Our Greatest Threat
Sea-level rise figures prominently among the consequences of climate change. It impacts settlements and ecosystems both through permanent inundation of the lowest-lying areas and by increasing the frequency and/or severity of storm surge over a much larger region.Kopp et al. Probabilistic 21st and 22nd century sea-level projections at a global network of tidegauge sites.1
The impacts of multi-metre sea level rise would be irreversible and may leave global civilisation ungovernable.2 The world’s coastal cities and ports would be permanently flooded, devastating high population areas, international trade and finance. Shown below is land area in 2050 projected to be below annual flood level, due to 26cm sea level rise relative to year 2000 (click on maps to enlarge).
In the past, Earth’s climate has alternated between ice ages and warm periods. Civilisation developed during the most recent warm period, known as the Holocene, that lasted 11,700 years. The prior warm period, known as the Eemian, occurred between 130,000 to 115,000 years ago.7 The best estimate of the maximum temperature of the Eemian, relative to preindustrial time, is between +0.5℃ and +1.0℃.7 The global surface temperature averaged over 2014–2018 wrt 1850-1900 was +1.04℃ according to the WMO,8 and in 2019 wrt 1880-1920 was +1.2˚C according to NASA GISS (Goddard Institute for Space Studies) global temperature analysis (GISTEMP).9 Temperature data of the Holocene (smoothed over centennial time periods) does not exceed +0.5℃.
We conclude that the modern trend line of global temperature crossed the early Holocene (smoothed) temperature maximum (+0.5℃) in about 1985.Hansen et al. (2017), Young people’s burden: requirement of negative CO2 emissions.10
Therefore we have warmed our climate beyond the temperature range of the Holocene, and are about to warm it beyond that of the Eemian.
During the Eemian, seas were 6 to 9 metres (20 to 30 feet) higher than today, indicating that multi-metre sea level rise will be a consequence of global warming beyond 1.0˚C. This would result from the collapse of Earth’s ice sheets, of which there are three – the Greenland, West Antarctic and East Antarctic ice sheets. Therefore the principal question is not how much sea level rise, but how fast?
The Holocene, over 11,700 years in duration, had relatively stable climate, prior to the remarkable warming in the past half century. The Eemian, which lasted from about 130,000 to 115,000 years ago, was moderately warmer than the Holocene and experienced sea level rise to heights 6–9 m (20–30 ft) greater than today.Hansen et al. (2017), Young people’s burden: requirement of negative CO2 emissions.10
Our GHG Emissions
Cumulative quantities of long lived greenhouse gases (GHGs) solely determine our long-term warming commitment, simply because of their long lifetimes in the atmosphere11 and they can’t be removed and sequestered at necessary scale. Cumulative carbon dioxide (CO2) is almost the singular cause, and is increasing rapidly12 13 (nitrous oxide (N2O) is the only other significant contributor but only accountable for about one tenth that of CO2, and changing slowly).
Half of all anthropogenic CO2 emissions have been emitted in the last 40 years and almost all (83%) by the world’s energy system.14 The most rapid increase of CO2 was emitted during 2015, second fastest 2016, and 2018 tied with 1998 as the third fastest.14 15
The trend of global average atmospheric CO2 concentration now exceeds 410 ppm.16
In the mid-Pliocene, 3–5 million years ago, the last time that the Earth’s atmosphere contained 400 ppm of CO2, global mean surface temperature was 2–3℃ warmer than today, the Greenland and West Antarctic ice sheets melted and even some of the East Antarctic ice was lost, leading to sea levels that were 10–20m higher than they are today.WMO, 2017, State of the Global Climate in 2017.17
The progress of international climate negotiations has been astonishingly slow, and the process ineffective. In 2020, 5 years since the Paris Agreement, 26 years since the UNFCCC came into force, 32 years since the formation of the IPCC, and 40 years since the first joint scientific meeting about atmospheric CO2, annual CO2 emissions from fossil fuels and industry have soared. Between the first joint scientific meeting in 1980, and 2019, CO2 emissions from fossil fuels and industry increased 90%, and increased 57% since COP1 in 1995.18 19 To make matters worse: (i) as explained above, limiting warming to 1.5℃ (the goal of the Paris Agreement) is not safe because when earth was last warmed 1.5℃, seas were 6 to 9m (20 to 30ft) higher; (ii) the scale of emission reductions prescribed are beyond any historic precedent;20 (iii) prescribed emission reductions depend on concurrent massive CO2 removal;21 (iv) the annual increase of CO2 emissions is near record rate;22 (v) 1.5℃ is imminent;23 and (vi) carbon offsetting is spurious.24
Charts 1 and 2 show we are still carbonising.
Chart 3 shows that ‘every year energy use increases, & most of the increases come from fossil fuels.’27
What To Do?
Two prominent efforts have pursued solutions: the UN’s climate treaty relying on the science of the IPCC (explained in 1.5˚C) and the efforts of Dr James Hansen30 (explained in 350 ppm). The most important finding of both is that it’s too late for decarbonisaton alone, and now ‘negative emission technologies’ (NETs), also known as ‘carbon dioxide removal’ (CDR) methods, are also required (this includes measures to increase natural carbon sinks such as reforestation).
Negative emissions are a burden being imposed on young people.Hansen, Young people’s burden: requirement of negative CO2 emissions.31
To stabilise temperature, CO2 emissions need to be made net-zero as shown below. This can only be achieved by rapidly reducing CO2 emissions (i.e. decarbonising) to the lowest level possible, and by using negative emissions. The faster emissions are reduced, the smaller the negative emissions burden.
The scale of the decarbonisation challenge to meet the Paris Agreement is underplayed in the public arena. It will require precipitous emissions reductions within 40 years and a new carbon sink on the scale of the ocean sink. Even then, the world is extremely likely to overshoot. A catastrophic failure of policy, for example, waiting another decade for transformative policy and full commitments to fossil-free economies, will have irreversible and deleterious repercussions for humanity’s remaining time on Earth. Only a global zero carbon roadmap will put the world on a course to phase-out greenhouse gas emissions and create the essential carbon sinks for Earth-system stability, without which, world prosperity is not possible.Rockström, J. et al. (2016), The world’s biggest gamble.33
The IPCC’s 1.5℃ pathways demand that CO2 emissions are roughly halved by 2030,34 and over the next 30 years negative emissions are ramped up so that by 2050, an amount of CO2 will have been removed from the atmosphere equivalent to that removed by the world’s ocean over a period of 15 years (about 150GtCO2). By this time, these negative emissions will need to be so vast that that the total annual removal of CO2 will be equivalent to that annually removed by the global ocean.21 These pathways have only a 50% to 66% chance of success35 and large uncertainties remain concerning the feasibility and impact of large-scale deployment of negative emission technologies.36
Dr James Hansen30 prescribes changes needed to reduce atmospheric CO2 to less than 350 ppm, in order to limit global temperature close to the Holocene range of +1℃ maximum. CO2 emissions must be reduced by one third by 2030 and the negative emissions burden is the same as that prescribed by the IPCC above.37 Despite decarbonisation being slower than in the IPCC’s 1.5˚C pathways, Hansen’s modelling results in warming being limited to less than +1°C, hopefully averting multi-metre sea level rise.
What Time Remains?
Cumulative CO2 solely determines our long-term warming commitment, and the less CO2 emitted, the smaller the future burden of negative emissions.
The remaining carbon budget from 2018 onwards is 580GtCO2 for a 50% chance of keeping warming below 1.5C. This is less than 15 years of global emissions at current rates.38
So, what does that mean?
This means that if we start reducing emissions steeply now and by the time we reach net-zero levels we have not emitted more than 580GtCO2, our best scientific understanding tells us have we expect a one-in-two chance that warming would be kept to 1.5C.
Moreover, if we want to be sure that this is also true until the end of the century, we’d have to aim to emit only 480GtCO2 until we reach net-zero instead. This is under 12 years of current emissions.‘A new approach for understanding the remaining carbon budget’, Dr Joeri Rogelj, Prof Piers Forster, CarbonBrief 2019.39
The remaining carbon budgets from 2020 onwards are listed below in table 1, calculated by subtracting the annual emissions of years 2018 and 2019 (shown in the supporting information below), away from the IPCC’s budgets shown in table 3. Also shown are the same remaining budgets with 50% of total uncertainties applied in either direction (50% was chosen because it would be unlikely for all uncertainties to align at an extremity). Note that 50th percentile of climate sensitivity refers to the most likely value.
Table 1 shows the IPCC’s remaining carbon budget for 1.5˚C may be spent in 12 years at the current rate of emissions, but if 50% of total uncertainties are applied in either direction, the same budget is spent sometime in the next 2 to 23 years. The usefulness of IPCC budgets is questionable because associated with each budget are very large uncertainties and warming that will trigger rapid and large sea level rise. The exceedance of any budget doesn’t herald the end of our chance to limit and reduce warming. Instead it should be a very loud alarm that reminds us our greenhouse gas emissions remain dangerously excessive and unmanaged.
Sadly, the world’s energy system remains intensively fossil fuelled,40 ‘every year energy use increases, & most of the increases come from fossil fuels’,27 emission offsets are spurious to avoid adequate action now, and there is no time left for half-measures; ‘winning slowly is the same as losing.'41
Supporting information –
- Kopp, R. E., R. M. Horton, C. M. Little, J. X. Mitrovica, M. Oppenheimer, D. J. Rasmussen, B. H. Strauss, and C. Tebaldi (2014), Probabilistic 21st and 22nd century sea-level projections at a global network of tide-gauge sites, Earth’s Future, 2, 383–406, doi:10.1002/2014EF000239, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014EF000239
- section titled ‘Sea Level Rise’, https://www.worldenergydata.org/existential-threat-pt1/
- table 2,1. p. 60, AR5 Synthesis Report – Climate Change 2014
IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp., https://www.ipcc.ch/report/ar5/syr/
- Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616, https://doi.org/10.5194/esd-8-577-2017, 2017
- p6, https://library.wmo.int/doc_num.php?explnum_id=5789
- Global Temperature in 2019, http://www.columbia.edu/~jeh1/mailings/2020/20200115_Temperature2019.pdf
- p 581, Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616, https://doi.org/10.5194/esd-8-577-2017, 2017
- chart 4, https://www.worldenergydata.org/ghgs/
- Matthews, H.D., Gillett, N.P., Stott, P.A. and Zickfeld, K., 2009. The proportionality of global warming to cumulative carbon emissions. Nature, 459(7248), p.829., http://indiaenvironmentportal.org.in/files/The proportionality of global warming.pdf
- charts 1 and 3, https://www.worldenergydata.org/ghgs/
- Black trend line at https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html, accessed 22 Jan 2020.
- WMO Statement on the State of the Global Climate in 2017, http://public.wmo.int/en/our-mandate/climate/wmo-statement-state-of-global-climate
- CO2 emissions from fossil fuel combustion and cement: (i) in 1980 = 19.4GtCO2; (ii) in 1995 = 23.39GtCO2; and (iii) in 2019 = 36.8GtCO2. 2019 with respect to 1980 = 36.8/19.4 = +90%, and 2019 with respect to 1995 = 36.8/23.39 = +57%
- Global Carbon Project. (2019). Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project, https://www.icos-cp.eu/GCP/2019, download labelled ‘2019 Global Budget v1.0’. Value for 2019 is projection shown in Global Carbon Budget, https://www.globalcarbonproject.org/carbonbudget/19/files/GCP_CarbonBudget_2019.pdf
- p.16, C.2.1, https://report.ipcc.ch/sr15/pdf/sr15_spm_final.pdf
- chart 6, https://www.worldenergydata.org/1-5c/
- chart 7, https://www.worldenergydata.org/ghgs/
- Global Carbon Project. (2019). Supplemental data of Global Carbon Budget 2019 (Version 1.0) [Data set]. Global Carbon Project, https://www.icos-cp.eu/GCP/2019, download labelled ‘2019 Global Budget v1.0’.
- Glen Peters, Research Director at Center for International Climate Research, https://twitter.com/peters_glen/status/1149219271236415489
- p. 1, Rockström, J. et al. (2016), The world’s biggest gamble, Earth’s Future, 4, 465 – 470, doi:10.1002/2016EF000392, https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016EF000392
- chart 3, https://www.worldenergydata.org/1-5c/
- table 1, https://www.worldenergydata.org/1-5c/
- p. 121, IPCC Special Report on 1.5°C, J. Rogelj, D. Shindell, K. Jiang, S. Fifita, P. Forster, V. Ginzburg, C. Handa, H. Kheshgi, S. Kobayashi, E. Kriegler, L. Mundaca, R. Séférian, M. V. Vilariño, 2018, Mitigation pathways compatible with 1.5°C in the context of sustainable development. In: Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P. R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)]. In Press. https://www.ipcc.ch/site/assets/uploads/sites/2/2018/11/SR15_Chapter2_Low_Res.pdf
- p. 108, table 2.2, IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press. https://www.ipcc.ch/sr15/