There is now little doubt that anthropogenic greenhouse gas emissions are causing the global temperature to rise and if emissions continue to grow at their current rate, the consequences for humanity could be catastrophic. The international community appears to be reaching agreement on the importance of taking action to limit the global temperature rise to 2°C above the pre-industrial level and have acknowledged that failing to do so will risk potentially irreversible damage to the environment and the climatic system.
Evidence suggests that even with a determined effort to meet this 2°C target, it will be hard to achieve without seriously impairing global economic growth. This is something the international community is unlikely to agree to. If the global temperature exceeds the 2°C target, models of the climate system predict that natural processes will not significantly lower the temperature for thousands of years. On this timescale, slower climate feedback processes like the melting of the planet's ice sheets will come into play, possibly causing the global temperature to rise further. Under these circumstances even 2°C of warming may not be safe.
It is argued that the only safe and permanent option available to reverse a dangerous increase in the global temperature is the direct capture of CO2 from the atmosphere. If the atmospheric CO2 concentration is reduced quickly once it has peaked, the oceans will act as a huge heat reservoir, cooling the atmosphere and causing the global temperature to fall. To ensure that the direct CO2 capture process brings about this rapid decline in the global temperature the atmospheric CO2 concentration will have to be reduced within a relatively short timeframe, perhaps by the end of the century.
Biological direct capture relies on plants or algae to capture CO2 from the atmosphere by photosynthesis and then store it, usually as biomass. It is estimated that combining all of the available methods of biological direct capture, a CO2 drawdown of between 50ppm and 100ppm could be achieve by the end of the century.
Chemical direct capture would involve building a man-made device to chemically capture CO2 from the atmosphere. This technology is still in the development phase, but it is estimated that, with a successful research program, it could achieve a CO2 drawdown of around 100ppm by the end of the century. However, this option may be prohibitively expensive.
Combining all of the available methods of direct CO2 capture and using them in conjunction with a strict emissions reduction regime, it may be possible to return the atmospheric CO2 concentration to 350ppm by the end of the century. If only biological direct capture methods are used, this reduction is likely to take until the middle or end of the next century. If used proactively, a program of direct CO2 capture has the potential to prevent warming and help make the 2°C target more achievable.