Last week in Paris, I watched happily as the world passed the final text of a truly global and surprisingly ambitious climate change agreement. Through clever geopolitical maneuvering by vulnerable, low-lying states — buttressed by social movements, activist groups, and NGOs — the world agreed to a deal stating that global temperature increases will be held, “well below 2°C ” above pre-industrial levels, and that countries will go further, “pursuing efforts to limit the temperature increase to 1.5 °C.” It has been a long time coming...
The question now, of course, is: What next? The implications of such an agreement are profound. Long-term temperature changes are largely driven by the cumulative emissions of carbon dioxide; to stabilize temperatures requires decreasing these emissions to zero. The current list of commitments by countries is nowhere near sufficient – if fully implemented, they would lead to about 3 degrees of warming; and even the built-in mechanism of 5-year reviews, intended to set increasingly ambitious targets, still leaves major challenges associated with limiting temperature increases to these levels.
Chief among them is that we are, essentially, almost at our global temperature increase limit already. The Earth’s average temperature has risen almost 0.9 degrees above pre-industrial levels. There is probably no escaping at least another 0.5 degrees of warming, due to the emissions we have already released. Clearly, the carbon budget is nearly expended. Thus, published scenarios to limit increases to 1.5 degrees require an immediate steep decline in global carbon dioxide emissions, down to essentially zero, by about 2050.
But that still won’t cut it. We would still probably overshoot 1.5 degrees. Under what has been sketched out in Paris, it is clear that we will have to go even further, to negative emissions — that is, using technologies to remove carbon dioxide from the atmosphere and store it somewhere else. Such technologies are already discussed: All 344 possible emissions scenarios from the most recent Intergovernmental Panel on Climate Change (IPCC) assume that negative emissions technologies (NETs) will be successfully deployed in order to give a 50:50 chance of remaining below 2 degrees. For 1.5 degrees, negative emissions are essential. That is to say, the Paris agreement is banking on us not only rapidly reducing emissions to zero, but also on humans’ capacity to subsequently remove carbon dioxide and other greenhouses gases from the atmosphere.
For many areas of life, near-zero emissions is an attainable, if ambitious, goal. Zero emissions buildings, energy efficiency standards and a roll-out of low carbon energy sources from the sun, wind, and waves to power electric vehicles all utilize technologies that exist. But what about removing carbon dioxide from the atmosphere, to store elsewhere? This is technically possible, but it’s a possibility that comes with many caveats.
One potential route calls for human intervention to recreate, enhance, or restore the processes that naturally remove carbon dioxide from the atmosphere. Only about half of the carbon dioxide that human activity releases into the atmosphere stays there and impacts the climate; the other half is naturally removed, going into the oceans, trees, and soil. These carbon sinks, as scientists like me refer to them, do work to remove harmful warming gases — and we can intervene, to a degree, to help supplement them. Those who advocate going this route call for solutions like careful forest restoration and slightly more technical efforts, which include using plant biomass to produce charcoal that can then be added to soil, locking carbon there for the long term or breaking up and spreading around silicate rocks, which remove carbon dioxide from the air, enhancing a natural chemical process.
Then there are the more ambitious engineering approaches based on removing carbon dioxide from the atmosphere and storing it somewhere safe, such as underground in geological deposits such as saline aquifers or old oil and gas reservoirs, known as carbon capture and storage (CCS). The most commonly discussed method is to cultivate crops or trees, which remove carbon from the atmosphere as they grow, then using this biomass in power plants to generate electricity, followed by CCS disposal of the waste carbon dioxide. This would create a power source that is not only carbon neutral, but carbon negative. Alternatively, some have discussed creating artificial trees via engineering chemical reactions to strip carbon dioxide directly from the air itself, again to stash it somewhere for safekeeping, using CCS.
All of these technologies have serious limits, however. The amount of land available for growing crops and trees for energy use is finite. Growing trees for CCS purposes comes into conflict with using land for food production and even other environmental projects like ecosystem restoration. Even restoring forests has pitfalls: In cold regions, for instance, planting dark-colored conifer trees can actually cause more global warming. (Their reflectance levels — which scientists call “albedo” — mean their darker surfaces heat up more than the lighter colored surface they replace, outweighing the amount of warming they prevent by removing carbon from the atmosphere.)
CCS has been the subject of sizeable research. It’s been seen as a potential lifeline for the fossil fuel industry: If we can simply safely lock away emissions, we can keep creating them. But so far, practical experiments have yielded poorer results than have hoped for. It is both more expensive and, technically, much more difficult than initially thought. During one industrial experiment in Algeria, for example, oil giant BP and others had to stop injecting CO2 underground in 2011 after concerns that the gas may begin leaking out; this, for a project whose total cost came to $2.7 billion.
A recent review of all these types of NETs, published in the journal Nature Climate Change, concluded that “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 greenhouse gas emissions.”
There is a danger that policymakers will not reduce emissions now and increasingly bet on NETs to come to the rescue later this century. It is a very dangerous bet. Dramatic emissions cuts — maybe even more dramatic than those already anticipated — will then be required to limit temperature rises to 1.5 degrees. Enhancing natural processes to remove carbon dioxide from the atmosphere will also be necessary, particularly in offsetting hard-to-reduce agricultural greenhouse gas emissions, but in practice will be limited. The use of CCS seems unlikely to deliver the really big negative emissions once expected, given current progress, and should certainly not be used as an excuse to extend the life of the fossil fuel industry.
We can expect to suck some carbon out of the atmosphere later this century, but the quantities will be limited. To keep to 1.5 degrees requires attaining zero emissions over the next 30 years, and then negative emissions. That requires tackling climate change on all fronts. With just 10 percent of the world’s people responsible for 50 percent of emissions immediate radical reductions are certainly possible. To achieve them, however, requires not only rapid changes in energy production, but also confronting the complex social and political challenges that have made grappling with climate change so difficult. In other words, there is no one technological solution that can make this vastly easier. Technology will be critical — but there is no magic negative emissions bullet.
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