At the beginning of week two of the Paris climate talks, an international group of scientists is calling on the world's industrial powers to aggressively and immediately reduce greenhouse gas emissions, stressing that overreliance on so-called negative emissions technologies may prove too costly and disruptive to keep Earth from overheating.
In an article published today in Nature Climate Change, 40 climate experts outline the environmental, economic and energy effects of using carbon-capture technologies to keep the planet within the widely accepted threshold of 2 degrees Celsius of global average temperature increase between now and 2050.
Simple applications of NETs include planting trees to consume CO2 as they grow or covering the ground with crushed carbon-absorbing rocks. More complicated approaches center on chemical processes to absorb CO2 from the atmosphere or burning plants for energy, capturing the carbon dioxide that would otherwise be released, and sequestering it deep below ground or in the ocean (a method known as bioenergy with carbon capture and storage).
While the Nature Climate Change article provides a detailed discussion of the pros and cons of NETs, the scientists conclude that on their own, the technologies will not be enough to solve the climate change dilemma. Humans must curb their use of greenhouse gas-emitting fossil fuels to have a real impact on global warming, they say.
"The bottom line is that we shouldn't imagine we can make up for poor decisions today by buying negative emissions tomorrow," said co-author Steven J. Davis, assistant professor of Earth system science at the University of California, Irvine. "It's easier, cheaper and less risky to tackle the challenge of fossil fuel CO2 before it's in the atmosphere."
The experts point to numerous factors that may lessen the effectiveness of NETs in combating global climate change. Approaches such as reforestation and growing crops as a fuel source require large tracts of land and potentially copious amounts of water. Planting trees above a certain latitude may limit the amount of sunlight reflected away from Earth. Machines to remove CO2 will be expensive to deploy and maintain. It's also uncertain how the public will respond to proposals involving the use of NETs in their locales.
"We show that all negative emissions technologies have significant limitations, and whilst we need to invest in research and development to try and overcome these limitations, the key message from our study is that we should not rely on these as-yet-unproven technologies to save us in the future," said lead author Pete Smith, a professor in the School of Biological Sciences at Scotland's University of Aberdeen. "Rather, swift and aggressive cuts in greenhouse gas emissions are needed now. The window of opportunity is closing rapidly, so it's imperative to get a global accord to move forward in Paris this month."
The review article in Nature Climate Change resulted from a collaboration under the Global Carbon Project.