According to a recently published Briefing Note of the Australian Breakthrough - National Center for Climate Restoration, there is no carbon budget left for the 1.5 °C goal. It states that the "IPCC carbon budgets underestimate current and future warming, omit important climate system feedback mechanisms, and make dangerous assumptions about risk-management." In this article, I review and synthesize several key scientific publications, asking whether there is really no carbon budget left to limit global warming to 1.5 °C above pre-industrial levels. I conclude that the situation is in fact severe - without unprecedented action within the next five to ten years and absolute zero emissions on a global scale already until 2030, the only option left to prevent dangerous climate change and thus a new 'hothouse Earth' will be Solar Radiation Management and thus a dangerous dependency on a technology which will not only make our beautiful blue skies disappear, but which will also even worsen the current climate catastrophe by far.
In its Special Report on Global Warming of 1.5°C from 2018, the Intergovernmental Panel on Climate Change (IPCC) states that "[h]uman activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels, with a likely range of 0.8°Cto 1.2°C." (IPCC, 2018: 4)
Last year, the World Meteorological Association (WMO) has published a multi-organizational high-level compilation of the latest climate science information (WMO et al., 2020). In this compilation, the WMO argues that "[t]he 5-year period from 2016-2020 is expected to be the warmest on record with an average global mean surface temperature [GMST] of 1.1 °C above pre-industrial era (1850-1900)." (WMO et al., 2020: 2) Even more, the WMO says that "[t]here is a growing chance of annual global mean near surface temperature temporarily exceeding1.5 °C above the 1850-1900 pre-industrial level, being ~20% in the 5-year period ending in2024." (ibid.: 3)
Given the likely range mentioned in the IPCC Special Report (from 2018), the analysis and the forecast of the WMO and the fact that annual global greenhouse gas emissions haven't declined since 2018, makes it very likely that global average warming has even already passed1.2 °C above pre-industrial levels (Spratt & Dunlop, 2021).
The interim drop in global GHG emissions due to the COVID-19 pandemic does not make a noteworthy difference, as GHG emissions has already returned to pre-pandemic levels (Tollefson, 2021). Regarding the possible remaining carbon budget, the IPCC concludes that "using GMST gives estimates of 770 and 570 GtCO2, for 50% and 66% probabilities, respectively(medium confidence)." (IPCC, 2018: 12) What is especially remarkable in this context is the following section from the IPCC Special Report:
“Uncertainties in the size of these estimated remaining carbon budgets are substantial and depend on several factors. Uncertainties in the climate response to CO2 and non-CO2emissions contribute ±400 GtCO2 and the level of historic warming contributes ±250 GtCO2(medium confidence). Potential additional carbon release from future permafrost thawing and methane release from wetlands would reduce budgets by up to 100 GtCO2 over the course of this century and more thereafter (medium confidence). In addition, the level of non-CO2 mitigation in the future could alter the remaining carbon budget by 250 GtCO2 in either
direction (medium confidence).” (Ibid.).
Given the first uncertainty, it seems as if the previous generation of climate sensitivity models were correct and thus the quantification of the possible remaining carbon budget (Zhu et al., 2020). The second uncertainty regarding the level of historic warming is hard to assess and is not further discussed in this article. The third uncertainty – the additional release of carbon due to permafrost thawing as well as due to other climate tipping points – and the fourth uncertainty – the level of non-CO2 emissions –, however, appear to be way more urgent than previously thought. In this context, Lenton et al. (2019: 594) argue that “[i]f forests are close to tipping points, Amazon dieback could release another 90 Gt CO2 and boreal forests a further 110 Gt CO2 [...]. With global total CO2 emissions still at more than 40 Gt per year, the remaining budget could be all but erased already.” They further conclude that “the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency
of the situation are acute [...].” (Lenton et al., 2019: 595)
As we already experience dramatic changes in our climate system, all these uncertainties should be a tremendous warning to us to step up real climate action immediately on a global scale (WMO et al., 2020). The situation is urgent. That’s for sure. But is it still possible to limit global warming to 1.5 °C and thus avoid a dramatic shift of the global climate system into a new ‘hothouse Earth’ (Steffen et al., 2018)?
In the 2018 Special Report on Global Warming of 1.5 °C, the IPCC has analyzed 4 model pathways (P1, P2, P3, P4) that would limit global warming to 1.5 °C until 2100 with no or limited overshoot (IPCC, 2018). Each pathway uses Carbon Dioxide Removal (CDR) to realize negative emissions in the second half of the 21st century. P1, the most optimistic pathway, is a “scenario in which social, business and technological innovations result in lower energy demand up to 2050 while living standards rise, especially in the global south.” (IPCC, 2018: 14) P1 is the only scenario that does not use BECCS or fossil fuels with CCS but relies only on Afforestation to decrease atmospheric carbon dioxide concentration. This scenario assumes e.g. that Kyoto-GHG emissions decrease by 50% until 2030 and by 82% until 2050 and that final energy demand will decrease by 15% and by 32% respectively (both relative to 2010 levels). The share of renewable electricity is assumed to be 60% in 2030 and 77% in 2050. To follow each path requires “far-reaching transitions in energy, land, urban and infrastructure (including transport and buildings) and industrial systems [...]” which are “unprecedented in terms of scale, but not necessarily in terms of speed [...]” (IPCC, 2018: 15).
Even if the underlying assumptions of the most optimistic scenario P1 would hold true, we do not follow the required path now but do even quite the opposite. According to the 2020 UNEP Production Gap Report, it would be necessary to “decrease fossil fuel production by roughly 6% per year between 2020 and 2030” to stay within 1.5 °C of global warming (SEI et al., 2020: 2). However, “[c]ountries are instead planning and projecting an average annual increase of 2%, which would result in more than double the production consistent with the 1.5°C limit.” (Ibid.) And according to the 2020 UNEP Emissions Gap Report, “[c]urrent NDCs [Nationally Detemined Contributions] remain seriously inadequate to achieve the climate goals of the Paris Agreement and would lead to a temperature increase of at least 3°C by the end of the century.” (UNEP, 2020: XXI) Even if “[r]ecently announced net-zero emissions goals could reduce this by about 0.5°C”, the global climate system would still change dramatically (Steffen
et al., 2018). A study from Anderson et al. (2020: 1290) finds that “[w]ithout a belief in the successful deployment of planetary scale negative emissions technologies, double-digit annual mitigation rates [about 12%] are required of developed countries, from 2020, if they are to align their policies with the Paris Agreement’s temperature commitments and principles of equity.”
All of this is well-known and has been highlighted many times by the global Fridays for Future movement. Yet, political leaders from all around the world still do not care and do not act as they should do to prevent irreversible harm and far-reaching injustice. Climate neutrality by 2050 is just not enough. The assumption that negative emissions technologies (NETs) will be available on a global scale from the second half of the 21st century is false and irresponsible (Minx et al., 2018). As Anderson & Peters (2016: 183) summarize it perfectly: “Negative-emission technologies are not an insurance policy, but rather an unjust and high-stakes gamble.”
As global NETs will not be available at a scale necessary to limit global warming to 1.5 °C, the only possible option left to prevent irreversible climate tipping points and thus a shift into a new 'hothouse Earth' would be Solar Radiation Management (SRM) (Shepherd et al., 2009). SRM is both scientifically and ethically very controversial because it does not address increasing atmospheric CO2 but only reduces radiative forcing by reflecting some sunlight away from Earth (Preston, 2013). Thus, adverse side effects of increasing atmospheric CO2 concentrations, such as air pollution or ocean acidification, are not addressed by SRM. Numerous SRM methods are considered of which the injection of aerosols, particularly sulphate aerosols, into the stratosphere, commonly termed Stratospheric Aerosol Injection (SAI), is the most prominent approach (Caldeira et al., 2013). A deployment of SAI would not only whiten the sky and make the beautiful blue disappear, it would also result in a dangerous lock-in effect, lead to unsolved governance issues and would thus worsen the current climate catastrophe by far (Preston, 2013). To avoid a future in which large parts of the Earth will be uninhabitable, in which billions of climate migrants will search for new places to live and in which wars for water and food are likely, the only option left is a systemic change until 2030.
Climate neutrality by 2050 is not an option as it would only worsen the current climate catastrophe. The stakes are high, but this is our only real option left.
Patrick Hohlwegler, Energy and Climate Policy Officer, ansvar 2030 & The Climate Task Force
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