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Climate and energy challenges for materials science Dolf Gielen, Francisco Boshell and Deger Saygin The Paris agreement on climate change represents an important step in the design of a new global framework for the mitigation of greenhouse gas emissions. Energy efficiency and renewable energy are keys for the success of this ambitious agreement.

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he risks of irreversible, catastrophic consequences of human-induced global warming have concerned scientists in the past decades, and have called for a concerted action from policymakers to prevent them. A clear sign of the impact that greenhouse gas (GHG) emissions are having on the earth’s climate is that the average global temperature of the most recent years is the warmest on record. As a result, glaciers are rapidly disappearing and sea levels are rising more than 0.2 cm per year. In September 2015, the average global temperature over land and ocean surfaces was 0.9 °C above the twentieth-century average1. This temperature rise is in line with the projections of the Intergovernmental Panel on Climate Change (IPCC) forecasting models, which relate these effects to the concentration of CO2 in the atmosphere. At the start of 2015, the CO2 concentration was 400 parts per million (ppm). The IPCC estimates that a CO2 concentration of 450 ppm — which, at the current rates of emission, will be reached in 2040 — would result in a global temperature rise of 2 °C above pre-industrial levels by 2100. Beyond 2 °C, experts predict serious consequences for the Earth’s climate. In 1997, the United Nations Framework Convention on Climate Change (UNFCCC) adopted the Kyoto protocol. The protocol, which came into force in 2005, is an international treaty that defines mandatory targets for the signatory countries to reduce GHG emissions. In 2013, the nations that ratified the treaty covered only 61.6% of the base-year emissions — the starting-point emission levels used to track progress of the emission reductions — generated by developed countries and economies in transition, and around 30% of the total global emissions. The success of the Kyoto protocol was limited. An alternative, voluntary approach based on Intended Nationally Determined

Contributions (INDCs) was considered the only viable way ahead for a more effective action. In preparation of the twenty-first Conference of the Parties (COP21) to the UNFCCC, which took place in Paris in December 2015, countries were asked to publicly declare what actions they intend to take under a new global agreement through their INDCs, which account for national circumstances, capabilities and priorities, with the aim to reduce global GHG emissions to restrict global temperature rise to 2 °C. The INDCs cover mitigation and adaptation, and describe what support each country needs or will provide to address climate change. As a result of the COP21 talks, which were mainly focused on reaching common consensus on specific solution areas2 and financial aspects between the representatives of 195 countries, a second international agreement on climate action, called the Paris agreement, has been adopted, and will come into force in 2020. This new treaty includes: an objective to limit climate change well below 2 °C, with rapid peaking of emissions and balancing of emissions and absorption in the second half of the twenty-first century; the formalization of the INDCs as an implementing means, to be reviewed every five years; common but differentiated responsibility of developed and developing countries; and a new market mechanism to accelerate implementation of low-carbon technology projects. The 2 °C goal translates into an expected amount of global GHG emissions no higher than 31–44 Gt CO2-eq by 20303 (where Gt CO2-eq represents the gigatonnes of CO2 having equivalent global warming potential when a time horizon of 100 years is considered). However, in 2010, they amounted to 49.8 Gt CO2-eq (ref. 4) and by 2030, the submitted INDCs could yield a total of 53–59 Gt CO2-eq GHG emissions5. Hence, a substantial gap with the expected targets remains, which needs to be addressed

NATURE MATERIALS | VOL 15 | FEBRUARY 2016 | www.nature.com/naturematerials

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through structural changes in energy production and utilization. In particular, access to modern lowcarbon energy services, energy efficiency and renewable energy are considered the three pillars of the required energy transition. These three directions are in line with the 2030 Sustainable Development Goals (SDGs) of the United Nations that call for access to affordable, reliable, sustainable and modern energy for all as well as for a substantial rise in the share of renewable energy in the global energy mix. In particular, SDG-7 pleads for enhanced international cooperation to facilitate access to clean-energy research and technology (related to renewable energy, energy efficiency and advanced and cleaner fossil-fuel technology), and promotes investment in energy infrastructures. In the following sections, an analysis of the trends in energy demand and of the strategies to decouple energy production from GHG emissions is provided from the perspective of the innovative materials and technology solutions that are required to make the transition to clean energy a reality.

Commodities perspective

The term anthropocene has been coined for a new geological era where mankind is impacting the lithosphere and ecology on a planetary scale6. The huge volumes of commodities that are produced, transported and consumed bear witness to this phenomenon. Energy production has amounted to 29 Gt of coal, 4 Gt of crude oil and 2.4 Gt of natural gas in 2013, which translates into a materials volume of around 36 Gt. Fossil fuels represent the largest volume of commodities that are produced and consumed. In comparison, around 8 Gt of wood and other biomass, 2.5 Gt of cereals, 1.7 Gt of steel and 4.1 Gt of cement are produced every year 7–9. Around half of global energy use is related to commodity-production activities: 117

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Climate and energy challenges for materials science.

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