The record-breaking heat of 2023 ushered in the first recorded 12-month period of temperatures exceeding 1.5°C over pre-industrial levels. The extreme heat we are facing today and the more punishing 2-3°C expected by the end of the century mean that the climate crisis is made up of not one but two climate emergencies – today’s still manageable emergency and tomorrow’s catastrophic and far less manageable emergency. Addressing both climate emergencies requires understanding that there are two distinct categories of climate pollutants that are warming the planet: long-lived carbon dioxide (CO2), responsible for half of historic warming, and non-CO2 super climate pollutants responsible for the other half, including the short-lived climate pollutants: methane, hydrofluorocarbons, tropospheric ozone, and black carbon. Reigning in the temperature in the near-term of the next 10 to 20 years – which is critical for slowing self-amplifying feedbacks and avoiding or slowing irreversible tipping points – is only possible if we cut the super pollutants in addition to CO2. Reigning in warming in the longer term later this century requires phasing out fossil fuels to get to zero CO2 emissions by mid-century. But the most aggressive cuts to CO2 cannot alone address the near-term climate emergency, in large part because shifting to clean energy reduces cooling sulfates that are emitted along with CO2 and the net effect only bends the warming curve by a modest amount through 2050. To address both climate emergencies, leaders and institutions must take actions to make deep cuts to both CO2 and these other super climate pollutants. These actions are complementary and not interchangeable. They are also essential to protecting human rights and the natural systems we depend on.
We are facing two climate emergencies
Most attention has been focused on the level of global warming at the end of the century. There has been some progress in this regard. Projections based on current policies and commitments suggest that human-caused emissions of carbon dioxide from burning fossil fuels and cutting down forests put us on the path for a warming of around 2.7°C by the end of the century, compared with around 4°C prior to the Paris Agreement.[1] This is progress. But even warming of 2.7°C would place nearly a third of humanity outside the corridor of life that human civilization has existed in for the past 6000 years,[2] while pushing the planet past a series of expected tipping points where impacts will be abrupt in some cases, non-linear, irreversible, and catastrophic.[3]
It is this end-of-century climate emergency that demands we stop adding carbon dioxide (CO2) to the atmosphere. Every ton of CO2 we emit is a legacy of committed warming for our children and future generations. This is the marathon that we are running now to free our economies from fossil fuel addiction.
But there is another climate emergency. This is the climate emergency we are facing today. We are already experiencing the extreme heatwaves, storms, floods, and droughts of a changed climate. Billion-dollar disasters are becoming more frequent,[4] and these financial costs only hint at the human costs and suffering as extreme weather, failed crops, and sea level rise drive tens of millions from their homes and fuel a global climate migration crisis.[5]
As an example of how climate change is affecting our lives, consider the state of California where I grew up. When I was a child, fire season lasted a few months. Now there are years where every month is fire season, and smoke turns the sky orange.[6] The impacts of climate trauma go beyond physical and financial to mental health and cognitive function.[7] And these impacts will grow as fossil-fueled fires burn more of western North America.[8]
Human-caused climate change is already reshaping our world. Indeed, experts in climate attribution science tell us that the “current climate has changed so significantly that the pre-industrial world [has become] a poor basis of comparison.”[9]
And this is with current warming of about 1.2°C above pre-industrial levels. Under current rates of warming, we are expected to lock in 1.5°C by 2030 when we exhaust the carbon budget.[10] Every tenth of a degree of warming increases the severity and frequency of extreme climate and weather disasters, as laid out starkly by the 2018 Special Report on 1.5°C by the Intergovernmental Panel on Climate Change and the more recent Sixth Assessment Report by the IPCC. We also know that beyond 1.5°C the risks of crossing irreversible tipping points in the climate system increase (Table 1).[11]
If cutting carbon dioxide is the marathon we are running today to limit the climate emergency at the end of the century, then cutting methane and the other super climate pollutants is the sprint we also need to run this decade to slow warming and limit today’s climate emergency.
Table 1. Summary of key climate tipping points and feedbacks. Adapted from The Global Tipping Points Report https://global-tipping-points.org/ (2023) and sources summarized in Zaelke, Picolotti, and Dreyfus (2024) The Need for Fast Near-Term Climate Mitigation to Slow Feedbacks and Avoid Tipping Points https://www.igsd.org/wp-content/uploads/2024/01/IGSD-Background-Note.pdf
The other half of warming
Nearly half of today’s warming from greenhouse gases is caused by super climate pollutants other than carbon dioxide (Figure 1). Super climate pollutants include methane, ground-level ozone smog, fluorinated gases, and nitrous oxide. There’s also black carbon soot, which isn’t a greenhouse gas but a particle that both contributes to warming and is a major air pollutant. These pollutants are tens to thousands of times more potent than CO2 at trapping heat in our atmosphere. And most of them are short-lived, meaning they only last for days to 15 years in the atmosphere.
Figure 1. Contributions to warming by climate pollutant. Data from Szopa, S.; Smith, C.; Blichner, S.; Berntsen, T.; Collins, B.; Gillett, N.; Thorne, P.; Trewin, B. (2021): Summary for Policymakers of the Working Group I Contribution to the IPCC Sixth Assessment Report – data for Figure SPM.2 (v20210809). NERC EDS Centre for Environmental Data Analysis, 09 August 2021. doi:10.5285/c1eb6dad1598427f8f9f3eae346ece2f.
If carbon dioxide is the slow burner, methane and the other super climate pollutants are the blow torch, together bringing our climate to a boil.[12]
According to the IPCC, carbon dioxide emissions are contributing about 0.8°C of present warming, while these other super climate pollutants are contributing a similar level of warming (Figure 1).
But that alone adds up to over 1.5°C, you might be thinking, and you’re right.
In addition to greenhouse gases, human activity also creates particle pollution or aerosols. Some of these contribute to warming, like black carbon, but others like sulfate aerosols are reflective and have a cooling effect. The cooling effect from these reflective aerosols is masking some of the warming from greenhouse gases. The main source of the cooling aerosols is burning fossil fuels. This means that as we run the marathon to cut carbon dioxide by transitioning our economies, there will be less of these cooling aerosols and more of the warming will be unmasked.[13] As highlighted by Jos Lelieveld and Örjan Gustafsson in their presentations, in addition to net warming, the pollution from burning fossil fuels is responsible for millions of premature deaths annually, so this unmasking effect is no reason to slow the fossil fuel phaseout. Instead, it is a reason to slash short-lived climate pollutants like methane alongside the energy transition.
Running two (or three) races simultaneously
To tackle both climate emergencies, we need our leaders and institutions to take actions to make deep cuts to both carbon dioxide and these other super climate pollutants. We can and we must bend down the temperature curve as soon and as quickly as possible to stay within the limits of the capacities of human and natural systems to adapt.
The sprint to cut methane and the other super climate pollutants is the best way we know to slow warming in the near term. While the marathon to phase out fossil fuels is the only way to limit warming in the longer term. Both these strategies are essential. They are complementary and they are not interchangeable. Cutting CO2 faster won’t slow warming in the near term because of the sulfate unmasking. At the same time, only cutting methane and the super climate pollutants without stopping CO2 emissions locks in long-term warming.
This is why we are in a dual race – two races at the same time – a sprint and a marathon. The good news is that there are many mitigation options available that are low-cost (Table 2). In addition to renewable energy from solar and wind, protecting forests and other existing carbon sinks, plugging methane leaks and using the Montreal Protocol to speed a transition to energy efficient and climate-friendly cooling are readily available strategies. And we are also starting a third race: the ultramarathon to develop approaches to remove carbon dioxide and possibly methane from the atmosphere.
Table 2. Overview of super climate pollutants and their mitigation potential, sources, atmospheric lifetimes, 20-year global warming potential, and mitigation strategies. Adapted from sources summarized in Zaelke, Picolotti, and Dreyfus (2024) The Need for Fast Near-Term Climate Mitigation to Slow Feedbacks and Avoid Tipping Points https://www.igsd.org/wp-content/uploads/2024/01/IGSD-Background-Note.pdf
The barriers to winning these races are generally not a question of science or knowledge, but the laws and policies that govern who gets to pollute and who pays for the impacts of that pollution. The science and scientists have done their job to make clear the risks we face. Now it is time for policy makers and the private sector to do their job and implement the known technologies at speed and scale so we can win both the sprint and the marathon.
Acknowledgements
Thank you to Alyssa Hull for preparing the tables and Julie Miller for making the figure. Review and helpful comments from Durwood Zaelke are gratefully acknowledged. I also thank the organizers of the summit and hosts for this timely Call to Action.
References
[1] UNFCCC (2023) Technical dialogue of the first global stocktake. Synthesis report by the co-facilitators on the technical dialogue, 13. See https://unfccc.int/documents/631600
[2] Lenton T.M., Xu C., Abrams J.F., Ghadiali A., Loriani S., Sakschewski B., Zimm C., Ebi K.L., Dunn R.R., Svenning J.-C., & Scheffer M. (2023) Quantifying the human cost of global warming, Nat Sustain 1-11, 7.
[3] Lenton T.M., et al. (2023) The Global Tipping Points Report 2023, University of Exeter, Exeter, UK. See https://global-tipping-points.org/
[4] See National Oceanic and Atmospheric Administration (last accessed 1 October 2023) Billion-Dollar Weather and Climate Disasters. See https://www.ncei.noaa.gov/access/billions/
[5] Kumari Rigaud K., de Sherbinin A., Jones B., Bergmann J., Clement V., Ober K., Schewe J., Adamo S., McCusker B., Heuser S., & Midgley A. (2018) Groundswell: Preparing for Internal Climate Migration, The World Bank, XXI. See https://openknowledge.worldbank.org/entities/publication/2be91c76-d023-5809-9c94-d41b71c25635
[6] BBC News (10 September 2020) California wildfires: Smoke turns skies orange. https://www.bbc.com/news/world-us-canada-54096319 See also Frontline Wildfire Defense (last accessed 14 July 2024) Statistics and historical data (“Many experts agree that a year-round fire season is a new norm.”). https://www.frontlinewildfire.com/california-wildfire-map/
[7] Grennan G.K., Withers M.C., Ramanathan D.S., & Mishra J. (2023) Differences in interference processing and frontal brain function with climate trauma from California’s deadliest wildfire, PLOS Climate 2(1): e0000125. See https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000125
[8] Dahl K.A., Abatzoglou J.T., Phillips C.A., Ortiz-Partida J.P., Licker R., Merner L.D., & Ekwurzel B. (2023) Quantifying the contribution of major carbon producers to increases in vapor pressure deficit and burned area in western US and southwestern Canadian forests, Environ. Res. Lett. 18(6): 064011. See https://iopscience.iop.org/article/10.1088/1748-9326/acbce8
[9] Harrington L.J., Ebi K.L., Frame D.J., & Otto F.E.L. (2022) Integrating attribution with adaptation for unprecedented future heatwaves, Clim. Change 172(2): 1-7, 3. See https://link.springer.com/article/10.1007/s10584-022-03357-4
[10] Forster P. M., et al. (2024) Indicators of Global Climate Change 2023: annual update of key indicators of the state of the climate system and human influence, Earth System Science Data 16(6): 2625-58. See https://essd.copernicus.org/articles/16/2625/2024/
[11] Armstrong McKay D.I., Staal A., Abrams J.F., Winkelmann R., Sakschewski B., Loriani S., Fetzer I., Cornell S.E., Rockström J., & Lenton T.M. (2022) Exceeding 1.5°C global warming could trigger multiple climate tipping points, Science 377(6611): 1-10, 7. See https://www.science.org/doi/10.1126/science.abn7950
[12] Harvey F. (29 November 2023) ‘Enough of endless delays’: Will COP28 force a course change for the world?, The Guardian. See https://www.theguardian.com/environment/ng-interactive/2023/nov/29/cop28-what-could-climate-conference-achieve
[13] Dreyfus G.B., Xu Y., Shindell D.T., Zaelke D., & Ramanathan V. (2022) Mitigating climate disruption in time: A self-consistent approach for avoiding both near-term and long-term global warming, Proc. Natl. Acad. Sci. 119(22): e2123536119. See https://www.pnas.org/doi/10.1073/pnas.2123536119