select search filters
briefings
roundups & rapid reactions
before the headlines
Fiona fox's blog
A study published in Nature Climate Change assesses remaining carbon budgets.
Prof Pierre Friedlingstein FRS, Chair in Mathematical Modelling of Climate Systems, University of Exeter, said:
“This new study suggests that the remaining carbon budget for 1.5°C is smaller than the one assessed by IPCC (which was already very small). The study also shows that minor changes in methodology can have major knock-on effects on this budget estimate. What it really means is that we are now in the “too close to call” territory for 1.5°C.”
The following quotes were supplied by our friends at the German SMC:
Prof. Dr. Niklas Höhne, Director and CEO, New Climate Institute, Cologne, and Professor of Mitigation of greenhouse gas emissions, Wageningen University, Netherlands
“The current study shows one thing above all: it will be very, very tight for the 1.5-degree limit. It is almost irrelevant whether the budget is used up in six years – as this study suggests – or in ten years, as previously thought, if emissions remain the same. It’s extremely tight either way. And that’s not a new finding.
“But that in no way means we should give up, quite the opposite. It shows that every ton of carbon dioxide saved is all the more important because the budget is so extremely tight. And even if the multi-year average temperature increase exceeds 1.5 degrees, it’s good to have saved as many emissions as possible beforehand, because every ton saved leads to less global temperature increase and therefore less damage.
“Even if the 1.5 degrees is exceeded for a period of time, the global mean temperature could decline down once emissions are reduced to zero and more CO2 is removed from the atmosphere. For that, too, it’s beneficial if less was emitted beforehand.
“The extreme temperatures and accompanying droughts, storms and extreme weather events of this past year in particular have shown that we simply cannot adapt to uncontrolled climate change. This study is another call to go into emergency mode and do everything we can to reduce greenhouse gas emissions as quickly as possible.”
Dr. Oliver Geden, Senior Fellow, EU/Europe Department, Stiftung Wissenschaft und Politik – German Institute for International and Security Affairs (SWP), Berlin
“Above all, the study shows that the calculation of remaining CO2 budgets depends on many assumptions and that these are not immutable quantities. Most importantly, it is necessary to understand that the budgets only include carbon dioxide and that these CO2 quantities are dependent on assumptions about future mitigation of other important greenhouse gases – such as methane and nitrous oxide – and it is these assumptions that have changed. The problem is that the budgets in the Working Group I contribution to the IPCC Assessment Report, published in 2021, were still calculated without knowledge of the emission reduction pathways from the Working Group III contribution, published in 2022. In the IPCC synthesis report, these two strands of knowledge were not allowed to be combined due to strict IPCC regulations. Basically, this current study now catches up with that. A similar recalculation can already be found in [1]. That is why the downward readjustment of the CO2 budgets does not come as a surprise to experts.
“The accompanying communication states, this calculation method halves the remaining CO2 budget. That is grossly misleading. The budget in the IPCC report of 500 gigatons of CO2 starts counting on Jan. 1, 2020, while the budget recalculated in this current study starts counting on Jan. 1, 2023, three years later. A rough estimate shows that 120 to 125 gigatons of CO2 emissions have been emitted in between. The difference between this study and the result of the IPCC Report is therefore only 125 to 130 gigatons. So of the 375 to 380 gigatons of the IPCC budget that was left at the beginning of 2023, it’s about one-third.
“Without attracting particular attention, the remaining CO2 budgets during the preparation of the IPCC reports, the 1.5-degree Special Report and the Working Group I contribution to the Assessment Report, have grown strongly due to methodological improvements, by several hundred gigatons – in [2] you will find graphs showing this vividly. For the 2 degree target, even in the current study from 2023 the budget is still about the same as calculated in the previous one, the Fifth IPCC Assessment Report for the period from 2011: the famous 1000 gigatons left for the world. All these changes of direction show that while the calculation of remaining CO2 budgets provides important indications, the figures themselves must not be set in absolute terms. Due to their permanent recalculation, global residual budgets are also not suitable for deriving national or European residual budgets from them, because these changes in direction and size cannot be implemented in the short term for German and European climate policy and undermine any climate policy certainty of expectation.”
[1] Forster PM et al. (2023): Indicators of Global Climate Change 2022: annual update of large-scale indicators of the state of the climate system and human influence. Earth System Science Data. DOI: 10.5194/essd-15-2295-2023.
[2] Knopf B et al. (2022): Ist Deutschland auf dem 1,5-Grad-Pfad? Eine Einordnung der Diskussion über ein nationales CO2-Budget. Publikation des Mercator Research Institute on Global Commons and Climate Change (MCC).
Gabriel Abrahão, Ph.D., Research Fellow in the Department of Transformation Pathways, Potsdam Institute for Climate Impact Research (PIK)
“Although the results of the study may at first glance seem like a radical deviation from the AR6 results, the downwards revision in budgets are mostly a consequence of new data and methodological updates that were in many ways expected since that work commenced. A large part of the reduction in the carbon budgets comes from the simple fact that humanity kept emitting around 40 gigatonnes of CO2 every year since the publication of the previous budget. Since the budget estimates for 1.5C were already very tight, every year emissions do not go down makes a large difference.
“Most of the remaining revision comes indirectly from updating the data and harmonization procedures on historical aerosol emissions. Aerosol emissions are largely associated with fossil fuel burning and are mostly undesirable for humankind due to their detrimental effect on regional air quality, but they do have a cooling effect on climate. As air quality standards improve and fossil fuels are phased out this cooling effect will reduce, having an effect of increasing warming and therefore reducing the remaining carbon budget. This effect has always been accounted for in the budget and had an appreciable impact on it. But the revised data on past aerosol emissions indicates that their cooling effect was somewhat underestimated, and this revised knowledge ultimately meant a smaller budget.
“It’s important to note that the actual understanding of how climate responds to CO2 or other GHG emissions did not change substantially with this revision of the carbon budget. The large relative changes to the remaining budget for staying below 1.5C are simply due to the fact that it was very small in the first place. The bulk of the effect of the revisions come from recent emissions and the effect of aerosols, both of which lead to large absolute reductions of the budgets. This makes the relative differences much larger in the case of the already small 1.5C budgets compared to 2C budgets.
“There is a very real possibility that we will overshoot the 1.5C target already in this decade. The Paris Agreement covers this overshoot possibility, but only if efforts are pursued to reach it in the long-term. Thus, the public debate and the international climate negotiations should already be discussing how to return to 1.5C after an overshoot, so that the provisional exceedance doesn’t end up being permanent.
“The fact that we are exhausting the budget so fast exposes how slow changes in human systems to reduce emissions can be. This means that even if the climate doesn’t change visibly and tangibly from one year to another, or with the crossing of a given target, action to reduce emissions has to be swift and decisive to avoid the worst impacts of climate change a few years down the line.”
Prof. Dr. Klaus Hubacek, Professor in Science, Technology and Society, Faculty of Science and Engineering, Universität Groningen, Netherlands
“Calculation refinements based on the latest data and robustness checks, such as those presented in this study are important to build confidence in the results and the process. The process does not end with each round of the IPCC, but must continue, and this study makes an important contribution to that process.
“An important problem remains on how to best communicate to the public the inevitable uncertainties in model results. On the other hand, the overall message does not change much because all estimates point to increasing urgency, global emissions are still rising, and the remaining emissions budget is rapidly declining, contributing to new temperature records and catastrophic events.
“Indeed, the inertia is in the response and resistance in the political system. Science, politics, and oil companies have been aware of the problem for many decades, but insufficient action has been taken, and the emissions are still rising. The focus therefore needs to be on how to achieve emissions reductions, and an increased focus therefore needs to be on social, economic, and political science, strengthening our understanding on how to best decarbonize the economy and society as quickly as possible.
“While studies like this one are important and academically interesting, they should no longer make a difference politically given the overwhelming evidence and urgency. Research needs to focus on mechanisms that prevent us from taking urgent action. Unfortunately, we also need more and more research on proper adaptation now that we have failed to stop climate change and have already triggered potentially irreversible changes.”
Prof. Dr. Carl-Friedrich Schleussner, Head of the Research Group Temporal Evolution of Adaptation Barriers and their Relevance for Climate-related Loss and Damage, Integrative Research Institute on Human-Environment Systems Change (IRI THESys), Humboldt University Berlin, and Head of Climate Science and Impacts, Climate Analytics, Berlin, said:
“This is a profound study that updates the methodology for calculating the remaining CO2 budget. However, the results should not be over-interpreted. The remaining CO2 budget, especially for a 50 percent chance of exceeding the 1.5-degree limit, is unfortunately very small and decreases with each year of ongoing emissions. That’s also one of the main reasons for the smaller budget in this study – the emissions between 2020 and 2022. Moreover, small changes in our scientific understanding led to very large relative effects with such a small remaining budget.
“Such changes are part of the scientific process. Methodological changes between the IPCC Special Report on 1.5 degrees and the Working Group I contribution to the Sixth Assessment Report resulted in a larger budget (see Figure 3 in [3]). Further updates in Working Group III and now this study again point to a smaller budget. However, these are all within the ranges of uncertainty.
“The CO2 budget is one of the most important concepts in climate research and climate communication. However, like other core variables of the climate system it is subject to uncertainty. Focusing on a single number has therefore always been problematic. This is especially the case now when the uncertainties become larger than the remaining budget. The likely range for the CO2 budget for the 1.5-degree target – with a probability of 17 to 83 percent – is estimated in this current study to be minus 200 to 830 gigatons of CO2. The range of uncertainty is thus four times larger than the remaining budget of 250 gigatons of CO2 from 2023 as a best estimate. The role of non-CO2 greenhouse gases and other climate forcers such as aerosols is becoming increasingly important. As a result, the warming level of 1.5 degrees might be reached years after the budget is exhausted. An ever smaller budget therefore does not automatically mean that the 1.5 degree target can no longer be achieved. It means however that our chances are diminishing.
“At the same time – and this is also important to mention – the authors of the current study also provide an estimate for the budget to ‘stay well below 2 degrees’ (with a 90% probability): The residual budget for this is 500 gigatons of CO2.
“Using the CO2 budget as ‘a fixed number’ rather than a general concept has clear limits. So, the study shows once again: the remaining CO2 budget for reaching the goals of the Paris Agreement is very small – no matter which method is used. And the message is always the same: In order to still meet the goals, emissions must be reduced as quickly as possible. The current decade is the critical decade. The results of this study are largely consistent with the analysis of the emissions pathways in the IPCC Working Group III paper. Working Group III clearly shows how we can keep within reach the goal of limiting global warming to 1.5 degrees: Global greenhouse gas emissions must peak before 2025, be on track for an approximate halving in 2030 and net zero emissions in 2050. That must be the focus of the upcoming climate conference in Dubai.”
[3] Constrain (2022): Zero in on the crotocal decade: Insights from the latest IPCC reports on the Paris Agreement, 1.5°C, and climate impacts. DOI: 10.5281/zenodo.711315.
‘Assessing the size and uncertainty of remaining carbon budgets’ by Robin D. Lamboll et al. was published in Nature Climate Change at 4pm UK TIME on Monday 30 October 2023.
DOI: https://doi.org/10.1038/s41558-023-01848-5
Declared interests
No interests declared