Optimism and pessimism on climate

A little optimism on climate

The recent publication in Nature, Reaching peak emissions, by Jackson et. al., has been echoed by hopeful messages in the media. Carbon Brief is more measured. Their piece Decrease in China’s coal use sees global emissions fall in 2015 starts optimistically:

The rapid growth in global carbon dioxide emissions over the last decade appears to have stalled, a new study says. Early estimates suggest that global emissions will decrease by 0.6% this year, following a small increase of 0.6% in 2014.

But Carbon Brief also reports

The break in the emissions trend is mainly down to a drop in coal use in China, the study says, which drives a projected decrease in Chinese emissions of around 4% compared to 2014.

However, this is unlikely to signal a peak in global emissions just yet, the researchers say.

But I’m more pessimistic on climate

Previously Carbon Brief reported the remaining carbon budget to give a 66% chance of keeping global warming below 1.5˚C. This year it is 243 billion tonnes. That means, if 243 billion tonnes more CO2e is emitted, global temperature will rise to 1.5˚C above the global temerature in 1750 – before modern industrialisation started. Using the same calculations, the remaining carbon budget to keep the rise below 2˚C is 843 billion tonnes.

For the sake of argument, in Is Green Growth a fantasy? I accepted the IPCC’s optimism that by the second half of the century, it will be possible to extract large amounts of carbon from the atmosphere, probably using Bioenergy Carbon Capture and Storage (BECCS).

The emissions before large scale BECCS is available  should be below the  “remaining carbon budget” so that it is possible to reach the “BECCS safety zone”, when carbon extraction can save the climate and carbon emissions to the atmosphere will be balanced by carbon extraction.

Taking the optimistic view and assuming that a global temperature rise of 2˚C is safe enough, I shall examine the figures of Jackson et. al. and see if the carbon extraction safety time zone can be reached within a remaining carbon budget of 843 billion tonnes of CO2e.

The main, fairly simple, calculation is to find the decrease in the yearly rate of CO2e emissions, which will eke out the budget of 843 billion tonnes until 2050 – the beginning of the BECCS safety zone. According to their Figure 1, emissions are now about 36.5 billion tonnes a year. A decrease in carbon emissions of 2.75% per year will mean the total emissions – from now to 2050 – will be just less than 843 billion tonnes.

The “emissions intensity of production” is the quantity of emissions (measured in tonnes of CO2e) created by a unit of production (measured in, say, time-standardised dollars). For the world, emissions intensity is global emissions divided by the global GDP. Jackson et. al. say:

…global GDP grew at a stable rate of 3.3–3.4% yr −1 during 2012, 2013 and 2014, and is projected to grow a further 3.1% in 2015 (IMF 14). The decoupling of fossil-fuel emissions and global GDP reduced the carbon intensity of the global economy by 2.7% in 2014; our projection for 2015 indicates a 3.7% reduction, compared with an average 1.1% for the decade before (Fig. 1). Two questions naturally arise from these new data: what is causing the break and does it signal the beginning of a reversal in global emissions growth?

Again, for the sake of this argument, let us assume that this year’s 3.7% decrease in carbon intensity can be continued indefinitely. Also assume that global GDP is growing at 1% per year. This tracks the increase in world population, which is 1% per year. This means – more or less – that average consumption per person remains the same as it is now.

With a growth rate for global GDP of 1% and a reduction of carbon intensity of 3.7%, carbon emissions should fall by 2.7% a year. The world will have reached the safety zone with a few months to spare.

Of course, this is a highly idealised piece of applied mathematics. (Remember the anecdote of the two avalanches connected by a light inextensible string”?) However, most of the assumptions err on the dangerous side. For the 1.5˚C budget, it will be exhausted by 2021.

More pessimism on climate

1. As As Jackson et. al. conjecture, the rate of decarbonisation may not (probably won’t) be maintained. Over the past 25 years it has been 1.3%. Relying on the year 2015 may turn out to be cherry picking.

2. Even at a their rate of decarbonisation (3.7%) there is no room for growth and to keep within the remaining carbon budget.

3. The IPCC budgets are too generous because of missing feedbacks in the climate models.

4. BECCS might not work at scale in time.

5. A global temperature rise of 2˚C might not be safe but the budget to keep within 1.5˚C runs out in 2021.

One cause for optimism on climate?

The affluent might listen to a higher power, cut the growth of polluting consumption and enjoy a better life. Are these the new commandments?:

Kill My world as little as you can
Don’t drve a car,
Don’t fly in planes
And leave that corned beef in the can

Kill My world as little as you’re able
Don’t build with bricks
Don’t build with steel
And eat the horse that’s in your stable.

Don’t kill My world by rushing all the time
Gaze at the stars
Breathe in the air
Praise all creation that is Mine


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