An international team of 18 scientists presents a couple of interesting and contrasting forest figures in their publication in Science.

Whenever you hear someone say ‘the world’s forests are a net carbon sink,’ you know something is out of balance. They shouldn’t be. In a natural equilibrium state, built around a stable climate, the atmospheric carbon cycle should be closed*. For forests this means growth of leaves, twigs and trunks equals decomposing biomass and CO2 uptake should be close to natural CO2 emissions.

Lots of carbon, enormous amounts of CO2

So when research by 14 different scientific institutions on 4 different continents indicates forests have between 1990 and 2007 stored 40.8 gigatonnes of pure carbon, or some 150 gigatonnes of CO2, you would hope to understand why – because this is substantial.

The mean annual rate of net absorption, the researchers say, is 2.4 gigatonnes of carbon, which translates to almost 9 gigatonnes CO2. If that annual average had also occurred [we don’t know yet] in record emissions year 2010 forests would have compensated almost 30 percent of our fossil fuel CO2 emissions.

Is it the boreal forests?

Apparently forests have switched to a higher gear and have grown higher, or denser, with growth temporarily outpacing biomass die-off. We are reminded of the soil nutrients negative forest-climate feedback and the temperature negative forest-climate feedback – so we would expect this takes place at high latitudes, the Russian taiga, Canadian boreal forests, in the northern US, in Scandinavia.

But we are mistaken, the researchers say. They found our planet’s vast boreal forests have ‘only’ accounted for 22 percent of carbon sequestration between 1990 and 2007. In fact they explicitly warn the boreal carbon reservoir is unstable, as it is threatened by positive climate feedbacks, like pine beetles, droughts and wildfires, all of which can substantially contribute to CO2 emissions.

More rain, more forest?

So something magical must be going on in the tropics. Magical to us, because we only ever think in climate feedbacks – and can’t imagine rainforests benefitting from higher temperatures. But, we theorise, they could of course benefit from an increase in the Earth’s general circulation.

The same phenomenon that increases the risk of droughts in the subtropics could intensify annual precipitation around the equator. And of course rainforests do like their rain. Strange thing though we witness the exact opposite happening in the Amazon. Perhaps the researchers have not done such a good job weighing in the entire nineties in their data range. What would results have looked like if they had focused on the naughties, including the record Amazon drought years of 2005 and 2010?

From forests to deforestation

Anyway, before you cheer at a hidden negative climate-forest feedback in the tropics, please remember burning fossil fuels is not the only way we disturb the Earth’s carbon cycle. There is land use change too, which is a far bigger carbon player in the tropics than at other latitudes

The bad news of course is tropical deforestation. Carbon emissions from this are enormous: 2.9 gigatonnes of pure carbon per year, averaged over 1990 to 2007. That is 10.7 gigatonnes of CO2, or [comparing to 2010 emissions] adding 35 percent to the CO2 emissions from burning fossil fuels.

The big forest equation

Fortunately deforestation is not the same thing as net deforestation. There is tropical afforestation too. The researchers estimate this is an annual carbon sink of 1.6 Gt carbon. The net carbon balance of tropical deforestation would therefore amount to emissions of 1.3 GtC per year, or 4.8 Gt of CO2.

Comparing to fossil fuel emissions of a couple of years ago this translates to roughly 20 percent, an often-cited number. Comparing to 2010 fossil emissions the percentage has declined – to [depending on how you prefer to express relative numbers] either adding 16 percent CO2 emissions, or amounting 14 percent of total CO2 emissions. (This decline of relative deforestation emissions is not a result of deforestation being tackled, but of continued growth of the world’s energy-related emissions.)

Forests are good

Altogether, according to the Science publication, even including net deforestation, the world’s forested areas would still – over 1990 to 2007 – have been a net carbon sink, of 1.1 gigatonne of carbon, or 4.07 gigatonnes of CO2, compensating almost 14 percent of 2010 energy emissions.

That is good news, but with quite a margin of uncertainty. Ignoring the rules of statistics and simply summing up the standard deviations of net deforestation (±0.7 GtC/y) and natural forest carbon absorption (±0.4 GtC/y) we could get to exactly the same number as above: 1.1 GtC.

But in a way the whole debate about whether forests are or are not a net sink is rather irrelevant. As shown above, the numbers run in the billions of tonnes of carbon, cycling through the forest systems each year.

From this we can simply conclude forests are of enormous importance to the Earth’s carbon cycle and climate – and having more of them is good, having less is bad.

[*) The natural forest carbon cycle is just a bit different to be more precise. The atmospheric cycle isn’t our planet’s only carbon cycle. Just like in the oceans, on land too there is also a deep, geological cycle, the one we are disturbing by digging up and burning all that fossil carbon. In a way forests are the gatekeepers of this cycle. In structural basins they provide the carbon to sequester in first peat and then coal and other forms of lithified carbon. From this we can conclude forests are indeed, in the natural state, a net carbon sink – although that sink is not larger than the natural emissions from the terrestrial geological carbon cycle – mostly places where tectonic forces push coal layers back the surface, where the fossil carbon oxidises, very slowly.]

© Rolf Schuttenhelm | www.bitsofscience.org