Headlines yesterday heralded global “greening” as having slowed the rise of carbon dioxide in the atmosphere, as reported by a new study. The idea is that increased plant growth fertilised by higher CO2 levels is partly offsetting our greenhouse gas emissions.
This pause in carbon dioxide going into the atmosphere is great news, isn’t it?
It would be if there really were a pause. In fact, in 2016 the level of CO2 in the atmosphere is set to go up by a record amount. Levels are shooting up at the fastest rate recorded since measurements first began in the 1950s – and probably at their quickest rate for hundreds of millions of years.
Hold on, isn’t there a new study saying there is a pause?
Not quite. The study says there was a pause in the growth rate. By that, the researchers mean that although the amount of CO2 in the atmosphere has been increasing every year, the rate at which it is going up each year has not.
The annual increase in CO2 levels varies a lot, but in the 1950s it averaged less than 1 part per million (ppm) a year. It increased to around 2 ppm in the early 2000s as CO2 emissions soared. Then between 2002 and 2014 the annual rise remained fairly constant, at just over 2 parts per million.
So CO2 is rising each year, but that amount is not increasing?
Er, almost. That was the case up until 2014. That’s another confusing thing about this study: the pause in the growth rate already appears to be over.
This year we are on track for the largest ever jump in atmospheric CO2 levels, of about 3.15 ppm. Total CO2 is now above the 400 ppm mark and it could be hundreds or even thousands of years before levels fall below this concentration again.
OK, but why did the growth rate pause between 2002 and 2014?
Not because of us, unfortunately. Emissions from human activities are rising again after falling slightly following the 2007 financial crisis. Instead, land plants took up even more CO2 than usual between 2002 and 2014.
We already knew that plants and the oceans soak up around half the extra CO2 we emit. The new study suggests that plants sucked up more than normal between 2002 and 2014.
And that is good news, isn’t it?
Yes, because CO2 levels would otherwise be even higher now. Unfortunately, plants are not going to continue to soak up ever more CO2 in the future.
Why won’t plants save us?
There are two main things going on. Firstly, most plants thrive with more CO2 in the atmosphere, so there has been an overall greening of the planet as levels rise. But plant growth will not continue to rise as CO2 levels keep climbing: experiments with plants grown at high CO2 levels show this fertilisation effect tails off.
Secondly, rising CO2 levels lead to higher temperatures, which increase the rate at which plants burn energy, or metabolise. When it’s warmer, plants “breathe out” more CO2.
Between 2002 and 2014, CO2 levels rose fast but global temperatures did not, so the CO2 fertilisation effect dominated. Now global temperatures are shooting up again, we are likely to see a period where plants take up less CO2 than they have been.
In the long term, the worry is that plants and the oceans will not keep on soaking up ever more CO2. If that happens, carbon dioxide levels could climb rapidly even if we start to get emissions under control.
Journal reference: Nature Communications, DOI: 10.1038/NCOMMS13428
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