Net zero is not enough

To mitigate the most serious impacts of climate change, many nations are focused on achieving net zero – the equilibrium between the emission and extraction of greenhouses to and from the atmosphere – by 2050. This is a herculean and costly task. Sadly, net zero is nowhere near enough to save us from the impacts of climate change because it still leaves us with sharply elevated atmospheric CO₂ levels, and does not fully account for the loss of Earth’s refrigerator (the ice stored at our poles, in mountain glaciers and in permafrost), or for the release of CO₂ stored in the ocean once atmospheric levels are stabilized. With these factors accounted for, temperatures could rapidly escalate, within a few hundred years, to those achieved naturally over a few hundreds of thousands of years in the mid Pliocene, when ice sheets were much reduced and global sea level may have been 10-15 m or more above those of today (cf. Dowsett et al., 2011) .

Rather than net zero, we must achieve negative emissions. While this will cost trillions, the future is not all doom and gloom because we know how to make a start on this. As President Biden has made abundantly clear in the USA (Browne 2023), we must act now to get where we need to be.

Net zero

Net zero is a term used to describe the notion that, to stop global warming, we should be aiming globally to achieve a state in which anthropogenic emissions of CO₂ are balanced by extractions of CO₂ from the atmosphere. This will require the adoption of technologies capable of extracting the emitted CO₂ and storing it by some means, largely through processes broadly described as ‘carbon capture and storage’ – very little of which exists currently. The storage could be underground in disused oil or gas reservoirs, much as happens with the excess CO₂ that comes up the drill pipe in Norway’s Sleipner Field, a natural gas field in the North Sea. However, capturing carbon from multiple, widely distributed point sources, such as those in agriculture, transport or home heating and cooling, and piping it into such reservoirs is a challenge orders of magnitude greater – one that will be costly. The best way to control emissions is to eliminate them entirely by replacing fossil fuels with electricity from renewable or nuclear sources.

The concept of net zero was loosely aired first in 2013 in the 5^th Assessment Report by the Intergovernmental Panel on Climate Change (IPCC). By December 2015, at the 21^st Conference of the Parties (COP-21) to the UN Framework Convention on Climate Change (UNFCCC for short), national governments agreed to aim  to achieve net zero by the second half of this century. In 2017, Sweden became the first nation to enshrine into law a mid-century net-zero target and in 2018 these ambitions received a further push from the IPCC’s  Special Report on 1.5°C, which concluded that “limiting [global average] temperature rise to around 1.5°C and preventing the worst impacts of climate change implies reaching net-zero emissions of CO₂ by mid-century along with deep reductions in non-CO₂ emissions” (which means greenhouse gases like methane, CH₄, and nitrous oxide, N₂O; see Box ‘CO₂ equivalent’). By 2019, national pledges to achieve net zero by mid-century covered about 16% of the global economy, a number that increased to 68% by 2021, and to 98% by 2022. The UK government was among the leaders in the push for net zero (HM Government 2021).

The significance of 1.5°C

Bearing in mind the scientific evaluations of climate change by the IPCC, the Parties present at COP-21 in 2015 decided that to avoid what they called “dangerous interference with the climate system” (meaning for example growing aridity in dry areas, expanding flooding in humid areas, heat increasing so much in some places that it would be impossible to work outside, increasing numbers of wildfires, and rising sea level) we should not let average global temperatures rise above a guardrail of 2°C, and preferably not above a guardrail of 1.5°C. These values represent the extent by which temperatures exceed the average for the period 1850 – 1900. By 2022/23 we had already reached an average global temperature rise of between 1.1 and 1.2°C, and by late 2023 we were headed for an average global temperature rise of 1.5°C (Nature, September 22, 2023) (in admittedly what was proving to be an El Niño year – such years usually involve an emission of some of the heat stored in the upper ocean, which has a small positive effect on the global average temperature). July 2023 was the world’s hottest month in recorded history (Nature, 18 August 2023).

July 2023 was the world’s hottest month in recorded history (Nature, 18 August 2023)

In recent years climate scientists have realized that focusing on the global average temperature hides the facts that: (i) average land temperatures now reach about 1.6°C above the 1850 – 1900 level (i.e. substantially more than the global average of ~1.1-1.2°C); (ii) average ocean temperatures reach about 0.9°C; and (iii) average Arctic temperatures are higher than the global average by as much as a factor of four (Rantanen et al 2022), a phenomenon known as polar amplification, which means rapidly proceeding ice melt. Climate scientists have also learned that 90% of the global warming signal (the increase in warming since 1850 – 1900) lies in the ocean, not in the atmosphere (Cheng et al., 2023). It is also clear that the ocean is the main repository for CO₂, storing about 25% annually of the anthropogenic emissions. These add to the vast amount of CO₂ stored naturally in the ocean, amounting to 38,000 Gigatonnes of carbon (16 times as much carbon as the terrestrial biosphere).

BOX | CO₂ equivalent

The focus on CO₂ as the main culprit in anthropogenic global warming ignores some important caveats. Firstly, although methane is less abundant than CO₂, it is vastly more potent as a greenhouse gas (by a factor of 28 – 36 over 100 years) (Browne, 2021), as too is nitrous oxide. Hence what we must consider is the CO₂ equivalent, the sum of all greenhouse gas emissions converted to their CO₂ equivalence. While the abundance of CO₂ in the air in 2023 reached 420 ppm, the CO₂ equivalent was already at 500 ppm (NOAA, 2021) – a value not seen in geological history since the natural warming experienced during the mid Pliocene, which was achieved over hundreds of thousands of years and peaked 3 million years ago. This natural warming is thought to have been caused largely by elevated CO₂ levels, with lesser contributions from declining ice sheets, orographic changes, and variations in surface albedo and atmospheric emissivity (Lunt et al., 2012), and resulted in a global climate that was very much warmer, with forests growing on the Arctic’s Ellesmere Island, and sea levels 10 to 15 m higher than they are today.

Additionally, for every 1°C of warming, the ocean emits 7% more water vapour, which is itself a potent greenhouse gas. Its main difference from CO₂ is that water vapour is frozen out of the air in the cold of the lower stratosphere.

A lack of progress

At the conclusion of the UNFCCC’s COP-26 in Glasgow in 2021, the signatories of the Climate Pact expressed “alarm and utmost concern that human activities have caused around 1.1°C of warming to date, that impacts are already being felt in every region, and that carbon budgets consistent with achieving the Paris Agreement temperature goal are now small and being rapidly depleted” (UNFCCC, 2021). The carbon budget refers to how much more CO₂ we can afford to emit before warming becomes catastrophic; it amounts to about 11 years of emissions at current rates.

The pact reaffirmed the need to keep temperatures well below 2°C above pre-industrial levels and the importance of continuing to pursue 1.5°C – a target that requires “rapid, deep and sustained reductions in global greenhouse gas emissions”, including the reduction of CO₂ emissions by 45% (relative to levels emitted in 2010) by 2030. The pact recognized the urgency of the matter, emphasizing the need for the Parties to collectively accelerate action, and implement mitigation measures. The pact also called on the Parties to accelerate development, deployment and dissemination of technologies that facilitate the transition to low-emission energy systems and, specifically, to accelerate efforts to “phase-down unabated coal power and inefficient fossil fuel subsidies” – the first time that coal and fossil fuel were mentioned in a COP report. Interestingly, the pact also emphasized the need to protect, conserve and restore nature, ecosystems and biodiversity in order to achieve the Paris Agreement temperature goal, highlighting the essential role for forests and other terrestrial and marine ecosystems as sinks for greenhouse gases. This was the first time that the Parties formally recognized the link between global warming and biodiversity and the need to act on both.

 Are we making progress? Climate Action Tracker estimated that with the policies presently in place, we are on course for 2.7°C warming by 2100 (Climate Action Tracker, 2021) – a slight improvement on the 2020 forecast from the UN Environment Programme’s Emissions Gap Report which concluded that we are “heading for at least a 3°C temperature rise this century” (UNEP, 2020). In critically examining the various national plans for climate change mitigation and reductions in greenhouse gas emission, the Washington Post identified a shortfall of 8.5 to 13.2 billion tons of annual greenhouse gas emissions (Mooney et al., 2021). Many of these national plans were unsupported, based on wishful thinking and came from inadequate national reporting of fossil fuel use. Critically, as explained below, the discussions so far have not fully taken into account either the loss of Earth’s refrigerator or the future release of CO₂ from Earth’s oceans.

Earth’s refrigerator

Earth’s climate system is being hit not just by warming from greenhouse gas emissions, but also by the melting of ice, especially sea ice. It has long been known that sea ice cover was declining in the Arctic, but in 2023 we learned that it is also in significant decline around Antarctica with data from the National Snow and Ice Data Center in Boulder, Colorado, USA, logging a record low (Fig. 1).

Figure 1 | Sea ice cover in the Antarctic. Data from 2023 highlight a significant decline in sea ice. Blue, 2023; green, 2022; orange, 2021; brown, 2020; magenta, 2019; dashed brown, 2014; median from 1981 to 2010, dark grey; lighter grey areas around the median illustrate the interquartile (or middle 50%) and interdecile (between 10% and 90%) ranges of the data. Sea ice extent is determined by adding up satellite image pixels with at least 15% sea ice concentration. The threshold is chosen because it is easier to determine whether a pixel contains at least 15% ice cover than, say, 75% cover, reducing uncertainty. The graph is based on the Sea Ice Index data (https://nsidc.org/data/seaice_index) collected by the National Snow and Ice Data Center, Colorado, USA. (Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder.)

Antarctica sea ice extent 1979 – 2023. Between 2013 and 2023, Antarctic sea ice has exhibited its highest and lowest extents in the satellite record. The record high winter maximum was recorded in 2014. The record lowest minimum occurred in March 2023 and the record lowest maximum in September 2023. (Credit: Animation by Michon Scott, based on NSIDC’s Charctic Interactive Sea Ice Graph (https://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/), and reproduced by courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder.)

Ice and snow are the main reflectors of solar energy back into space (Siegert et al., 2023), which helps to keep the planet moderately cool – they act as Earth’s refrigerator. Through their melt we are losing Earth’s refrigerator (Summerhayes, 2023) and the planet’s albedo, which means the Sun’s rays are now warming large areas of dark-coloured and newly exposed land and sea, accelerating warming.

Losing land ice also raises sea level. However, the rates of warming, ice loss and sea level rise are not yet in equilibrium. Temperatures are rising rapidly, but sea level is rising slowly (albeit at a rate that has increased gradually since 1900 to 4mm/yr) (WMO, 2021). Eventually the two will reach a new equilibrium in which we will have a significantly warmer climate and a significantly higher sea level. The loss of ice and snowpack from mountain ranges provides a further concern in that these currently control the water supply for many nations around the world.

When we consider the slow components of the climate system, such as ice melt, we can likely add a further degree of warming, taking us to a global average of close to 3.5 – 4°C by 2100, which, in turn, through polar amplification, implies about 7 – 8°C in polar regions. The mid Pliocene is our best guide currently to what is likely. In such a climate we would expect subtropical conditions to reach even the Canadian Arctic and sea levels to reach maxima of 10 to 15 m, rising after 2100 at rates of about 1 m (3.3ft) per century or more (Rohling, 2019). Given these profound implications, why do we not hear our decision-makers talking about the polar amplification of the global warming signal?

Ocean release

A complicating factor when it comes to trying to control our CO₂ emissions is that not only has the ocean absorbed heat, it has also absorbed CO₂ to enable it to remain chemically as closely as possible in equilibrium with the atmospheric load of CO₂. When we start reducing our CO₂ emissions from the air, CO₂ must inevitably be released back into the atmosphere from the CO₂-rich ocean to maintain the ocean’s physicochemical equilibrium with the atmosphere as the latter begins to lose its CO₂. This feedback will keep the CO₂ content of the atmosphere from declining as rapidly as our emissions do, helping to maintain our present warmth for a very long time – way beyond 100 years (Archer et al., 2009).

This outgassing of CO₂ from the ocean means that 1.6 – 1.7 times more CO₂ than expected must be captured from the atmosphere to achieve the levels of CO₂ reductions we’re aiming for (Rohling, 2021). So, while a 1 ppm reduction in the concentration of CO₂ in the atmosphere alone equates to a mass change of about 7.81 GtCO₂ , due to the ocean release of CO₂, a 1 ppm reduction will in fact require the removal of roughly 12.9 GtCO₂. This unfortunate fact is not widely appreciated.

Even if we achieve net zero by 2050, oceanic release of CO₂ will result in the retention in the atmosphere by 2050 of a large amount of our prior CO₂ emissions, meaning a sustained level of warmth (or more likely a slow increase in warmth as more ice melts and albedo decreases further as a result). We are already losing substantial amounts of ice at an average global temperature of 1.1 – 1.2°C above pre-industrial temperatures, so the persistence of warmth at 1.5°C or 2.0°C will (a) sustain high levels of evaporation, exacerbating global warming through the emission of yet more water vapour from the ocean; (b) further warm the ocean, exacerbating sea level rise; and (c) further exacerbate ice melt, contributing both to more sea-level rise, and to further loss of albedo, which will in turn warm the climate more (Summerhayes, 2023).

In addition, our polluting emissions of CO₂ have made the ocean less alkaline (hence the descriptor ‘ocean acidification’). In 5,000 to 10,000 years the ocean’s natural chemistry will begin to compensate for that acidification, but it will take a great deal longer for the ocean to revert to close to its originally more alkaline equilibrium position. In the meantime, the ocean’s ecological systems will remain under pressure, with marine organisms having to adapt to these new conditions, and possibly failing to do so. Particular threats exist for organisms forming their skeletons from CaCO₃, such as the pteropods at the base of the Southern Ocean food chain, and the corals of our spectacular and iconic reefs, which are the nurseries for food fish. This is not a desirable outcome.

To ensure that our children and grandchildren live in a climate that is not much different to the one we enjoyed for much of the 20^th century, requires an absolute reduction in the amount of CO₂ in the atmosphere (Shuckmann et al., 2020; Hansen et al., 2016). This demands not simply an equilibrium between CO₂ supply and extraction (i.e. net zero), but rather ‘negative emissions’, meaning that we must rapidly pull CO₂ out of the air until its value falls to some appropriate level – say 350 ppm, which is what it was in 1988. This is a far more massive challenge than net zero and amounts to extractions in the region of as much as 20 GtCO₂/year by 2100, or between 100 and 1,000 GtCO₂ removed from the atmosphere over the course of the 21^st century (IPCC, 2018). Taking excess CO₂ out of the air is the only way to save our refrigerator, and we must start now.

Time to act

The challenge before us is enormous, but we must resist the inertia in the incumbent energy infrastructure. For instance, the International Energy Agency declared that to meet net zero by 2050, the exploitation and development of all new oil and gas fields must stop in 2021 and no new coal-fired power stations can be built (Harvey, 2021), while Welsby and colleagues (2021) showed that nearly 60% of all oil and fossil methane gas, as well as 90% of coal, must remain unextracted if we are to have even a 50% chance of limiting warming to 1.5 °C . Clearly, we are already missing those suggested targets.

Make no mistake, we do have it within our power to change. As an example of what is possible, South Australia now generates enough energy from solar panels on house roofs to meet virtually all its electricity needs (Morton, Guardian, 1 Nov. 2023). As noted in Simon Sharpe’s 2023 book, Five Times Faster, with the rise in renewable power, electric vehicles, and energy efficiency the world has become less emissions intensive by about 1.5% per year. To keep average global warming below 1.5°C by 2050 we need that number to be 8% per year, i.e. five times faster, which we can achieve by replacing fossil fuels with cleaner alternatives at much faster rates. Sharpe castigates economists for assuming that the worldwide supply and demand of goods and services is like a machine operating at an equilibrium, which grossly underestimates the impacts of global overheating on lives and livelihoods, and overestimates the cost of replacing fossil fuels as the primary energy source.

Make no mistake, we do have it within our power to change

Renewable energy sources are already cheaper or close to cheaper than fossil fuels. As one result, coal has been essentially phased out in the UK, and Norway has used subsidies to stimulate sales of electric cars at a rate ten times faster than other countries, to help phase out petrol and diesel fuelled vehicles. As new technologies come on stream in volume, their costs fall, as we have seen with offshore wind farms. Battery technology is rapidly advancing. And on the horizon lurks the potentially cheap production of the gas needed for the proposed hydrogen economy (New Scientist, October 2, 2023).

Neither the general public, the media, nor our politicians seem to realise the enormity of the challenge before us. Net zero is merely a sticking plaster; it will not save the world from what is coming. Negative emissions are needed, and it will cost trillions. It’s time to bite the bullet and act.

Author

DR COLIN SUMMERHAYES

Scott Polar Research Institute, University of Cambridge, UK

Further reading

• Archer, D. et al. (2009)  Atmospheric Lifetime of Fossil Fuel Carbon Dioxide. Annual Reviews of Earth and Planetary Science 37, 117–34; https://doi.org/10.1146/annurev.earth.031208.100206
• Browne, E. (2023) Methane Vs CO2: Which Is the Most Potent Greenhouse Gas As White House Unveils New Pledge. Newsweek 11/2/2021; https://www.whitehouse.gov/briefing-room/
• Cheng, L. (2023) Another Year of Record Heat for the Oceans. Advances in Atmospheric Science. 40, 963–974; https://doi.org/10.1007/s00376-023-2385-2
• Climate Action Tracker (2021) Temperatures: addressing global warming; https://climateactiontracker.org/global/temperatures/. November 9th update.
• Dowsett, H.J. et al. (2011) Sea surface temperatures of the mid-Piacenzian Warm Period: A comparison of PRISM3 and HadCM3. Palaeogeography, Palaeoclimatology, Palaeoecology 309, 83–91; https://doi.org/10.1016/j.palaeo.2011.03.016
• Hansen, J. et al. (2016) Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2°C global warming could be dangerous. Atmospheric Chemistry and Physics 16, 3761–3812; https://doi.org/10.5194/acp-16-3761-2016
• HM Government (2021) Net Zero Strategy: Build Back Greener. Presented to Parliament pursuant to Section 14 of the Climate Change Act 2008. Crown Copyright 2021; www.gov.uk/official-documents
• IPCC (2013) Fifth Assessment Report; www.ipcc.ch/assessment-report/ar5/
• IPCC (2018) Global Warming of 1.5°C. An IPCC Special Report; https://www.ipcc.ch/sr15/
• Lunt, D.J. et al. (2012) On the causes of mid-Pliocene warmth and polar amplification. Earth and Planetary Science Letters 321-322, 128–138; https://doi.org/10.1016/j.epsl.2011.12.042
• Mooney, C. et al. (2021) Countries’ climate pledges built on flawed data, Post investigation finds. Washington Post, November 7; https://www.washingtonpost.com/climate-environment/interactive/2021/greenhouse-gas-emissions-pledges-data/
• Morton, A. (2023) ‘Go hard and go big’: How Australia got solar panels onto one in every three Houses. The Guardian, 1 November; https://www.theguardian.com/environment/2023/nov/01/how-generous-subsidies-helped-australia-to-become-a-leader-in-solar-power
• National Snow and Ice Data Center (2023) Antarctic sea ice hits record low maximum extent for 2023. NSIDC Boulder, Colorado, USA. News Release 25 September; https://nsidc.org/news-analyses/news-stories/antarctic-sea-ice-hits-record-low-maximum-extent-2023
• National Snow and Ice Data Center (2023) Antarctic sets a record low maximum by wide margin. NSIDC Boulder, Colorado, USA. News Release 25 September; https://nsidc.org/arcticseaicenews/2023/09/antarctic-sets-a-record-low-maximum-by-wide-margin/
• National Snow and Ice Data Center (2023) Charctic Interactive Sea Ice Graph; https://nsidc.org/arcticseaicenews/charctic-interactive-sea-ice-graph/
• NOAA (2021) The NOAA Annual Greenhouse Gas Index (AGGI); https://gml.noaa.gov/aggi/aggi.html [Accessed 3 December 2021]
• Rantanen, M. et al. (2022) The Arctic has warmed nearly four times faster than the globe since 1979. Communications Earth & Environment 3:168; https://doi.org/10.1038/s43247-022-00498-3
• Rohling, E.J. (2019) The Climate Question: Natural Cycles, Human Impact, Future Outlook. Oxford University Press, 162 pp.
• Rohling, E.J. (2021) Rebalancing Our Climate: The Future Starts Today. Oxford University Press, 296 pp.
• Schuckmann, K.v. et al. (2020) Heat stored in the Earth system: where does the energy go? Earth System Science Data 12, 2013–2041; https://doi.org/10.5194/essd-12-2013-2020
• Sharpe, S. (2023) Five Times Faster: Rethinking the Science, Economics, and Diplomacy of Climate Change. Cambridge University Press, 350 pp.
• Siegert M.J. et al. (2023) Antarctic extreme events. Frontiers in Environmental Science 11:1229283; https://doi.org/10.3389/fenvs.2023.1229283
• Summerhayes, C.P. (2023) The Icy Planet: Saving Earth’s Refrigerator. Oxford University Press, 772 pp.
• UNEP (2020) Emissions Gap Report 2020. UNEP, Nairobi; https://www.unep.org/emissions-gap-report-2020.
• UNFCC (2021) Glasgow Climate Pact; https://unfccc.int/documents/310475 [Accessed 5 December 2021]
• WMO (2021) State of the Global Climate 2021. Provisional report, World Meteorological Organization, Geneva; https://public.wmo.int/en/our-mandate/climate/wmo-statement-state-of-global-climate

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