Some climate scientists choose not to talk specifically about emissions reductions. Dr James Hansen of NASA’s Goddard Institute for Space Studies and Columbia University is one of them. Prof John Schellnhuber of the Potsdam Institute for Climate Impact Research is another. Instead they refer to a level of atmospheric CO2 (plus other greenhouse gases that together constitute a total forcing) that would be required to re-establish a ‘safe’ climate.
But surely targeting a safe level of CO2 is basically the same thing as aiming for an emissions cut of X% of some baseline year in Y years (e.g. 80% global reduction in emissions compared to 1990 levels by 2050)? Well, sort of, but not really. Both approaches certainly aim for a reduction in human-caused climate forcing, as the figure to the right indicates. Yet the difference between the two strategies is subtle but important.
To once again use an archery analogy (yes, I like these), aiming for a target CO2 concentration is like fixing your sights directly on the bullseye. That gold circle is what you want your arrow to hit, so you shoot for it. There’s a chance that you’ll miss, of course, but you use your past shooting experience, your training, and some good old ‘gut feeling’ to estimate where to aim – and are prepared to adjust your shooting hand up or down as the wind changes.
Alternatively, aiming for a given level of emissions reduction is like estimating the distance to the target butt, knowing the draw weight of the bow, the shaft stiffness of the arrow, and so on, and then using some ballistics theory to calculate the angle of launch you require in order for the arrow trajectory to rise and descend to the target according to the appropriate mathematical parabola. You don’t look at the target when you shoot, and have no real chance to adjust your aim should the wind direction or speed change.
Spot the difference? One is an explicit aim, the other is implicit. That difference might matter, it might not.
There is an article in the Guardian today which explains Schellnhuber’s position in some detail. To quote:
Roll back time to safeguard climate, expert warns
[A return to pre-industrial levels of carbon dioxide urged as the only way to prevent the worst impacts of global warming]
Scientists may have to turn back time and clean the atmosphere of all man-made carbon dioxide to prevent the worst impacts of global warming, one of Europe’s most senior climate scientists has warned.
Professor John Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany, told the Guardian that only a return to pre-industrial levels of CO2 would be enough to guarantee a safe future for the planet. He said that current political targets to slow the growth in emissions and stabilise carbon levels were insufficient, and that ways may have to be found to actively remove CO2 from the air.
Schellnhuber said: “We have to start pondering that it might not be enough to stabilise carbon levels. We should not rule out that it might be necessary to bring them down again.”
Carbon levels have fluctuated over the last few hundred thousand years, but have rarely gone much beyond 280 parts per million (ppm), which is commonly referred to as the pre-industrial concentration. Over the last few centuries, human emissions of greenhouse gases have forced that concentration up as high as 387ppm, and it is rising at more than 2ppm each year.
World governments are currently trying to agree a deal that would restrict emissions and stabilise carbon levels at 450ppm, in an effort to limit global temperatures to 2C warmer than pre-industrial times.
Schellnhuber, who has advised the German government and European Commission on climate, said: “It is a compromise between ambition and feasibility. A rise of 2C could avoid some of the big environmental disasters, but it is still only a compromise.”
He said even a small increase in temperature could trigger one of several climatic tipping points, such as methane released from melting permafrost, and bring much more severe global warming.
“It is a very sweeping argument, but nobody can say for sure that 330ppm is safe,” he said. “Perhaps it will not matter whether we have 270ppm or 320ppm, but operating well outside the [historic] realm of carbon dioxide concentrations is risky as long as we have not fully understood the relevant feedback mechanisms.”
To read the full article, click here.
A recent study in the final throes of peer review, by Hansen and 9 co-authors, undertakes a detailed analysis of past climate responses to greenhouse gas forcing to show that the safe ‘target CO2’ to aim at is between 300 and 350 ppm CO2. Now given that we are already at ~387ppm in 2008 and rising at 2ppm per year, it’s clear on this basis that we’ve already overshot. All logic says that it’s time to nock another arrow to the bowstring and shoot again at the target – as quickly as possible – and be prepared to shoot again if we miss.
But instead, we’ve turned our back on the archery range and are running in the opposite direction…
31 replies on “Target atmospheric CO2 levels, not vague emissions reductions”
If a safe target is 300-350, and we are already at 387ppm… and you look at the rate of growth of CO2 emissions, and the political realities of the planet… well it certainly makes you hope like buggery that the other mob are right after all;)
I agree Matt.
Unless the sceptics are right, the momentum in both the global system and in human reluctance to address emissions are indeed likely to herald the ‘alarmist’ consequences that many have described.
And just because it is our grandkids and their decendants who will cop the brunt of our indecision, doesn’t make it any less alarming…
On the other hand the denialists’ laissez faire economic approach, which they so vociferously promote, doesn’t seem to be living up to its wonderful reputation if the last day’s financial woes are any indication.
It may not be your intention, but your words suggest that economists are at the head of the ‘do nothing’ school of climate thought. This couldn’t be further from the truth. Most of the economists I know (and being an economics phd student, I know quite a few) consider responding to climate change the central issue of the day (and likely, the next 50 years). Similarly, many of them are devoting most of their energy to combating the ‘human reluctance’ you identify.
That’s what economists do. They work out what rules and institutions are required to encourage self serving people to act in socially optimal ways.
Target setting is a critical part of climate policy that has been largely ignored in the past in Australia. Setting clear targets is a standard practice in government policy-making but it is something that the previous Australian Government simply refused to do.
The current Australian Government has its emissions reduction target of 60% by 2050 (based on year 2000 emissions) but also refuses to set any targets for stabilising atmospheric greenhouse gases or temperature rises.
David Spratt made a good analysis of Labor’s target of 60% reduction by 2050, pointing out that it appears to be based on the Stern Review and stabilising greenhouse gases at 550ppm and mean global temperature rises at 3°C (Spratt D (2007), “Is Labor’s climate policy ‘backed by the science’?”, http://www.carbonequity.info/docs/alppolicy.html).
Stabilising greenhouse gases and aerosols around 350 ppm CO2-eq and allowing a rise in mean global temperature of 1°C appear to be the highest targets that should be set if the Great Barrier Reef is to be protected from serious degradation.
Hansen et al (2008) recommend a target of stabilising atmospheric CO2 at 350 ppm based on saving coral reefs and avoiding major climate tipping points (see http://arxiv.org/abs/0804.1126). Note that Hansen prefers to use CO2-only levels rather than carbon dioxide equivalents (CO2-eq) because he believes the latter term has introduced more confusion than it is worth (Hansen pers. com. that will hopefully be explained in the final paper).
Whether it is technologically or economically feasible to return to 350 ppm CO2 or CO2-eq and stabilise the mean global temperature at 1°C or less should not be determiniative of whether Australia should set this target for its climate change policies.
Policy-makers should set targets based on what we want to achieve. We should not set targets that will produce unacceptable outcomes. To illustrate this point: if we want to build a bridge across a river that is 1km wide we would not ask our engineers and scientists to build us a bridge that was 500m long. We should apply the same logic to climate change policy and set targets for our engineers and scientists to achieve that produce results that we want to achieve. In this way protecting coral reefs such as the Great Barrier Reef can be used as a flagship ecosystem and a yardstick against which to measure dangerous climate change and, conversely, acceptable climate change.
I have made a more detailed explanation of these points available at http://cmsdata.iucn.org/downloads/cel_op_mcgrath.pdf
Barry, your archery analogy is a good one.
Setting clear targets for stabilising atmospheric greenhouse gases and temperature rises is an essential part of the policy response that the previous Australian government refused to do and the current government is very reluctant to do.
David Spratt makes a good argument that Labor’s target of reducing Austalia’s emissions by 60% by 2050 was based on the Stern Review and stabilising greenhouse gases at 550 ppm and global temperature rises at 3°C: http://www.carbonequity.info/docs/alppolicy.html.
I believe these targets are far too high and we should aim to for targets that we expect will save the Great Barrier Reef. Based on the IPCC AR4 and associated literature that requires us to aim to stabilise greenhouse gases at 350 ppm CO2-eq and allow a mean global temperature rise of 1°C: http://cmsdata.iucn.org/downloads/cel_op_mcgrath.pdf.
Hansen et al’s (2008) paper under review argues for returning the atmosphere to less than 350 ppm CO2 to avoid major tipping points “if humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted”: http://arxiv.org/abs/0804.1126.
[Sorry Chris, these got snagged by the SPAM filter due to lots of links – I’m still ‘honing’ the beast]
Sorry, about the duplicated comments in 4 and 5. I thought comment 4 had been lost in the email ether so I re-wrote it as comment 5.
If I wanted to lose weight am I better off stating that I want to be a size 8, or looking at my diet and exercise routine and setting achievable goals with small measurable timescales that may or may not be related to the ultimate concept of being a size 8…
I’m sure out there somewhere are world reknowned experts in the psychology and effictiveness of setting goals to achieve results. As climate scientists sure the end result is your focus, but sometimes you ahve to let go and t=let the experts in behaviour change, business management, psychology etc make sure that the “targets” we use are worlds best practice not the opinion/hunch of climate scientists…
As an example (and sorry I’m always the token nay-sayer!) the analogy with the bridge length sounds a lot like a lot of denialist arguments… they roll off the tongue and they are quite convincing… but do they actually stack up to the poke of a stick.
Anyway – at this stage I’d settle for a 500m bridge – by the time they get 500m hopefully everyone is convinced that the next 500m is a sensible idea…
by opinion/hunch I’m talking about the types of targets set… not the scientific opinion on what CO2eq levels need to be… I am NOT suggesting that the CO2 levels desired are just a hunch;)
I’m not even saying Barry is wrong… or that I’m right… just that out there somewhere is someone who IS right, and they need to be brought in to the fold.
Trouble is MattB, if you aim to be size 8 but keep on over-eating, or at least still sneak that choccy snack, then you hardly shed any weight at all. Then diabetes or a heart attack (read: carbon-cycle feedbacks) gets you and it no longer really matters whether you are still vainly pretending to diet or not. You have bigger, more immediate problems.
But at this stage I pragmatically prefer no bridge and no target butt at all, as I briefly mentioned here:
and will detail more some time soonish.
Just found this: http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/behavior.htm
“Behaviors That Will Help You Lose Weight and Maintain It
Set The Right Goals
Setting the right goals is an important first step. Most people trying to lose weight focus on just that one goal: weight loss. However, the most productive areas to focus on are the dietary and exercise changes that will lead to that long-term weight change. Successful weight managers are those who select two or three goals at a time that they are willing to take on, that meet the following criteria of useful goals:
Effective goals are 1) specific; 2) attainable; and 3) forgiving (less than perfect). “Exercise more” is a commendable ideal, but it’s not specific. “Walk five miles everyday” is specific and measurable, but is it attainable if you ‘re just starting out?” Walk 30 minutes every day” is more attainable, but what happens if you’re held up at work one day and there’s a thunderstorm during your walking time another day? “Walk 30 minutes, five days each week” is specific, attainable, and forgiving. In short, a great goal!”
It may appear totally random… but to me important. It randomly ties in with my weight loss example… but that was a coincidence.
As an aside is the goal 350ppm, or is the goal sending our massive economies past the tipping point of being carbon dependent and over in to the realms of liberation from carbon addiction. If we achieve that goal, then 350ppm is easy… And maybe the far easier target of 10-20% by 2020 and 60% by 2050 is enough to send us over the tipping point. I can only guess, but you may well find that an economy that has reduced by 20% by 2020 can;t help but thrive on even deeper cuts as we have realigned the economic and environmental benefits…
Hi Barry. Thanks for your blog. Would you consider a post about the variety of ways CO2 concentrations are reported and how they stand in relation to the common targets (e.g. 450ppm and 550ppm in Garnaut)?
I ask because Garnaut says we’re already at 455ppm CO2-e as a way of claiming the unlikelihood of a global deal based on 450ppm. On the other hand, this post refers to a new paper indicating a ‘safe’ level of 300-350 ppm CO2, while CO2 currently sits at around 387 ppm – does that include all greenhouse gases? If so, who is right? If it’s not a CO2-e figure, why not?
I think the different ways emissions are reported is clouding the debate, and you might just be the person to clarify matters for us non-scientists!
Barry @ 9 – I thought that was my point? I guess it depends on how close to the heart attack we already are. the article suggests we are already there I suppose… Is the politics pretending we are a size 16 hoping to be a size 8 when in reality we are a 400kg brute who can’t get out of bed without using a small crane and widening the door;)
Darren: Well, it’s both misleading and correct to say we are at 455ppm CO2-e! Why? Well, you’re right – it needs a blog post to clarify. I’ll do that soon, thanks for the prompt.
As comment #4 illustrates, the emissions reductions targets now being discussed, as the Kyoto protocol approaches its end, appear to be implicitly based on target levels of CO2 or CO2e anyway. (Where the Kyoto targets themselves came from, I don’t exactly know.) The way I prefer to think about these things is in terms of 350, 450, and 550 ppm CO2, as defining three different futures. Thus, the Labor target is implicitly a 550 target. Garnaut advocates aiming at 550 in the short term, but becoming incrementally more ambitious. The G-8 commissioned a study from the IEA on how to achieve 450. And Hansen advocates 350.
Okay MattB, let’s agree that all analogies are ultimately flawed!
Jane O’Sullivan of the University of Queensland has drawn my attention to an article in this month’s Phil. Trans. R. Soc. A (special edition on geoengineering), by Anderson and Bows, which has a way of talking about emissions trajectories that I haven’t seen before: if emissions peak in year Y, stabilization at X ppm requires subsequent annual reductions of …% in CO2e emissions, or …% in “energy and process emissions”.
For example, with peak emissions occurring in 2015, a target of 450 ppm CO2e (we really need to get clear on this CO2 vs CO2e thing) requires 6.5% annual reductions in energy and process emissions.
As a real-world policy, that might not be the best way to proceed, since it requires the biggest absolute reductions immediately after the peak. But there’s no need to assume constant annual reductions. For example, they could follow a Bell curve shape, with the early reductions starting small, the rate of reduction then rising to its maximum, and then tailing off again as an asymptotically safe level of emissions is reached.
I find that this way of thinking can take you a long way towards imagining realistic emissions trajectories. Thus, the 450 scenario outlined by the IEA works a bit like the “stabilization wedge game”, with some of the reduction coming from renewables, some from nuclear, some from carbon capture, and some from efficiency. Throw in that famous I=PAT equation (Impact = Population x Affluence x Technology), the idea that economic growth = growth in energy use x growth in energy efficiency, ideas about differentiated responsibility… and you can pretty much construct your own blueprint for the content of a Copenhagen 2009 agreement, even a 350-compliant one.
I don’t like the archery analogy Barry. The target rotted years ago and there is nothing left to hit. It is becoming more evident by the day that slow feedback mechanisms are already in play. The summer arctic sea ice volume is at its lowest level, and a floating island, for the first time in recorded history and this is happening when the climate is in a La Nina event – must be underwater volcanoes. Atmospheric methane rose in 2007. If its source is the tundra and sea bed clathrates then carbon reduction measures are academic. That aside I would argue that AGW is only a symptom. The primary problem is human population. It has well and truly overshot the earth’s sustainable carrying capacity, a capacity that humanity is significantly reducing as it consumes the planet’s fresh water, biota and easily extractable mineral resources while, at the same time, polluting the land, water and air with industrial waste. This site, http://weheartworld.com/climate-change/global-devastation-hotspot/ provides some stark satellite photos of detrimental human impact.We are facing multiple crises; financial; ageing infrastructure; the increasing complexity of our civilisation; a staggering number of serious environmental problems; and, rapidly diminishing returns on investments in resources and energy. The days of plentiful cheap carbon energy and minerals are rapidly coming to an end while the alternatives remain little more than mirages. Check out the lockstep prices of oil, food and minerals.AGW will be compounded if there is a decline in oil production in the near future? The world will turn to carbon intensive fuels such as coal and gas, and synthetic oil (from tar sands, coal and gas)? That’s a no brainer.The very complexity of our civilisation that needs cheap energy-rich liquid fuels to function works against mitigation. Without it it ceases to function and most alive will starve. People have a religious belief in technology solving the energy problem. The empirical and measurable evidence of climate change now being gathered points to AGW that may already be unstoppable now. If we had twenty years of burning carbon fuels at twice the present rate before we reached 350ppm CO2 equivalent; could successfully introduce global population control policies (say 1 child per woman) to commence decreasing the human population; cheap plentiful oil and other resources; and, no significant wars, environmental or financial problems then maybe.Does anyone seriously believe that any serious efforts will be made to stop the rise in AGGs in the near future? Maybe planet wide experimental geoengineering, if we have the resources, when it is apparent to all that it is ‘all over red rover’. Hello acid rain, dead forests, oceans of jellyfish and cyanobacteria, the scent of rotting eggs and the Great Anthropecene Extinction Event. Humanity will have answered the Fermi paradox. Barry #8 Your analogy about continuing to overeat leading to diabetes followed by a MI is spot on.
Thanks, Barry. That will be very useful as the Rudd position evolves.
The great insight of Hansen et al’s Target CO2 paper and previously their Trace Gases paper is that it doesn’t much matter too much when in the next several decades or so that we burn the remaining fossil fuels, enough CO2 will remain from that burning to cook the planet. So if we set targets on how much of which fossil fuels we will leave in the ground, then, to a first approximation, the emission trajectories will take care of themselves. If somebody comes up with good CCS, then these targets can be revised.
Of course, countries with economies built around coal will go out of their way to ignore this, but it is one of those insights that is so blindingly obviously correct that it’s a marvel that nobody thought of it before — probably they did. Spencer Weart will probably come up with someone who worked this out in 1897!
A real nasty is that the IPCC have defined two different forms if CO2-equivalence
(see glossary of WG1 AR4) and both these can have variants — you should be able to tell which by the units (but occasionally this gets confused).
Concentration-equivalence (in ppm CO2-e) is defined by equal radiative forcing. The two main variants are whether you include short-lived contributions (especially the negative contribution form aerosols). My opinion is that if you want to interpret what is happening, then you need to include everything, while if you want to consider the long-term changes to which we are committed, then it is better to just look at the long-lived gases.
Emission-equivalence (in mass of CO2-e per unit time — e.g. Mt CO2-e per year) is defined in terms of the cumulative radiative forcing over some time horizon, taking into account the natural loss of the gas from the atmosphere. Usually the time horizon is taken as 100 years. This is the definition that is used for the National Inventories and the Kyoto Protocol (and most other analysis, e.g. Garnaut). My own view is that emission-equivalence is really just a summary statistic — certainly you can’t use it in climate modelling. Russell, Singer and Brook argue for using a shorter time horizon. I have two problems with this:
(i) moving towards stabilisation reqires using a longer time horizon; (ii) I think their justification of short-term fixes to avoid “tipping points” is weak — there is most likely a sequence of tipping points to be avoided and in the case of arctic summers being ice-free, the time of the “tipping point” is more likely 2000 (i.e now too late) rather than 2015.
I didn’t intend to blanket all, or indeed many at all, economists in my comment.
I did refer to the “denialists’ laissez faire economic approach”, and in doing so I was speaking of (generally non-economist) deniers of AGW who argue that leaving solutions to an unregulated market is the best answer if warming does actually exist. My reference to the last several days of stock market activity simply reflected that much of the current credit crunch seems to stem from the Bush administration’s enthusiastic laissez faire deregulation that is in line with this ‘leave-it-to-the-free-market’ thinking.
I think that we would both agree that this would not help with successfully mitigating AGW.
Bernard J. (#21)
You are correct not to imply economists typically take a laissez-faire attitude to greenhouse gas mitigation. Any economist knows sometimes markets work and sometimes they don’t. Anyone who doesn’t know what market failure is, and doesn’t understand why unrestrainted greenhouse gas emissions are a classsic example of it, doesn’t know much about economics.
I don’t notice many economists sticking their heads in the sand on this issue.
Garnaut is the obvious example of someone taking an orthodox approach to addressing market failure.
Here is another, one you might normally associate with the leave-it-to-the-market philosophy, John Hewson:
“What I would hope, I guess, is that Turnbull should take a harder line on climate change. The suggestion from Garnaut that we can start softly, softly, with 10 or 15 per cent for his target in 2020 is nonsense, against the sort of targets Australia has to meet by 2050. In those terms, I think Turnbull should be taking a harder line, pushing Rudd to do more, setting a high jump bar if you like, against which Rudd will be measured and they would have more significant consequences for business.”
It’s in an interview at:
#20, there are plenty of coal seams and
pipes leaking methane around the world. Are you proposing that we
shouldn’t flare and fix them? Are you proposing that we shouldn’t
research ways of reducing NOx emissions from fertilizer application?
Maybe we can squeeze past a tipping point without having to do these
things, maybe not. Is anybody that certain about where these tipping
Neither Barry, Peter or myself are
suggesting that we should co-opt CCS design engineers or solar
panel engineers for these tasks, we are not proposing any reduction
in efforts to control CO2. Our proposal is simply that people should change their diet to reduce its forcing footprint.
The social impact
on livestock industries would be large, but the social impacts of
larger hurricanes or longer El-Ninos would be far greater.
In any event, these industries already have massive adverse
throughout the community. 1 million of the current Australian
population will get colorectal cancer in their lives and about
500,000 of those will be caused by pig, sheep and cattle meat. That
estimate is based on the consensus scientific view of the 150+ authors
of the World Cancer Research Fund 2007 report (rather similar in
nature to the IPCC AR4) and uses calculations of Prof Graham
Giles of Cancer Council Victoria based on their research cohort of
about 37000 Victorians and assuming that people cut back
to one red meat meal per week. So when you think about the impact on
the livestock sector, make sure you balance this with 500,000 fewer
colorectal cancer cases … and that’s just for starters… there is
also heart disease and erectile disfunction to consider … not
to mention the biodiversity impacts of cattle production.
Darren (#11), RealClimate gives a good explanation of carbon dioxide equivalents when used in terms of atmospheric concentrations rather than emissions at http://www.realclimate.org/index.php/archives/2007/10/co2-equivalents/.
Garnaut was referring to 455 ppm CO2-eq in terms of the effect of all GHGs currently in the atmosphere without any reduction for the cooling effects of aerosols.
The IPCC (2007: 102) summarised the effects of GHGs and aerosols as follows:
“Atmospheric CO2 concentrations [reached] 379 ppm in 2005 … The direct effect of all the long-lived GHGs is substantial, with the total CO2 equivalent concentration of these gases [in 2005] estimated to be around 455 ppm CO2-eq (range: 433-477 ppm CO2-eq). The effects of aerosols and landuse changes reduce radiative forcing so that the net forcing of human activities is in the range of 311 to 435 ppm CO2-eq, with a central estimate of about 375 ppm CO2-eq.”
Here is my explanation of the background to these figures and the term “carbon dioxide equivalents” (Nb. I am lawyer, not a climate-scientist like Barry or Ian so take this with a grain of salt. It comes from an article I wrote in a law journal last year):
For ease of comparison and modelling greenhouse gas emissions and atmospheric concentrations are commonly measured in a standard unit known as “carbon dioxide equivalents” (CO2-e or CO2-eq). This term is defined and used in slightly different ways in the context of emissions and atmospheric concentrations of greenhouse gases. The unifying theme for the different uses is that they allow the effect of different greenhouse gases to be compared using carbon dioxide as a standard unit for reference. It may be noted also that some authors and inventories refer to “carbon equivalents” when discussing quantities or atmospheric concentrations of greenhouse gases. Figures for “carbon equivalents” can be converted to “carbon dioxide equivalents” by multiplying by 44/12 to take account of the different molecular weights. Carbon equivalents can be a more meaningful term when considering carbon not held in the form of CO2, such as coal. However, the IPCC generally uses “carbon dioxide equivalents”.
When referring to greenhouse gas emissions, “carbon dioxide equivalent” refers to the amount of carbon dioxide that would give the same warming effect as the effect of the greenhouse gas or greenhouse gases being emitted. It is normally used when attributing aggregate emissions from a particular source over a specified timeframe. It is used in this way at national and international levels to account for greenhouse emissions and reductions over time. Article 3 of the Kyoto Protocol states targets for emissions reductions in terms of “aggregate anthropogenic carbon dioxide equivalent emissions of the greenhouse gases listed in Annex A.” Using this approach, Australia’s net greenhouse gas emissions across all sectors in 2004 totalled 564.7 million tonnes of carbon dioxide equivalent. The expected carbon dioxide equivalent emissions from burning different fuels can also be calculated using a standard methodology (see http://www.climatechange.gov.au/workbook/pubs/workbook-feb2008.pdf).
When referring to atmospheric concentrations of greenhouse gases, “carbon dioxide equivalent” refers to the concentration of carbon dioxide that would give the same warming effect as the collective effect of all of the greenhouse gases in the atmosphere. Put in a more technical way, this means the atmospheric concentration of carbon dioxide that gives a radiative forcing equal to the sum of the forcings from all of the individual greenhouse gas in the atmosphere.
Houghton (2004: 259) explains that when converting from carbon dioxide only concentrations to carbon dioxide equivalent concentrations, the amount that needs to be added varies with different concentrations of greenhouse gases as the relationship between radiative forcing and concentration is non-linear. For example, setting stabilisation targets of atmospheric carbon dioxide at 450 or 550 ppm would become about 520 or 640 ppm carbon dioxide equivalents, respectively, due to the additional warming effect of other greenhouse gases. Stern (2007) used the term in this manner. These approaches exclude the cooling effect of aerosols.
However, the use of this term is not uniform when discussing stabilisation targets as some authors define carbon dioxide equivalent concentrations as the net forcing of all anthropogenic radiative forcing agents including greenhouse gases, tropospheric ozone, and aerosols but not natural forcings. Hare and Meinshausen (2006) is an example of this approach. The inclusion of aerosols alters the meaning considerably. As noted earlier, the IPCC’s latest report indicates that the current radiative forcing of non-carbon dioxide greenhouse gases and aerosols effectively cancel each other, so that the net effect of all radiative forcing components is currently roughly equal to the effect of carbon dioxide alone. However, this offsetting effect is unlikely to remain in the future as improved pollution controls are expected to significantly reduce the cooling effect of aerosols over the course of coming decades: Meinshausen et al (2006).
With this context explained, it is understandable why Hansen et al (2008) prefer to use CO2 only targets and avoid the use of CO2-eq targets but for non-climate scientists (such as myself) we have to largely work with the approach adopted by the IPCC and international framework so we cannot avoid using CO2-eq. It is important to understand exactly what people mean when they refer to the term. I look forward to Barry’s explanation about it.
Hansen et al (2008) “Target CO2 – Where should humanity aim?” (in review – see draft at http://arxiv.org/abs/0804.1126).
Hare B and Meinshausen M, “How much warming are we committed to and how much can be avoided?” (2006) 75 Climatic Change 111.
Houghton J (2004), Global Warming: The Complete Briefing (3rd ed, Cambridge University Press, Cambridge)
IPCC (2007), Climate change 2007: Mitigation. Contribution of Working group III to the Fourth Assessment Report of the IPCC (Cambridge University Press, Cambridge), http://www.ipcc.ch/ipccreports/ar4-wg3.htm
Meinshausen M, Hare B, Wigley TML, van Vuuren D, den Elsen MGJ, and Swart R (2006), “Multi-gas emissions pathways to meet climate targets” 75 Climatic Change 151.
Stern N (2007), The Stern Review on the Economics of Climate Change (Cambridge University Press, Cambrige).
Barry – have you been in Kev07s ear again?
“Kevin Rudd was explaining the benefits of clean coal technology during a meeting with the coal industry today when he said the necessary stabilising point to tackle climate change is an emissions level of 450 parts per million by 2050.”
referencing a CO2 ppm target! very impressive!
[…] McGrath has made an excellent attempt to explain these concepts in simple terms, in comments posted here and here. I’ll reproduce them below (with some edited corrections), as they are very clear […]
[…] authorTop 10 ways to reduce your CO2 emissions footprintThinking big and fast on renewable energyTarget atmospheric CO2 levels, not vague emissions reductionsWhat if the sun got stuck?Garnaut Climate Change Review Final Report – open threadThe global food […]
James Hansen’s article, “Target CO2 – Where Should Humanity Aim?”, referred to in this post has now been accepted by Open Atmospheric Science Journal and is now in-press for publication. The final version following peer review is available at http://arxiv.org/abs/0804.1126
Am I missing a glaringly obviously point that all of the above arguments seemed to have missed? Climate is something that occurs over a long period of time and as such can be repaired over a period of time. The measurements of carbon in the atmosphere whilst looking intimidating now could soon stablise over 30 – 40 years if we ALL start behaving and even begin dropping after 50years?
There is about 20% of the carbon that stays in the active carbon cycle ‘forever’ — millennia to hundreds of millennia. So, without active intervention to draw down carbon (in addition to eliminating emissions), it will stabilise and drop, but slowly. Also, carbon sinks are more effective as the pCO2 (partial pressure) rises, so as we take CO2 out of the air we can expect the effectiveness of natural sinks to decline.’
Related to the draw down issue, there is this interesting new post by Monbiot on ‘biochar’ — a pretty harsh critique:
[…] Target atmospheric CO2 levels, not vague carbon emissions […]