Appearing in the earliest IPCC reports, the Kaya Identity is a venerable feature of climate change discussions.
Based on the earlier IPAT approach developed by Barry Commoner, Paul Ehrlich and others during the 1970s debates on resource exhaustion, the Kaya Identity – named after Yoichi Kaya, a Japanese economist – provides a remarkably powerful distillation of the emissions problem.
It presents energy-related CO2 emissions as being the product of four variables: population, economic output (GDP) per capita, energy intensity (the energy required per unit of GDP) and the carbon intensity of energy generation (the amount of CO2 produced per unit of energy).
This, in turn, leads to an approximation for emissions growth:
In the real world, interdependencies between these variables makes disentangling their individual effects challenging. A statistical technique known as decomposition analysis can, subject to various mathematical assumptions, provide reasonable (or, at least, defensible) estimates.
At the global level, CO2 emissions from the combustion of fossil fuels are growing by about 2% per year. The Kaya breakdown looks, stylistically, like this:
Thus, two of the four drivers of global CO2 emissions – population and per capita GDP – are positive (i.e. growing over time), one – carbon intensity – is barely changing, and only one – energy intensity – is unambiguously falling. The net effect is positive growth in CO2 emissions.
Decomposition of global CO2 emissions from fossil fuel composition, 1970-2010. Credit: IPCC
Immediately, then, one can grasp a sense of the mitigation challenge.
Population is forecast to continue increasing, from 7.2 billion today to 9.7 billion by 2050. But population control is essentially ruled out for ethical and political reasons – noting in passing, though, that the Chinese ‘One Child’ policy, recently relaxed, was one of the single-largest (albeit inadvertent) emissions-reduction policies ever deployed.
Economic growth is still largely unquestioned orthodoxy, a policy objective of all governments of all stripes and colours. The pursuit of GDP growth is unlikely to change any time soon.
So that leaves reductions in energy intensity (through energy efficiency and energy conservation) and carbon intensity (through switching to renewable energy, less carbon-intensive fossil fuels and nuclear, and capturing emissions before they vent into the atmosphere – CCS) to do the mitigation heavy-lifting.
In some regions of the world, we’re already seeing that happen. Energy intensity has declined by more than one-third, and in some cases one-half, across the OECD, Eastern Europe and Asia in the past 40 years. All regions outside Asia have registered reductions in carbon intensity.
But this hasn’t been enough at the global level.
If global energy-sector CO2 emissions are to be stabilised, energy and carbon intensity need to decline, collectively, by 3% per year, roughly three times their current rate. If CO2 emissions are to actually be reduced, the decline needs to be steeper than 3% per year. And if the world is to stay within a 2-degrees Celsius carbon budget, the rate of decline needs to be more than 6% per year.
Decarbonisation trends in the world economy (combined energy intensity and carbon intensity). Credit: PriceWaterhouseCoopers
This is theoretically possible, but it would require a historically unprecedented rate of technological adoption, not to mention premature closure of literally billions of dollars’ worth of contemporary energy generation infrastructure.
And the challenge doesn’t stop there. The Kaya Identity, in its typical form at least, addresses only energy-related CO2 emissions.
CO2 contributes three-quarters of the gaseous radiative forcing responsible for anthropogenic climate change. About half of this CO2 stems from energy-related emissions.
That leaves an awful lot of other emissions that need to be tackled – the likes of ‘chemical’ (non-energy) CO2 emissions from cement and industrial production, for example, or methane emissions from gas flaring and agriculture.
So, as daunting as the Kaya story is, it’s not daunting enough.
It should also be noted that the Kaya perspective has its detractors. The key lies in the name. The Kaya Identity is an identity, a tautology – it’s a mathematical statement that is true by definition. Cancel out the constituent terms and one is left with the not-so-profound truism that emissions are equal to emissions.
This has generated fevered debates in some corners of the internet about how much insight it can genuinely provide.
After all, one could include new components in the equation, such as profound – but utterly irrelevant – insights about the climate impacts of eating soup, and the identity would remain true:
The maths would continue to work but the equation would have no scientific or policy relevance. And if croutons can be there but shouldn’t be, what does that mean for the original – ‘real’ – variables: are they similarly irrelevant?
This debate, however, rather misses the point. For a field that features all the complex science and mathematical equations one could ask for, the Kaya Identity plays an admirable role in thinking about and conceptually simplifying the climate mitigation challenge.
It doesn’t say anything about how emissions can be brought down, and it glosses over the many inter-linkages and feedbacks between its constituent variables. But it crystallises, in a way few other approaches do, the stark decisions and trade-offs that need to be made.
And stark they are, for we live on a planet with a growing population and a growing economy. We face a classic Red Queen effect: we must ‘run’ (mitigate) ever faster merely to stand still.
Are we up to the challenge?