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Scrutinizing negative emissions technologies

Published online 16 December 2015

Negative emissions technologies are promising but have limits. 

Moheb Costandi

A new review assesses the global impact of different negative emissions technologies (NETs) to estimate the limits and costs of applying them widely.

NETs remove greenhouse gases from the atmosphere. They include bioenergy with carbon capture and storage, direct air capture of environmental carbon dioxide by engineered chemical reactions, enhanced weathering of minerals, afforestation and reforestation, and manipulation of oceanic carbon intake.   

The review comes as climate change negotiators agreed, earlier this week, to limit global warming to “well below” 2 °C relative to pre-industrial times. 

Pete Smith of the University of Aberdeen and his colleagues use data from recent literature to assess usage of resources such as land, water and energy when a range of different NETs are implemented together1.   

According to their analysis, direct air capture is expensive and inefficient and therefore likely to be deployed slowly, and enhanced weathering of minerals requires large areas of land, so its logistical costs might be prohibitive for widespread deployment

By contrast, afforestation and reforestation is relatively cheap, but several factors may limit its effectiveness. It reduces the fraction of the Sun’s radiation that is reflected at high altitude, for example, and increases atmospheric water vapour content. It also increases water requirements, which could be problematic in dry areas. 

“Some [of these] technologies have been less well researched than others, [so] our sources of information have different levels of confidence,” says Smith. Nevertheless, their review suggests that despite some advantages, heavy reliance on NETs in the future could be risky. 

“In 10 years' time we should have better data upon which to make decisions, and we are [collecting] some of this under the Magnet Initiative of the Global Carbon Project.”


  1. Smith, P. et al. Biophysical and economic limits to negative CO2 emissions. Nature Climate Change (2015).