land-resources.com

The Global Assessment of Land Degradation (GLADA)

A Global Assessment of Land Degradation, monitored from satellite imagery over a 22-year period, has recently been made. The results are unexpected -- to say the least! They cast doubt on widely-held assumptions about where soil and land degradation.

For nearly 30 years, the world has been dependent on the Global Assessment of Soil Degradation (GLASOD). This was an attempt to estimate the state of degradation in about 1990, resulting from the cumulative effects throughout the history of land use. It was based on subjective assessment, estimates by soil scientists who knew conditions in their countries.

GLADA differs in two respects. First, it measures recent trends in degradation, over the period 1981-2003. Secondly, it is based on an objective method, monitoring the Normalized Difference Vegetation Index (NDVI) obtained annually from satellite data, based on an 8 km grid. NDVI is well-established as a way of estimating net primary production by plants (NPP), or plant growth. The main climatic variables are allowed for by correcting the data for rainfall and temperature. Changes in land use could not be allowed for, because there is no time-series of data. Where adjusted NPP has fallen, this is assumed to be due to adverse soil changes, land degradatiton. Where it has increased, it results from land improvement.

For GLADA results see the Full Report, an account on the FAO LADA site, and a guide to country studies on the ISRIC site.

Results: where is degradation taking place?

GLASOD estimated degraded land, cumulative over the entire history of land use. GLADA measures degrading land, trends over the period 1981-2003. These two sets of results would not be expected to follow the same pattern. Even so, the results are highly unexpected.

Where is it widely believed that the state of the land is really bad? The Sahel (semi-arid) belt of Africa is one such area, where degradation is commonly referred to by the widely misused term 'desertification'. Another area is the Amazon rain forest, where widespread forest clearance is certainly taking place. More generally, it was supposed that drylands are the worst affected. Indeed, the present study was commissioned as part of the LADA project, Land Degradation in Drylands; the humid regions were included for comparison.

But the African Sahel and the Amazon rain forest largely show land improvement, an increase in climate-adjusted plant growth. So do most of the world's drylands. 78% of the currently-degrading land is in the humid regions -- not a finding expected by a study directed at drylands!

The most severely degrading areas are:

• The Central African (Congo) forest, and savanna areas to the east and south of it, including Transvaal and Natal in South Africa.
• South East Asia, both mainland and insular, including Indo-China, Malaysia and Indonesia, together with South China.

Other regions where plant growth has decreased include much of Mexico, the Argentinian Pampas, the high-latitude forest belt of Canada, and parts of the Great Dividing Range of Australia. As GLADA puts it, "the usual suspects", such as around the African Sahel, the Mediterranean and the Middle East, are only lightly affected. Most of India appears to have recent land improvement, although Bangladesh and Nepal show degradation.

The worst affected countries, with >50% of their territory degrading, are, in Africa the Congo, Zaire, Equatorial Guinea, Gabon, Sierra Leone, Zambia, and the most affected (95% degrading) Swaziland; in Asia, Myanmar, Malaysia, Thailand, Laos, Korea (N and S), and Indonesia; plus a surprise, New Zealand.

Pundits will want to debate the validity of these findings. As they stand, they challenge current notions and open up debate. The scientific cat is among the pigeons of long-held assumptions.

The authors of this study themselves pose the question, "Is the biomass trend indicated by GLADA real?", but do not attempt to answer this. They expect to set up a GLADA web site for discussion.

Another surprise: is rising atmospheric carbon dioxide increasing world plant growth?

There is another unexpected, and unintended, finding. Over the 22-year period, world plant growth has increased by 3.8%. Graphs of annual data show fluctuations with 2-5 year periods, but a steady upward trend. This trend is found for all six continents.

The authors of GLADA note this result, but refrain from commenting upon it. but there is an intriguing possibility. Could the rise in atmospheric carbon dioxide (CO2) be leading to faster plant growth? Plants grow by photosynthesis, taking up carbon dioxide and giving out oxygen. The benefits of adding supplementary CO2 to horticultural greenhouses are well known. A steady rise in CO2 has taken place over the period of the study.

Could we be witnessing a 'greenhouse effect' - in a different sense! - on a world scale? If this were the case, there might be the makings of a global self-adjusting mechanism, taking more CO2 out of the atmosphere. It could well be that we are not seeing this damping mechanism because at present it is swamped by rising emissions.

An earlier standard account, not taking account of the above results, is given in Chapter 7, Land Degradation, of Land Resources: Now and for the Future.

October 2008

* * * * *