Climate targets and 1 tonne

One might well ask how the One Tonne Life project came to the conclusion that it is OK to release one tonne of carbon dioxide equivalents per person and year. After all, the project could have picked a different figure. The answer lies in the fact that there are three main considerations determining the extent of emissions we could have here in Sweden without “destroying” the climate.

The first consideration is just what extent of climate change we feel is acceptable. Many of the world’s nations have backed the 2-degree target, that is to say that the global average temperature may not increase by more than 2 degrees over pre-industrial levels. This means the time before mankind started burning fossil fuels, around 1750. To date the global average temperature has already increased by about 0.7 degrees from that base-line. However, the 2-degree target is not a scientific limit, it is actually a political decision. There are experts who suggest we should impose a 1.5-degree limit instead, and others who say that a higher temperature increase would not cause major problems.

The second consideration is how the climate will respond to a higher concentration of greenhouse gases in the atmosphere. This is a scientific issue, but science does not yet have a clear answer. The climate system is highly complex and it has been a couple of million years since the last time Earth experienced the high levels of greenhouse gases we are seeing today. For this reason we do not really know what is likely to happen. Climate sensitivity is a measure we use to estimate how responsive the planet’s climate is to certain factors. It is expressed as the temperature increase that we will have in the long term if the carbon dioxide concentration of Earth’s atmosphere is doubled over pre-industrial levels. Scientific literature indicates that climate sensitivity is probably between 1.5 and 4.5 degrees.

The third consideration is how many people will be living on Earth and how our emissions are distributed. We believe that the planet’s population will stabilise at about 10 billion around the year 2050, but will everyone be producing the same amount of emissions? At present, the average American produces about 20 times the emissions of the average Indian. Does this mean that the American will be allowed to emit more than the Indian in 2050 too, or that by then it will be India’s time to release more per head of population than the USA? Once again a question that science cannot answer but that is instead a political issue.

Using the Chalmers Climate Calculator (CCC) it is possible to see what emission reductions are going to be needed in order to meet various climate targets. Go to CCC and under Emission Scenario enter the year 2010. After that, at Rate of Reduction write 2. Now press the Generate Scenario button. The graph on the left will show that emissions towards the end of the century will be about 10 billion tonnes of carbon dioxide, so if we have a world population of 10 billion people that means emissions of roughly 1 tonne of carbon dioxide per person and year. The graphic on the far left shows that the global average temperature will have increased by less than 2 degrees. This is a rough explanation of how the target figure in One Tonne Life was decided.

However, if you now key in that Climate Sensitivity is instead 4.5 degrees, and press the Generate Scenario button once more, you will see that the temperature will rise by more than 2 degrees. Use this tool to test by how much emissions will now have to be reduced to meet the 2-degree target.

Fredrik Hedenus, Chalmers University of Technology