全球变化知识资源中心

Biophysical, biogeochemical (that is, nutrients), energy and economic resource implications of large-scale implementation of NETs differ significantly. For DAC, costs and energy requirements are currently prohibitive and can be anticipated to slow deployment. Research and development is needed to reduce costs and energy requirements. For EW, the land areas required for spreading and/or burying crushed olivine are large, such that the logistical costs may represent an important barrier, compounded by the fact that the plausible potential for carbon removal is lower than for other NETs. In contrast, AR is relatively inexpensive, but the unintended impacts on radiative forcing through decreased albedo at high latitudes, and increased evapotranspiration increasing the atmospheric water vapour content, could limit effectiveness; likewise, increased water requirements could be an important trade-off, particularly in dry regions. Competition for land is also a potential issue, as it is for BECCS^{50, 88, 89}. BECCS may also be limited by nutrient demand, or by increased water use, particularly if feedstocks are irrigated and when the additional water required for CCS is considered. These biophysical and economic resource implications may directly impose limits on the implementation of NETs in the future, but they may also indirectly constrain NETs by interacting with a number of societal challenges facing humanity in the coming decades, such as food, water and energy security, and thereby sustainable development. In addition to the biophysical and economic limits to NETs considered here, social, educational and institutional barriers, such as public acceptance of and safety concerns about new technologies and related deployment policies, could limit implementation. The drivers, risks, and limitations of the supply of NETs, showing activities thought to increase the potential supply of NETs, as well as the risks and geophysical and societal limits to the potential of NETs, are shown in Supplementary Fig. 1. Commercialization and deployment at larger scales will also allow more to be learnt about these technologies, in order to improve their efficiency and reduce cost.

To inform society of the potential risks and opportunities afforded by all mitigation options available, more research on NETs is clearly required. Although we have collated the best available data on NET impacts and have reflected changes related to deployment scale as accurately as possible, it is clear that common modelling frameworks are required to implement learning, cost, supply and efficiency curves for all NETs. By implementing such curves, future models will be able to develop portfolios of trajectories of NET development, allowing least-cost options to be selected, and learning and efficiency improvements to be reflected. The inconsistency in coverage of NETs and their impacts highlights this key knowledge gap; this analysis will help to frame these developments in the modelling community.

For BECCS, research and development is required to deliver high-efficiency energy conversion and distribution processes for the lowest-impact CCS, and the cost of infrastructure to transport CO₂ from BECCS production areas to storage locations needs to be further evaluated. To this end, early deployment of CCS would enhance understanding of the risks and possible improvements of the technology. Integrated pilot plants need to be built (storing ~1 Mt CO₂ per year) to examine how combined BECCS functions⁹⁰; the capital cost of 5–10 full-size demonstrations of BECCS or CCS would require the investment of approximately US$5–10 billion⁹⁰. There is also a need to develop socio-economic governance systems for all NETs, to provide incentives to fund this research and development, and implementation of infrastructure in the most sustainable manner, to limit adverse impacts in the transition to low-carbon energy systems, and to manage the risks associated with CCS (such as leakage, seismic action and environmental impacts)⁹¹. Priorities include investing in renewable and low-carbon technologies, efficiency and the integration of energy systems (to make the most of waste heat, excess electrons from photovoltaic panels and wind, and to close the carbon cycle of fossil sources by capturing and reusing CO₂ by catalysis), and the realization of additional environmental benefits. In the meantime, emission reductions must continue to be the central goal for addressing climate change.

Addressing climate change remains a fundamental challenge for humanity, but there are risks associated with relying heavily on any technology that has adverse impacts on other aspects of regional or planetary sustainability. Although deep and rapid decarbonization may yet allow us to meet the <2 °C climate goal through emissions reduction alone⁸, this window of opportunity is rapidly closing^{8, 92} and so there is likely to be some need for NETs in the future^{41, 93}. Our analysis indicates that there are numerous resource implications associated with the widespread implementation of NETs that vary between technologies and that need to be satisfactorily addressed before NETs can play a significant role in achieving climate change goals. Although some NETs could offer added environmental benefits (for example, improved soil carbon storage²⁸), a heavy reliance on NETs in the future, if used as a means to allow continued use of fossil fuels in the present, is extremely risky, as our ability to stabilize the climate at <2 °C declines as cumulative emissions increase^{8, 35, 92}. A failure of NETs to deliver expected mitigation in the future, due to any combination of biophysical and economic limits examined here, leaves us with no 'plan B'⁴⁵. As this study shows, there is no NET (or combination of NETs) currently available that could be implemented to meet the <2 °C target without significant impact on either land, energy, water, nutrient, albedo or cost, and so 'plan A' must be to immediately and aggressively reduce GHG emissions.