Why children must emit eight times less CO2 than their grandparents

This is a re-post from Carbon Brief

Global emissions of CO2 need to decline precipitously over the next few decades, if the world is to meet the Paris Agreement goals of limiting global warming to “well below 2C” and, ideally, below 1.5C.

If these goals are to be met, young people would have to live the greater part of their lives without contributing significantly to global emissions. Essentially, they would have fewer “allowable” CO2 emissions during their lifetime, compared with older generations.

To determine just how much smaller their personal CO2 limits would be, Carbon Brief has combined historical data on emissions and population with projections for the future. In a world where warming is limited to 1.5C, the average person born today can emit only an eighth of the lifetime emissions of someone born in 1950.

The interactive tool, below, shows the size of each person’s “carbon budget” during their  lifetime – based on when and where they were born.

It looks at two different scenarios: one where the world limits warming to well below 2C above pre-industrial levels by 2100; and one were warming is limited to 1.5C.

It also considers two different ways of sharing future allowable emissions: one where each country tracks “optimal” pathways taken from models; and another, focused on equality, where each person can use the same portion of future emissions, no matter where they live.

In all cases, younger generations will have to make do with substantially smaller lifetime carbon budgets than older generations, if the Paris limits are to be respected. This is because most of the allowable emissions have already been used up, meaning young people will not have the luxury of unmitigated emissions enjoyed by older generations.

The idea for this analysis was first proposed to Carbon Brief by Dr Ben Caldecott at the University of Oxford. The methodology used – and its limitations – are explained in detail at the end of this article. Carbon Brief is now working to further develop the analysis with Dr Caldecott and his colleagues.

The global picture

Global emissions must peak in the next decade and quickly decline for the world to stay below its Paris Agreement limits, according to the UN. In the scenarios examined in this article (see methodology at the end for details), global emissions peak around 2020, decline around 50% by 2045 and then fall below zero around 2075 in order to hold global warming to below 2C.

Emissions have to fall even faster for warming to be kept below 1.5C – falling around 50% by 2030 and to below zero by 2055. In the 1.5C scenarios examined here, large amounts of negative emissions are deployed by the end of the century, removing carbon from the atmosphere equivalent to roughly a third of today’s emissions.

These emissions pathways can be divided up into average “lifetime carbon budgets” that depend on an individual’s year of birth. This allocation is based on the changing global population and emissions during each individual’s lifetime.

The figure below shows the global average lifetime carbon budget for people born in each year between 1900 and 2017, in scenarios where warming is kept below 1.5C (dark blue) or 2C (light blue).

Global average lifetime carbon budgets per-capita by birth year for 1.5C and 2C scenarios, assuming a lifespan of 85 years. Based on historical emissions data from the Global Carbon Project, historical and future projected population from the United Nations and global emission projections from MESSAGE-GLOBIOM. Generation birth years shown at the bottom from the Pew Research Center. See the methodology section for details. Chart by Carbon Brief using Highcharts.

As the chart above shows, if warming is limited to well below 2C the global average lifetime carbon budget for someone born in 2017 is 122 tonnes of CO2, only about a third as large as the budget for someone born in 1950. If warming is to be limited to 1.5C, the remaining budget is only 43 tonnes of CO2 and the difference is eight times as large.

Current per-capita global emissions are around 4.9 tonnes per person per year. This means that the lifetime carbon budget of someone born today is equal to 25 years of current emissions if warming is limited to well below 2C – and only nine years of current emissions if warming is limited to 1.5C.

Divvying up emissions

The analysis above uses a global average carbon budget. However, in reality, there is no such thing as a “global average” person and each country’s emissions will follow a slightly different trajectory in “well below” 2C and 1.5C worlds.

In general, emission reductions will need to be proportionally larger in developed, wealthier countries, such as the US, where per-capita emissions are very high. Developing nations, such as India, already have much lower per-capita emissions.

To put the difference into perspective, the average Indian had emissions of 1.9 tonnes of CO2 in 2017, whereas the figure in the US was 16.9 tonnes of CO2.

Moreover, historical emissions vary greatly between countries, with the likes of the US and UK responsible for a far larger share of cumulative emissions since the industrial revolution. This poses an open question as to how the fixed global carbon budgets set by the Paris Agreement should be divided between different countries.

There are lots of different ways to allocating future emissions between countries. Integrated assessment models (IAMs) – energy system models that examine what mix of different technologies and choices are needed to meet climate targets – provide one set of budget allocations, reporting future emissions for each region of the world.

The figure below is based on the allocations in 1.5C scenarios from IAMs. It shows how lifetime carbon budgets vary based on birth year, for four major countries and regions that are responsible for the bulk of global CO2 emissions. These are the US (light blue line), Europe (dark blue), China (red), and India (yellow).

Lifetime carbon budgets by birth year based on historical emissions and future IAM 1.5C scenarios, assuming a lifespan of 85 years. Based on historical emissions data from the Global Carbon Project, historical and future projected population from the United Nations and regional emission projections from MESSAGE-GLOBIOM. Generation birth years shown at the bottom from the Pew Research Center. See the methodology section for details. Chart by Carbon Brief using Highcharts.

If the remaining carbon budget is divided up in this way, based on IAM pathways, then national  allowable lifetime emissions are much more similar for someone born in 2017 than in 1950 – but there are still large differences between countries.

For example, someone born today in the US would still be allocated a lifetime carbon budget some 15 times larger than someone born in India. Their budget would be four times larger than someone born in China and around twice as large as in Europe.

The table below shows the lifetime carbon budget in a 1.5C world (2C world) both globally and by major country/region, broken down by generation:

Lifetime carbon budgets in tonnes of CO2 by birth year based on historical emissions and future IAM 1.5C (and 2C) scenarios. Pre-Boomer generations have identical 1.5C and 2C carbon budgets. Using generation periods from the Pew Research Center and averaging the lifetime budget of all the birth years of each generation.

This approach raises obvious questions about equity, as it implies that countries with high historical emissions will also receive a larger share of the proverbial pie in the future. There are lots of different ways to define equity – and little agreement – regarding which approaches would be both possible and “fair” for allocating future emissions.

One alternative would be to allocate the remaining budget equally between all people, wherever they live. This might be hard to achieve in practice as, say, per-capita US emissions would need to fall rapidly towards the global average while those in India would immediately rise.

But it provides a useful thought experiment that can be contrasted to the lifetime carbon budget allocation set out above. Even this might not be truly equal, is it neglects responsibility for historical emissions.

The figure below shows the effect of this allocation on lifetime carbon budgets by birth year for the same four major countries and regions. It is based on historical per-capita emissions and equal per-capita shares of the remaining carbon budget from 2018 onwards, in a scenario where warming is limited to 1.5C.

Same as the prior figure, but using global emission projections from MESSAGE-GLOBIOM to calculate future global per person emissions. See the methodology section for details. Chart by Carbon Brief using Highcharts.

The chart above shows that lifetime carbon budgets converge much more quickly when future emissions are divided equally, even though historical differences between countries remain. As a result, someone born in 2017 would have a similar lifetime carbon budget no matter where they are born.

Some limitations

Calculating lifetime carbon budgets is necessarily imperfect and relies on a series of unrealistic assumptions. Every person is different and, in practice, individual emissions will be strongly affected by income, behaviour and other factors.

While the average 1.5C lifetime carbon budget of someone, say, born in the US around 1995 might be 696 tonnes of CO2, people in that generation will, in practice, have widely varying individual emissions.

The approach taken here – dividing national emissions by population – also glosses over the fact that a sizable portion of emissions for some countries are the result of industrial and commercial activity producing goods for trade that are not consumed at home. These “consumption footprints” can differ significantly from national emission estimates, as Carbon Brief has previously examined.

For simplicity, a constant lifespan of 85 years is assumed when calculating lifetime carbon footprints. This is higher than the current average lifespan in most countries, but may be more realistic for younger generations today given expected advances in medical science and access to healthcare. However, in practice, lifespan differences between countries will likely persist into the future and could impact these calculations.

Finally, this approach assumes that emissions in a given year can be assigned equally across the population regardless of age. In reality, people are probably responsible for considerably lower emissions when they are children than adults, as they are not, say, driving cars and are often consuming less.

That said, this analysis provides a first look at how lifetime carbon budgets vary by age. It suggests that the allowable lifetime emissions for young people today is a fraction of that of previous generations, as the global budget for avoiding warming of 1.5C or 2C has already been mostly used up.

Methodology

Lifetime carbon budgets were calculated by adding the historical and projected future per-capita emissions for each year that an individual is expected to live – assuming a constant lifespan of 85 years since a given birth year for simplicity. This is higher than the current global average lifespan (it is typical of Japan today), but may be more typical for the lifespan of younger people today given continuing medical advances.

For example, if someone were born in the year 2000 in India, their lifetime carbon footprint would be the sum of historical per-capita emissions in India from 2000 to 2017, plus forecast per-capita emissions in India between 2018 and 2085.

The end of 2017 serves as the demarcation between historical and future emissions because 2018 emission and population values are not yet available for all countries.

Carbon budgets were calculated for all possible birth years from 1900 to 2017 for major countries and each of the world regions where UN population projections were available: Africa, Europe, Latin America and the Caribbean, North America, Oceania and Asia.

Historical CO2 emission estimates for each country from 1751-2017 were obtained from the Global Carbon Project. Historical population data from 1950-2017 and future population projections from 2018-2100 were obtained for each country from the UN World Population Prospects 2017. The “medium” scenario was chosen for future population projections, as it matches reasonably well with the population assumptions in the Shared Socioeconomic Pathway (SSP2) world used for IAM emission scenarios.

Future emissions by country for both 1.5C and 2C targets were based on IAM runs from the International Institute for Applied Systems Analysis (IIASA) MESSAGE-GLOBIOM model using the SSP2 world. SSP2 is a world where current economic and population trends broadly continue and MESSAGE-GLOBIOM was the model chosen to represent SSP2. MESSAGE-GLOBIOM emissions by region – and globally – were taken from the IAMC 1.5C Scenario Explorer.

As IAM runs in recent years lack country-specific values, regional emission estimates were used to estimate country-specific trajectories by scaling current country emissions by the percent reduction in regional emissions from the IAM runs. For example, if the IAM runs showed OECD countries reducing emissions by 50% by 2040 in a 1.5C scenario, emissions in each OECD country were estimated to decrease by 50% by 2040.

Net future emissions were used for per-capita emission estimates. This means that in many countries future per-capita emissions go negative in the second half of the 21st century, particularly in 1.5C scenarios. The distribution of negative emissions in MESSAGE-GLOBIOM varies regionally, with a particularly high concentration of negative emissions in Latin America and the Caribbean.

Finally, as both emission and population projections are only available through to 2100, but people born after 2015 will still be alive post-2100, per-capita emissions are assumed to remain constant at 2100 values in subsequent years.

Two future emission allocation scenarios are provided: one based on the regional MESSAGE-GLOBIOM emission pathways and one where the global MESSAGE-GLOBIOM projected emissions are distributed evenly to every country on a per-capita basis after 2017. The latter shows how a more equitable distribution of remaining emissions would affect lifetime carbon budgets, compared to the allocation in IAMs.

The countries featured in the interactive tool are a subset of those with the largest populations. However, major regions are also included, so if there is a country not featured on the list its region should provide a reasonable estimate. The “North America” region is not shown as all member countries appear on the list.

Posted by Zeke Hausfather on Friday, 14 August, 2020

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