In the end, the results were mixed. First, the good news: biobased plastics are good for climate change. During their lifetime, they emit two times fewer greenhouse gases than those made from oil. A tonne of ‘classic’ high-density polyethylene releases four tonnes of CO₂ equivalent, while a tonne from agricultural sources will generate less than two. Bio polyethylene also consumes less fossil resources: 500 kg oil equivalent for sugar beet and 400 kg for wheat, compared with approximately 1,200 kg for oil.

So that’s good, right? Not so fast! Now for the bad news. There are two points: eutrophication and acidification. The first one is mainly due to the use of fertilisers in the fields. The earth is boosted with nitrates and phosphates, which then percolate into the water. The result isn’t very pretty: algae that proliferate so well and so extensively that they absorb all the oxygen, to the detriment of the fauna and the flora, which are destroyed. The production of biobased plastic leads to terrestrial eutrophication that is six to seven times greater, and even seven to eight times more in aquatic environments.

The results aren’t any better regarding acidification either, with an excess of nitrogen oxides, sulphur oxides, etc. This is caused by significant consumption of fossil fuels and the emission of pollutants owing to the use of fertilisers that generate acid rain, affecting fauna and flora as well as building facades. Acidification caused by bioplastic is two to three times worse than for products based on fossil fuels.

Side effects that are hardly negligible. So if we were to weigh up oil against agriculture, what downsides are we prepared to accept to produce polyethylene? Sandra Belboom offers a careful answer. “Life cycle assessment is a tool to help make decisions, it provides extra information in relation to the economic and technical points of view. But it isn’t a decision-making tool”. At the very least, it allows us to put an end to a popular belief: everything that comes from natural materials isn’t necessarily eco-friendly. “This is a cliché we have to get rid of so that we don’t become the victims of ‘greenwashing’”.

The result is that the decision to choose oil or natural products is a political issue. Is it preferable to reduce greenhouse gas emissions, even if this has other harmful effects on the environment? CO₂ is a global problem. One kilo of carbon dioxide less, wherever it has been captured, will benefit the whole world. As for eutrophication and acidification, they are part of a more local context. It comes down to choosing between the well-being of the planet and the well-being of our own environment, which can also be achieved through reasoned farming which helps to reduce these effects through the optimised use of fertilisers…

Especially as plastic based on sugar beet or wheat would have other repercussions. Space is needed to grow these raw materials. And yet, arable land isn’t extendible and current production almost certainly won’t meet all our needs. We would there for have to ‘evict’ the cattle or abandon other types of cultivation. Where would we put the animals? Which products would we have to import more of? What would the economic consequences be? Of course, modifying the use of a few hectares will go unnoticed. But if this change were to occur countrywide or across the continents… we would experience a domino effect. “If there were to be a wide-scale political decision, we would have to go one step further and envisage a consequential analysis, even if it included quite major uncertainties. No-one has a crystal ball! In any case, integrating an economic approach means that you can try to deduce the type of changes that might occur and avoid causing adverse effects”.

The cultivation of sugar cane in Brazil is a good example. In essence, using this raw material to make plastic seems like a very good idea. The yield is very high (up to 200 tonnes per hectare, according to certain publications!) and nothing is thrown away: the bagasse – the fibrous residue obtained after crushing the sugar cane – can be burnt, therefore the producers are self-sufficient in terms of energy and consume hardly any fossil fuels. As for CO₂, the benefit is even greater than with sugar beet or wheat. The gain is said to be 3 to 4. Since the beginning of this cultivation in the 1970s, the land devoted to sugar cane has been considerably extended. Thankfully, because of the type of soil required, there has been no need for deforestation to free up the space. Instead, it is pastures that have provided this space. The cattle has been moved, in particular towards the Amazon forest. Subsequently, sugar cane is suspected of indirectly causing deforestation. “If we take this parameter into account”, Sandra Belboom emphasises, “the CO₂ impact becomes greater than that caused by fossil fuels. Or you would have to compensate by a payback time (the number of years during which you would have to use bioproducts to counterbalance this deforestation), which varies between 5 and 100 years, depending on the percentage of deforestation taken into account”.