Chris Jones, from the UK Met Office Hadley Centre, presented a paper at EGU 2009 yesterday on The Trillionth Tonne. The analysis shows that the key driver of temperature change is the total cumulative amount of carbon emissions. To keep below the 2°C global average temperature rise generally regarded as the threshold for preventing dangerous warming, we need to keep total cumulative emissions below a trillion tonnes. And the world is already halfway there.

Which is why the latest news about Canada’s carbon emissions are so embarrassing. Canada is now top among the G8 nations for emissions growth. Let’s look at the numbers: 747 megatonnes in 2007, up from 592 megatonnes in 1990. Using the figures in the Environment Canada report, I calculated the Canada has emitted over 12 gigatonnes since 1990. That’s 12 billion tonnes. So, in 17 years we burnt though more than 1.2% of the entire world’s total budget of carbon emissions. A total budget that has to last from the dawn of industrialization to the point at which the whole world become carbon-neutral. Oh, and Canada has 0.5% of the world’s population.

Disclaimer: I have to check whether the Hadley Centre’s target is 1 trillion tonnes of CO2-equivalent, or 1 trillion tonnes of Carbon (they are different!). The EnvCanada report numbers refer to the former.

Update: I checked with Chris, and as I feared, I got the wrong units – it’s a trillion tonnes of carbon. The conversion factor is about 3.66, so that gives us about 3.66 trillion tonnes of carbon dioxide to play with. [Note: Emissions targets are usually phrased in terms of “Carbon dioxide equivalent”, which is a bit hard to calculate as different greenhouse gases have both different molecular weights and different warming factors].

So my revised figures are that Canada burnt through only about 0.33% of the world’s total budget in the last 17 years. Which looks a little better, until you consider:

  • by population, that’s 2/3 of Canada’s entire share. 
  • Using the cumulative totals from 1900-2002. plus the figures for the more recent years from the Environment Canada report (and assuming 2008 was similar to 2007) we’ve emitted 27 gigatonnes of CO2 since 1900. Which is about 0.73% of the world’s budget, or about 147% of our fair share per head. 
  • By population, our fair share of the world’s budget is about 18 gigatonnes CO2 (=5 gigatonnes Carbon). We’d burnt through that by 1997. Everything since then is someone else’s share.

There is no silver bullet for climate change, just as there’s no silver bullet for software engineering. To understand why this is, you need to understand the magnitude of the problem.

Firstly, there’s the question of what a “safe” temperature rise would be. There’s a broad consensus among climate scientists that about a rise of around 2°C (above pre-industrial levels) is a sensible upper limit. I’ve asked a number of climate scientists why this threshold, and the answer is that above this level, scary feedback effects start to kick in, and then we’re in serious trouble. If you look at the assessments from the IPCC, the lowest stabilization level they consider is 450 ppm (parts per million), but its clear from their figures that even at this level, we would overshoot the 2°C threshold. Since that report, some scientists have argued this is way too high, and 350ppm would be a better target. Worryingly, the last IPCC assessment was based on climate models that did not include feedback effects.

Then, there’s the question of how to get there. Stabilizing at 350-450ppm requires a reduction of greenhouse emissions of around 80% in industrialized nations by the year 2050. Monbiot argues that if you think in terms of a reduction per capita, you have to allow for population growth. So that really means a reduction more like 90% per person. And again, due to our uncertainty about feedback effects, the emissions targets may need to be even lower.

How do reduce emissions by 90% per person? The problem is that our emissions of greenhouse gases come from everything we do, and no one activity or industry dominates. I was looking for a good graphic for my ICSE talk, to illustrate this point, when I came across this chart of sources of emissions:


World Greenhouse Gas Emissions by Sector

World Greenhouse Gas Emissions by Sector



I think that’s enough on it’s own to show there is not likely to be a silver bullet. The only way to solve the problem is a systemic analysis of sources of emissions, and we have to take into account a huge number of different options. If you want more detail on the figures, Jon Rynn at Grist has started to put together some spreadsheets to add up all the sources of emissions, and some specific contributors.

BTW, the IPCC’s frequently asked questions is a great primer for anyone new to the physics of climate change.