The Science
Carbon Maths


Target Concentration for CO2 in the atmosphere:

Since 1751, approximately 356 billion metric tonnes (gigatons) of carbon has been released into the atmosphere from burning fossil fuels (and cement production, which accounts for about 5% of global CO2 emissions because CO2 is a by-product of converting calcium carbonate into lime).   Half of all fossil fuel emissions have occurred since 1980. (Biogeosciences, 9, 1845-1871, 2012)

In 2010, at UN climate talks in Mexico, 167 countries, together responsible for more than 87% of global carbon emissions, committed to prevent average temperatures rising more than 2˙C above pre-industrial levels.  "Two degrees" has been adopted as the threshold which we must not pass.

To keep temperature rise within 2˙C, climate scientists calculate that human activity can add no more than another 565 gigatons of carbon dioxide into the atmosphere by 2050.  At present rates, (31 gigatons globally during 2011 and rising) this figure will be reached by 2028. (Bill McKibben, Rolling Stone 2/08/12

Average temperatures have so far risen by about 0.8˙C above pre-industrial levels, but even if carbon emissions from human activity stopped overnight, temperature would rise by another 0.8˙C, because it takes decades for the heat absorbed by the oceans to find its way to the atmosphere.  Scientists calculate that there is a 53 - 87% probability that the 2 degree target will be exceeded if global greenhouse gas emissions stay more than 25% above 2000 levels in 2020. 

Computer models indicate that current trends in emissions put us on track for an average temperature increase of 6˙C by the end of this century - a devastating change.   The 2012 Report for the World Bank by the Potsdam Institute "Turn Down the Heat" predicts that current emissions put us on track for a temperature increase between 4˙ and 6˙C by 2100.

The relationship between CO2 emissions and temperature is not immediately apparent.  For this reason, the concentration of CO2 in the atmosphere provides a more useful metric for framing policy


The 2007 Stern Review warned about the consequences of reaching a concentration of 550ppm CO2(e): "The annual flow of emissions is accelerating, as fast-growing economies invest in high carbon infrastructure and as demand for energy and transport increases around the world. The level of 550ppm CO2e could be reached as early as 2035. At this level there is at least a 77% chance - and perhaps up to a 99% chance, depending on the climate model used - of a global average temperature rise exceeding 2˙C by the end of the century, giving at least a 50% risk of exceeding 5˙C global average temperature change during the following decades. This would take humans into unknown territory. An illustration of the scale of such an increase is that we are now only around 5˙C warmer than in the last ice age. Such changes would transform the physical geography of the world. A radical change in the physical geography of the world must have powerful implications for the human geography - where people live, and how they live their lives."

"In the long term, annual global emissions will need to be reduced to below 5 Gigatonnes (Gt) CO2e, the level that the earth can absorb without adding to the concentration of  green house gasses (GHGs) in the atmosphere. This is more than 80% below the absolute level of current annual emissions." (Stern Review, 2007)

The Stern Review indicated that it will be very difficult to stabilise CO2 in the atmosphere at a level below 450ppm: "Anything lower would certainly impose very high adjustment costs in the near term for small gains and might not even be feasible, not least because of past delays in taking strong action". "The Economics of Climate Change, The Stern Review", 2007


More recent research by climate scientist James Hansen indicates that carbon dioxide in the atmosphere is already at dangerous levels and must fall from the current 394ppm to at most 350 parts per million.  Hansen warns  "if the present overshoot of this target concentration is not brief, there is the possibility of seeding irreversible catastrophic effects". (Abstract of ‘Target Atmospheric CO2:  Where Should Humanity Aim?, James Hansen et al :

Hansen's key insight is that it will only be possible to stabilise at 350ppm if we stop burning coal, which releases more CO2 than gas or oil.  A major grass roots campaign in the USA ( is focussed on this aim.  It also campaigns on key issues such as tar sands, but is up fighting well funded lobbying on behalf of fossil fuel companies (e.g. which deny climate change is a problem.

In 2008, the United Kingdom became the first country to pass legislation with binding targets for its future carbon emissions.  The independent Committee on Climate Change (CCC) was established to set national targets and monitor progress.  It determined that UK carbon emissions should be cut by 2050 compared to 1990, based on the principle that each person is entitled to the same per capita share of a finite global carbon budget and that global emissions need to be be 50% less in 2050 than 1990. 

The Committee on Climate Change's 2012 (4th annual) report to Parliament expressed growing concern at the UK's failure to meet unambitious initial carbon reduction targets, but is also relevant to other nations: "There are major challenges sustaining and increasing the pace of investment in low-carbon power generation, building fabric measures and other energy efficiency improvements, renewable heat, electric vehicles and travel behaviour change.  Whereas when we first highlighted the need for a step change, there was a lead-time of several years, this has now elapsed.  Therefore the step change is needed urgently if we are to remain on track to meeting future carbon budgets.  In other words, it is crucial now to move from policy development to delivery.

Our View
The warnings from scientists have become increasingly urgent and make an unanswerable case for keeping future emissions within a precautionary, science-based, finite global carbon budget of 565 gigatons.  However, we are addicted to carbon and most governments are currently more concerned about economic growth than cutting carbon emissions. It will be decades before renewable energy can replace fossil fuels, so it is vital that we waste less energy.   However, we do not believe national measures to cut carbon emissions can succeed without a binding global climate deal which creates international peer pressure for all countries to act; requires industrialised countries to meet tough emissions targets and creates economic incentives for emerging and developing nations to build low carbon economies. 

A recent report by the Carbon Tracker Initiative highlights the obstacles to a low carbon global economy.   It shows that the proven fossil fuel reserves of coal, oil and gas owned by governments and fossil fuel companies have a total market value of around 27 trillion dollars.   The stock value of major corporations depends on burning these.  However, this would produce an estimated 2,795 gigatons of carbon emissions - five time the 'safe' limit of 565 gigatons - and take Earth's climate to uncharted territory.

In short, the world needs to end its addiction to fossil fuels in time to keep these carbon assets in the ground.  We must therefore find a way to make the transition to a low carbon world in order to ensure future economic and social well-being - and do so before a low carbon infrastructure is in place.  We believe this will depend on harnessing market forces to share a finite global carbon budget in accordance with overarching scientific and ethical principles.


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