The 350 ppm carbon dioxide challenge and how to achieve it

By Renfrey Clarke

January 14, 2010 -- The target posed by leading NASA climate scientist James Hansen of stabilising atmospheric carbon dioxide at 350 parts per million (ppm) is increasingly understood in conjunction with the need to keep cumulative emissions within a tight global “budget”. While the point at which budgeted emissions occur is not in theory crucial, in practice there is a need to ensure that emissions peak early and decline swiftly thereafter.

Low or zero-emissions energy generating plant needs to be built at an accelerated rate, with the timeframe seeming to exclude nuclear power. Since it appears impossible for high-emitting developed countries to stay within their national carbon budgets for a 350 ppm outcome, international agreements must ensure that developing countries are paid for the use of their carbon entitlements. For reasons of humanity and historical justice, developing countries must also be helped to achieve a rapid net expansion of their energy supplies. The cost to developed countries of meeting these obligations would resemble that borne by leading Western states in waging the Cold War. This cost can be met through a system of international levies on military budgets and other unproductive economic activity.

James Hansen’s early 2008 paper, “Target Atmospheric CO2: Where Should Humanity Aim?” deserves to be seen as the first satisfactorily researched attempt to set a goal for the atmospheric carbon content that can be permitted if, as Hansen put it, “humanity wishes to preserve a planet similar to the one on which civilization developed and to which life on earth is adapted.”[1] From a CO2 concentration at the time of 385 pppm, Hansen argued, the level needed to be cut to 350 ppm at most.

Hansen acknowledged that 350 ppm would likely serve only as an initial target, noting that “CO2 ~300-325 ppm” might be needed to “restore sea ice to its area of 25 years ago”. Later in 2008 Professor Hans Joachim Schellnhuber, director of the Potsdam Institute for Climate Impact Research in Germany, commented to the British Guardian:

…nobody can say that 330 ppm is safe. Perhaps it will not matter whether we have 270 ppm or 320 ppm, 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.[2]

Since 2008 further research has in part modified the view of how emissions targets should be set. In particular, two related studies led by Myles Allen and Malte Meinshausen have concluded that the most accurate predictor of the degree of global warming is the total, cumulative quantity of greenhouse gases that enters the atmosphere over time.[3] The practice of citing “350”-style stabilisation targets remains accepted for various purposes, but scientists now reject as misleading the practice – still widely employed by governments – of posing targets in terms of percentage cuts by particular dates. An 80 per cent reduction of annual emissions by 2050 will have sharply different outcomes depending on whether most of the cuts are made early or late; if cuts are delayed for decades, the tonnage of gases emitted by 2050 will be dramatically greater.

Budgeting emissions

With the studies by Allen and Meinshausen, humanity has acquired a “carbon budget”, in the form of cumulative tonnages of carbon dioxide that correspond to specific levels of warming. To provide a 75 per cent chance of keeping temperature rises to less than 2 degrees Celsius above pre-industrial levels, Meinshausen calculates that emissions of CO2 between 2000 and 2050 must be limited to 1000 gigatonnes (billion tonnes), with releases of other greenhouse gases restricted in proportion. Of this 1000 Gt of CO2, more than 330 Gt are already in the atmosphere, having been released between 2000 and 2009. If present emission rates continue, a simple calculation shows, Meinshausen’s budget for the period until 2050 will be exhausted more than 20 years early.

The political movement for “350” has taken on remarkable force, and to de-emphasise its key demand in favour of a figure for budgeted tonnes of CO2 would confuse the message for little real gain. Still, campaigners for “350” need to grasp how their target should be understood in terms of cumulative emissions. In addition, the “carbon budget” allows a range of insights, especially as regards the obligations of rich countries to the developing world, that “350” does not.

Unfortunately, the findings of Hansen and Meinshausen are not easily juxtaposed. Hansen in specifying a goal of 350 ppm sought to give climate change campaigners a target they could cite as being likely to keep the existing environment relatively intact. By contrast, Meinshausen uses 2ºC of warming essentially as a benchmark, acknowledging that it “cannot be regarded as a ‘safe level’.”[4]

The task of collating and comparing the two approaches is taken up in a Greenhouse Development Rights brief entitled A 350 ppm Emergency Pathway.[5] As one would expect, the “350” target emerges as far more challenging than Meinshausen’s 2ºC. Compared with cumulative emissions for 2ºC of 1000 Gt between 2000 and 2050, only 750 Gt can be released if progress toward 350 ppm is to stay on track. Moreover, emissions must eventually be made negative, using methods such as sequestration in forests and soils. Hansen, the authors of the Pathway explain, projects that net global CO2 emissions will have to decline to zero by 2050, with sequestration of a further 150 Gt between 2050 and 2100, if atmospheric CO2 is to fall to 350 ppm by the end of the century.

From the allowable 750 Gt by 2050, we have to subtract the 330 Gt that was released between 2000 and 2009. That leaves a total of only about 420 Gt that can be emitted in the period until mid-century. If emissions continue at current rates, this indicates, the world’s carbon budget for an eventual return to 350 ppm will be exhausted around 2021.

Clearly, global greenhouse emissions will need to peak in the next few years, then begin a rapid fall. Annual cuts after the peak will need to be above 5 per cent of the peak emissions figure.[6] At the same time, the legitimate demand of people in the developing world for improvements in their lives will have to be met. To achieve this, net energy supplies in developing countries will need to increase at annual rates at least approaching 5 per cent, at the same time as fossil-fuel generating plant is retired.

Throughout the developing world, new generating capacity – all from renewable sources – will have to come on stream at rates around 10 per cent of existing capacity per year, similar to the pace at which generating capacity was increasing in China in the boom years between 2000 and 2008. Meanwhile, renewable generating plant will have to replace fossil-fuelled installations in the developed countries.

The cost will be steep, and the political obstacles daunting. Should activists therefore pursue less demanding goals, which today’s political decision makers might be more inclined to implement?

The answer is implicit in the often-heard observation that nature does not negotiate. The price which the laws of physics put on preserving a recognisable world is high, but the cost of paying any less will be catastrophic. Nor are arguments that centre on “political practicality” compelling. No policy course is practical if it promises, with a high degree of likelihood, to destroy the natural environment on which modern civilisation, with its political life and all, ultimately rests. “Incrementalist” strategies, which would have us arguing for partial and inadequate goals in the hope of strengthening them later, run up against the fact that time is acutely short.

There is simply no choice for climate activists but to put the facts before the population -- and to call on people to do what they must.       

The need to act promptly

Provided total emissions are kept within budget, Allen and Meinshausen note, it should not in theory matter when the emitting takes place. But in practice there is a definite catch: if “business as usual” releases are not quickly reined in, staying inside the budget limits will eventually require cuts so rapid as to be effectively impossible. Even in the relatively undemanding context of his “two degrees” projection, Meinshausen stresses the need to make a quick start and to concentrate big cuts within the early years:

…in terms of the ability to adapt the system towards a zero carbon future, in terms of the shorter lived non-CO2 gases, and in terms of intergenerational equity, the timing of emissions is of high importance. A further delay in emission reductions might render the more precautionary climate targets practically infeasible…[7]

For a return to 350 ppm, it will be necessary to focus heavily on building the renewable energy technologies that are available now, or that are likely to become widely deployable within the next few years. Wind power, hydro, conventional geothermal, biomass, solar photovoltaic and solar thermal are already in use. The CETO and Pelamis wave power technologies are about to join them. “Hot dry rock” geothermal is at the demonstration-plant stage.

One technology which will be severely restricted by this time frame, if not ruled out entirely, is nuclear power. The performance of the nuclear industry in bringing new plants, even those of familiar configuration, into operation in less than a decade has not been good. Today’s most advanced, “third-generation” plants retain the fundamental drawbacks of previous designs, and are still regarded as pre-commercial. The most notable example of the type, at Olkiluoto in Finland, remains uncompleted, at least three and a half years behind schedule and plagued by cost overruns of more than 50 per cent. “Fourth-generation” integrated fast-breeder reactors, proposed as a solution to many of the dangers of today’s plants, exist only as a concept and are unlikely to come on stream in significant numbers in less than 15-20 years.

Meanwhile, the high cost and complexity of nuclear plants has to be weighed against the cheapness and robust simplicity of technology such as CETO wave-power farms – which on good sites such as Southern Ocean [south of Australia] coastlines promise to generate base-load power more predictably and reliably than nuclear.  

The cost of profligacy

Citizens of wealthy countries that emit carbon at high per capita rates – in the case of Australia, more than four times the world average, even without taking coal exports into account – should not demand favours from the rest of humanity. The individual ration for Australians under the “carbon budget” should be the same as for the rest of the global population – neither more nor less.

Over the period from now to 2050, Australia’s population is likely to average about 29 million, compared to a global figure of about 8 billion. This puts our country’s share of the allowable 420 Gt of carbon dioxide emissions for the “350” target at about 1520 million tonnes.

Not even the most strenuous efforts will allow us to end emissions before this budget allowance is used up. Australian emissions of carbon dioxide in 2007 were 456 million tonnes.[8] At that rate, our budget will be blown early in 2013. For much of the remaining period until our net emissions cease, Australians will be carbon spongers on the rest of humanity – and in particular, on the great majority of people whose emissions are only a fraction of the world average.

In a just world, profligacy has a price. In the case of wealthy countries, it is the obligation to pay others for the use of their carbon entitlements, now being forcibly expropriated via our hair dryers, plasma screens and SUVs. Massive transfers of wealth and technology will need to take place from the developed to the developing world.

Developed-world citizens should not flatter themselves that these transfers will in any sense represent foreign aid. An appropriate term would be reparations, for the theft from poor countries of the chance to develop their economies along the cheap, technologically familiar fossil-fuel route.

The key mechanism for these transfers will need to be state-to-state agreements, under international direction. To minimise corruption and waste, governments will no doubt want most of the payments to be in kind, in the form of equipment and expert services needed to help developing countries to raise their energy-use efficiency and create modern energy grids powered by renewables. Training for engineers could be a big item. So too could the construction in developing countries of solar panel or wave-power manufacturing plants. As grids develop and are interconnected, another item could be renewably-generated electricity.

The likely price

Research into the likely price of the “350” project has not been extensive, but in an October 2009 survey Frank Ackerman and his collaborators concluded that when studies performed for environmental advocates and business groups are filtered out, a body of findings exists to support the view that “the …ambitious reductions in emissions required to reach 350 ppm CO2 might have net costs of 1 to 3 percent of world output.”[9] To put this in perspective, the sum involved is comparable to global military spending, currently about 2.5 per cent of gross world product. It should be noted, however, that US military spending during the Second World War reached as high as 37 per cent of gross domestic product – and that the US economy grew strongly over the course of that conflict.[10]

Where, and how, might the needed investment funds be obtained? Since this article deals with what must be done, not with what conventional political discourse reckons is feasible, the observation will be made: throughout world capitalism there are large sectors that produce essentially nothing that is seriously necessary to human beings. These sectors could be curtailed, permitting big savings.

The proposal here is that an international clean energy authority be set up, with the power to place levies on currently ill-used (or completely unproductive) capital and to direct it toward achieving Hansen’s goal of 350 ppm by the end of the century.

What kind of sums would be involved? Gross world product (GWP) in 2008 was calculated by the International Monetary Fund at US$61 trillion.[11] At 3 per cent of GWP (here, we shall use the least optimistic of Ackerman’s figures), the cost of stabilising atmospheric CO2 at 350 ppm would come to about US$1800 billion per year. According to the US Environmental Protection Agency, total emissions from the developing world are now approaching those from the developed countries,[12] so the following annual spending suggests itself:

Cost of ending developed world emissions: US$900 billion

                     developing                      : US$900 billion

Beyond this, provision has to be made for the rapid expansion of energy availability in the developing world. An International Energy Agency press release in November 2008 spoke of the need to spend more than US$1 trillion per year, the great bulk of it in the developing world, to meet global incremental energy demand.[13] Let us assume the following figure for the developing-world component of this sum:

Additional developing-world generating capacity (renewables): US$900 billion.

In this scheme, the total investment needed each year to clean up and augment the world’s energy supplies would thus be around US$2700 billion, of which US$1800 billion would be spent in or for the developing countries. Of this US$1800 billion, a certain portion would be covered by funds which developing countries would in any case assign to expanding their energy complexes. The remainder, it is argued here, ought to come from the developed world.

How much might the transfer of wealth to the developing countries then come to? A sum of in the region of US$1300-1500 billion per year would seem realistic. Developing-country representatives, to be sure, have suggested a larger figure. At the Copenhagen climate talks in December 2009, a draft text from the African Group of 50 countries proposed that rich countries pay 5 per cent of their GDPs to help developing countries combat climate change.[14] This would amount to about US$2000 billion per year.

How to pay for it

World military spending in 2007, meanwhile, was about US$1500 billion.[15] A global levy of two-thirds of military budgets would provide much of the sum needed annually for “greening” the developing world and expanding its energy sector. International tensions would be blunted by the fact that the disarmament was uniform. The curtailing of weapons production would free up industrial capacity and skilled personnel whose scarcity, some argue, stands in the way of rapid decarbonisation of energy generation.

Taxes could also be placed on speculative financial activity, which does not create value but merely redistributes it – typically, in favour of the already wealthy. Internationally, the measures employed could include a “Tobin tax”, levied on cross-border financial transfers, or a stamp duty on conversion of the world’s currencies. Such imposts might raise a further US$50 billion per year before beginning to suppress the activity feeding them. Taxation at the national level of securities speculation might be counted on to raise a further US$50 billion, with the added benefit of helping to avoid further global financial crises.

Luxury consumption by the rich is another unproductive, essentially parasitic area of capitalist economies. A graduated international tax could be placed on the incomes of the richest 1 per cent of the world’s households, corresponding to perhaps 10 per cent of households in developed countries. If it yielded an annual US$5000 per well-off individual (the bulk of this sum would need to be extracted from the very wealthiest), this tax could raise perhaps US$300 billion.

Together, these measures would raise around US$1400 billion a year. Where else might savings be found? In a levy, say, on the advertising industry, whose revenues for 2004 were estimated at almost US$350 billion?[16] With the volume of trade on world currency markets in 2007 running at US$3200 billion per day,[17] it is hard to imagine that the potential for large additional savings does not exist.

Needless to say, the most privileged groups – along with people used to profiting from military aggression – would campaign furiously against such initiatives. Protests at “confiscation” measures would be accompanied by investment strikes. But it is worth recalling that in the US during the Second World War the tax rate levied on incomes of more than a million dollars a year was 94 per cent,[18] and the millionaires continued to invest. None of them would have doubted that if they had refused to invest, the wartime state had the will to make confiscation a reality.

Meanwhile, there is good reason to think that if global clean energy levies took large amounts of capital from the world’s rich, and turned it to productive purposes, consumption by the great bulk of the population would hold up even in the countries where the burden was heaviest. As well as providing jobs directly, state-directed economic mobilisation would have a broad stimulatory effect. Because of such factors popular living standards in the US during the Second World War appear to have fallen only marginally, if at all, despite the vast costs of the war effort.[19]

Why aren’t governments doing this?

Compared to the exactions of the Second World War period, the 5 per cent of developed-country gross product sought by the African Group at Copenhagen seems almost minor. It must be remembered, though, that even in the US the huge boost to industrial production during the war years was a form of economic self-cannibalism, obtained at the cost of running down existing industrial plant and contracting huge state debts to the population. Even in today’s most advanced countries, the long-term diversion of 5 per cent of GDP to dealing with the climate emergency would impose strains.

These strains, though, would not be unendurable, or without precedent in the “peacetime” years since 1945. US military spending, currently about 4 per cent of GDP, was above 7 per cent for each of the years between 1951 and 1971. Between 1952 and 1959 it remained above 10 per cent.[20] British military spending peaked at almost 10 per cent of GDP in the early 1950s, and until the mid-1960s remained above 6 per cent.[21] The French military consumed 14 per cent of GDP during the Indochina war in 1953, and still accounted for 8 per cent in 1957 before settling back to 5.2 per cent in 1965.[22] The 1950s and 1960s in the advanced countries are remembered as a time of prosperity and expansion, when industrial plant was renewed and when debts remaining from the Second World War were paid off. This was despite the booming Cold War arms plants being unproductive by definition, with the value embodied in their output being taken out and destroyed.

To argue that the capitalist system cannot afford to deal with climate change is thus at least technically wrong. The system has paid a similar – or much greater – cost in order to meet previous challenges. The central reason why the nettle of climate change is not being grasped is that private capital is exactly that – private, required to produce profit for specific people and corporations. Combating climate change means profits foregone, in the case of oil left in the ground and stranded assets, in the case of coal export facilities made idle. Both the oil and the coal trains are owned by particular corporations, able to lobby politicians, influence media outlets and fund political parties and candidates.

In the long term, no capitalist has an interest in climate change making modern economic life impossible. But the point when today’s inaction will result in thorough catastrophe is still decades off, and for most entrepreneurs, prosperity or bankruptcy is a question to be resolved in a much briefer timeframe. Even a single decade is a near eternity for people who typically, must invest money and realise profits within a three-to-five year cycle. While relatively few company executives are climate-change deniers, the temptation is strong for them to regard the costs of carbon abatement as a dangerous drag on already uncertain profits, and hence as something to be postponed or foisted onto others. If key positions within political power structures are captured by forces that have a direct stake in ignoring the climate danger – notoriously, large energy firms – there are many people in corporate life who will passively follow.

Perspectives for activists

If meeting the challenge of the “350” target were economically impossible, the political tasks of today’s rulers would be much simpler. As it is, governments face the task of explaining why the costs of waging the Cold War had to be met, while to pay the broadly comparable price of staving off an environmental holocaust is simply inconceivable.

When the challenge is posed in these terms, the arguments of today’s national leaders are seen to be beneath disdain. But of course, the arguments are not put in this fashion. If progress toward saving the environment is blocked, that is always attributed to the irresponsibility and intransigence of someone else.

In recent Western commentaries, the blame has regularly been directed at China. And indeed, the conduct of the China’s leaders has been reprehensible. Whatever developed countries might do, there is no way China’s rulers can maintain their current emissions policies without placing nature and civilisation under threat.

The Chinese government has a point, though, when it counters that per capita emissions in developed countries remain several times their own, and that of the increment of atmospheric carbon dioxide since the beginning of the industrial revolution, around 80 per cent, was put there by the developed world.[23]

So what if the developed countries, on the basis of global agreements, undertook to pay the costs of cleaning up China’s energy use, and to contribute a large share of the additional funds needed to create a fully modern energy sector based on renewables? Would China’s leadership, for all its national sensitivities, hold out?

If such a course is not being pursued, the reasons do not have to do with its practicality. The problem is that so far, the costs to the world’s ruling groups of not pursuing this course have been insufficient. The political price of this inaction must be raised exponentially. Achieving this is not a job for politicians who quake at the thought of how energy corporations would respond. In essence, it is not a job for politicians at all.

Nature and civilisation will be saved only if masses of the world’s population – the people who have no stake in continued greenhouse pollution – enter the political process directly. The lies of the spinmeisters must be answered forcefully and in detail. Equivocators on climate action must be made unelectable. The politics of carbon abatement must be taken into the schools, the universities, the workplaces and the streets. And if need be, the path to catastrophe must be blocked by the living bodies of citizens, chained to the coal loaders and thronging the tracks.

[Renfrey Clarke is a climate activist from Adelaide, South Australia, and a member of the Socialist Alliance.]


1. Posted online early in 2008; published later that year in The Open Atmospheric Science Journal, v. 2 pp. 217-231.

2. The Guardian, 15 Sept. 2008.

3. Myles R. Allen et al., “Warming caused by cumulative carbon emissions: towards the trillionth tonne”. Nature 458, 1163-1166 (30 April 2009), Malte Meinshausen et al. “Greenhouse-gas emission targets for limiting global warming to 2˚C”. Nature 458, 1158-1162 (30 April 2009).

4. Meinshausen op. cit. See also David Spratt and Damien Lawson in December 2009: “…2 degrees is not a safe target but a catastrophe. The research tells us that a 2-degree warming will initiate large carbon feedbacks on land and in the oceans, on sea-ice and mountain tundras, taking the earth well past significant tipping points. Likely impacts will include large-scale disintegration of the Greenland and West Antarctic ice sheets; sea-level rises; the extinction of an estimated 15 to 40 per cent of plant and animal species; dangerous ocean acidification and widespread drought, desertification and malnutrition in Africa, Australia, Mediterranean Europe, and the Western USA.” (Carbon Equity. David Spratt and Damien Lawson, “A Climate Con: Analysis of the ‘Copenhagen Accord’”,

5. EcoEquity, A 350 ppm Emergency Pathway,

6. For a sobering account of the speed with which cuts must be made, see Kevin Anderson and Alice Bows, “Reframing the climate change challenge in light of post-2000 emission trends,”

7.  Nature, 30 April 2009. Informal Background Q&A on Meinshausen et al.“GHG targets & 2˚C”.

8.  Google docs,

9. Economics for Equity and Environment. Frank Ackerman et. al., “The Economics of 350: The Benefits and Costs of Climate Stabilization”,

10. Eh.Net Encyclopedia. Christopher Tassava, “The American Economy during World War II”,




14. COP 15 Copenhagen, 12 Dec. 2009,


16.  High Beam Research,


18.  Tassava op. cit.

19.  Tassava. op. cit.

20.   Truth And Politics, “Relative Size of US Military Spending, 1940-2003”,


22. Jean-Marcel Jeanney, “L’économie française pendant la présidence du general de Gaulle”. Revue de l’OFCE, 1992 v. 39 pp. 7-29, p. 9. Keith Hartley and Todd Sandler, The Economics of defence spending: an international survey, p. 153. London, Routledge, 1990.

23.  See Global Carbon Project: GCP Carbon Budget. “From a historical perspective, developing countries with 80% of the world’s population still [2008] account for about 20% of the cumulative emissions since 1751; the poorest countries in the world, with 800 million people, have contributed less than 1% of these cumulative emissions”,


Submitted by Terry Townsend on Mon, 01/18/2010 - 18:15


Hi All,
Overall I appreciated reading Renfrey's paper because it makes many good points but I have one main problem with it and that is its antipathy towards the idea of sanctioning CO2 emissions cuts by certain dates. Renfrey says:
The practice of citing “350”-style stabilisation targets remains accepted for various purposes, but scientists now reject as misleading the practice – still widely employed by governments – of posing targets in terms of percentage cuts by particular dates.
would appreciate knowing who these scientists are and what they have said because I am not aware of any climatologist who advocates rejecting the idea of having "[GHG emission] targets in terms of percentage cuts by particular dates". There may be some but I am not aware of them, leave alone the idea that scientists now reject such 'rate-by-date' cuts as a "misleading the practice". For example, James Hansen said after he and his colleagues had their paper, Target Atmospheric CO2: Where Should Humanity Aim?, published in April 2008 that the target of 25% to 40% below 1990 levels for developed nations was "a guaranteed disaster". (I have taken this as Hansen implying that a much more ambitious target is needed.) The Potsdam Institute for Climate Impact Research news release, regarding the April 2009 paper by Malte Meinshausen et al, Greenhouse gas emission targets for limiting global warming to 2°C, said:
The study concluded that greenhouse gas emissions must be cut by more than 50 percent by 2050 relative to 1990 levels, if the risk of exceeding 2°C is to be limited to 25 percent.
To my mind, a carbon budget should go hand-in-hand with cuts in emissions by certain dates and not be characterised as being inconsistent with them. Once we have determined what is a safe level of total CO2 emissions that we must not exceed, then we need to set ourselves a series of targets for reducing our emissions by certain dates to keep within that budget. I can see no other sensible alternative because if we say that we must not emit more than 420 Gt CO2 from now, how do we translate that into a practical plan for reducing emissions (i.e. a plan does not ask too much nor too little of us)? Baldy saying that we cannot emit more than 420 Gt CO2 by 2050 and that we must reduce our emissions soon is not likely to save humanity or the biosphere, in my opinion. Without specifying 'rates by dates', humanity is very likely to blow a reasonably tough budget within two or three decades because demonstrably the usual human way of dealing with anthropogenic global warming for more than 20 years, as typified by the inaction of the developed nations of the world, is to constantly put off till tomorrow what should be done today. Therefore any excuse for delaying the necessary work and expense of transforming the world's economy into a sustainable one, which must include very significantly cutting CO2 emissions, is usually very welcome by governments such as our own. If we agree on what level of total emissions that we must not exceed but refuse to have a program of deep and targeted (preferably annual) cuts in emissions, we run the risk of fatally leaving to too late all of the hard work that must be done. For example, it would be crazy to try to go from say 20% below 1990 levels in 2030 (assuming some reductions in our rate of emissions are made from now till then) to zero in 2031 and thereafter, to keep within a total of 750 Gt of CO2 emissions this century.
Renfrey says:
While the point at which budgeted emissions occur is not in theory crucial, in practice there is a need to ensure that emissions peak early and decline swiftly thereafter.
That is correct but what is not said in Renfrey's paper is what milestones must be achieved in emissions reductions to ensure the early peak and the swift decline. In his paper, Renfrey mentioned the excellent paper by Paul Baer et al called A 350ppm Emergency Pathway, which can be accessed at and it contains the following graph which is a study of three emissions pathways. The 350ppm pathway proposed by James Hansen et al is the trajectory along the right-hand side boundary (or 'frontier') of the red area and the red area itself represents the total emissions allowed up until 2050 (420 Gt CO2).


Along the frontier of the red area, emissions for 2020 are budgeted to be at about 15 Gt CO2 (which represents a cut of about 42% below 1990 emissions). Likewise, from all years covered by the curve, we can work out each year's required emissions reductions if we follow the reduction path that the smooth 'reverse S-curve' shape defines from the peak level of emissions. Of course, the chances of an emissions reductions trajectory following something very close to a smooth reverse S-curve are very small indeed, because a 'stepped' trajectory is likely to occur if a suitable carbon budget is actioned, which involves an abrupt end to the destruction of 'old growth' forests and a practical plan for large-scale green energy power stations to come 'on line' after the closure of CO2-emitting power stations.

Baer et al provide the following table for the "350ppm" and "2C" pathways:


This table defines three main points for the trajectory for the 350ppm pathway (after the proposed start year for reductions, this year): a peaking of emissions in 2011; a cut of 42% below 1990 levels by 2020; and a cut of 100% below 1990 levels (i.e. zero) emissions by 2050. I suggest that these points help to fundamentally define the type of trajectory required for a 750 Gt CO2 budget when showing or making a graphical representation is not practicable. Once our trajectory 'curve' is defined, so is our rate-by-date guide to what our targets should be.
In summary, as I see it, keeping within a safe carbon budget is the long term imperative but target emission rate cuts by certain dates are required to keep us on the necessary pathway to 350ppm or lower.
All the best,

Most of what Mark says strikes me as perfectly unexceptionable. But when people criticise what I write, I appreciate it if they quote the whole of the relevant passage. In this case, the next sentence reads:

"An 80 per cent reduction of annual emissions by 2050 will have sharply different outcomes depending on whether most of the cuts are made early or late; if cuts are delayed for decades, the tonnage of gases emitted by 2050 will be dramatically greater."

From this it should be clear that what I object to is the practice, essentially of governments, of posing emissions targets in terms of percentage reductions by remote end-points. It’s been a long time since I noticed a scientist - or any mathematically literate person - joining with Brown, Rudd and Wong in this exceedingly dodgy practice. The purpose of playing with percentages in this fashion is, of course, to appear deep green while leaving the way open to continue pumping lethal quantities of carbon dioxide into the atmosphere during the very decades when steep cuts are indispensable.

If "80 by 2050"-type formulations have gone out of fashion (and except among the more slippery kind of poseur, I’d argue, they definitely have), this is undoubtedly due in some degree to a lacerating analysis published by George Monbiot last August. It’s at, and still makes compelling reading.

In my paper, I stress the need for emissions to peak early and decline swiftly thereafter, reaching zero by 2050. Whether the resultant curve is plotted in absolute tonnes or as a percentage of emissions is not important, except perhaps insofar as using percentages might seem to vindicate the Browns, Rudds, etc. Where activists pose the need for near-term cuts in formulae such as "40 per cent by 2020" I’ve no special objection, though as I explain, a "carbon budget" approach has the advantage of making clear the obligation of the developed world to pay for emissions reduction in developing countries.

The point, of course, is that anyone who supports rapid, massive emissions cuts, however quantified, has assimilated the essentials of the science and become part of the solution. Such people are doing something fundamentally different from developed-world governments, whose position ("Lord give me chastity, but do not give it to me yet!") puts them at the core of the problem.

Submitted by Talha Hassan (not verified) on Thu, 05/19/2011 - 14:02


Nice article.I'll have to agree with Renfrey as a whole but some of the mArk's arguments are correct.Like what he said below
"That is correct but what is not said in Renfrey's paper is what milestones must be achieved in emissions reductions to ensure the early peak and the swift decline."