The Paris Climate Agreement envisages limiting global warming to 1.5 degrees Celsius. This triggers the urgent need to scale back carbon dioxide emissions and drive greenhouse gas emissions down to nearly zero by 2050. The main means for achieving this are phasing out fossil fuel such as coal and oil and switching to renewable energy. To hit the climate targets set for 2050, most secnerios in climate research identify the additional need en route to net-zero emissions to actively withdraw carbon dioxide from the atmosphere and put it in permanent storage. A recent study by Canada’s Simon Fraser University has taken a closer look at the efficacy of such measures.
Removing carbon dioxide from the atmosphere is a process referred to as ‘carbon capture’ and often paraphrased as ‘negative emissions’. There are various ways to go about this, including extracting carbon dioxide directly from the exhaust gases of industrial processes and placing it in long-term storage, e.g. in underground caverns. Indirect measures like large scale tree planting, which stores carbon naturally, are another option.
So far, scientists have worked on the assumption that the greenhouse gas effect of releasing carbon dioxide could be neutralised by capturing the same amount of carbon dioxide afterwards. However, a study by a research team headed by physicist Kirsten Zickfeld from Simon Fraser University in Canada reveals that this assumption may be false. Zickfeld and her team’s project involved examining the impact of positive and negative carbon dioxide emissions on the atmosphere based on the simulation of large amounts of sequestered carbon in an earth system model.
This delivered a clear verdict: The simulation proves that the earth system displays varying reactions to emissions with matching withdrawals of carbon dioxide. Carbon dioxide released increases levels of this greenhouse gas in the atmosphere more than a withdrawal of the same amount reduces them. This means that the atmosphere’s reaction to the stimulus provided by positive and negative emissions is asymmetrical. In addition, the greater the amount of carbon dioxide released, the greater the differences in reaction.
Another conclusion was that emissions of carbon dioxide coupled with matching withdrawals have effects on the climate that differ from those of a direct reduction of emissions by the same amount. What this means in concrete terms is that, to neutralise emissions, a greater amount of carbon dioxide must be captured than emitted in the first place. Therefore, capturing greenhouse gases is less efficient than reducing carbon dioxide emissions directly from the outset.