Looking at past changes in each driver of total emissions emphasises the necessity of deploying very low carbon technologies.
There has been some recent discussion about the Kaya identity, which breaks down emissions from energy use into four factors: population, income per capita, energy intensity of the economy (energy used to produce each unit of GDP), and carbon intensity of energy supply (carbon emissions per unit of energy), so:
Carbon emissions from energy use =
population x (GDP/capita) x (energy/GDP) x (carbon/energy)
This applies to energy, the largest source of man-made greenhouse gas emissions, but it’s not a useful tool for looking other sources of emissions such as deforestation, industrial process emissions, and non-energy activities producing other greenhouse gases.
Emissions from energy use have continued to grow to date because the growth in the first two factors in the identity has been greater than reduction in the last two. The chart below shows the breakdown into the four factors of the change each decade in total global CO2 emissions from fossil fuel combustion. Changes are measured in absolute terms – gigatonnes (Gt) of CO2 emissions per decade – rather than percentages. The bar segments show the changes associated with each factor alone, holding the other factors constant. Total changes, the net effect of the four factors, are marked by a triangle. In each decade population has grown, as has income per capita. This has been partially offset by a reduction in energy intensity. The smallest effect to date has been the change in the carbon intensity of energy use. There was a slight reduction in the emissions intensity of energy supply for three decades, reflecting in large part the growth of gas use, which has lower emissions per unit of energy than other fossil fuels, and, for part of the period, growth in electricity generation from nuclear. However this trend reversed in the first decade of this century, mainly due to the growth of coal use in China. This reversal, together with the larger effect of very strong per capita GDP growth and continuing population growth, resulted in an acceleration of the growth of emissions after the turn of the century.
Source: IPCC (see notes )
Globally, population will continue to grow in the coming decades, as will income per head, with the world economy likely to roughly triple in size by 2050 . If historic trends continue then falls in energy intensity energy will balance some of this growth of GDP, resulting in total energy use growing by about 70% over the period to 2050. There is doubtless more that can be done to reduce energy intensity, but even doubling this rate from its past level, which would be an extraordinary achievement, would only approximately stabilise energy use, and thus emissions if there is no change in carbon intensity.
This leads a huge amount needing to be done by reductions in carbon intensity to achieve emissions reduction targets. Emissions need to more than halve globally by 2050 if the agreed target of limiting temperature rise to two degrees centigrade is to be reached, and emissions will need to continue reducing thereafter. Global average carbon intensity needs to fall by a corresponding amount, even if huge gains in energy efficiency are sufficient to keep total energy use constant, which seems unlikely. Less ambitious emissions reductions targets still in practice will require large reductions in carbon intensity, especially if progress on reducing energy intensity is slower. Such reductions are made all the more challenging by the huge inertia in the energy system due to the scale and long lifetimes of existing infrastructure, with high emission existing energy sources often lasting decades. This in turn implies that increasingly most new infrastructure will need to be very low carbon if the required changes to carbon intensity are to be made in only a few decades
However carbon intensity of energy use is currently almost unchanged from its level a quarter of a century ago. Of the four factors driving emissions this has had by far the least effect – the net effect of the red bars in the above chart is much smaller than that of the other three factors. If emissions are to be reduced, this factor needs to become as large as, or larger than, the other three factors. A transformation of the energy system towards much lower carbon intensity really is necessary. In this respect the falling cost of renewables, especially solar, and progress in delivering batteries with improved technology and lower costs represent some of the most encouraging developments in securing a low carbon economy that we have yet seen.
Those who point out the urgency of the transformation to low carbon energy systems are right. Those who point out the difficulty of achieving the huge required trend break in carbon intensity are also right. The combination of urgency and scale of required reductions in carbon intensity, along with the need for faster improvements in the efficiency of energy use, is, at its simplest, what makes the problem of reducing emissions from the energy sector so challenging.
Notes and references
 The chart is from IPCC, Fifth Assessment Report Working Group 3, and can be found here:
Income is converted into common units using purchasing power parity (PPP) exchange rates.
 The extrapolation of the formula to 2050 is indicatively as follows. World GDP is expected grow at roughly 3% p.a., roughly tripling by 2050. This comprises a 30% growth in population to just over 9 billion over the period, with GDP per capita growing at around 2.3% p.a. to more than double by 2050. Data is from OECD modelling see http://www.oecd-ilibrary.org/environment/an-economic-projection-to-2050-the-oecd-env-linkages-model36-baseline_5kg0ndkjvfhf-en
With no change in energy intensity, energy use would also triple. However, energy intensity has historically reduced at about 1.5% p.a. over the past quarter century or so (based on World Bank data – other sources give broadly similar figures). This would reduce growth in energy demand to around 70%. Doubling the rate of energy intensity reduction to 3.0%, a huge increase, would only succeed in reducing the growth rate of emissions to around zero, thus stabilising emissions at their current level. Any reductions in emissions would still need to come from changes in carbon intensity.