-term behavior of the earth’s atmosphere and oceans have known for several years that their models allow at least one dramatically different equilibrium. During the entire geological past, this alternative climate has never existed, but it could be an equally valid solution to the system of equations governing the earth. It is what some climatologists call the White Earth climate: an earth whose continents are covered by snow and whose oceans are covered by ice. A glaciated earth would reflect seventy percent of the incoming solar radiation and so would stay extremely cold. The lowest layer of the atmosphere, the troposphere, would be much thinner. The storms that would blow across the frozen surface would be much smaller than the storms we know. In general, the climate would be less hospitable to life as we know it. Computer models have such a strong tendency to fall into the White Earth equilibrium that climatologists find themselves wondering why it has never come about. It may simply be a matter of chance.”

“To push the earth’s climate into the glaciated state would require a huge kick from some external source. But Lorenz described yet another plausible kind of behavior called “almost-intransitivity.” An almost-intransitive system displays one sort of average behavior for a very long time, fluctuating within certain bounds. Then, for no reason whatsoever, it shifts into a different sort of behavior, still fluctuating but producing a different average. The people who design computer models are aware of Lorenz’s discovery, but they try at all costs to avoid almost-intransitivity. It is too unpredictable. Their natural bias is to make models with a strong tendency to return to the equilibrium we measure every day on the real planet. Then, to explain large changes in climate, they look for external causes—changes in the earth’s orbit around the sun, for example. Yet it takes no great imagination for a climatologist to see that almost-intransitivity might well explain why the earth’s climate has drifted in and out of long Ice Ages at mysterious, irregular intervals. If so, no physical cause need be found for the timing. The Ice Ages may simply be a byproduct of chaos.”

Chaos: Making a New Science, James Gleick

Probability Distributon Functions are Basins of Attraction

It occurred to me that a PDF is an inverted basin of attraction. If we are experiencing more extreme weather, the PDF is shifting. The basin of attraction is changing. A steep basin in the past would be consistent with low sensitivity. There is little variability and few chances of a regime change. A flattened basin like a flattened PDF is going to give us lots of variability. So we have the question, do GHGs flatten the basin? With a flattened basin, is it possible for the ball (the climate) to shift to another regime, perhaps like Trenberth’s step changes? Are past regime shifts such as a PDO shift, a flattening of the basin, and do they also explain the transitions in and out of glacials and interglacials?

PDFs and basins