There must be something in the air in the spring, because it seems to be the time of year when I gain the energy to review a bit of GW science. It is almost exactly a year ago that I wrote briefly about how difficult it is going to be to prevent dangerous climate change (CO2 > 450ppm) if we don’t increase the amount of carbon stored in the terrestrial biosphere (shorthand: “forests”).
I’m in the process of preparing a presentation provisionally titled “Save the Forests: Fixing Global Warming for Dummies”. So I suppose it is serendipitous that my New Scientist magazine (dated 4th April 2009) fell open a couple of hours ago at a Fred Pearce article titled “Keeping the planet’s heart pumping“. I say I “suppose” it is serendipitous, because the article presages some of the ideas I was going to include in my presentation. I guess the reinforcement of my point by the publication of this article outweighs the reduction in its originality.
I’ve started to get a little ratty when anyone suggests that reforestation may be an ineffective policy. The problem is that many people realise that carbon offsetting is a sham. But the principle that we should preserve and increase the area of natural forest and preserve its integrity is absolutely correct. Right policy, wrong financial instrument (and in the case of monoculture plantations, poor execution). I intend to go into this point in more detail, and even have a title for the blog post (I’m telling you now in case I never get round to this one): “Don’t throw the forest out with the trees!”. Play on words is for children. Real men play on idioms. And eat quips!
Gorshkov and Makarieva point out that forests generate rising air (low pressure) not just because they are dark (absorbing heat, expanding air, making it less dense and causing it to rise) but also because of what they call the “biotic pump”. That is, the trees pump moisture into the air (cooling themselves) which condenses at higher altitude. When the resulting water drops fall through the air column (my interpretation) – even to the ground as rain – the airmass becomes less dense and rises. Condensation not only reduces volume, but also releases heat, again causing airmasses to expand and rise. Rising air draws up air below it and other air rushes in from the sides and even from above. This process happens on (within reason) all scales of airmass. The effect can be clearly seen in billowing cumulus clouds. The early part of “A Cloudspotter’s Guide” describes the experience of a parachutist in a storm cloud, alternately falling and being carried up in rising pockets of air within the cloud. I don’t see how this could be explained without something like the “biotic pump”.
What really strikes me about the article, though, is that NS reports that Makarieva claims that:
“Nobody has looked at the pressure drop caused by water vapour turning to water.”
And the article – written, remember, by Fred Pearce, who has been reporting environmental issues and GW in particular for decades – goes on to note that:
“…because forest models do not include the biotic pump, it is impossible to say what wiping the Amazon off the map would mean for rainfall worldwide.”
I’ve recently been wondering whether our understanding of the climate is quantitatively strong, but qualitatively weak. Too much reliance on those computer models – remember, it’s garbage in, garbage out.
Now, the climate and weather models should be foolproof because they are held to rely on the laws of physics. But if they fail to capture accurately the process of lowering of air pressure due to the condensation of water vapour they could, I suppose, be systematically in error.
Even if this mechanism is implicit in the models, and it’s just the humans who fail to recognise it (quite feasible if the models correctly implement the laws of physics), they definitely fail (because they don’t implement feedbacks from climate to vegetation) to capture the positive feedback that causes forests to spread across continents. That is:
1. Moist forests create low pressure air masses (the rising air may directly result in rainfall over the forest and surrounding areas, in particular inland);
2. Drawing in moist air from the ocean (hence the importance of coastal forests emphasised by Gorshkov and Makarieva);
3. Creating airflow (at least seasonally) from the coast;
4. Providing rainfall to maintain and increase the area of the forest.
So, once established, a rainforest is self-sustaining, and indeed will tend to grow until it fills the continent at least over a latitudinal band or some other process or natural obstacle (e.g. mountain range) keeps it in check. Deforestation creates the reverse feedback. Once a tipping point is reached, the drying-out of a forest may become unstoppable.
I find it hard to believe that Gorshkov and Makarieva’s idea is new. Indeed, some commenters on the NS article note antecedents, notably something called the Permaculture movement, a 1970s idea of Bill Mollison and David Holmgren, though the “biotic pump” doesn’t seem at first glance to be central to the Permaculture philosophy. But NS also reports “that current theory doesn’t explain clearly how the lowlands in continental interiors maintain wet climates.”
I’m rather puzzled, since I’d always assumed that this mechanism explained cloud formation, storms, hurricanes, monsoons and why there is no forest in North Africa and air pressure there is predominantly high. I thought the problem was communication, or rather the lack of it, by the scientists. If Fred Pearce’s article can be taken at face value, it seems that the problem may instead be one of understanding, or rather the lack of it.