Eric: “Planting trees could be the number one super-weapon in tackling climate change.” This was the headline-grabbing summary of a research paper published in the journal Science recently. Greta Thunberg put the conclusions in context: we need trees, but we really need to keep fossil fuels in the ground. Trees and methane are two notable omissions from our discussion so far. Let's start with trees, and then take on cows. So far, there is a clear structure to our argument: work out how significant a contribution every sector is to man-made emissions, assess how tractable it is to solutions, and then apply EPICs, smart regulations or other smart policies. So, what contribution do forests make to global emissions?

Corinne: Everything we have argued for so far involves reducing how much CO2 we emit into the atmosphere every year – restricting sources of carbon. There is a separate class of factors which can absorb CO2 from the atmosphere. These are natural “carbon sinks”, such as trees, soil and oceans, and man-made technologies, such as carbon capture and storage. Forests sit in this category. Everyone knows we need to stop the deforestation of the tropics. If you have ever purchased a “carbon offset” when you book a flight, this may well have involved planting trees.

Eric: How should we think about nature-based carbon sinks compared to all the emissions we are trying to abate?

Corinne: It is very hard to measure the effect of carbon sinks with accuracy, so the range of estimates is very wide. Oceans are estimated to capture between 25–30 per cent of annual emissions. Forests absorb somewhere between 5 and 15 per cent of man-made emissions. To put both in context, the steel industry alone generates 8 per cent of global emissions. So Greta's summary is correct. Trees are good, but we cannot rely on capture, we need to keep the fossil fuels in the ground.

Eric: If forests absorb so much less than oceans, why do we hear so much more about planting trees than protecting the oceans?

Corinne: There is a huge amount we can and should be doing to protect oceans: eradicating plastic and microplastic pollution, heavy metal pollution, overfishing. These are extremely important to the health of the oceans and the preservation of biodiversity, but the ocean's specific ability to absorb carbon emissions is largely out of our control.