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Speaking science to climate policy

Posted on 15 June 2011 by John Cook

Reposted from The Conversation. This is the third part in a two-week series Clearing up the Climate Debate.

CLEARING UP THE CLIMATE DEBATE: CSIRO’s James Risbey explains why it’s not “alarmist” to describe the threat of climate change to the public and how the climate system will respond to half measures.

With many issues to be considered in setting a climate policy one can end up wondering what the role of climate science is in all this.

After all, climate science doesn’t tell us what to do. It doesn’t tell us whether to have a carbon price or where it should be set. Those decisions ultimately involve a range of normative and deliberative issues which are beyond the scope of climatology.

Climatology can tell us, however, what is likely to happen if we don’t act, or if we don’t act with sufficient speed to keep total emissions within specific carbon allocations.

There is no single threshold above which climate change is dangerous and below which it is safe. There is a spectrum of impacts. But some of the largest impacts are effectively irreversible and the thresholds for them are very near.

In particular, the melting and breakdown of polar ice sheets seems to be in the vicinity of a couple of degrees warming. This expectation is based on current high rates of mass loss from the ice sheets compared to relative stability through the Holocene (the past 10,000 years) and on past ice sheet response in periods such as the Pliocene (a few million years ago) when the Earth was a couple of degrees warmer than preindustrial times (and sea level up to 25m higher).

We have already had about 0.8°C warming globally, with another third of a degree locked in by the inertia of the climate system.

That leaves, somewhat optimistically, perhaps a degree or so of wiggle room. Translating that into carbon emissions, if we wish to keep the total warming below about 2°C (with 50% chance), then we have a total global carbon emission allocation of between about 800 and 1000Gt carbon.

We have already emitted about 550Gt, leaving perhaps another 250–450Gt. Current global emissions are about 10Gt per year, growing at roughly 3% per year.

That leaves a few decades at present rates before having committed to 2°C warming and crossing the expected thresholds for ice sheet disintegration. And that is for a 50% chance of not crossing the 2°C threshold. For more comfortable odds of staying within the threshold, the total carbon allocation drops and so the time to threshold is even shorter.

Surely this estimate is vastly uncertain?

Everything has some uncertainty, but the uncertainty in this case lies mostly in the timing, not in the essential result. Ice sheets are sensitive to warming somewhere in this vicinity of temperature change and the climate system will yield 2°C warming somewhere in the vicinity of 800–1000Gt of carbon emissions.

If the climate is a bit less sensitive than we think then we might have a little bit more wiggle room than the 250–450Gt allotment, but not much, and we’ll exceed that allotment very soon thereafter anyway.

We’re only a few decades away from a major tipping point, plus or minus only about a decade. The rate at which the ice sheets would melt is fairly uncertain, but not the result that says we are very close to a tipping point committing to such melt and breakdown.

If we were to keep remaining emissions inside the 250–450Gt carbon allocation, we would need to take account of the inertia in energy systems and infrastructure, which set some limits on the maximum rate that emissions can be reduced.

To stay within the budget, we can’t hope to emit 10Gt a year (the present emissions rate) for the next thirty years and then reduce emissions suddenly to zero. Rather, net emissions would need to be phased down to zero to stay within the budget.

The longer stringent emissions reductions are delayed, the more drastic they must be to stay within the 250–450Gt budget. With more than a small delay, the reductions needed are faster than can be achieved in turning over the stock of emitting infrastructure.

Thus, if we were to stay within this budget, dramatic emission reductions would have to begin now. Delayed action on stringent emissions reductions almost certainly implies overshooting the thresholds and locking in vast long term impacts.

Is it irresponsible or “alarmist” of climatologists to point this out? The science brief for policy is not to prescribe policies, but to point out the implications of pursuing or not pursuing particular courses of action.

Pointing out that we are close to one of the largest tipping points imaginable in the climate system is well within the remit of science. It’s not alarmist to describe the threat accurately; it’s alarming if the political and social culture can’t absorb this.

Sociologists tell us that it is easier to motivate people for climate policies by focusing on the benefits of acting (the carrot) rather than on the costs of not acting (the stick).

As such, they suggest focusing on a positive vision and the good outcomes associated with addressing climate change. While this may be the right strategy, the appeal to benefit comes with no timetable and no particular sense of urgency.

It is the knowledge of climate thresholds and emission rates that sets the timing issues. Science provides the stick, which is the statement of consequence of not reducing carbon dioxide emissions in rapid order. The carrot might be the best way to get us moving, but the stick sets the beat.

Whatever motivations we use to enact climate policy, the climate will respond to our emissions. Emissions policies must therefore be measured by the effects they will have on the climate (among other things).

Policy measures that do not provide the ability for a stringent draw down of carbon emissions on the short time scales implied by the 250–450Gt carbon budget vastly increase the likelihood of crossing critical climate thresholds.

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Comments 1 to 24:

  1. If you separate out the (long lived) greenhouse forcing and the (short lived) aerosol forcing, the 2 degree target suddenly looks a lot closer. The total radiative forcing by greenhouse gases (CO2, CH4, CFC's, N2O, O3) is around 3 W/m2, with which we have ‘committed’ the planet to warm up by 2.4 °C (1.6-3.6 °C), according to a climate sensitivity of 3 °C (2-4.5 °C) for a doubling of CO2. The observed amount of warming thus far has been less than this, because part of the excess energy is stored in the oceans (amounting to ~0.5 °C), and the remainder (~1.3 °C) has been masked by the cooling effect of anthropogenic aerosols (Ramanathan and Feng, 2009). This simple analysis shows that the ‘2 degree target’ of maximum acceptable warming is looming on the horizon, as the climate equilibrates and aerosol pollution is cleaned up.
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  2. Bart, how did you arrived at a current "committed" value of 2.4C? It appears that you are attributing higher warmer to the gases other than CO2. Is this correct?
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  3. The numbers and reasoning is taken from Ramanathan and Feng (2009). Total greenhouse forcing is 3 W/m2, which exists approx half of CO2 and half of the other GHG's. With a sensitivity of 3 deg per doubling (or 0.8 deg per W/m2) this comes to 2.4 deg.
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  4. What's the residence time of aerosols compared with that of the other GHGs? If they're not too dissimilar, then it'll even out even if we cut emissions of aerosols. Assuming, of course, that human emissions are the only source of the other GHGs, which seems perhaps incorrect given the permafrost melt going on. Either way, just considering the CO2 forcing (and the fast feedbacks) would give a lower bound for the committed warming, right?
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  5. I like this article, except for the comment that we're committed to another third of a degree warming. It's twice that, and as bart notes, even more once atmospheric aerosols are reduced. But the comments about risk thresholds and needing to cut emissions immediately are spot on. Bern - aerosols have a very short atmospheric residence time, just a couple of years I believe. As we reduce aerosol emissions, atmospheric concentrations will drop rapidly. If you just consider GHGs and ignore the aerosol forcing, that gives the upper bound for committed warming. That's bart's 2.4°C from current GHG levels, although technically it's not an upper bound because he's using the most likely climate sensitivity value (3°C), not the upper bound (4.5°C or more).
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  6. The residence time for aerosols depends, I think, on where the aerosols are located. Sulfates from volcanic eruptions can end up in the stratosphere, and stay there for a few months or years. Aersols from non-volcanic sources (dust, sulfates, nitrates, organic aerosols, black carbon, etc) stay in the lower atmosphere on the order of days.
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  7. The aerosols are Hansen's Faustian Bargian. Hansen estimates the forcing as -1.3 W/m2. According to Bart's 0.8 C/W/m2 @3, that is over 1C of committed warming in addition to what is in the pipeline due to ocean thermal inertia. As dmccubbi says, they have a residence time of only a few days or weeks. They are continually renewed (they are one of the major components of the asian brown cloud and acid rain in the US Northeast). When they are cleaned up (eventually they must be cleaned up) the climate forcing will immediately increase. The bargain is that the aerosol pollution today will keep temperatures from rising untill the aerosols are no longer released. This committed warming is hidden for the present.
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  8. totally agree!
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  9. Does anyone know the source of Risbey's reference to "a total global carbon emission allocation of between about 800 and 1000Gt carbon" and that "we have already emitted about 550Gt, leaving perhaps another 250–450Gt". Meinshausen et al (2009) “Greenhouse-gas emission targets for limiting global warming to 2°C” Nature 458:1158-1162 stated that, "Limiting cumulative CO2 emissions over 2000–50 to 1,000 Gt CO2 yields a 25% probability of warming exceeding 2 6C—and a limit of 1,440 Gt CO2 yields a 50% probability—given a representative estimate of the distribution of climate system properties." Meinshausen et al's figures are in CO2 while Risbey appears to be using carbon-only figures (Nb. We can convert from carbon to CO2 by multiplying by 44/12). Meinshausen et al are also talking about emissions between 2000-2050 while Risbey does not define the time interval so it appears he may be speaking about emissions since the beginning of the Industrial revolution. Can anyone clarify these figures?
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  10. Michael Sweet #7 One of the oft used reasons for the post-WW2 cooling up to about 1975 was the masking effect of Aerosols. Do you remember the argument that the 'Clean Air Acts' - cleaned up North American and European sulphate and other emissions - had unmasked that CO2GHG forcing - hence the increase in temperatures since 1975? Well - Hansen has brought them back to explain another masking of CO2GHG forcings - the last 10 years or so. While it is feasible that unmeasured Aerosols from the Chinese and Indian and other Asian economies have taken the place of the 1945-75 lot, one wonders why the window 1975-2000 ever existed. Surely emissions from Asia are not a new thing and have been steadily increasing for the last 30 years. "According to Bart's 0.8 C/W/m2 @3, that is over 1C of committed warming in addition to what is in the pipeline due to ocean thermal inertia." What is in the pipeline? Temperature rise or a rise in sequestered heat energy? Ocean thermal inertia can only redistribute heat already sequestered as temperature rise (or ice melt) in many places and temperature drops somewhere else. This is an 'internal' forcing effect. At any instant in time, the storage of heat energy will be represented by a mass phase change (WV or Ice) and a mass temperaure increase somewhere. If Hansen's rediscovered Aerosols are masking CO2GHG forcing by closing the imbalance gap - less heat energy is being sequestered for the oceans thermal inertia to redistribute. The 'warming' is not in the pipeline - its in the time tunnel - dependent on future imbalance.
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  11. Ken Lambert @10 asks rhetorically:
    "While it is feasible that unmeasured Aerosols from the Chinese and Indian and other Asian economies have taken the place of the 1945-75 lot, one wonders why the window 1975-2000 ever existed. Surely emissions from Asia are not a new thing and have been steadily increasing for the last 30 years."
    If he tried for a little less rhetoric and a little more understanding, he would realise that Chinese coal consumption went through the roof after 2001, and that aerosol emissions have risen with it.
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  12. Tropospheric aerosols have a lifetime of several days or weeks (dependent on their size). In sharp contrast to the lifetime of greenhouse gases, ranging from 12 years (methane), via nitrous oxide (120 years) to CO2 (100's to 10(0),000's of years). Stratospheric aerosols (e.g. as a consequence of large Tropical volcanic eruptions, but also in-situ formation) have a lifetime of a couple of years. See also this background on atmospheric aerosols that I wrote on my blog.
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  13. Ken, I suggest you read the linked paper. Hansen has been calling for a satelite to measure aerosol contributions since at least 1990 and estimates in the paper I linked at 7 that such a mission would cost only $100,000,000. For various political reasons it has never been launched. (The Bush adminsitration removed "protect the home planet" from the NASA mission statement, but Clinton did not launch it either). Tom's graph indicates that your description of a lag between when the West (partially) cleaned up their aerosols and when China started emitting large amounts correlated with the temperature rise you described. Perhaps if you read the background you would see that this is a coherent narative. This is a critical issue since if in fact aerosols have hidden 1C of warming we are in for a big shock when China cleans up its act, as it eventually must. The ocean's thermal inertia holds back the temperature over the ocean and nearby land. A quick type of "ocean thermal inertia" in the search function yields Has Earth warmed as much as expected? which describes how the ocean holds back the temperature of the Earth in the first section labeled "Thermal Inertia". The search has a large number of additional hits. Your argument that "Ocean thermal inertia can only redistribute heat" is simply incorrect. You have been on this site long enough to have read this material, what are you trying to show with such a naive argument?
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  14. Correction: the Bush Administration actually removed "to understand and defend our home planet." from the NASA mission statement, not "protect" the home planet.
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  15. #13 "Hansen has been calling for a satelite to measure aerosol contributions since at least 1990 and estimates in the paper I linked at 7 that such a mission would cost only $100,000,000. For various political reasons it has never been launched." Wasn't it the mission of the crashed Glory satellite though ?
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  16. Tom Curtis #11 Your coal use chart is a useful illustration Tom, but not the full story of Aerosols. While coal consumption by China has risen sharply in the last 10 years, so has the building of cleaner plant - the whole of Australia's coal fired electricity capacity every year for the next 10 years is planned. Closing of China's older dirtier plant is also happening as PM Gillard is wont to tell us (which of course is less than half the story). So what we really need is a global chart of Aerosol releases into the atmosphere. I will look around for one.
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  17. Michael Sweet #13 "Your argument that "Ocean thermal inertia can only redistribute heat" is simply incorrect. You have been on this site long enough to have read this material, what are you trying to show with such a naive argument?" Why it my argument incorrect? It is consistent with the first law, and does not confuse temperature rise with increased energy absorption.
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  18. Ken Lambert @16, China's efforts to reduce air pollution, and as a result, aerosol production have only recently started. The first five year plan to reduce aerosols began in 2008, so from 2001 to 2008, the increase in coal consumption probably maps very closely to increased aerosol production. These twin images from NASA will help put China's emissions into context. The first shows the average aerosol optical depth over the period 2000-2007. The second shows the 2007 anomaly with respect to that average.
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  19. Ken, Many arguments that are consistent with the first law are incorrect, there are many other issues that have to be considered. I am not sure what you are confusing, you are not being clear on your problem. Increased energy absorption does cause temperature rise. The heat capacity of the ocean (which is what relates to the thermal inertia) means that it absorbs heat from the atmosphere and cools the atmosphere down. This heat is then mixed around the ocean. The ocean will absorb heat as long as it is not in equilibrium with the atmosphere. Search ocean heat content for discussion of this heat. Your statement "Ocean thermal inertia can only redistribute heat" is incorrect. Thermal inertia and heat distribution are different issues. This has been extensively discussed on Skeptical Science before. Please read the citation I provided to you above and use the search function if you are still confused. It is not my responsiblity to spoon feed you information that is readily available. If you have a question about the thermal inertia of the ocean post it on a more suitable thread. The aerosol forcing has been a critical hole in the information to determine the forcings of Global climate change for decades. If one satelite did not make orbit another should have been launched.
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  20. It's not that simple, as described in this article : "One way to look at this is that we have a football team with only one player at most positions, and none at a few positions. When one of the players we do have gets hurt: there are no replacements. You play without him and wait until he heals. The time to heal a lost space mission is typically 3 to 7 years depending on budgets and how many spare parts remain from the last instrument builds."
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  21. Papy, This mission has been delayed for over 20 years already. That is more than 6 times your three year turn-around time. It is a scandal that such basic information on the Climate is still not known. If Hansen's estimate of 1.3 W/m2 is correct we are in for a world of pain.
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  22. Michael Sweet #21 "If Hansen's estimate of 1.3 W/m2 is correct we are in for a world of pain." Hansen has upped the aerosol cooling estimate as one of the factors to explain the reduced theoretical warming imbalance ie. 0.9 down to 0.59W/sq.m. I don't recall seeing anything in his paper about raising the warming forcings for CO2GHG which should theroetically be about 1.77W/sq.m for a 390ppmv concentration. Why would this forbode a 'world of pain'?
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  23. Michael Sweet #19 I am not confused at all. Thermal inertia has everything to do with heat flow and temperature distribution.
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  24. Ken, Apparently the link I provided for you in 13 did not work for you. here it is again. After you have read the section on thermal inertia and unrealized warming we can discuss this further. It would be better to post your questions on that thread since it is on topic there.
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