Sunday, April 28, 2013

The better question is what should the global average temperature revert to after stabilizing

An argument I've seen more than once from climate inactivists sometimes comes in the form of a question, "what is the ideal average global temperature," as if the question has a deep implication. In mid-gallop from "there's no warming; the warming is all natural; humans have little contribution," this is the step, "the warming gets us to a better temperature anyway," before they move on to "the overall negative effect isn't that bad; it's too soon to take action; it's too late to take action."

The first naive thought would be that places like Alaska should welcome some warmth, and a lot of the world's land mass is polar. What they miss is how melting permafrost results in sinking roads and buildings, forests die because insect pests survive mild winters more easily, and coastlines disappear with the loss of sea-ice protection from waves. If you put Hawaii's climate in Alaska, then Alaska would suffer. Both the biological and human environments are adapted for the climates they have.

So here's my hypothetical alternative:  assume, very optimistically, that in the year 2050, gross CO2 and equivalent emissions have been reduced 95% from present through a variety of technological and behavioral changes, and that carbon-negative technologies like biochar and biomass-plus-sequestration balance out the remaining 5%. What do you do next year and the following years?

Simplest answer is do even better, if you can. The rule that when you find yourself in a hole, the first thing to do is stop digging, hasn't yet been satisfied. The oceans will be transferring back latent heat for decades after 2050, so even zeroing out emissions won't be enough to stop further warming. If you can get an increase in carbon-negative activities so that effect, plus annual ocean absorption of CO2, means the reduced atmospheric CO2 warming balances out the latent heat release from oceans, then at that point we'll have stopped digging deeper. And then, what next?

A further increase in carbon-negative actions will mean anthropogenic forcing is slightly net negative compared to the previous year. Continuing that year after year would start to raise the question, when do we stop? What average temperature are we aiming for? I don't think it's the 1850 average - neither we humans nor many ecosystems will function most naturally at that level.

I don't really have the answer; I just think it's an interesting question. Maybe more of a science fiction question, but our children will (hopefully) have to deal with it someday. As a policy question, the most recent, highest temperature will not be the one that people or ecologies are most adapted to, and neither will a temperature from a century or two earlier. People probably adapt faster than ecosystems, so if we choose a human-biased priority then the aimed-for cooling will be less pronounced than one prioritizing ecosystem recovery. Different societies and different ecosystems will have different ideal stabilizing temperatures, but unless they're really good with geo-engineering, then we only get one level of net forcing.

Maybe there won't still be millions of subsistence farmers on the edge of malnutrition 50 years from now, but I wouldn't count on that. Stabilizing their precipitation patterns probably should rank in the highest priority, but we'll have to see how much political pull they'll have to make that happen.

UPDATE:  Good comments, esp from Tom Curtis who says it's not correct to call the heat transfer back from oceans "latent heat". I'm not sure I agree though that the optimal temperature for humans or nature would be a pre-industrial temperature, either the 1850 temp I discuss above or Tom's reference to typical (warmer) Holocene temps. Natural ecosystems will have spent the previous 150 years moving in response to climate change - trying to get them to move again when 9 billion people are in the way could be a recipe for even further losses. Humans will be even more adapted to the existing climate.

A chosen temperature would eventually have to be low enough to stabilize the Greenland and West Antarctic ice sheets, although I assume we've got many additional decades or longer to do that.

I think in the very long term we would want to return to something like Tom's preferred temperature level.


Russell Seitz said...

"assume, very optimistically, that in the year 2050, gross CO2 and equivalent emissions have been reduced 95% from present through a variety of technological and behavioral changes,"

While legislating a 95% anthropic breathing behavior cut would reduce CO2 emissions by thousands of teragrams, I doubt the measure's proponents could survive the necessary filibuster.

David B. Benson said...

I advocate reducing atmospheric CO2 to 270 ppm. That will clearly take many decades, if not centuries, giving an opportunity to experience many different regional climates.

After some time at such low levels sea level will go down and then, if desired, the concentration can go back up to about 285 ppm.

Richard Mercer said...

"what is the ideal average global temperature,"

I see that argument all the time in comments at MSM articles on climate change. What I tell them is that something within the temperature range of the wonderful Holocene would be nice. And explain a few of the alternatives to that.

Tom Curtis said...

First, "better" always needs a qualifier. Better for cockroaches in not better for humans, and better for polar bears is probably bad for humans (and brown and black bears) as well. Assuming that "better" is better for the flourishing of human civilization, or better for the flourishing of currently existing ecosystems or some combination of the two, better means closer to the mean global surface temperature over the last 10 thousand years or so. Following Marcot et al (2013), that is about 0.2 C above the 1961-1990 average. There are probably fewer short term problems if it is dropped the 1961-1990 average, and a slight long term benefit if we lift it just above it. Anything above the 2000-2009 average exceeds levels that could be considered beneficial.

Second, the ocean has warmed less the the GMST, so there is no latent heat waiting to return. Rather, there is a TOA energy imbalance that can be restored by increasing surface temperature. Stopping the increase in GHG concentrations will not restore that balance, which will only be fully restored once the ocean is in thermal equilibrium with a surface at the right temperature. For a CO2 concentration of 400 ppmv, that temperature is approximately 1.5 C above the preindustrial average, or 1 C above the 1961-1990 average.

Third, however, once we eliminate net emissions, the atmospheric CO2 will rapidly reduce as an equilibrium of pCO2 is restored between atmosphere, surface ocean and deep ocean. That process will approximately halve the change in CO2 concentration from pre-industrial levels over about two hundred years (after which it will take thousands of years for additional reductions). By coincidence, the rate of fall CO2 concentration is close to the rate of achieving temperature equilibrium, so that the net effect is that temperatures are fairly stable over the period as we change from the transient climate response to the peak concentration to the equilibrium response to the stabilized CO2 concentration. Consequently a stabilization concentration of about 330 ppmv is about right for a stable temperature at about 0.2 C above the 1961-1990 average; and a peak CO2 concentration of 380 ppmv followed by zero net human emissions is required for an "optimal" final outcome.

Whoops! Blew right past that one, didn't we.

So some artificial sequestration is required for an optimal outcome, and if we only reach zero net emissions in 2050, the amount we need to sequester will approach the amount we have already emitted to date. However, attempting to reduce atmospheric concentrations to 280 ppmv again absent considerable refinement of the calculation above would be a mistake.

Richard Mercer said...

With so many of the present GOP congress already mistakenly believing that human breathing has anything to do with climate change or that any scientists think such absurd things, like you mistakenly believe, they would filibuster it anyway.

You don't add or subtract CO2 from the active carbon cycle when you breathe. Burning fossil fuels Adds to that carbon cycle and is overwhelming the natural processes that help keep it in balance.

if plants needed more CO2, they would take up all the excess CO2 we are emitting, and it wouldn't be accumulating in the atmosphere - up 40% in 100 years. -about 30% higher than any time in at least the last 800,000 years

The oceans are taking up 30% of our excess CO2.
- and are becoming acidified as a result. CO2 dissolved in sea water becomes carbonic acid.

And yet it keeps accumulating in the atmosphere. That is nature telling you that we are overwhelming the natural carbon cycle with too much carbon.

Fredb said...

I think its the wrong question ... asking what the optimum temperature is only has meaning against a subjective metric ... for which different sectors of society will have vastly differing opinions.

The question we should be examining, I suggest, is whether we are appropriately adapted in our infrastructure to and activities to whatever the temperature is / will be?

The difficulties arise when temperature is unstable and our activities cannot equilibrate to a known envelope of expected variability.

Martin Vermeer said...

By coincidence, the rate of fall CO2 concentration is close to the rate of achieving temperature equilibrium

Yep, I missed that too in Brian's piece

Jay said...

"what is the ideal average global temperature,"

A stable one.

Anonymous said...

I'm currently in the belief it should stabilize to +4C and c.400ppm CO2 with about 20m of sea level rise. This will allow me to continue my current life style and have some hope the work put on the yard will still have some effect in 3000AD, though likely even the progeny of the elm I moved about a 100 miles north of normal range will have died by then :-\. (think this as normal climate 'hysteria' if your prone to such exaggeration)

Russell Seitz said...

Richard mercer's grasp of the carbon cycle seems only marginally better than his sense of irony - it takes fossil fuel to put the food he respires on his table too.

Tom Curtis said...

Russel Seitz' knowledge of chemistry appears to be lacking. While natural gas is used to reduce nitrogen into ammonia used to make fertilizer, the carbon from the natural gas does not enter the food cycle. Hence respiration remains the simple recycling of carbon already in the atmosphere.

I have seen suggestions that bacteria used to absorb CO2 from industrial processes could then be used as fertilizer, a process that would introduce fossil carbon into the food cycle, not to mention eliminate any sequestration of carbon attempted in the first place; but currently while fossil fuels are used to produce food, they do not enter the food cycle.

Tom Curtis said...

Brian raises the interesting (and very complicted) issue of migration in an update.

My feeling is that in the short term (next 200 years), those species able to migrate to adapt to a warming climate will also be able to migrate back to adapt to a cooling climate if we draw down CO2 by artificial means. In contrast, those species that have difficulty migrating will, if still surviving, be very stressed by the warmer climate in their habitat, either directly or by increased competition from other species. For those species a more rapid reduction in temperature will be beneficial, whereas for the more migratory species it will not be harmful (or as harmful) because they are in fact able to change their range in response to changing climate.

Over the course of 200 years, adaption other than by migration is not a significant factor.

Having said that, I would certainly settle for eliminating net emissions by 2050 over what seems the more likely prospect for our future.

David B. Benson said...

Brian's update fails to take into account the changes in precipitation, globally increasing as the square of the temperature increase. [Source: Ray Pierrehumbert's "Principles of Planetary Climate"] We need a climate which is best for the agriculture we practice now. Nor is it at all clear that 'optimal' agriculture can be practiced at a more elevated temperature.

Prudence suggests now attempting to return to the global regime of around 1850. Future generations may wish to adjust the adjustment.

Jeffrey Davis said...

At 2C of warming not only will roads sink etc but CO2 and methane will outgas from tundra and taiga at levels equal to current human contributions. Meaning we just wave at 2C of warming as we pass it. So, some ideal temp would be seriously less than 2C warmer than before AGW.

Hank Roberts said...

Wrong idea.

Aim to reduce the rate of change, to kill less of the biosphere, less quickly, than we're doing now.

The Silent Mass Extinction of Insect Herbivores in Biodiversity Hotspots
DOI: 10.1111/j.1523-1739.2009.01327.x

What can you do about it?

Find a vacant lot, an ignored ditch, a bit of nature within walking or bicycling/skateboarding distance of kids, and -- keep it alive.

Hank Roberts said...

Saving a Million Species
Extinction Risk from Climate Change
ISBN: 978-1-61091-182-5 (Online)

"In this first section of the book, we look at the 2004 publication that sparked worldwide interest in extinction risk from climate change. The overall purpose and scope of the book are described in chapter 1, with an overview of the chapter structures and the reason for each part of the book. Chris D. Thomas, lead author on the 2004 study, then explains the research, the limitations of the methods used at the time, and the importance of the findings. The final chapter of this part reviews the policy implications of extinction risk from climate change, from the UK House of Commons and the US Senate to international
climate treaties and beyond."


Hank Roberts said...

shorter: the effects of change are different in different places; we aren't going to get back to the same place by reducing co2. Local attention everywhere to reduce the rate of loss might help a bit.,width=280,height=280,appearanceId=4.png/i-think-you-ll-find-it-s-a-bit-more-white-text_design.png

Brian said...

Tom - sounds like we need data.

David - no I'm thinking about precip, and I think precip is the key issue for subsistence farmers, much more than a few tenths of degree of temp change. Subsistence farmers aren't and won't be used to 1850 precip patterns. My guess (just a guess) is they'll be best adjusted to whatever the climate was from 20 years before the inflection point. Maybe earlier if the climate had been changing rapidly.

David B. Benson said...

Assuming quasi-stability is reached, a decline would be quite slow. Plenty of time to adjust.

Subsistence farmers surely would prefer predictability of precipitation. That is obtained by lowering the temperature slight;y.

raypierre said...

The point labeled "Third" by Tom Curtis above is based on a fundamental misconception regarding the carbon cycle. The excess CO2 will most emphatically not go down by a factor of two within two hundred years, because CO2 is taken up on a range of different time scales ranging into the millennia, and we have already "used up" the fast sink. In reality, if CO2 peaks at 625 ppm at the time we stop emitting, it only goes down to 525 PPM by the year 3000. And the more CO2 you pump out, the worse the situation gets, since you saturate the ocean's ability to take up CO2, by acidifying it. Take a look at Susan Solomon's paper on "irreversibility" in PNAS, or the WCRP review article we wrote.

raypierre said...

Davis Benson is quoting the result on precip in my book more or less correctly, but we have to attach a few caveats to that calculation. First, it's at best global mean, and since land is only 1/3 of the globe, it's hard to constrain land precip by energetic arguments alone. Second, even with regard to the global mean, the results from the averaged energy balance overestimate the precip/temperature slope relative to what you see in GCm's, indicating that something in the dynamics is changing boundary layer parameters in a way to offset some of the anticipated precip increase. Finally, max precip rates to seem to scale nonlinearly as the simple arguments imply, even though global mean precip increases more weakly than anticipated.

Lots to learn about precip, still, lots still not understood.

raypierre said...

Though Tom Curtis overstated the CO2 reduction during the two centuries following the cessation of emisson, his statement about temperatures remaining pretty flat over a thousand years, owing to the competition between the deep ocean warming up and the CO2 being drawn down, is correct, at least insofar as idealized models like the UVic model show. This flat period is what you get instead of "committed warming." It hasn't been extensively studied in full ocean-atmosphere GCM's though, and there are indications that contrary to the simplified models, there may be some continued warming following cessation of emissions. Stay tuned.

Tom Curtis said...

raypierre, I based my claim on the rule of thumb espoused by David Archer that atmospheric CO2 will reduce to about 25% of total emissions over 200 years, with that level being then maintained "forever" in practical terms. Given that about 50% of CO2 stays in the atmosphere after a year (or that current atmospheric CO2 has increased over pre-industrial levels by about half of total emissions to date), it follows that over the 200 years following the cessation of emissions, the increase in atmospheric concentration will approximately halve.

Of course, as total emissions climb, the fraction retained by the atmosphere and the time to equilibrium between surface and deep ocean increases (See Archer et al 2009); but for a trillion tonnes C emissions, the 25% of total emissions (or approximately 50% of peak increase in concentration) is close enough. Specifically, greater accuracy regarding CO2 concentrations will do little to improve our predictions of future temperatures at that level given uncertainties about climate sensitivity.

Of course, ceasing net emissions by 2050 will closely approximate to total emissions of about a trillion tonnes of Carbon.

Given the above, are you disagreeing with me because you think my approximation is not close enough for the purposes of this discussion? Or are you disagreeing with me because you disagree with Archer's estimates of the future trajectory of CO2 concentration? And if the later, on what basis?

raypierre said...

Tom you are double counting. The air fraction of 50% is due to the past atmosphere-ocean disequilibrium, and the uptake does not continue at the same rate after emissions cease. I'm not "disagreeing" with you, I'm pointing out correct calculations of how rapidly the CO2 falls relative to its peak value, once emissions cease. For a good discussion of this point, leading off from the basic ocean chemistry and mixing dynamics Archer has been writing about for years, see Susan Solomon's paper on "irreversibility"

Relative to its peak value, CO2 goes down rather little over the millennium following cessation of emissions.

Also, I don't know where you are getting your 200 year figure. Excess CO2 declines to about 20% of the cumulative emissions over the deep ocean mixing time, which is more like 1000 years than 200 years. And because of the "fast uptake" part of the CO2, the 50% of net emissions that have disappeared from the atmosphere at the time emissions cease is already counted toward that run down to 20%, so you don't get to count it again when figuring out how rapidly CO2 declines from its peak value. All very well explained in the Solomon et al PNAS article.

Tom Curtis said...


1) You are "disagreeing" with me. You initial posted started:

"The point labeled "Third" by Tom Curtis above is based on a fundamental misconception regarding the carbon cycle. The excess CO2 will most emphatically not go down by a factor of two within two hundred years, because CO2 is taken up on a range of different time scales ranging into the millennia, and we have already "used up" the fast sink."
(My emphasis)

If you were not disagreeing with me you would have written something along the lines of, "While Tom's formula is a useful rule of thumb; a more accurate formula is ...". If indeed you are now not disagreeing with me, you need to retract those earlier emphatic claims.

2) Going to your cited source (Solomon et al 2008), I took their figure 1 a, and plotted lines corresponding to my rule of thumb regarding the decline of CO2, marking the point of 200 years since cessation of emissions with a cross bar. The resulting figure is here, and shows my estimate agrees with Solomon et al for peak concentrations of 450 and 550 ppmv, and is fairly close at 650 ppmv. It will become increasingly inaccurate at higher concentrations.

I also marked the 525 ppmv level mentioned in your original post, which is shown to be above the concentration reached for an 850 ppmv peak by the end of the milenium. Your claim that a 625 ppmv peak would result in 525 ppmv at the end of the milenium is also shown to be in disagreement with Solomon et al.

3) I am not double counting. At the time when emissions cease (assuming no tail of), the CO2 in the atmosphere is approximately 50% of cumulative emissions. For approx one trillion tonnes C of emissions, the CO2 excess over pre-industrial levels represents approx 25% of cumulative emissions, and hence 50% of the excess at the time of cessation of emissions.

Solomon et al disagree with that approximation in that they cite 40% rather than 50% of peak concentration at the time of stabilization. As they say:

"Fig. 1 shows that a quasi-equilibrium amount of CO2 is expected to be retained in the atmosphere by the end of the millennium that is surprisingly large: typically ≈40% of the peak concentration enhancement over preindustrial values (≈280 ppmv). This can be easily understood on the basis of the observed instantaneous airborne fraction (AFpeak) of ≈50% of anthropogenic carbon emissions retained during their buildup in the atmosphere, together with well-established ocean chemistry and physics that require ≈20% of the emitted carbon to remain in the atmosphere on thousand-year timescales"

Where appear to disagree with Solomon is in placing the interval to effective stabilization as being only 200 years; but for lower realizable total emissions (on trillion tonnes), that is a reasonable approximation as shown above.

Anonymous said...

Can we just get back to a world in which we have May snowstorms in Texas again?

Oh, wait...

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