Another forest study points to negative climate feedback

And this one may really present some good news for the climate, as it isn’t a Petri dish case, but very much in vivo. But don’t get overexcited.

A group of Finnish researchers studied forest sites in 68 nations and found net carbon sink increasing, without forest area expansion, they write in PLoS ONE.

Especially in colder northern regions a temperature rise may [initially] promote tree growth and forest density, as illustrated by the photo comparison of a Finnish boreal forest site with visably lower tree height in the left image, from 1893. The right picture (1997) shows the same spot more than a century later, with (presumably) increased biomass carbon storage. Image derived from PLoS ONE publication.

The researchers best studied forests in the United States, where they had access to detailed information provided by the Forest Service (dating back to 1953) – and also made use of an FAO inventory that is mostly based on remote sensing of forested areas (over 1990 to 2010) in various lands and continents.

Northern forests improving

It shows forest quality is clearly improving in the US. Total forest area over the examined period grew by just 1 percent, but forest density, [sort of] equalling carbon storage, increased 51 percent. Just how much carbon was sequestered remains to be researched. Also in similar high-latitude northern hemisphere forests the density clearly increased, for instance in Europe.

Tropical forests

Globally the picture is less clear cut. Indonesian forest degradation over the last decennia drove the entire Asian forest carbon balance into a negative state. Meanwhile countries in Africa and South America saw a slight increase in forest density, although continued deforestation led to a negative carbon balance there too. Altogether the forest density increased in 45 of 68 nations [so it decreased in 23].

Climate feedbacks?

Just how much of the carbon uptake of forest density increases should actually be contributed to a climate feedback is also unclear. In tropical regions it is very unlikely temperature rises will promote any tree growth. Locally a precipitation increase around the ITCZ seems an option, although such theories easily evaporate when confronted with recent news on Amazon droughts, which last year led to a negative carbon balance of 8 Gt of CO2, adding an extra 25 percent to 2010’s CO2 emissions record, and that -talking of remote sensing- is indeed still visible from space.

In northerly regions a negative climate feedback is quite possible, as earlier springs and later autumns could increase the duration of the growth season.

Second biological winter

This may well be a short-lived phase though. Spring and autumn may favour additional biomass growth – but at the peak of summer, when you would least expect it, nature may well add a ‘second winter season’ to the biological year, with an exhaling carbon flux, as research showed this happened during several of the 21^st century hot years. [According to French ecologist Philippe Ciais the consecutive heat waves of the West European hot summer of 2003 not only killed 50,000 people, but also led to the estimated release of 0.5 Gt of pure carbon (1.8 billion tonnes of CO2) from organic soils, crops and forests.]

With summer temperatures in these regions increasing dramatically over the next decades [Stanford research released yesterday, publication upcoming in Climatic Change] and adding the risk of raging wildfires, it is difficult to imagine the Canadian boreal forests or the Siberian taiga will be able to do much to save the climate – if they will not present themselves as positive feedbacks anyway.

Pine beetles

In 2008 Canadian researchers in Nature suggested CO2 emissions associated with the pine beetle outbreaks – a direct result of milder winters – will amount to 270 megatonnes of CO2 up to the year 2020. That outweighs the entire fossil fuel carbon emissions reduction target of the Canadian government over the period of 1990 [base year Kyoto] to 2020 [focus new UNFCCC treaty].

This is to make two points: it is hard to see any good climate news within the complete carbon balance context of the Earth. Forests’ carbon behaviour is complex – and has the potential to dwarf us when we keep engaging a planetary challenge small scale.