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The improbable 2°C global warming target

Mitigating global warning is a pressing and daunting task for the world’s major economies. This column says that the 2°C target set by G8 leaders is both politically and technologically unrealistic. It argues they must adopt more realistic targets and long-term commitments to adaptation plans.

In L’Aquila – the earthquake hit Italian city that hosted the 2009 G8 meeting – the Major Economies Forum (MEF) leaders committed to restrict global warming to 2 degrees Centigrade (2°C) and have agreed to halve emissions by 2050 to achieve this target (MEF 2009).1

Can the 2°C target actually be achieved? Is the medium-term 2050 target for emissions reductions consistent with the long-term temperature goal of +2°C?

GHG implications of the leaders’ 2°C ceiling

Global warming is caused by growing concentrations in the atmosphere of gases – the greenhouse gases (GHGs) – that are transparent to the inflow of ultraviolet solar radiation and reflect instead the ultrared radiation returned by the earth surface. With higher concentrations of GHGs, the solar radiation trapped below the atmosphere increases and so does the world average temperature, bringing along climate change worldwide. Although the precise relationship between temperature and concentrations of GHGs is not yet completely understood, scientists have defined confidence intervals for temperature changes as a function of different levels of GHG concentrations (IPCC 2007, see Table 1).

Table 1. Predicted mean global temperature increase from pre-industrial levels


Greenhouse gas concentration (ppm CO2-equivalent) Most likely Very likely above (>90%) Likely in the range (>66%)
350 1.0 0.5 0.6 - 1.4
450 2.1 1.0 1.4 - 3.1
550 2.9 1.5 1.9 - 4.4
650 3.6 1.8 2.4 - 5.5
750 4.3 2.1 2.8 - 6.4
1000 5.5 2.8 3.7 - 8.3
1200 6.3 3.1 4.2 - 9.4

Source: IPCC Fourth Assessment Report, WG I, Chapter 10, Table 10.8

According to IPCC experts, in order to keep the temperature increase below 2°C with high probability, concentrations of GHGs should not exceed 380-390 ppm CO2-eq. If we accept the possibility of overshooting the target, the level of concentrations can be a bit higher but no greater than 450 ppm. It is instead unlikely that global warming will be contained below +2°C if concentrations grow above 550 ppm CO2-eq.

Understandably, the MEF declaration was not a precise scientific document, and it omitted to mention the deep structural uncertainty that dominates the climate problem. We assume here that the MEF leaders set the +2°C goal as a best guess. As a consequence, the corresponding concentration target is roughly 450 ppm CO2-equivalent and possibly lower.

Can this concentration target be achieved by the end of the century?

The present level of GHG concentration is 430 ppm CO2-eq (390 CO2 only), well above the 380-390 ppm level necessary to make a temperature increase above 2°C unlikely. 430 ppm is also very close to 450 ppm, a threshold value that will be reached in a few years, whatever world leaders decided in L’Aquila and will decide in the coming years.
In the absence of technologies that can absorb large fractions of the stock – not just the flow – of GHG emissions, the 2°C target could be achieved with a median likelihood only if emissions are cut to virtually zero worldwide, starting from 2012 onward. This is clearly unrealistic.

Interestingly, even the emission reduction path proposed at L’Aquila is not fully consistent with the 2°C target. If we assume that emissions will halve by 2050, declining at a constant pace from 2010, concentrations of CO2 in the atmosphere will be 40 ppm higher in 2050 (Bosetti et al. 2006).2 This implies that all GHG concentrations will reach 470 ppm in 2050, assuming that emissions of non-CO2 gases are heroically cut to zero from 2010 onwards. The emissions path envisaged by MEF leaders is thus more in line with a 525-550 ppm target by the end of the century, which corresponds to a global warming of about a +2.5C° / +3.0C°.

Carbon-absorbing technologies

Therefore, the only way to reach with certainty the 2°C target is to implement, at a planetary scale, technologies that absorb the stock of GHGs from the atmosphere and store them either in soils, underground or in the oceans, forever. Ideally, with scrubbing technologies capable of operating at very large scale, any concentration level is achievable and any temperature target can be met. Emission reduction efforts would not be an issue anyway.

At small scale, it is already possible to absorb CO2 from the atmosphere and store it underground. For example, carbon dioxide can be absorbed by growing biomasses and burying them either in the upper soil or deeply underground, using power plants equipped with carbon capture and storage. The scale at which this activity must take place would be huge. Rough calculations show that, in order to achieve the 2°C target, at least 20-30 giga-tonnes of carbon dioxide stock should be absorbed from the atmosphere and stored underground, in case of a full and immediate world cooperation effort to reduce the flow of GHG emissions. The scale of sequestration would be much larger in case of delayed or incomplete world mitigation efforts.

This is a gigantic task that is unachievable with current technology. For example, there are still doubts that it will be possible to store carbon by means of coal-fired power plants with capture and storage at the scale of mega-tonnes per year, something on a scale thousands of time smaller than what is needed for the +2C° target. Also unclear is the availability of accessible geological deposits capable of storing vast amounts of carbon dioxide, safely and forever. A leakage rate on the order of a fraction of percentage point would be sufficient to offset all sequestration efforts in a couple of centuries.

Other important questions arise with regard to the possibility and convenience of devoting enormous amounts of land and energy to grow, harvest, and process biomasses. The implications for land use, soil dynamics, and the effective geographical availability of the required biomasses have to be carefully assessed. And this is without mentioning the importance of assessing the economic costs of this vast absorption and storage activity.

As for geo-engineering technologies – which would allow us to keep temperature increases under control if the concentration target is not met – it is important to invest in research to assess their technical and economic feasibility.3 However, the massive adoption of geo-engineering technologies to control temperature remains, for the moment, purely hypothetical.

Implications of delayed action

Things get even more complicated if we acknowledge that no delay and virtually full world cooperation are needed to attain the less stringent 525-550 ppm CO2-equivalent concentration target. Indeed, if action is delayed until 2030, it will hardly be possible to achieve the 525-550 ppm CO2-equivalent target (450 ppm CO2). Even in the case in which delayed action comes only from non-OECD countries, the 525-550 ppm CO2-equivalent stabilisation target would quickly become unattainable (Bosetti et al. 2009a).

Figure 1 clarifies this last statement. The bold green line plots the emission pathway of GHG emissions consistent with a 550 ppm CO2-equivalent long-term stabilisation target; the other lines plot the path of GHG emissions of non-OECD countries, in both their reference scenario and for different targets in 2050. When the bold green line crosses the other lines, emissions in OECD countries should be zero for the 550 ppm CO2-eq. target to be attainable. Without mitigation efforts in non-OECD countries, the OECD group would need to reach zero emissions as early as 2030 and quickly move towards negative emissions. A mild degree of commitment from non-OECD countries would also be insufficient; it would force OECD countries to reach zero emissions between 2030 and 2050, as detailed in Table 2 (Bosetti et al. 2009b).

Figure 1. Global emission pathway achieving the 550 ppm CO2 eq target in 2100 (in green) and Non-OECD countries' emissions trajectories for various levels of future emission reductions.

Source: WITCH model, FEEM.

Table 2. Non-OECD and OECD emission reductions to achieve 550ppm in 2100

  Emissions reduction from current path by non-OECD countries at 2050
  0% -10% -20% -30% -40% -50%
Year in which OECD emissions must be zero 2030  2033 2035 2040  2045  2052 

Source: WITCH model, FEEM.

The +2°C target is therefore an unlikely option without technologies that yield large amounts of negative emissions, a possibility that now appears highly speculative. In addition, in the case of delays in adopting mitigation policies, limited participation in a global climate agreement, or both, even the more modest 525-550ppm CO2-eq target would not be attainable (Bosetti et al. 2009c). In terms of temperature rise, this means that a 2.5-3°C target is much more likely than 2°C.

An immediate consequence of this conclusion is that an optimally designed climate policy should entail a detailed and credible plan for both mitigation, and adaptation. A realistic target – technologically feasible, politically achievable, and economically inexpensive – would be to stabilise concentrations of all GHGs at about 600 ppm by the end of the century. This would anyway represent a remarkable commitment, if we consider that without controls GHG emissions can easily increase to 800-900 ppm CO2-equivalent by 2100. At the same time, long-term commitments to adaptation plans, particularly in the more vulnerable developing countries’ regions, should be adopted, the related funding should be made available, and their implementation linked to development programs in non-OECD regions.


Bosetti V., Carraro, C. and M. Tavoni (2009a), ‘Climate Change Mitigation Strategies in Fast-Growing Countries: The Benefits of Early Action’, CEPR Discussion Paper 5732.
Bosetti V., Carraro C., De Cian E., Duval R., Massetti E. and M. Tavoni (2009), "The Incentives to Participate in and the Stability of International Climate Coalitions: A Game-Theoretic Approach Using the WITCH Model", OECD Economics Department Working Papers, No. 702, June 2009.
Bosetti V., Carraro, C. and M. Tavoni (2009c), ‘Delayed Participation of Developing Countries to Climate Agreements: Should Action in the EU and US be Postponed?’, CEPR Discussion Paper 6967.
Bosetti, V.,C. Carraro, M. Galeotti, E. Massetti and M. Tavoni (2006). “WITCH: A World Induced Technical Change Hybrid Model,” The Energy Journal, December 2006: 13-38.
MEF (2009) Statement on the Economy and Climate Change, Coppito (AQ), 10 July 2009.
IPCC (2007), Fourth Assessment Report.

1 The 17 major economies participating in the Major Economies Forum are Australia, Brazil, Canada, China, the European Union, France, Germany, India, Indonesia, Italy, Japan, Korea, Mexico, Russia, South Africa, the UK, and the US. Denmark, in its capacity as the President of the December 2009 Conference of the Parties to the UN Framework Convention on Climate Change, and the UN have also been invited to participate in this dialogue. Emissions reductions are computed with respect to an unspecified reference year that is likely 2005.
2 These values have been computed using the three-boxes reduced-form MAGIC model parameterised in the WITCH model.
3 Geo-engineering options are meant to contain the increase of temperature, at local or global level, if concentrations will exceed safe levels and temperature change will be fast and uncontrolled. One of the options most widely discussed is to inject in the atmosphere vast quantities of aerosols to increase the amount of solar radiation reflected out of the atmosphere.


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