本文转自《Forbes》发表的题为“Negative Emissions Won't Rescue Us From Climate Change”的报道。

作者：Andy Stone

日期：2018.10.29

原文链接：https://www.forbes.com/sites/andystone/2018/10/29/negative-emissions-wont-rescue-us-from-climate-change/#4c23e40b6362

Flooded parking lot, Miami, Florida. (Credit: Getty Royalty Free)

The Intergovernmental Panel on Climate Change’s interim report on global warming, released earlier this month, is jarring stuff. It describes the extensive environmental and economic damage that will befall Earth as global warming approaches 1.5 degrees Celsius, even as we speed on a trajectory that will warm the planet twice as much by the end of this century.

But the most important contribution of the IPCC’s report may be that it describes in very clear terms exactly what we need to do to hold warming to 1.5 degrees and minimize climate change and its impacts. Summing up, the path to least climate impact will require nations to work together to cut global carbon emissions by 45% in just over a decade.

Such a cut in emissions will require an unprecedented degree of political will and global cooperation. Highlighting the challenge, at the September climate talks in Bangkok, the warm up to December’s major global climate summit in Katowice, Poland, climate negotiators maintained “deep differences” on perpetual issues of climate finance and rules that will govern implementation of the Paris Climate Accord. Bangkok ended with few concrete accomplishments, adding to the sense of near inevitability that carbon emissions will continue to rise at a robust clip well beyond 2030, and that the atmosphere will in fact surpass the end-of-century warming limit of 1.5 degrees before 2050.

Yet, despite the major political barriers to dramatic near-term emissions cuts, a terrifying realization is that such action is, in fact, the most realistic option available to hold climate change in check. Of the climate action pathways modeled by the IPCC, the scenario that requires boldest action in the near term is the only one that doesn’t also require a leap of faith that a suite of uneconomic, logistically challenging, and ultimately unproven negative emissions technologies will in fact deliver us from our collective peril.

The gulf between the promise of these technologies (a multitude of variations on carbon capture and storage) and their real ability to offset a future of high carbon emissions appears insurmountable, if not in terms technological viability, then almost surely in terms of magnitude and scale.

In its report, the IPCC offers four illustrative model pathways for limiting warming to 1.5 degrees with little or no “overshoot”, or excess emissions of carbon dioxide that would subsequently need to be pulled from the air to bring temperatures back down to the year 2100 target. Inscenario “P1” — the fastest track modeled by IPCC — “social, business and technological innovations” cut emissions to the necessary level in the decades to come, even as global standards of living continue to rise. Coal and oil use fall by 78% and 37%, respectively, by 2030, while nuclear capacity grows by 59% and non-biomass renewable energy by 430%. Those are dramatic numbers, but theoretically doable if nations were to ratchet up the clean energy and energy efficiency tools they already have on their collective plates.

Conspicuously absent from the move-fast scenario are carbon capture and storage (CCS) and bioenergy with carbon capture and storage (BECCS), the two most frequently mentioned methods of reducing and eliminating emissions. They wouldn’t be necessary. The only required negative emissions technology is good old afforestation, the planting of trees.

But the remaining three IPCC model pathways assume progressively less global success in curbing emissions in the near term, and increasing reliance on negative emissions to mop up the overshoot in carbon emissions that would result.

The most politically tenable, and least ambitious, of the 1.5 degree scenarios would allow oil use to grow 86% over the coming decade, with coal use falling 59%. In turn, 1200 gigatons of carbon would need to be sucked from the atmosphere by the century’s end. That’s equal to 32 years of annual anthropogenic carbon emissions at today’s levels, an astounding target given that BECCS is for practical purposes nonexistent today. What’s more, BECCS at scale would require 724 million arable hectares of Earth’s surface to be dedicated to bioenergy crops, approximately double the size of India. BECCS of such magnitude looks wildly improbable on a planet expected to house 2.5 billion more people by midcentury, with greater demand for food and cropland.

The prospects for other negative emissions realities are just as discouraging.  Garden variety carbon capture and storage, the solution that would allow coal and natural gas to continue to be burned well into the future without sending carbon dioxide into the atmosphere, counts just one working scale example in the United States. NRG Energy’s Petra Nova coal power plant in Texas captures 1.4 million tons of CO2 per year. For perspective, that’s about four millionths (4 x 10-6) of the 37 gigatons of global yearly anthropogenic emissions. Captured CO2 from this coal plant is sent via 80 miles of bespoke pipeline to an oil field where it is injected into oil wells, helping to push out more oil and, in clear irony, perpetuating the fossil fuel cycle.

Ultimately, the carbon capture process itself uses lots of energy. As the U.S. Energy Information Administration points out, Petra Nova “requires a dedicated natural gas unit to accommodate the energy requirements of the carbon capture process,” meaning that more CO2 is produced in the service of preventing CO2 from being released into the atmosphere. Powerplants fitted with carbon capture technology would need to be connected by pipeline to distant reservoirs, just as the Texas plant, but on a scale much more vast if CCS is to matter to the climate, creating a need for an entire reverse pipeline infrastructure to transport CO2, the economics of which are just beginning to be explored.

These challenges are understood within the scientific community. That’s why, on October 24, the National Academies of Sciences published areport highlighting the urgency that government invest to speed development of negative emissions technologies.

And none of this is to say negative emissions aren’t possible or a worthwhile pursuit. It’s just that they’re only going to be a partial solution to the climate problem, akin to an insurance policy whose payout is likely to prove woefully inadequate when disaster strikes. We place our bets on the promise of negative emissions, and the dangerous political complacency that such faith engenders, at our global peril.