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The Paris Agreement in 2015 saw the international community agree to limit warming to within 2°C. The Hansen team argue that the much safer approach is to reduce atmospheric concentrations of CO₂ from the current annual average of more than 400ppm (parts per million) back to 1980s levels of 350ppm. This is a moderately more ambitious goal than the aspiration announced in Paris to further attempt to limit warming to no more than 1.5°C. Many climate scientists and policymakers believe that either the 2°C or 1.5°C limits will only be possible with negative emissions because the international community will be unable to make the required reductions in time.

Putting carbon back in the ground


Other energy sources are at best carbon-neutral, but BECCS removes more than it emits. Elrapto, CC BY-SA

BECCS has important limits, such as the sheer amount of land, water and fertiliser required to satisfy our energy demand. Perhaps more importantly, it doesn’t exist at anything like the scale required of it. Thus far only small pilot projects have demonstrated its feasibility. Other negative emissions approaches involve fertilising the ocean to increase photosynthesis, or direct air capture which sucks CO₂ out of the air and converts it into plastics or other products.

An ethanol production plant in South Dakota, US. We’ll need many more of these – equipped with carbon capture tech – to have an impact on global emissions. Jim Parkin / shutterstock



Scenarios for future carbon dioxide emissions and extraction.



To give these numbers some context, the entire US federal budget is about US$4 trillion, while annual spending by all countries on military and defence is US$1.7 trillion.

A climate balancing act

Humans have pumped over 1.5 trillion tonnes of CO₂ into the atmosphere since 1750. It is not just the amount, but the rate at which this CO₂ has been added. The oceans can absorb extra CO₂ but not fast enough to remove all human inputs and so it has been progressively building up in the atmosphere. This extra CO₂ traps more heat than would otherwise escape out into space. More energy is therefore entering the climate system than leaving it.



Climate change isn’t instant. Even if carbon emissions ceased today, ice caps would keep melting for decades. Bernhard Staehli / shutterstock

One argument against making drastic cuts to greenhouse gas emissions is that it will harm economies as our industries are still largely fossil fuelled. Responding to climate change needs to balance the desire to continue to grow economies today with avoiding disastrous climate change or prohibitively expensive remedies tomorrow.


By delaying significant carbon emission reductions we risk handing both an impossible financial and technological burden to future generations. Our children and grandchildren may be unable to understand how we negotiated such an arrangement on their behalf.