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100 Months

Tuesday, August 5, 2008
Posted in Global Warming | Tagged Global Warming

A new report from a UK economics think-tank is suggesting we’ve got about 100 months to make some serious inroads into reducing our carbon emissions before we get past a critical tipping point. The estimates from the New Economics Foundation are based on an assumption the atmosphere currently contains 377 ppm of greenhouse gases which would need to be stabilised to 400 ppm. The campaign is backed by several high-profile UK organisations, including organic food certifier The Soil Association, Greenpeace and the Institute for Public Policy Research. Link to the campaign - www.onehundredmonths.org

100 Months: Technical note

Summary

We calculate that 100 months from 1 August 2008, atmospheric concentrations of greenhouse gases will begin to exceed a point whereby it is no longer likely we will be able to avert potentially irreversible climate change. ‘Likely’ in this context refers to the definition of risk used by the Intergovernmental Panel on Climate Change (IPCC) to mean that, at that particular level of greenhouse gas concentration, there is only a 66 - 90 per cent chance of global average surface temperatures stabilising at 2º Celsius above pre-industrial levels.1

Once this concentration is exceeded, it becomes more and more likely that we will overshoot a 2º C level of warming. This is the maximum acceptable level of temperature rise agreed by the European Union and others as necessary to retain reasonable confidence of preventing uncontrollable and ultimately catastrophic warming. We also believe this calculation to be conservative. The reasons why and the assumptions behind our conclusion are detailed below.

Context: human-driven climate change

Present concentrations of carbon dioxide, the most prevalent greenhouse gas, are the highest they have been for the past 650,000 years. In the space of 250 years, the fossil fuel backed Industrial Revolution, and accompanying land-use changes, such as urbanisation and deforestation means we have released, cumulatively more than 1,800 billion tonnes of CO2 into the atmosphere.2 Currently, approximately 1,000 tonnes of carbon dioxide (CO2) are released into the Earths’ atmosphere every

second due to human (or ‘anthropogenic’) activity.3

Greenhouse gases trap incoming solar radiation. If there are more of these gases in the atmosphere, more heat is trapped causing the planet to warm. Once a certain atmospheric concentration of greenhouse gases is passed (often termed a ‘tipping point’), global warming could accelerate. A number of positive feedback loops amplify the warming effect by a physical process triggered by the initial warming itself or the increase in greenhouse gasses. One example is the melting of ice cover which reduces the reflective ability of the Earth’s surface and, by revealing a darker land surface, increases heat absorption. Other processes, such as a decreasing ability of oceans to absorb CO2 due to increasing wind strengths linked to climate change, have already been observed in the Southern and North Atlantic oceans.4

This increases the amount of CO2 in the atmosphere, causing further climate change. Because of such self-reinforcing feedbacks, once a critical greenhouse concentration threshold is passed, even if human beings stop releasing additional greenhouse gases into the atmosphere, global warming is likely to continue. The Earth’s climate may shift into a different state (i.e. different ocean circulation, wind and rainfall patterns) with potentially catastrophic implications for life on Earth. Such a change in the state of the climate system is often referred to as irreversible climate change.

100 months from August 2008

By using the best estimates of current greenhouse gas concentrations, emission growth rates, conservative estimates for the potentially damaging environmental feedbacks that accelerate global warming, and the maximum concentration of greenhouse gases that might prevent irreversible climate change, it is possible to estimate the length of time until this threshold is passed.3

CO2 is, of course, not the only gas that affects the climate. For this reason atmospheric concentration figures are often quoted to take account of other factors, including other greenhouse gasses. This is

the figure given as the carbon dioxide equivalent or, CO2e. Two different figures for CO2e are commonly given depending on whether it is expressing just those gases covered by the Kyoto Protocol, which is not comprehensive, or all radiative forcings5 that affect the amount of energy

received by the climate system and hence its warming or cooling.

Carbon dioxide equivalent (CO2e) is the amount of carbon dioxide that would be required to give the same global average radiative forcing as the sum of all other forcings. Most commonly, the six greenhouse gasses covered by the Kyoto Protocol have been used to calculate CO2e.6

However, if all anthropogenic driven radiative forcings are grouped together viz. not just those covered by the Kyoto Protocol, a more accurate estimate of the radiative forcings can be calculated.7 We have used the most up to date estimate from the latest Intergovernmental Panel on Climate Change report from Working Group One8 on total anthropogenic radiative forcings to calculate the current CO2e. This

approximation also includes some negative radiative forcings (forcings that result in cooling rather than warming, but which may be shorter-term in effect).9

In our calculation, we have taken the concentration threshold to be 400 parts per million volume (ppm) expressed as the more complete measure of carbon dioxide equivalent.10 Only by stabilising emissions at this concentration is it ‘likely’ that the global average temperature change will stabilise at 2º C above pre-industrial levels. In December 2007, the likely CO2e concentration is estimated to be just under 377 ppm, based on a CO2 concentration of 383ppm - this seemingly counter-intuitive

measure is explained by the proper inclusion in the CO2e figure of all emissions effecting radiative forcing - in other words both those with cooling and warming effects.

In our analysis, we have assumed a 3.3 per cent annual growth rate of emissions. This is based on the average growth rate of carbon dioxide emissions over the period 2000 through to 2006.11 We have assumed that the other radiative forcings remain constant. The 3.3 per cent growth rate includes carbon-cycle feedbacks (decrease in the effectiveness of the

land and ocean sinks in removing anthropogenic CO2) as well as direct anthropogenic emissions. Of the 3.3 per cent increase, 18 ± 15% of the annual growth rate is due to carbon-cycle feedbacks, while 17 ± 6% is due to the increasing carbon intensity of the global economy (ratio of carbon per unit of economic activity – i.e. GDP). The remaining 65% ± 16% is due to the increase in the global

economic activity.12

As the atmospheric concentration of CO2e increases, so will the strength of carbon-cycle feedbacks. Given this we have also included the conservative, lower bound estimate for acceleration of carboncycle

feedbacks.13 Our analysis shows that, assuming that other anthropogenic driven radiative forcings remain constant and the growth rate of carbon dioxide emissions (due to economic growth and increasing carbon

intensity of the economy) remains stable – by the end of December 2016 we will exceed an atmospheric CO2e concentration of 400ppmv.

Our estimate is cautious. We have used the lowest estimate of carbon-cycle feedbacks. Furthermore, historically, an increase in the Earth’s global average surface temperatures of just below 2oC has been considered a ‘safe’14 level of warming. But, with the advancement of global climate models to three-dimensional coupled entities, with ever increasing spatial resolutions, it is now known that the impacts of climate change will manifest in more extreme local changes in temperature. For example, collapse of the Greenland Ice Sheet is more than likely to be triggered by a local warming of 2.7 degrees, which could correspond to a global mean temperature increase of 2 degrees or less.15 The disintegration of the Greenland Ice Sheet could correspond to a sea level rise of up to 7 metres.

Endnotes

1 Hence, even at this level, there is still a one-third chance of exceeding the 2 degree C threshold.

2 A total of 488 billion tonnes of carbon has been emitted since the beginning of the Industrial Revolution. This has been

converted to carbon dioxide multiplying the units of carbon by 44 and dividing by 12 [Canadell J, Le Quéré C, Raupach M, Field

C, Buitenhuis E, Ciais P, Conway T, Gillett N, Houghton R, Marland G (2007) ‘Contributions to accelerating atmospheric CO2

growth from economic activity, carbon intensity, and efficiency of natural sinks’ Proceedings of the National Academy of

Sciences 104 (47) 18866-18870.]

3 Approximately 8 Gigatonnes of carbon are emitted each year. This has been converted to carbon dioxide by using the

standard conversion of multiplying the units of carbon by 44 and dividing by 12.

4 See for example Le Quéré C, Rödenbeck C, Buitenhuis E, Conway T, Langenfelds R, Gomez A, Labuschagne C, Ramonet

M, Nakazawa T, Metzl N, Gillett N and Heimann M (2007) ‘ Saturation of the Southern ocean CO2 sink due to recent climate

change’ Science 316 (5832): 1735-1738.

5 Radiative forcing is defined by the IPCC as a radiative flux (flow of energy) change evaluated at the tropopause. The

tropopause is the discontinuity between the troposphere (the portion of the atmosphere that reaches from the surface to a

height of between 10 and 20km where most of the weather occurs) and the stratosphere (the layer of the atmosphere that

extends from the tropopause to a height of approximately 50km and absorbs most of the harmful ultraviolet radiation from the

Sun). Positive radiative forcings lead to a global mean surface warming and negative radiative forcings lead to a global mean

surface cooling.

6 CO2 – Carbon Dioxide, CH4 – Methane, N2O – Nitrous Oxide, SF6 – Sulphur Hexafluoride, HFCs – Hydrofluorocarbons and

PFCs – Perfluorocarbons.

7 For example ozone, sulphate aerosols, and black carbon.

8 IPCC (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment

Report of the Intergovernmental Panel on Climate Change Cambridge: Cambridge University Press.

9 An important caveat is that particulate emissions which often have a net negative radiative forcing, such as the smogs over industrialising Asia, have a much shorter atmospheric lifetime than the dominant greenhouse gas, CO2 (which has a lifetime of 100 years or more).This means that the real extent of the warming is being masked. [See for example, Andreae M, Jones C and Cox P (2005) ‘Strong present-day aerosol cooling implies hot future’ Nature 435: 1187-1190.]

10 In 2006, an analysis by Malte Meinhausen suggested that stabilisation of greenhouse gas concentrations (defined as CO2e) at 550ppm is accompanied by the 68-99% risk of overshooting a warming of 2 degrees. According to the IPCC, this is defined as “likely” to “very likely”. His analysis also showed that only by stabilising emissions at 400ppm is it “likely” that global average temperature change will stabilise at 2 degrees. [Meinhausen, M. (2006), What does a 2°C target mean for greenhouse gas concentrations? A brief analysis based on multi-gas emission pathways and several climate sensitivity uncertainty estimates (

Avoiding dangerous climate change, in H.J. Schellnhuber et al. (eds.), Cambridge, Cambridge University Press, pp.265 – 280)].

This assessment has been supported by findings from two further analyses; an analysis by Baer P and Mastrandrea M (2006)

[High Stakes: Designing emissions pathways to reduce the risk of dangerous climate change London: Institute for Public Policy

Research]. However, top NASA scientist James Hansen and his colleagues form Columbia University in New York published a

paper in early 2008 http://www.citebase.org/abstract?id=oai:arXiv.org:0804.1126 to say that, rather than deciding on a future,

higher level at which to stabilise the amount of CO2 in the atmosphere, we need to reduce CO2 concentrations to 350ppm – the

level they were in 1988.

11 Canadell J, Le Quéré C, Raupach M, Field C, Buitenhuis E, Ciais P, Conway T, Gillett N, Houghton R, Marland G (2007)

‘Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks’

Proceedings of the National Academy of Sciences 104 (47) 18866-18870.

12 Ibid.

13 Friedlingstein P, Cox P, Betts R, Bopp L, von Bloh W, Brovkin V, Cadule P, Doney S, Eby M, Fung I, Bala G, John J, Jones

C, Joos F, Kato T, Kawamiya M, Knorr W, Lindsay H, Matthews H, Raddatz T, Rayer P, Reick C, Roeckner E, Schnitzler K-G,

Schnur R, Strassman K, Weaver A, Yoshikawa C and Zeng N (2006) ‘Climate-Carbon Cycle Feedback Analysis: Results from

the C4MIP Model Intercomparison’ Journal of Climate, 19:3337-3353.

Friedlingstein et al (2006) analysed the output of eleven climate-carbon cycle models (climate models linked to the biological

and geophysical carbon cycle) over the period 1850-2100. The models calculate the Airborne Fraction (AF), the ratio of

atmospheric CO2 increase in a given year to that year’s total emissions from fossil fuels and land-use change. AF is a function

of the biological and physical processes governing land-atmosphere and ocean- atmosphere CO2 exchanges. It has a large

interannual variability, and is linked to large ocean-atmosphere regimes such as ENSO (El Nino Southern Oscillation). This

ratio has been around 0.45 over the period 2000 through 2006; however, the efficiency of the natural sinks in recent years has

started to fall. The intercomparison analysis suggested a total increase of additional CO2 emitted due to carbon-cycle

feedbacks could lie in the range 19 and 220ppm by the end of the 21st century. While the majority of the eleven models

analysed suggest the increase will lie between 50 and 150ppm, we have used the output of the most conservative climatecarbon

cycle model output in our calculation – i.e. 19ppm additional CO2 by the end of the 21st century. We have used their

time series of data for our calculation.

14 A ‘safe’ level of warming although appears to be a technical quantity, in fact, this is a highly political figure due to the

asymmetries of the potential impacts of climate change.

15 For example, P. Huybrechts et al. (1991) ‘The Greenland Ice-Sheet and Greenhouse Warming’, Global and PlanetaryChange 89, 399–412, and J. Gregory et al., 2004, ‘Climatology: Threatened loss of the Greenland ice-sheet’, Nature 428, 616.