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Setting the agenda in research
Comment
AVIATION-IMAGES/UNIVERSAL IMAGES GROUP VIA GETTY
Industries such as aviation do not have clear road maps for tackling carbon dioxide emissions.
Three ways to improve
net-zero emissions targets
Joeri Rogelj, Oliver Geden, Annette Cowie & Andy Reisinger
Vague claims by countries is zero. In response, ever more countries, Important questions are being overlooked.
institutions and companies are announcing Should some sectors, such as electricity gener-
and companies will lull the net-zero targets1,2. Recent examples include ation, reach net zero earlier to counterbalance
world into missing its the United States, China, the European Union, harder-to-abate sectors including heavy indus-
the technology firm Microsoft and an alliance try? Is it fair to expect emerging economies
climate goal. of European airports. These welcome signs of to reach net zero on the same schedule as
intent come with difficulties. long-industrialized ones? Without careful
F
Plans are hard to compare, and definitions attention to such issues, individual achieve-
loose. The details behind ‘net-zero’ labels ments risk being too weak to deliver the
ive years ago, the United Nations Paris differ enormously. Some targets focus solely collective climate goal of the Paris agreement.
climate agreement set a ceiling for on carbon dioxide. Others cover all greenhouse Critics could argue that vague targets are
global warming at well below 2 °C, gases. Companies might consider only emis- better than none. But the stakes are too high
ideally 1.5 °C relative to pre-industrial sions under their direct control, or include to take comfort in mere announcements.
levels. World leaders also agreed to bal- those from their supply chains and from the Everyone need not make the same choices.
ance greenhouse-gas emissions in the second use or disposal of their products. Sometimes But without more clarity, strategies behind
half of the century, so that the sum of all green- the targets do not aim to reduce emissions, but net-zero targets cannot be understood; nor
house gases emitted from human activities compensate for them with offsets. can their impact be evaluated.
Nature | Vol 591 | 18 March 2021 | 365
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Comment
Here we call on nations, companies and the but the impact of CO2 already present in the determine this equivalent amount: the green-
researchers advising them to clarify three atmosphere will linger for centuries. By con- house gas Global Warming Potential over 100
aspects of their targets: their scope; how they trast, shorter-lived greenhouse gases, such as years, or GWP-100.
are deemed adequate and fair; and concrete methane, last for years to decades. Reducing All available Paris-compatible pathways to
road maps towards and beyond net zero. A key these would diminish their contribution to meet net-zero greenhouse-gas emissions use
date for this clarity will be the next UN climate warming relatively quickly. But eliminating a combination of three strategies. These are:
summit, in Glasgow, UK, in November, where their emissions is impossible at present, and rapid and large reductions in CO2; additional
countries will present new climate pledges. there are currently no technologies to actively deep reductions in non-CO2 greenhouse gases;
remove them from the atmosphere, unlike CO2. and a ramping up of strategies to remove CO2
Scope Each country or organization takes a differ- from the air. (One specific pathway is shown
Targets must specify which emission sources ent tack. The EU targets all greenhouse gases in ‘It’s all in the detail’). In the pathways for
and which gases are covered (CO2, all green- by 2050. China’s net-zero plan focuses on limiting warming to 1.5 °C presented by the
house gases, or a subset); when net zero will be balancing only CO2 emissions, by 2060. The Intergovernmental Panel on Climate Change
reached, and whether the intent is to reduce, current presidential administration’s climate (IPCC) net-zero CO2 emissions are reached
remove or offset the emissions. plan for the United States aims to reach net around 2050 (2046–55). However, net-zero
The choice of gases being tackled affects the zero by 2050, but has yet to specify which greenhouse-gas emissions would only be
outcome for the climate. CO2 is the main cause gases are covered. The Paris agreement con- reached one or two decades later (2061–84)3.
of rising global temperatures; it accumulates siders all greenhouse gases; emissions that Because CO2 removal is used to balance
and lasts for hundreds to thousands of years cannot be eliminated have to be balanced by other, shorter-lived greenhouse gases, the
in the atmosphere. Bringing CO2 emissions removing an equivalent amount of CO2 from Paris agreement’s net-zero target will achieve
down to net zero halts further warming, the atmosphere. The UN has agreed a metric to more than stabilizing global warming: temper-
atures will peak and slowly decline4,5 (see ‘It’s
all in the detail’). But the balance is delicate:
IT’S ALL IN THE DETAIL change the metric or the gases covered, the
Choosing different gases, different timing for net-zero emissions and different speed at which various gases are reduced or
methods of aggregating emissions can have very different outcomes. the proportion of reduction versus removal,
Global greenhouse-gas (GHG) emissions and the peak and rate of temperature decline
Illustrative pathway for reaching net-zero carbon dioxide and net-zero GHG emissions (from ref. 3). could be very different5. Temperature might
CO2 Non-CO2 (CH4, N2O and fluorinated gases in GWP-100*) not even decline at all.
60
Vague terms abound. For example, ‘carbon
Reaching a net-zero CO2 target
Global CO2-equivalent emissions
Residual CO2 emissions are balanced
neutral’ and ‘climate neutral’ are sometimes
(gigatonnes CO2-e per year)
by an equal amount of CO2 removal. synonyms for net-zero CO2 and net-zero green-
Reaching a net-zero GHG target
40 Residual CO2 and other GHG house-gas emissions, sometimes not. France’s
emissions are balanced by an strategy, for instance, speaks of carbon
equivalent amount of CO2 removal. neutrality but spans all greenhouse gases.
Consistency, clarity and accuracy are essential.
20 The scope of the sources of emissions cov-
ered also varies. Company targets might cover
only emissions resulting from their direct
activities, or they might also include emis-
0
sions across their value chains. For example,
CO2 removal
Swedish furniture giant IKEA has a net-zero
target that includes all emissions from its
–20 entire supply chain. So does Microsoft, which
2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 by 2050 also plans to neutralize all CO2 it has
Global-warming implications emitted since its founding in 1975. By contrast,
Estimated global temperature peaks (in pink) and ACI Europe, representing more than 500 Euro-
declines (arrows) under net-zero GHG emissions.
pean airports, has set a net-zero CO2 target for
2.5
2050 that covers just buildings and operations
on land, not emissions from aeroplanes. Only
Net-zero CO2 target Net-zero GHG target 2% of emissions of all aviation activities that
Global average temperature increase
2.0 Global warming Global warming
roughly stabilizes. peaks then declines. pass through these airports is addressed.
(°C relative to 1850–1900)
The anticipated role of CO2 removals also
1.5 needs more clarity. Target setters should
declare how they will combine emissions
reductions, direct removals and offsets. Direct
1.0 CO2 removals are those under the control of the
organization or entity. Offsets are purchased
reductions or removals fulfilled by someone
0.5 else, elsewhere. On the whole, direct emis-
Continued decline
accelerated by global
sions reductions are preferable. Strategies
net-negative GHG emissions. for removal or offsetting often have uncertain
SOURCE: REF. 3
0 effectiveness and could come with more risks.
2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
For example, some solutions require a lot
*GWP-100, Global Warming Potential over 100 years (United Nations metric
for transferring emissions of different gases to a common scale). of land. This can have knock-on impacts on
366 | Nature | Vol 591 | 18 March 2021
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.COSTFOTO/BARCROFT MEDIA VIA GETTY
Trees planted on barren mountains in China’s Hebei province will help to reduce carbon dioxide levels.
biodiversity, and the security of food and global warming. For example, Singapore is a emissions targets are a zero-sum game. If one
water supplies. Planting a forest, for example, small, densely populated country with limited country or company does less, others have to
might displace agriculture; planting mono- potential to deploy renewables, but is rich and do more to achieve the same global temperature
culture energy crops could reduce species has high capacity to finance action. The EU has outcome. If a country takes 2065 as its national
diversity. The permanence of such biological been contributing to global warming for more net-zero CO2 target for limiting global warming
removals remains uncertain. The long-term than a century and was heavily deforested in to 1.5 °C, for example, it implicitly expects that
carbon-storage capacity of forests and soils is the past, which means it now has significant all other countries together will reach net-zero
not well known, and there can be no guarantee potential for reforestation. Yemen has some CO2 before the global average of around 2050.
that a forest won’t later be logged, devastated of the best solar-energy resources in the world, Parties to the Paris agreement should pro-
by a forest fire or altered by climate change6. actively disclose why they consider their net-
Engineered solutions — such as using chem- “Target setters should zero targets to be fair and adequate. This is
ical filters to suck CO2 from the air to be buried currently requested for near-term national
underground — are still nascent. Deploying
declare how they will mitigation targets (the nationally determined
such technologies on a scale large enough combine emissions contributions, or NDCs) but not yet for longer-
to have a global impact remains speculative. reductions, direct term targets such as net zero.
Ultimately, costs and social acceptance might Fairness issues also arise from carbon off-
limit any CO2 removal option. For example,
removals and offsets.” sets. Cheap offsets can mean that a company
communities might oppose landscapes being makes limited effort to address its own emis-
transformed or CO2 being stored beneath their sions. Offsetting emissions through projects
homes6. Net-zero declarations should say how but, as a least-developed country experiencing in another country outsources any social
these risks will be managed. continuing unrest, it has little access to the nec- and environmental risks. Finally, avoiding
essary investments. Similar diversity applies to double-counting of offsets is a challenge —
Adequacy and fairness economic sectors. The agriculture and forestry for example, if a reforestation project in Sierra
Countries and companies that set net-zero sector has clear opportunities for CO2 removal; Leone sells an offset credit to Microsoft, this
targets generally assume, and sometimes aviation and metals industries don’t. emissions reduction is double-counted if it is
claim, that these will meet the global goals of Net-zero targets defined using metrics other also included towards Sierra Leone’s emission
the Paris agreement. However, this involves than GWP-100 also shift the mitigation burden target. The climate, obviously, sees the bene-
implicit assumptions about what a fair con- between gases, and thus between sectors and fit only once. Finally, trade in manufactured
tribution would be and what others should countries5,7–9. Giving less weight to methane goods and energy further complicates the
contribute. Ethical judgments are unavoidable, would make it easier for countries with high picture of who is responsible for what.
even if unvoiced. emissions from agriculture to claim that net On the whole, cutting emissions locally
What is considered ‘fair’ differs across coun- zero has been reached. However, this would is preferable to offsetting them. Continued
tries and communities. All of these are in dif- also result in more global warming unless reliance on offsets might become increasingly
ferent stages of development, with a variety other countries — with mainly CO2 emissions unrealistic and ultimately unfair to countries
of opportunities, financing and resources — agree to reach net zero sooner. that provide the offsets but cannot count
available, and differing contributions to Climate policies can be win–win, but those actions towards their own targets.
Nature | Vol 591 | 18 March 2021 | 367
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.Comment
Two key questions for adequacy and fairness greenhouse-gas emissions by 2035 and 2045, needed to help explore how fairness concepts
are: would the world still hit net zero if every- respectively, and going net negative thereafter. apply to today’s multinational corporations
one applied this logic? And would it be fair to In the context of an EU-wide net-zero target — which span multiple countries and sectors,
apply the same logic to all countries? for 2050, these intentions can compensate for and involve staff on incomes ranging from the
Self-interests cannot be avoided. Countries the weaker reductions of other nations with lowest to the highest. Equity must be made a
often choose fairness principles that favour greater technical or political obstacles. central part of the process, including in tools
their situation, overestimating how much their Furthermore, targets must be amenable to used to design net-zero targets so that the
actions contribute to the global goal. Modelling tightening. As we experience an increasingly wider implications of assumptions and deci-
has shown, for example, that if all countries warmer world, nations and companies might sions become clear.
were to pursue the targets they deem fair and in decide that action should be taken more For work towards a concrete road map, the
line with 1.5 °C, warming would hit 2 °C (ref. 10). rapidly12. Acceleration can be encouraged UN review process for detailing near-term
This is why rigour and transparency are needed by setting a regular schedule for revising NDCs — assessing and revising them every
to support conversations about what is and isn’t targets, such as those already in place for the five years — is a good starting point. That can
equitable in light of a collective, global target. shorter-term NDCs. be extended to include longer-term net-zero
Critics might question the value of discuss- targets. Companies should do the same and
Long-term road map ing a road map to net-negative emissions when apply a standardized review process, consid-
Net-zero targets are more credible if they the world is not even on track to achieve net ering the aspects we have outlined here.
include milestones, an implementation plan, zero. In our view, these conversations are Today’s targets are only the start of a long
and a statement about longer-term intent for crucial in shaping a sustainable long-term journey towards a safer world.
either maintaining net zero or going net nega- future for our shared planet.
tive. Leaving these out risks inaction, diversions
and failure. Next steps The authors
For example, the United Kingdom announced Countries and companies must add rigour and
in 2019 that it would reach net-zero green- clarity to their net-zero targets, to enable these Joeri Rogelj is director of research at the
house-gas emissions by 2050, but its near-term to be evaluated and assessed (see ‘Checklist’ Grantham Institute – Climate Change & the
policies were still off track. In 2020, it published and Supplementary information). Environment, Imperial College London,
its first NDC for 2030, together with other Research can support improvements in London, UK, and a senior research scholar
policy milestones. In combination, these now all three aspects. For example, in respect to at the International Institute for Applied
present a more coherent domestic plan. scope, nature-based carbon sinks are used for Systems Analysis, Laxenburg, Austria. Oliver
Net-zero targets are not end points. They are direct removals or offsetting. Studies should Geden is a senior fellow at the German
themselves milestones to meeting net-negative better characterize risks surrounding the Institute for International and Security Affairs,
emissions targets further down the road11. Few future reliability of such sinks, their potential Berlin, Germany. Annette Cowie is senior
targets explicitly consider this, but it deter- reversal under climate change and their wider principal research scientist at the New South
mines how net zero is best approached. social and ecological consequences. Wales Department of Primary Industries,
Those most able to reach a net-negative The area of adequacy and fairness is ripe NSW, Australia, and adjunct professor at the
future need to plan for it now — not out of for work across multiple disciplines. Setting University of New England, Armidale, NSW,
generosity, but out of necessity, to reach net-zero targets at a country or company level Australia. Andy Reisinger is a visiting fellow
net zero globally. Finland and Sweden, for cannot be done only by natural scientists or at the Climate Change Institute, Australian
example, have targets for reaching net-zero economists. Ethicists and social scientists are National University, Canberra, Australia.
A.C. and A.R. declare competing interests. Full
details and Supplementary information are
available at go.nature.com/3tglpa2.
The views expressed in this article are those
Checklist for rigorous of the authors and do not necessarily reflect
those of their institutions.
and clear net-zero plans e-mail: j.rogelj@imperial.ac.uk
Scope • Would the global climate goal be achieved 1. Rogelj, J. et al. Environ. Res. Lett. 10, 105007 (2015).
2. Geden, O. Nature Geosci. 9, 340–342 (2016).
• What global temperature goal does this if everyone did this? 3. Rogelj, J. et al. In Global Warming of 1.5 °C (eds
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temperature, or see it peak and decline)? these principles are applied universally? Panel on Climate Change & World Meteorological
Organization, 2018).
• What is the target date for net zero? • How will your target affect others’ capacity
4. Tanaka, K. & O’Neill, B. C. Nature Clim. Change 8, 319–324
• Which greenhouse gases are considered? to achieve net zero, and their pursuit of other (2018).
• How are greenhouse gases aggregated Sustainable Development Goals? 5. Fuglestvedt, J. et al. Phil. Trans. R. Soc. A 376, 20160445
(GWP-100 or another metric)? (2018).
• What is the extent of the emissions (over Road map 6. Intergovernmental Panel on Climate Change In
Climate Change and Land: an IPCC special report (eds
which territories, time frames or activities)? • What milestones and policies will support Shukla, P. R. et al.) 1–41 (IPCC, 2019).
• What are the relative contributions of achievement? 7. Rogelj, J. & Schleussner, C.-F. Environ. Res. Lett. 14,
reductions, removals and offsets? • What monitoring and review system will be 114039 (2019).
8. Brennan, M. E. & Zaitchik, B. F. Environ. Res. Lett. 8,
• How will risks be managed around used to assess progress and revise the target? 014033 (2013).
removals and offsets? • Will net zero be maintained, or is it a step 9. Lynch, J., Cain, M., Pierrehumbert, R. & Allen, M. Environ.
towards net negative? Res. Lett. 15, 044023 (2020).
Fairness 10. Robiou du Pont, Y. & Meinshausen, M. Nature Commun. 9,
4810 (2018).
• What principles are being applied? See Supplementary information for details. 11. Rogelj, J. et al. Nature 573, 357–363 (2019).
12. Obersteiner, M. et al. Science 294, 786–787 (2001).
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