We use global mean surface temperatures derived from the SSPs as the only proxy of climate change. Despite the temperature being key to a number of climate change impacts, we neglect spatial heterogeneity of global warming, the regionally different starting points of warming as well as the mechanisms related to tipping points of the climate system1,5. Weather extremes are highly dependent on the absolute temperature increase as predicted by the SSPs and have been proven to affect generations unequally4. The frequency and impact of weather extremes such as the number of heatwave days and annual maximum one-day precipitation co-vary with global mean temperatures16. We expect this to additionally increase generational as well as geographical inequality, seeing that the effects of rising temperatures are not spatially homogeneous1,5,16. Furthermore, we focus on per-capita lifetime emissions only, without considering the historical inequality of emissions as revealed through cumulative emissions. Thereby, we can observe more precisely the impact individual birth cohorts have compared to each other and between world regions, in relation to the relative amount of warming they experience. For all trends and developments we describe, it must be noted that here, we refer to warming over a lifetime, neglecting the underlying absolute warming (compared to preindustrial) experienced by birth cohorts. The later a birth cohort is born, the higher the initial level of warming that the lifetime temperature increase adds to. Incorporating the temperature compared to preindustrial experienced by birth cohorts notably emphasizes inequality between birth cohorts (Fig. S6, S9, S10).
The index presented in this study depends on two variables. ΣGHG affects the index linearly, while ΔT has a non-linear effect. As a result, extreme experienced temperature increases do not proportionally reduce the index value, this mostly comes to effect in the high-emission scenarios. Compared to its variance, the extent of the temperature increase in the evaluation period still ensures a rather stable effect on the index. To facilitate comprehensibility of the index the simple relationship between the variables is assumed to represent responsibility sufficiently to evaluate trends in the data while maintaining a focus on the emissions. The mathematical relationship of the variables must however be noted, and the index is interpreted with adequate caution, especially where ΔT reaches extreme values.
The data used for this study do not cover socio-economic variability and its relation to GHG emissions on a spatial scale smaller than the world regions. Thus, intra-world region differences are neglected, as well as variability within the population of an individual country. The world region of Africa includes the countries with the highest and lowest per-capita emissions simultaneously, exemplifying the need for improvement of spatial resolution or a different parameter of grouping countries (i.e., by per-capita emissions or GDP). The data used for this study do not cover intra-national socio-economic variability and its relation to GHG emissions. Significant differences exist between nations and socio-economic groups within nations for example regarding the income and individual emissions. In the EU alone, differences of a factor of 20 were found upon the investigation of household GHG emissions, where high emissions were found to correlate with high income17.
Emission-free technologies of power generation have been in use for centuries globally (water- and wind energy). The advancement of such technologies to an industrial-level efficiency and extent has only been possible due to high emissions throughout the process. Thus, initially higher GHG emissions of a generation (causing temperature increase and adding to the responsibility of the generation for future climate change) should arguably remedy some of the responsibility. In contrast, not exhausting such emission-free technologies despite the availability of financial, technological, and material resources could be understood as additional responsibility for the effects of climate change. Such political, technological, and socio-economic circumstances vary greatly across world regions and can change rapidly with major events happening, such as the onset of a pandemic or a war.
Despite these simplifications, our study clearly demonstrates the extensive geographical and generational inequalities of climate change and proposes a methodological approach to quantify these. Additionally, our approach can be applied to datasets of any temporal or spatial resolution, given that such data are available, promising a variety of future applications and additional results when applied to different input datasets. The attribution of GHG emissions to an individual state is rarely straightforward, just as much as the question of who has a right to emissions, historical responsibility, and future emissions budgets18. However, Ott et al.17 provide convincing arguments that it is possible to assign emissions and responsibility at a country-scale, and even with regard to single extreme events19.
Our analyses reveal an inequality of ΣGHG and ΔT between individual birth cohorts and regions, most prominently a clear distinction between high- and low emission regions. Arguably among the earliest and most prominent approaches to quantify generational inequality is the Generational Environmental Debt as described by Azar and Holmberg. Despite the common basis of generational justice, their study uses a much more economy-centered approach, whereas we focus on the real-lifetime emissions and experienced warming and thereby choose a less economic, more individuum-centered method20. Such economy-centered approaches have been refined lately, yielding results that match those presented here21,22. Van Houtan et al.23 have presented a method closely related to the one presented here. They introduce an index of local climate disparity, based on emissions and local warming. While achieving a higher spatial resolution, their approach omits the generational focus that we introduce. Generally, the results presented by Van Houtan et al. and ours are in very good agreement, especially with regard to the spatial inequality of emissions where an overwhelming majority of emissions originate from nations of the Global North23. Previous research has shown that the highest emitting countries are also the ones least vulnerable to the effects of climate change, an aspect that amplifies geographical inequality that is not accounted for in this study24.
The emissions caused by one birth cohort only unfold their impact on the climate with a certain time lag. The birth cohorts do not only experience the consequences of their own lifetime emissions, but of those that previous generations emitted. Hypothetically, a future birth cohort might be born without having emitted a single gram of CO2 but will continue to suffer from global warming caused by emissions attributed to past birth cohorts. Simultaneously, the obligation to lower emissions is subject to the same time lag in emissions. Despite suffering from increasing ΔT, birth cohorts must lower their ΣGHG so that later birth cohorts can experience decreasing ΔT. At the same time, each birth cohort determines the burden of future cohorts through their emissions as a cause of future temperature increase, irrespective of the consequences (ΔT) they are faced with themselves. Within this observation lies the central problem of generational inequality: There will be birth cohorts suffering from the effects of high ΣGHG of previous birth cohorts while carrying the burden of having to reduce their lifetime emissions to allow future birth cohorts to experience decreasing amounts of ΔT. The causes (high emissions causing temperature increase) as well as the effects (higher temperatures, the obligation to lower emissions) of climate responsibility extend across lifetimes and generations, impeding precise quantification and the attribution to individual birth cohorts. Successive birth cohorts either face or escape the consequences or the climate (ir-)responsible behavior of previous generations. Simultaneously, and irrespective of the consequences (ΔT) confronted with, each birth cohort determines the consequential burden future cohorts are faced with through their emissions as a cause of further temperature increase.
The implications deriving from our results can hardly be interpreted further without agreeing on an underlying concept of justice, raising the moral question if all people should have an equal right to emissions (distributive justice) and if that applies to all generations25? Finding an answer to the question of justice and responsibility is outside the aim and scope of this study and has been touched upon elsewhere, however, must be kept in mind when working with the implications of our results. In a wider sense, our results raise ethical and moral questions. Who is responsible for which emissions, how strongly should cumulative historical emissions be weighted, and who is obliged to reduce GHG emissions in the future? Can birth cohorts be held accountable for the GHG emissions caused while still underaged? We provide valuable context and quantitative results to support the debate.
The current generations are not only the first to feel the effects of anthropogenic climate change5. We show that they are also the first ones having a realistic chance of seeing decreasing amounts of warming compared to preceding generations26, given the commitment to a low-emission future in a global effort.