AT THE core of the Paris climate change agreement is the aspiration to “[hold] the increase in the global average temperature to well below 2°C above pre-industrial levels”. At current rates of greenhouse gas emissions, we have 20 years before such a rise is inevitable. To avoid it, we need emissions to peak as soon as possible – preferably by 2020 – before making their way to zero by about 2070.
There are some grounds for optimism: energy and industry emissions may already be peaking as the world moves away from the dirtiest of fossil fuels, coal (see “Living with climate change: Turning the corner“). But this needs to be seen in context. We are still emitting almost 42 gigatonnes of carbon dioxide each year. Change is not yet happening fast enough or on a large-enough scale to meet the world’s growing energy demand.
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Besides, closing coal mines and investing in renewables for electricity generation is the easy part. Generating electricity accounts for only a quarter of global greenhouse gas emissions, with emissions from agriculture, forestry, industry and transportation making up the rest.
Oil, the primary fuel for transport, is particularly difficult to replace. Cars and buses can be made to run on electricity, but powering planes will require the large-scale development of renewable, sustainable jet fuel. The current global production, mainly biofuel made from fermenting crops, is minuscule compared even with the annual consumption of 42 billion litres of jet fuel by US scheduled airlines.
Of course, we could choose to fly less and cut down on other behaviours that have high carbon footprints, such as eating meat. The question is whether we will – and how soon. The aspiration set out in the Paris agreement is just that: an aspiration. Based on the concrete commitments for emissions reductions by 2030 made by individual countries, it seems likely we will continue to overshoot the trajectory necessary to hit zero in 2070 by some way (see diagram).
This means that not only will we have to wean ourselves entirely off fossil fuels, but we may have to suck carbon dioxide out of the atmosphere on an industrial scale to hit the 2°C target. Assuming industry and agriculture continue to produce greenhouse gas emissions in the near term, we would have to suck about 600 gigatonnes of carbon out of the atmosphere this century.
The main contender for achieving this is bioenergy with carbon capture and storage (BECCS), in which plants are grown and then burned to produce energy. The CO2 they sucked out of the atmosphere while growing is released when they are burned but captured and shoved underground before it can escape back into the atmosphere.
On the upside, the biofuel could power aircraft. The catch is that to have a meaningful effect, BECCS would have to happen on a massive scale, with thousands of industrial facilities across the globe. Not only is the technology untested on those scales, but we don’t have enough spare land to grow the crops while continuing to feed ourselves. Staying below 2°C would mean planting crops solely for the purpose of CO2 removal on around 500 million hectares – equal to a third of Earth’s arable land surface or half the area of the US – competing with ecosystems and food crops. In short, the consequences of negative emissions technology could be worse than overshooting 2°C.
To stay below 2°C, we need a radical change in commitment, where nations make climate change their top priority and the transfer to renewables is accelerated. With our present policies, we are on track to reach 3.6°C of warming. Even with the promises made by world leaders in Paris, which have yet to materialise, we will exceed 3°C. Then again, Donald Trump may be the spur the world needs to galvanise action (see “Living with climate change: Turning the corner?”).
This article appeared in print under the headline “Living with climate change: Can we limit warming to 2°c?”
Article amended on 7 July 2017
Correction: The figure given for consumption of fuel by US airlines was wrong when this article was first published. It has now been changed.
