África reverdece desde hace 20 años

'Por la influencia del aumento del porcentaje de CO2 en la atmósfera, 

podemos esperar disfrutar de climas más ecuánimes y mejores' 

Arrhenius, padre de la teoría del efecto invernadero

Estas fotos las tomé en junio de 2008, ahora hará nueve años, desde una avioneta que sobrevolaba el desierto de Namibia, entre Sussusvlei y Walvis Bay, tras haber subido a las espectaculares dunas rojizas. Me sorprendió la cantidad de vegetación que, como una erupción de acné, salpicaba la mayor parte de las dunas que se divisaban en una extensión de miles de kilómetros cuadrados. Se lo comenté al sudafricano que pilotaba la Cessna y me dijo que, desde hacía unos años, el desierto estaba reverdeciendo pero que desconocía las causas.

Hasta 2012, que yo sepa, nadie se hizo eco del fenómeno. El primero en hacerlo fue, al parecer, Ranga Myneni de la Universidad de Boston en un par de conferencias en las que concluyó que en 30 años se había producido un aumento de un 14% de la vegetación en todo el planeta Tierra. El año siguiente, 2013, el Wall Street Journal publicaba el primer artículo sobre el tema en un medio de comunicación, firmado por Matt Ridley.

Ni decir tiene que se desencadenó una gran polémica. Y después el silencio. O casi. Hasta que ayer, via Plaza Moyua, encontré otro par de artículos sobre el tema. Uno, titulado 'África reverdece desde hace 20 años' publicado en Science Nordic -del que reproduzco un extracto a continuación-, y otro titulado 'Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa', publicado en Nature.

SCIENCENORDIC.- In Africa, a fight is happening. On one side natural forces are making the continent greener, and on the other, people are removing trees and bushes from the continent.

In densely populated regions, people are cutting down trees and forests, but elsewhere, where human populations are more thinly spread, bushes and scrub vegetation are thriving.

Now, scientists have quantified for the first time how vegetation across the continent has changed in the past 20 years.

Thirty six per cent of the continent has become greener, while 11 per cent is becoming less green.

The results show that not all is lost for Africa’s nature, say the scientists behind the new research.

“Our results are both positive and negative. Of course it’s not good that humans have had a negative influence on the distribution of trees and bushes in 11 per cent of Africa in the last 20 years, but it doesn’t come as a complete surprise,” says co-author Martin Brandt from the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Denmark.

“On the other hand it’s not all negative as an area—three times larger than the area where trees and bushes are disappearing—is becoming greener, which is positive, at least from a climate point of view,” he says.

The new study is published in the scientific journal Nature Ecology and Evolution.

África, pues, reverdece. Pero no solo África. Reverdece todo el planeta. Lo expuso el periodista y divulgador científico Matt Ridley el 17 de octubre de 2016 ante la conferencia anual de la Global Warming Policy Foundation en la Royal Society de Londres, tras años de ser vetada su presencia en la misma. Y esto es lo que dijo:

I want to talk about global greening, the gradual, but large, increase in green vegetation on the planet.

I think this is one of the most momentous discoveries of recent years and one that transforms the scientific background to climate policy, though you would never know it from the way it has been reported. And it is a story in which I have been both vilified and vindicated.

In December 2012, the environmental scientist Jesse Ausubel of Rockefeller University drew my attention to a video online of a lecture given by Ranga Myneni of Boston University.

In this and a subsequent lecture Myneni presented ingenious analysis of data from satellites proving that much of the vegetated area of the planet was getting greener, only a little bit was getting browner, and that overall in 30 years there had been a roughly 14% increase in green vegetation on planet Earth.

In this slide he argued that this was occurring in all vegetation types – tropical rain forests, subarctic taiga, grasslands, semi-deserts, farmland, everywhere.

What is more, Myneni argued that by various means he could calculate that about half of this greening was a direct result of rising carbon dioxide levels in the atmosphere, rather than the application of agricultural fertiliser, irrigation, warmer temperatures or increased rainfall.

Carbon dioxide, along with water, is the raw material that plants use to make carbohydrates, with the help of sunlight, so it stands to reason that raising its concentration should help plants grow.

I was startled by Myneni’s data. I knew that there had been thousands of so-called free-air concentration (FACE) experiments, in which levels of CO2 had been increased over crops or wild ecosystems to find out if it boosted their growth (it did), and that commercial greenhouse owners now routinely maintain CO2 levels in their greenhouses at more than double ambient levels – because it makes their tomatoes grow faster.

But the global effect of CO2 levels on the quantity of vegetation had not, as far as I could tell, been measured till now.

Other lines of evidence also pointed to this global greening:

* the increased rate of growth of forest trees,

* the increased amplitude of seasonal carbon dioxide variation measured in Hawaii and elsewhere,

* photographic surveys of vegetation,

* the increased growth rate of phytoplankton, marine plants and some corals, and so on.

I published an article in the Wall Street Journal in January 2013 on these various lines of evidence, including Myneni’s satellite analysis, pointing to the increase in green vegetation.

This was probably the very first article in the mainstream media on the satellite evidence for global greening.

For this I was subjected online to withering scorn by the usual climate spin doctors, but even they had to admit I was “factually accurate”.

Six months later Randall Donohue and colleagues in Australia published a paper using satellite data to conclude that the arid parts of the planet, such as western Australia and the Sahel region, had seen a net greening of 11% over 30 years – similar results to Myneni’s.

Myneni’s results, however, remained unpublished. I was puzzled by this. Then I realized that one of the IPCC’s periodic assessment reports was in preparation, and that probably Dr Myneni and colleagues might delay the publication of their results until after that report was published, lest “the skeptics have a field day” with it. (The team says there was other reasons for the delay.)

That last phrase, by the way, is from one of the Climategate emails, the one on 22 September 1999 in which Dr Michael Mann approves the deletion of inconvenient data.

Sure enough, Myneni’s results were eventually published three years later in April 2016 in a paper in Nature Climate Change, with 32 authors from 24 institutions in eight countries – when the IPCC report was safely in the public domain and the great Paris climate jamboree was over.

His results were now even stronger than he had concluded in his 2012 lecture. Now he said that 70% of the cause of greening was carbon dioxide – up from half.

As Myneni’s co-author Zaichun Zhu, of Beijing University, puts it, it’s equivalent to adding a green continent twice the size of mainland USA.

Frankly, I think this is big news. A new continent’s worth of green vegetation in a single human generation.

At the end of 2015, when his paper had been under peer review for eight months so he knew these results were coming, Dr Myneni, criticized me specifically, saying on a green blog that “[Ridley] falsely claims that CO2 fertilisation is responsible for the greening of the earth”. Yet a few months later he himself published evidence that “CO2 fertilisation explains 70% of the greening trend”.

In the press release accompanying the article in April 2016 he once again referred to me by name:

“The beneficial aspect of CO2 fertilization in promoting plant growth has been used by contrarians, notably Lord Ridley…to argue against cuts in carbon emissions to mitigate climate change…"

As Richard Tol commented: “The new paper vindicates what Matt Ridley and others have been saying all along — yet they apparently deserve to be kicked nonetheless.”

I wrote to Dr Myneni politely asking him to justify his criticism of me with specific examples. He was unable to do so. “There are no ‘up-sides’ to having too much CO2 in the air,” was all he said.

In the very same issue of the same journal was another paper from an international team about a further benefit of global greening, which concluded that CO2 fertilisation is likely to increase crop water productivity throughout the world, for example by up to 48% for rain-fed wheat in arid areas, and that “If realized in the fields, the effects of elevated [CO2] could considerably mitigate global yield losses whilst reducing agricultural consumptive water use (4–17%).”

Their chart shows that without CO2 fertilisation, crops will become more water-stressed during the current century; with it they will become LESS water-stressed.

These are huge benefits for the earth and for people. The CO2 fertilisation effect is already worth trillions of dollars, according to detailed calculations by Craig Idso.

At this point Steve McIntyre of Climate Audit drew attention to my vindication on twitter. Richard Betts, the Met Office’s twitter frequenter, protested that global greening was well known and had been referred to in the IPCC’s report.

This was misleading at best. The Summary for Policy Makers of Working Group 2 refers to global greening through carbon dioxide fertilisation not at all. The full report of WG2, published six months after the Summary for Policy Makers in that reprehensible fashion so beloved of the IPCC, does very gently hint at there being some evidence of greening, but in a dismissive way, and far too late to catch the attention of journalists. These are the only mentions I could find:

“Satellite observations from 1982–2010 show an 11% increase in green foliage cover in warm, arid environments…Higher CO2 concentrations enhance photosynthesis and growth (up to a point) and reduce water use by the plant…these effects are mostly beneficial; however, high CO2 also has negative effects.”

“In summary, there is high confidence that net terrestrial ecosystem productivity at the global scale has increased relative to the preindustrial era. There is low confidence in attribution of these trends to climate change. Most studies speculate that rising CO2 concentrations are contributing to this trend through stimulation of photosynthesis but there is no clear, consistent signal of a climate change contribution.”

The main text of Working Group 1 contains an even briefer statement:

“Warming (and possibly the CO2 fertilisation effect) has also been correlated with global trends in satellite greenness observations, which resulted in an estimated 6% increase of global NPP, or the accumulation of 3.4 PgC on land over the period 1982–1999 (Nemani et al., 2003).”

If that’s a clear and prominent statement that carbon dioxide emissions have increased green vegetation on the planet by 14% and are significantly reducing the water requirements of agriculture, then I’m the Queen of Sheba.

Back in 1908 Svante Arrhenius, the father of the greenhouse theory, said the following:

“By the influence of the increasing percentage of carbonic acid in the atmosphere, we may hope to enjoy ages with more equable and better climates.”

It appears he was not wrong.

Leer el texto completo de la conferencia de Matt Ridley 'Reverdecimiento global frente a calentamiento global'