How low do we want the amount of Carbon Dioxide in the atmosphere?
Mike Kearsley - 24th November 2025
I am writing in the hope that my current confusion might be cleared up by the membership of the NZ Skeptics. This concerns climate change, and more specifically the need to reduce carbon emissions – the level of CO2 in the atmosphere.
You have mentioned this several times in recent newsletters and I have heard this mantra on many occasions in the past. Essentially, the mantra goes, we humans are hell bent on destroying the planet through our increasing CO2 emissions, and we need drastic change right now while we still have some time. However, the issue I have is that, when asked (and I have asked many times), no speaker has ever said what level they want CO2 concentrations to be reduced to. Where are we now? Where do we want to be? For me, this seemed a simple but rather important question. So, I checked it out – and it is then that my confusion even increased.
Now, we all agree that plants are very important and without plants on earth we all die. They need sunshine, water, nutrients and CO2. They give out oxygen, and they feed all living creatures on the planet, directly or indirectly. Humans have been farming plants for a long while, and we have worked out very well what are the optimum levels for healthy plant growth for all of the above inputs. Of course different plants have slightly different needs at different times etc – but there is a general consistency for most plant life.
If you check online there is a lot of information available, with some variation between the information given. However, the percentage of CO2 which is optimal for plant growth is commonly given as between 1000 and 1200 parts per million (ppm). Yes, it is that small (one thousandth part of the atmosphere). Levels above 2000 ppm can become toxic for some plants, but levels below 400 ppm will limit growth for most plants. Higher concentrations of CO2 allow plants to thrive at higher temperatures. A level lower than 400ppm is not beneficial, and below 200ppm plants will die off.
So, what is the current level of CO2 in the atmosphere? Again, estimates vary a little, but the most common level given is about 425ppm, and this had grown during the industrial age from an all-time estimated low of 280ppm – a growth of 145ppm in say 300 years? So, at 280ppm the concentration of CO2 was getting dangerously low, and the current level is clearly between a third and a half of what is said to be the optimum for plant growth. Further, the reports say that, in the past, CO2 levels have reached as high as 4000 ppm – some say even more than this - and life appears to have survived – indeed flourished.
So, if the figures are correct and we had made no improvements in burning fossil fuels and the like (i.e. we just carried on as we have done for most of the last 300 years of industrial activity) we would need to increase CO2 by over 700ppm just to reach the optimum concentration for plant growth. If it took 300 years to increase by 145ppm, we would thus need some 1400 years of the worst industrial pollution just to get to the optimum level of CO2 for healthy plant growth, and a further 2,000 years of the worst pollution to reach potentially problem levels for plant growth. Of course, there are more of us on the planet, but we have also been tackling pollution one way or another for quite a while. This does not seem to fit well with the scenario that is presented by climate doomsayers.
The final nail of confusion was the repeated advertising online for methods to add CO2 to your plants to facilitate healthy growth – including CO2 bags, pumped CO2 gas and various farming techniques – yes, farmers are actively adding CO2 to try to bring CO2 levels to nearer the optimum for plants.
So, can the members please enlighten me where I am going wrong with the figures presented – because all of these figures are what is commonly presented online.