In a previous post, I raised the question how best to measure ‘Nature’, arguing the benefits of an overall Index including biodiversity, habitat diversity, human usage value and sheer volume of living organisms, biomass. Here a look is taken at whether biomass has been increasing or decreasing in recent decades.
The appeal of biomass as a measure of Nature is that it abstracts from anything innately human. Hence, if you asked an alien who has no innate affinity to any particular species to define the state of Earth’s ‘Nature’ then I can well imagine that alien defining it by biomass, which is the total mass of all life on earth. The trees, the plankton, the humans, the fungi, the fish, etc. Biologists talk of dry biomass, wet biomass, biomass with or without bacteria (who are hard to count), etc. One can argue that the more of all that there is, the more of ‘Nature’ there is. Biomass is then one measure of how much total ‘Nature’ the earth produces.
Now, the key thing of interest is the change in biomass: how much new life gets produced per period as the climate changes. The closest measured equivalent of this notion of Nature is ‘net primary production’ of biomass, which is the rate of new biomass produced. The question of how to divide biomass production into the bits benefitting this or that species is then a matter of inter-species politics that our imaginary alien might well just not be interested in. A living cell is a living cell, no matter what it is used for, the alien might well muse. So what if the humans use most of it for their benefit? To the victor the spoils the alien would argue.
By the way, humans constitute no more than 0.1 billion tonnes of dry biomass, making up only about 1 in ten-thousand of each kilogram of biomass on the planet. Our mass according to some estimates is half that of cattle or Antarctic krill, and only a third to one-thirtieth that of all ants. Clearly, we are not yet really on top of the inter-species biomass league!
So, what do we know about net primary production (NPP) over time? Well, probably the best study before 2000 (after which we have more comparable satellite data) is the 2003 Science study by Nemani et al. They essentially tried to count as much as they could what was happening to forests, wetlands, tundras, etc.
As their beautiful picture above indicates, most of the earth has seen a net increase in the 1982-1999 period (in green) and only a few areas saw a decrease (in brown). As one can see, the 1982-1999 period was good for Australian biomass production, as well as that of nearly all highly inhabited areas in the world. If you care about things that depend on our biomass production, such the potential for food production, then this is a very encouraging picture. If this was all you knew, you would want this trend to continue.
What about after 2000? Again the best evidence I could find comes via the Science magazine, albeit this time in response to an estimate. Zhao and Running in 2010 published an article talking about how drought in the Southern Hemisphere probably lead to a reduction in NPP there, counter-balancing the increases in the Northern hemisphere.
The problem with their study was that it was not actually based on data but on computer simulations. This opened the door to accusations of rigging and the reply posted a year later by a team of 6 scientists headed by Arindam Samanta said of this study that “the small trends, regional patterns, and interannual variations that they describe are artifacts of their NPP model. Satellite observations of vegetation activity show no statistically significant changes in more than 85% of the vegetated lands south of 70°N during the same 2000 to 2009 period.”
So where does this controversy, which still rages on and on, leave us in terms of best-guess NPP trends? Well, the picture below is the one produced for the 2000-2009 period by the 6 scientists basing themselves on as much data as they could find, mainly from satellites. It shows winners and losers, with central Australia this time a loser, but on a global level it is pretty much ‘no trend’. If anything, the consensus appears to be of an increase in NNP up till 2003 at least.
So, our imaginary alien would have to conclude that the climate change observed in this period had been good news for ‘Nature’ until the 2000s, after which there have been some Northern hemisphere winners and losers in Argentina and Australia, but no clear global trend on the surface.
As to the oceans, there are recent signs of greater net production in the Arctic (see http://www.agu.org/pubs/crossref/2011/2011JC007151.shtml) but also signs that ocean productivity as a whole is slightly reducing though it appears to be notoriously difficult to guess trends in the productivity of the oceans given how deep they are and how large areas are covered by ice. Generally speaking, cold water is able to hold more CO2 and is thus generally more life-abundant which would lead to less NPP as the oceans warm, but the major issue with oceans is the availability of nutrients in the water, not temperature, which is why warm coral reefs are full of life and the cold deep ocean is not. The amount of nutrients in turns is a complicated thing depending on currents and river systems and a whole host of other stuff, so we are awaiting better global data to tell us where the trend in ocean NPP is.
In conclusion, biomass production at the moment seems to be holding up for the world as a whole, at least to the extent that it is measurable. For an imaginary alien who cares nothing for species esthetically pleasing to humans and who would be solely interested in current ‘production’, this would mean Nature is not doing so badly.
Unlike the imaginary alien, I do think diversity of species and habitats is something worthwhile and even that some diversity is better than other diversity (diversity in higher order mammals is surely of greater aesthetic value to us than diversity in something like see-dwelling bacteria). So in a more composite index of Nature it would surely not be doing so well.
As to future trends in biomass, we will have to see if we are currently at the top of the mountain or merely at the top of the foothills in terms of world biomass production.