The Climate Science Global Warming Model, Part I: Today's Human-Caused Global Warming

To repeat:  In this series of blog posts, I attempt to give an overall view of the physics/chemistry-based climate science dealing with climate change and today’s global warming.  I do so because I can’t find an overall summary such as the one I’m about to try to create.  My hope is that readers will understand why this science makes me so alarmed and seemingly so pessimistic.  As always, misunderstandings and misstatements are my fault and do not reflect on the science itself.

In this post, we take a look at what climate science has to say about today’s human-caused global warming.  This episode of global warming beyond what would be expected from the Milankovitch cycle and underwater volcanism (that is, from a “Goldilocks” steady state that would soon begin to slowly decrease towards an Ice Age) has already taken us to global temperatures not seen for the last 1-5 million years. An additional 1.5 degrees C is already “baked in” – that is, the temperatures have not yet caught up to the atmospheric carbon level, but even with zero carbon emissions from now on, and without some technology that we do not presently have, we will still see this additional increase to a new “semi-steady state”.

The key difference between all previous episodes of global warming and this human-caused one is that for the first time ever, carbon stored in animal and vegetative matter beneath the Earth (including the shallow parts of the oceans) is being brought above ground and burned, injecting into the atmosphere much of the carbon so stored over the last 100 million to billion years of life on Earth.  Specifically, this means oil, natural gas, and coal, as well as the tar sands and oil shale that also contain “fossils”.  If all of the rest of this “energy reserve” were to be used in the same way over the next 100 years, then atmospheric carbon would probably reach more than 2000 ppm, and possibly as high as 4000 ppm.

As should be clear from my last blog post, the main difference in climate behavior from previous episodes of global warming is that atmospheric carbon injection is happening far faster:  about 100 times faster.  Thus, parts of the climate process that operate to slow global warming or return to a previous “steady state”, which operate much more slowly, have very little impact on this global warming.  More specifically, “weathering” has very little impact, and oceanic ice-creation mechanisms that operate to keep the Arctic filled with sea ice also cannot operate very well.

Let’s be a little more specific about the last point.  The Oceans in today’s Earth operate in a great surface/underwater loop or “conveyor belt”.  The Gulf Stream’s warm water moves up to a point near Greenland, cooling as it goes.  When it reaches that point, it dives down below the surface and begins a journey south to the Antarctic Ocean, and from thence up the Pacific to a point near Alaska and Siberia, where it resurfaces and completes the loop back to the Gulf Stream.  This is called a “conveyor belt” because it transmits surface ocean temperature changes to the deep Ocean over a period of about 100 years. 

When global warming causes sea ice to melt more at the “dive-down” point, that sea ice is fresher water, and therefore warmer (salt prevents water from freezing until it reaches about 29 degrees F, at which point the salt is expelled from the sea ice).  That warmer, fresher water slows or even stops the Gulf Stream from diving down at the dive-down point.  That, in turn, slows down or stops the rest of the Gulf Stream if it goes on too long.  If it doesn’t, the fresher water, being more easy to freeze, reforms near the dive point, and the “diving down” resumes.  In effect, up to a point, this circulation cycle acts to retard or reverse the loss-of-sea-ice albedo effects of global warming.  However, the faster the global warming, the less effect this mechanism has.

Climate Effects of Today’s Global Warming

Today’s global warming has and will have (remember, further temperature increases are “baked in”) effects on global climate especially in four ways:

1.       Temperatures in the Arctic and Antarctic warm perhaps 5-10 times as much as those nearer the Equator.  Also, winter temperatures warm more than summer ones, and night temperatures warm more than day ones.  The key effect from this on the Northern Hemisphere is that the temperature differential between Northern Eurasia and Northern America and the Arctic decreases, while weather patterns on both sides of that divide have more energy to stay on the “wrong side”.  The initial result is that weather systems on both sides linger longer, and are more extreme.  Boston saw almost-record cold last January for long periods of time, as Arctic weather came down and lingered; the Arctic saw several unprecedented warm spells from the south over this New Year, including a 22-hour period where the temperature was above freezing.  In the long run, however, the warmth from global warming will dominate the cold from the Arctic: a few million years ago, 360 ppm atmospheric carbon saw a subtropical Arctic.

2.       The atmosphere has more energy (heat) and more rain, making for stronger and windier extreme-weather events.  It appears that increased incidence of tornadoes and increased top winds in hurricanes are to some extent caused by these temperature increases.  Likewise, heavier rain/snow when it occurs and greater wind-driven oceanic “storm surges” onto land are definitely occurring and are effectively caused by global warming.

3.       Climate “zones” move poleward, i.e., northward/southward.  In particular, subtropical zones with high heat and massive droughts extend northward/southward.  In 2050-2070, America/southern Canada (except New England/NY), pretty much all of Europe, the Middle East, India, and 2/3 of China, as well as Australia and Tasmania, are projected to be suffering Dust-Bowl-like droughts, if things go on as they have.  This may or may not be true if very strong action is taken that reduces drastically human contributions to atmospheric carbon.  Because of the unusual speed of global warming, the majority of ecosystems in these area cannot move northward effectively and are faced with extinction in the next 100 years.  Also, a decreased availability of water for farming due to loss of snowpacks and overuse of aquifers combined with the drought conditions places perhaps ½ of all of today’s arable land under threat.

4.       Land ice and snow melts exceptionally rapidly.  Melting of sea ice only causes slight ocean rise, as ice displaces as much area as cold water; oceanic heat increases (slow to happen, as noted) expand and therefore raise the oceans less than 10%.  Land ice and snow melting, however, translates directly into sea-level rise.  The key areas here are Greenland, West Antarctica, and East Antarctica.  A domino effect in the Arctic, which has already started, removes the sea-ice barriers to increased flow of Greenland glaciers into the oceans surrounding it, leading to a doubling of net land-ice loss there every 10 years.  West Antarctica has already begun the same process, especially in the Antarctic Peninsula, and it appears that East Antarctica is doing so as well.  Greenland may contribute as much as 10 feet of sea-level rise this century, while West Antarctica may well also contribute that amount.  In the longer term (again, this may be “baked in” by a 360-ppm atmospheric carbon level), sea level rise may reach a maximum of 220-240 feet.  This would submerge 1/3 of the world’s present arable land in salt water.  If we add 30-foot-higher storm surges that “salt” water supplies inland, most of today’s coasts to 50-100 miles inland will be uninhabitable.  Thus, for example, in the US, Florida, Louisiana, Alabama, and Mississippi will effectively cease to exist.

There are also indirect “feedback loops” related to climate change that make temperature increases per atmospheric-carbon doubling 4 degrees C rather than 2.5 degrees C.  The three key effects there are increased albedo due to ice/snow replacement by water/rock/earth, melting of permafrost (which has already begun to occur) that leads to release of permafrost carbon into the air, which in turn increases temperature, and emergence of “black carbon” from the layers of melted ice, which “dirties” and thereby increases the albedo of remaining snow/ice as well as adding carbon to the atmosphere.

Finally, we should note some of the things that do not affect today’s climate change in the medium to long term:

1.       Above-water volcanism and “nuclear winter”.  These create winter-like conditions in which the temperature drops drastically.  In both cases, the particles that create these conditions linger for 1-7 years, but the net effect on underlying climate change is exactly zero:  if you double atmospheric carbon during the 1-7-year period after an eruption or nuclear-winter episode, at the end of 7 years a temperature increase of 4 degrees C will be “baked in.”

2.       Changes in the amount of cloud cover caused by climate change.  Studies have shown that, if anything, these increase temperatures slightly. 

3.       Aerosol pollution.  This is most frequently a side-effect of coal burning, and results in increased “smog”-related airborn particles that decrease temperatures significantly.  However, as China is now finding out, aerosol pollution can only counteract increased atmospheric carbon up to a point, after which people suffering aerosol pollution will die.  Thus, it now appears extremely likely that overall aerosol pollution will decrease over the next century, “uncovering” the temperature increases that were masked by the pollution increases.

4.       Methane emissions will increase sharply, but it now appears that their overall effect will be relatively small.

Summary

What worries climate scientists the most about today’s global warming is its unprecedented speed and extent, both of these being the result of humans rapidly and massively extracting and injecting into the atmosphere hundreds of millions of years of sequestered carbon primarily from dead animals and plants. The resulting feedback loops and processes, less checked than ever before by stabilization processes, enhance the immediate temperature effects of the new atmospheric carbon and have reached the point of oceanic saturation where much of the carbon now in the atmosphere will not be “cycled out” for thousands of years.

But there is another key implication of today’s process of global warming.  At every stage, the climate effects of doubling atmospheric carbon are 2-10 times greater – including the negative ones.  Thus, while storm surges once we enter the 500-1000 ppm atmospheric carbon stage, the frequency of 100-mph storms now equals that of 80- or 90-mph storms in the previous stage; and the damages from 100-mph winds is 10 times that of 80-90-mph winds. To fail to prevent 500 ppm atmospheric carbon is an argument for redoubling our efforts to prevent 1000 ppm atmospheric carbon, not for giving up, because the consequences of failure to prevent each stage get progressively worse.

What is the worst that can happen, and what does it require?  According to a recent study by Hansen et al, the “can’t get any worse” scenario happens when we burn a certain amount of today’s fossil fuel reserves within the next 100-200 years:

If we burn all the coal plus a very minor amount of everything else, we reach “worst consequences.”

If we burn all of the oil, 33% of the coal, and none of the gas/tar sands/oil shale we reach “worst consequences”.

If we burn 17% of the coal, 50% of the natural gas, and all the tar sands/oil shale/oil, we reach “worst consequences”.

What are the worst consequences?  Here’s a brief summary of those:

In Hansen’s worst-consequence world, humans could survive below the mountains during the day outside only for short periods of time during the summer, and there would be few if any places to grow grains. 

In my version of Hansen's worst-consequence world, we would try to survive on less than 10 % of today's farmland, less than 10 % of the animal and vegetable species (with disrupted ecosystems), and practically zero edible ocean species, in methane-heavy mosquito-infested peat swamps that must be developed before they can be farmed, in dangerous weather, for thousands of years.

My final blog post on this subject will examine certain implications of climate science on effective ways to combat today’s global warming.

ClimateChange and Harvard Magazine: My Jaw Dropped

For my sins – and especially the sin of having gone to Harvard University at one point – I used to get an alumnus copy of Harvard Magazine.  I cancelled my subscription last year, when a fatuous article by a Harvard professor of environmental science recommended that the US allow the Keystone XL pipeline without the slightest effort to explain why James Hansen (far more expert in climate science than the professor) had said “Keystone XL means game over for the environment”.  At that time, it was clear to me that Harvard Magazine’s editors had not the slightest idea that there was anything wrong with the article, and not the slightest intention of boning up on the subject because of the gravity of the issue.

However, other members of the family still get the rag, and I took a brief look at an article in the latest issue about “why the US may be on the cusp of a [sustainable} energy revolution.”  And, although I was braced for something clueless, my jaw still dropped.  Here’s the quote (condensed):

“Beyond the connection between long-term climate change and the use of energy … lies another stark reality: the total amount of fossil fuel on earth is finite, and so, inevitably, eventually, ‘We will run out.’”

Not sure why this is so awful a framing of the article’s issues?  Try this equivalent.  In the middle of global nuclear war, Harvard Magazine publishes the following:

“We need to think about beginning to substitute something else for atomic bombs, because, at this rate, in about twenty years (long after the rubble has stopped bouncing) we will run out of uranium.”

And the rest of the article shows absolutely no consciousness of what climate science is predicting.  The focus is on wind power.  Double the wind speed means four times the energy, so the middle (windiest) 1/3-1/6 of the US should be covered with turbines – no mention of the likelihood that average wind speed is likely to increase by 1/3 over the next 30 years, nor that wind patterns are likely to change (as they are already doing in winter because of the disruption of the jet stream), so that the middle third of the US may not be the windiest.  Offshore wind farms are good with regard to wind speed but more expensive to build – no mention of the likelihood that East Coast waters are likely to rise by a foot over the next 35 years, drowning low-lying turbines, or that higher-category hurricane patterns that might overstress the turbines are also likely.  And then there’s the assertion that biofuels are renewable – it’s not clear what the article is referring to, since corn ethanol production and use can in some cases involve more carbon emissions than oil, while cellulosic ethanol is not yet as widely used.

To summarize:  Harvard Magazine is by no means a set of climate change deniers – there is no sense that the truth is something to twist in order to make money for oneself, no matter the consequences.  Rather, Harvard Magazine is a representative of what I would call, in drug addiction terms, enablers.  Their minds are simply not open to the possibility that delay involves exponentially increasing costs and deaths.  And so, they allow deniers to carry the day by sheer weight of inertia, just as enablers multiply addiction’s costs simply by deciding that they don’t need to face the addiction problem just yet.

I am reminded of someone once saying of an outrageous act, “It wasn’t just immoral – it was downright stupid.”  But, heavens, how could someone like Harvard be stupid?  In the words of Westbrook Pegler, that last sentence “was writ ironical.”

James Hansen’s Climate Change Magnum Opus: Unsurpassed Horror and Sad Beauty

I warn you that this description of the latest draft paper by James Hansen and others should horrify you, if you are sane.  After Joe Romm first published its sound bite (30 degrees Fahrenheit increase if most fossil fuels are burned, 50 degrees at higher latitudes), I delayed reading it in detail. Now that I have, I find it builds on his (and others’) 40 years of work in the area and the latest research to provide an up-to-date climate change model whose implications are mostly more alarming than any I have seen elsewhere. 

What follows is my layperson’s attempt to summarize and draw further conclusions. I scant the discussion of new analyses of previous episodes of global warming (and cooling) that allow the development of the new model, focusing instead on the mechanisms and implications of the model.  Please note that, afaik, this is the first model that attempts to fully include the effects of methane and permafrost melting.

It’s About CO2

The first insight in the new model I summarize as follows:

As atmospheric CO2 increases or decreases, global average temperature increases or decreases proportionally, with a lag either way typically of a few decades.

This increase or decrease can be broken down into three parts:

1.       The immediate effect of the CO2 itself -- perhaps 60% of the total effect.

2.       The immediate and “over a few decades” effect of other “greenhouse gases” , or GHGs (here we are talking particularly about the methane in permafrost and methane hydrates on the continental shelves, released by warming, as well as GHGs such as nitrous oxide) – perhaps 20% of the total effect.

3.       The so-called “fast feedback” effects, in which the released CO2 and other factors (e.g., increased albedo) lead to additional warming “over a few decades”.

Two quick notes:  First, Hansen does not do my split; instead, he distinguishes between the effects of CO2 and the effects of other GHGs over the medium term (about 75-25) and then separately distinguishes between the immediate overall “climate sensitivity” and the medium-term or total “climate sensitivity” (again, about 75 % immediately and 100 % in the long term).  Second, the “over a few decades” is my interpretation of how quickly “X times CO2” seems to match global temperature data over more recent sets of data.  Hansen might very well say that this may or may not occur quite this rapidly, but it doesn’t matter to him because even with a thousand-year time frame for the full effect, CO2 will not be recycled out of the atmosphere for “few thousand years”, so we still reach the full “climate sensitivity”.

Just to get the usual objections out of the way, Hansen is not saying that CO2 always leads the way – on the contrary, in Ice Age scenarios in which a certain point in a Milankovitch cycle causes extreme winters in our northern hemisphere, leading to increased glaciation and therefore decreased albedo and CO2 release to the atmosphere, CO2 follows other factors.  Today, however, primarily because of fossil-fuel emissions, CO2 is leading the way.

A sub-finding, still important, is that there is a linear relationship (again, sometimes with a lag of decades) between deep-ocean temperature change and atmospheric temperature change (expressed as “the change in temp at the surface is somewhere between 1.5 and 2.5 times the change in temp of the deep ocean” – or, about 67% of the global temperature increase goes into surface temps, 33% into deep ocean temps).  I include this because it seems that the recent “slowdown” in global surface temperature ascent is primarily caused by increased accumulation in the deep ocean.  However, again in a relatively short time frame, we should go back to more rapid average global surface temperature increases, because we’re still increasing atmospheric CO2 rapidly and 2/3 of that will start again going back into surface temps.

The Effect of “CO2 Plus” Is Bigger Than We Thought

In the past, Hansen among others has seen the effect of doubled CO2 as somewhere in the 2-3 degrees Celsius range.  Now, he sees a range of 3-4 degrees C – apparently, primarily because he now takes into account “other GHGs”.  To put it more pointedly, in my own interpretation:

Each doubling of CO2 leads to a global temperature change of 2.25-3 degrees Celsius (4-5.4 degrees F) “over a few decades”, and to a change of 3-4 degrees C (5.4-7.2 degrees F) “over 1 or 2 centuries.”

I mention this not only because the consequences of today’s global warming are more dire than we thought (i.e., the effects of that warming, immediately and over the next century or two), but also because many of us are still hung up over that “stop emissions and hold the increase to 2 degrees C” target that was the main topic at recent global governmental summits.  The atmospheric CO2 level at the beginning of the Industrial Revolution was about 250 parts per million (ppm), and is now at about 400 ppm.  If you do the math, that means we have baked in at least 2.2-3 degrees C of global temperature increase already. After 15 years of inaction, that target now has zero chance of success.

At this point, I want to do a shout-out to those wonderful folks at the Arctic Sea Ice blog and forum.  Hansen specifically notes the data supporting melting of Arctic sea ice, plus collapse of the Greenland and West Antarctic ice sheets, at levels slightly below today’s CO2.  He also notes data supporting the idea that Greenland and West Antarctica can go pretty rapidly, “in a few centuries”, iirc – I interpret “in a few centuries” as within 250-450 years from now.

The Percent of Fossil Fuels We Need To Leave In The Ground Forever Is Greater Than We Thought

Before I get to the consequences if we don’t leave a percentage of fossil fuels in the ground, let’s see how the minimum amount of fossil fuels burned before we reach “worst consequences” has changed. Today’s estimate of total recoverable fossil-fuel reserves (coal, oil [primarily tar sands and oil shale], and natural gas) is about the equivalent of 15,000 Gt C (billions of tons of carbon emitted).  Of this, coal is about 7.3-11 Gt C, and the rest is split approximately equivalently between natural gas and tar sands/oil shale. Originally, we thought that burning 10,000 Gt C in the next century would get us to “worst consequences”.  Now, Hansen places the correct amount as somewhere between 5,000 Gt C and 10,000 Gt C.  Reading between the lines, I am placing the range as 6,000-7,000 Gt C, with 5,000 Gt C if we want to be ultra-safe, and I’m estimating coal as 60% of the emittable total, 20% tar sands/oil shale/oil, 20% natural gas.  Note, btw, that according to Hansen fossil-fuel emissions have increased consistently by about 3 % per year since 1950, including last year. At that rate, we’d reach 6,000-7,000 Gt C in about 65-70 years.

Again, note that Hansen breaks the fossil fuels down as coal, traditional oil/gas, and oil shale/tar sands/fracked gas, so I’m guesstimating the equivalents.

So here’s the way it works out:

If we burn all the coal plus a very minor amount of everything else, we reach “worst consequences.”

If we burn all of everything but coal and 33% of the coal, we reach “worst consequences”.

If we burn 17% of the coal, 50% of the natural gas, and all the tar sands/oil shale/oil, we reach “worst consequences”.

So this, imho, is why I agree with Hansen that allowing the Keystone XL pipeline is “game over” for the climate, as in “worst consequences almost inevitable”.  The Keystone XL pipeline is a “gateway drug” for tar sands and oil shale.  The source (Alberta, Canada) has a large part of the known tar sands oil, and presents similar difficulties in abstracting and processing to oil shale.  It’s the furthest along in terms of entering the world market.  If that source succeeds, as the saying goes, once the nose of the camel is in the tent, you may expect the rest of the camel to enter.  In this case, if Alberta succeeds in getting the Keystone XL pipeline, it is probably the case that most of the tar sands and oil shale will be used; if not, probably not.

Right now, Alberta has no real buyers except the US, and the US is not set up to accept the oil, nor Canada to ship it to them in bulk.  The pipeline would effectively create an infrastructure to ship it, primarily to the rest of the world, which presumably would accept it – especially China – creating a market that allows Alberta profitability.  Alternatives are much more costly, are susceptible to pressure from the US, and would probably not be undertaken at all.  Note that increased shipment via truck is more costly, and would probably require major investments in truck structure, to handle the more toxic tar-sands crude, so that it is probably not a large-scale alternative that would make the project a success.  Likewise, trains and tracks to the Canadian ports to ship directly to world markets would probably prove too costly.

Now go back to the model.  It’s pretty darn likely we’ll burn 17% of the coal no matter what, and the majority of the natural gas.  Now add the tar sands and oil shale.  Worst consequences, here we come.

The Worst Is Likelier Than We Thought, Arrives Sooner, Is Almost As Bad As Our Worst Nightmare, And Is More Inescapable Once We Get There Than We Hoped

We’ve already dealt with “likelier than we thought”, and we can guess from the rapidity of response to atmospheric CO2 rise and the increase in the estimated climate sensitivity to atmospheric CO2 that it arrives sooner than we had projected. But what is this “worst consequences almost as bad as our worst nightmare”, and “worst consequences, once arrived, more inescapable that we hoped”?

For us, the worst consequences are not “snowball Earth”, locked in eternal ice, but “runaway GHG Earth” a la Venus, with the surface and air too hot and too acid to support water or any life at all (water vapor in the atmosphere vaporizes from the heat long before it reaches the surface).  It’s an inescapable condition, since once the atmosphere locks in the heat, the Sun’s heat from outside trapped by the CO2 and other gases in the atmosphere balances escaping heat from the troposphere (top of the atmosphere).  Hansen’s model shows that we are still 100 million to 1 billion years from being able to reach that state, even by burning all fossil fuels in a gigantic funeral pyre. 

The worst consequence, as cited before, is therefore as cited at the very beginning, Joe Romm’s sound bite:  30 degrees F increase globally, 50 degrees in the high latitudes.  Here’s Hansen’s take on what that means:  it will take all areas of the Earth except the mountains above 35 degrees C “wet bulb temperature” during their summers.  That in turn, according to Hansen, would mean the following: 

In the worst-consequence world, humans could survive below the mountains during the day outside only for short periods of time during the summer, and there would be few if any places to grow grains. 

Effectively, most areas of the globe would be Death Valley-like or worse, at least during the summer.

Here I think Hansen, because he properly doesn’t diverge into movement polewards of weather patterns and the effects of high water and possible toxic blooms, underestimates the threat to humanity’s survival.  Recent research suggests that with global warming, tropic climates stretch northwards.  Thus, projections for the US (not to mention Europe below Scandinavia, Australia, southern Africa, and southern Russia) is for extreme drought.  How can this be, when there will be lots of increased water vapor in the air?  Answer: it will be rare in falling, and far more massive and violent when it does.  The heat will bake the ground hard, so that when it does rain, the rain will merely bounce off the ground and run off (with possible erosion), rather than irrigating anything.  Add depletion of aquifers and of ice-pack runoff, and it will be very hard to grow anything (I suppose, mountains partially excepted) below Siberia, northern Canada/Alaska, and Scandinavia. 

However, these have their own problems:  rains too massive (and violent) to support large-scale agriculture – which is why you don’t see farming on Seattle’s Olympic Peninsula.  The only “moderate-rainfall” areas projected as of now, away from the sea and the equator, are a strip in northern Canada, one in northern Argentina, one in Siberia, and possibly one in Manchuria. Most of this land is permafrost right now.  To even start farming there would require waiting until the permafrost melts, and moving in the meantime to “intermediate” farming areas.  Two moves, minimal farmland, and greater challenges from violent weather.  Oh, and if you want to turn to hunting you’ll be lucky if you have an ecosystem that supports top-level meat animals, not to mention the 90% of plant and animal species that will likely be extinct by then. As for the ocean, forget about it as a food source, unless you like jellyfish (according to research done for the UN recently).

In my version of Hansen's worst-consequence world, we would try to survive on less than 10 % of today's farmland, less than 10% of the animal and vegetable species with disrupted ecosystems, and practically zero edible ocean species, in territory that must be developed before it is usable, in dangerous weather, for thousands of years.

Hansen notes that one effective animal evolutionary response to past heat episodes has been hereditary dwarfism.  Or, as I like to think about it, we could all become hobbits.  However, because we are heading towards this excessive heat much faster than in those times, we can’t evolve fast enough; so that’s out.

What about inescapable?  Well, according to Hansen, CO2 levels would not get out of what he calls the “moderately moist greenhouse” area for thousands of years, and would not reach close to where we are now until 10,000-100,000 years hence.  By which time, not only will we be dead, but most of humanity, if not all.

Now, I had feared the Venus scenario, so the worst consequences are not as bad as I thought.  However, the increased estimate for temperatures in the moderately moist greenhouse and the wet bulb temperature consideration makes the next-worst scenario more likely than before to end humanity altogether. 

Snowball Earth:  Sad Beauty of a Sidelight

Having said all this, Hansen at least gives a beautiful analysis of why we don’t wind up a “snowball Earth” (the opposite scenario from a “runaway greenhouse”).  He notes that once the Earth is covered with ice, carbon can’t be recycled to the Earth via “weathering” (absorption from the atmosphere by rocks whose surfaces are abraded by wind and water).  So volcanic emissions and the like put more and more carbon dioxide in the atmosphere, until the temperature warms up enough and melting of the ice begins.  Apparently, evidence suggests that this may have happened once or twice in the past, when the Sun was delivering less light and hence heat.

Envoi

The usual caveats apply.  Primarily, they fall in the category of “I was reading Hansen out of fear, and so I may be stretching the outer limits of what may happen, just as Hansen may be understating out of scientific conservatism.”  Make up your own mind.

I am reminded of a British Beyond the Fringe comedy skit about WW II, suitably amended:

“Go up in the air, carbon. Don’t come back.”

“Goodbye, sir.  Or perhaps it’s ‘au revoir’?”

“No, carbon.”

And what will it take for humanity to really start listening to Hansen, and to the science?