Recently I and two co-authors – Kwesi Quagraine and Erle Ellis – proposed a new metric for how to measure and track the progress of the clean energy transition worldwide. This was published as a Commentary in Nature on 26 January 2026 under the title: ‘As we breach 1.5 °C, we must replace temperature limits with clean-energy targets’.

We’ve had a lot of interest in the Clean Energy Shift, and lots of questions to field via email and social media. In order to address them better for a wider audience, below you’ll find an FAQ on the whole concept addressing some of the issues and challenges that have been raised. I hope you find it useful.

To start with, here’s a few of the key paragraphs from the initial paper:

In 2024, Earth’s global mean surface temperature averaged 1.55 °C above pre-industrial levels2, and the average for 2023–25 is 1.48 °C, perilously close to the limit. Keeping to the Paris target now looks impossible by any realistic measure. Yet this moment should not invite despair. Instead, it demands an urgent reframing of how climate progress is measured and mobilized.

The world today looks very different from that in 2015 when the Paris goal was framed. Although emissions are still rising and global actions on climate change are slow, a lot of progress has been made. Clean energy is expanding rapidly and decarbonization, not fossil fuels, is the new ‘business as usual’. In the first three quarters of 2025, growth in clean electricity generation outpaced that in energy demand for the first time, implying that fossil fuels are being displaced.

We argue that the main focus of climate action in 2026 and beyond should be on accelerating the clean-energy revolution. And the rate at which clean energy displaces fossil fuels in the global economy should become the key measure of climate progress. Here we describe how such progress can be tracked and incentivized using a metric we call the clean-energy shift. Unlike chasing intangible temperature targets, cleaning up the energy sector is a more-focused battle that the world can win.

FAQ on the Clean Energy Shift metric

• What is the exact definition of the CES metric?

  CES is defined as the difference between the percentage annual growth rate of clean energy supply and the percentage annual growth rate of total energy demand over the same time interval. A positive CES indicates that clean energy is expanding faster than overall energy use. If sustained for long enough and at a high enough value, the economy will decarbonise with mathematical certainty.

• Are you suggesting CES replaces existing climate policy, including the Paris targets like 1.5C?

  CES is a different kind of metric that we hope policymakers will find useful as the 1.5C Paris limit is passed in realtime. Temperature targets remain useful normative guideposts, but their achievement can only be assessed probabilistically and over long time horizons, making them poorly suited as near-term performance indicators.CES provides a near-term, operational metric of transition speed. By focusing on annual structural change in the energy system, it offers policymakers a real-time indicator of whether the clean energy transition is accelerating sufficiently to improve long-term temperature outcomes.

• Why do we need a new metric like CES when COP28 already agreed actionable targets like tripling renewables and doubling energy efficiency by 2030?

  CES is complementary to these measures, and helps understand and guide their implementation. But tripling renewables in and of itself does not guarantee any reduction in fossil fuel use if demand rises faster than increased renewable energy supply. This is why CES matters: keeping the metric positive requires that the rate at which renewable and other clean energy increase their supply outpaces increases in overall demand. It is this differential – not increases in renewable capacity and efficiency per se – that guarantee the reduction and eventual elimination of fossil fuels and any delivery of the Paris Agreement’s goals.

• What are the future CES values needed for a net-zero/2 degrees/1.5 degrees pathway to guide policymakers?

In our paper we provide historical CES values and indicate the direction and approximate magnitude needed: CES would need to rise substantially from its recent level of around 4% and keep climbing through the 2030s and 2040s to eliminate fossil fuels by 2050. However, in our paper we chose not to present future pathway numbers – for example CES values guiding towards a 2050 net zero date – because of problems in the way that energy datasets are currently presented. The most widely available datasets use primary energy, which has decreasing relevance for an economy that is moving away from fossil fuels and towards electricity. A more appropriate dataset would be for useful energy. See question below on primary energy vs useful energy.

• What about primary energy vs useful energy?

Most global energy statistics are presented in primary energy terms. Primary energy embeds large thermodynamic losses — for example, roughly two-thirds of the energy content of coal is lost as waste heat in a conventional power station, whereas solar photovoltaics generate electricity directly. As economies electrify, many of these losses disappear.

Primary energy accounting can therefore overstate the total energy input needed to deliver equivalent energy services as the system electrifies. A more economically meaningful metric would be “useful energy” — such as lumens of light or kilometres travelled — but economy-wide useful-energy datasets are not yet available in open, harmonised form. In our paper we therefore call for systematic research to develop accessible, useful-energy accounts to inform future transition modelling.

• If CES is already positive why aren’t we already past peak emissions?

A positive CES does not automatically imply falling emissions because the effect depends on the relative size of the clean and fossil portions of the energy system. CES measures relative growth rates; emissions depend on absolute quantities.

Consider a system with 1000 EJ total energy and 100 EJ clean energy. If total energy grows at 3% and clean energy grows at 6%, CES equals 3%. Total energy rises to 1030 EJ and clean energy to 106 EJ. Fossil energy therefore still increases in absolute terms.

If instead clean energy represents 500 EJ of a 1000 EJ system, the same 3% CES would stabilise fossil use in absolute terms. Once the clean share is sufficiently large relative to the growth differential, a sustained positive CES drives fossil energy downward in absolute terms. The timing of peak emissions therefore depends on both the growth differential and the starting shares.

• What about hard to abate sector (aviation, chemicals, steel, agriculture)?

CES measures the balance between clean and fossil energy growth. It does not by itself solve non-energy emissions such as those arising from agriculture or land-use change, which require sector-specific strategies. Hard-to-abate sectors such as aviation, cement, steel and chemicals currently constrain the upper bound of achievable decarbonisation. Whether CES slows once electricity is largely decarbonised depends on the rate of technological substitution in these sectors. Historically, sectors once considered intractable — such as road transport — have shifted rapidly once cost and policy conditions aligned. The frontier of achievable CES therefore shifts as innovation and policy expand the set of sectors that can be decarbonised.

• What about other gases (methane, NO2 etc)?

CES is an energy transition metric. It does not directly measure emissions or differentiate between greenhouse gases. Some gases — such as nitrous oxide from agriculture — require independent mitigation strategies. Others, such as fossil methane, decline as a co-benefit of reducing fossil fuel production and use. Biogenic methane and land-use emissions remain outside the scope of the metric and must be addressed separately.

• What does CES tell us about emissions? Does it differentiate between different types of fossil fuel?

CES does not differentiate between coal, oil or gas within the fossil portion of the energy system. A system dominated by coal has a very different emissions profile from one dominated by gas, even at identical CES values. Emissions accounting and fuel-specific policies therefore remain essential complements to the metric.

• What about the carbon costs of manufacturing clean energy sources like renewables?

Energy used to manufacture clean technologies forms part of total energy demand. If that manufacturing relies on fossil fuels, it will temporarily increase fossil energy use. A positive CES therefore requires clean generation growth to exceed both baseline demand growth and energy used to build the transition itself.

• What about data centres and other potentially large future sources of energy demand? Are they factored in?

CES is agnostic regarding the source of demand growth. Any increase in energy use — including from data centres — enters the total energy term. Rapid demand growth therefore requires proportionally faster clean supply growth to maintain a positive CES. In this sense, CES embeds demand-side discipline within the metric itself.

• Does CES assume that markets are driving a clean energy transition that is already inevitable?

CES does not assume any particular driver of change. It is a descriptive metric of growth differentials within the energy system. That said, sustaining a high and positive CES over time, however, is unlikely to occur without deliberate policy support, infrastructure build-out, capital investment and regulatory signals that enable clean energy to outpace total demand growth.

• What do you do with biomass: do you consider it clean or not?

In our analysis we classify traditional biomass with fossil fuels and modern bioenergy with clean energy. This avoids creating a perverse incentive to count subsistence biomass use — particularly wood and charcoal for cooking — as evidence of transition progress.

We recognise ongoing debate regarding land use, food competition and ecological impacts of bioenergy. Our classification is designed to avoid overstating transition progress by excluding traditional biomass from the clean category, while acknowledging that some modern bioenergy may play a transitional role despite unresolved sustainability concerns.

• Who came up with the CES concept?

The conceptual foundation was articulated by Michael Liebreich in his essay “The Pragmatic Climate Reset, Part 1.” Our Nature paper formalises and operationalises the concept into a tractable policy metric. We are grateful for his constructive input during the drafting process.