9

BBC states the following

Approximately 12 million hectares (46,000 sq miles) are lost around the world each year as a direct consequence of drought and desertification. That's the equivalent of 2,000 American football fields every hour. To put that into context, if those fields were lined up end to end, you would have to drive at 130 mph (210km/h) just to keep pace with desertification's spread.

The BBC article refers to an article by the UN, which in turn refers to a broken link by the UNCCD (United Nations Convention to Combat Desertification).

If the rate of desertification and drought were true, a landmass equivalent to the Amazonas rainforest would be lost within 70 years if the area of the rainforest is assumed as 5.5km^2. (5.5 x 10^12)m^2/(91.44m x 48.8m x 2000 x 24 x 365))

Is there any factual evidence that can support the claim that BBC made?

9
  • Here is the correct link for the UNCCD source: www2.unccd.int/actions/…
    – LShaver
    Jun 29 at 14:19
  • 1
    Following the train all the way down. UNCCD links this paper which references this brochure which would appear to be the source of the "12 million hectares" figure, without a citation. A case of citogenesis?
    – LShaver
    Jun 29 at 15:02
  • Makes me wonder about their definition of "lost" etc. Would agriculture land turned into wild forest be "lost"? If not, why is land turned into desert "lost"? I'm sure the various desert creatures don't consider it lost.
    – pipe
    Jun 30 at 5:21
  • Seems bizarre the figure they quote leads back to a brochure published in 2001! Are there no more recent studies?
    – Showsni
    Jun 30 at 9:37
  • The wikipedia article on desertification cites 650,000 km^2 over 50 years or around 13,000km^2 per year just for the Sahel zone and a further 3,600km^2 per year for the Gobi desert. That is less but a comparable order of magnitude. So the general gist of hundreds of football fields per hour seems correct.
    – quarague
    Jun 30 at 10:59

1 Answer 1

5

According to Nature Communications July 2020:

Globally, of the 44.5 million km^2 of drylands, 6% of these areas experienced desertification (i.e., significant negative change in NDVImax), 41% showed significant greening (i.e., significant positive change), and 53% had no significant change between 1982 and 2015 (Fig. 1a). The mean (±1 SD) of the area-weighted dryland vegetation change, as represented by the change in NDVImax was 0.031 ± 0.053. We estimated the scale of desertification to be 2.70 million km^2, which is significantly below a previous estimate of ~10.5 million km^2 over the same region, but over a different time window (1982 and 2003)^1. A large part of this discrepancy can be attributed to climatic differences in the end dates of the studies (2003 vs. 2015), with increased rainfall over regions including the Sahel and India.

(NDVI means "Normalized Difference Vegetation Index")

So overall there was more area with an increase in vegetation, not a decrease.

Counting areas with decreased vegetation only, and disregarding the much greater area with increased vegetation, 270 million hectares had less vegetation 33 years later, about 8.2 million hectares per year.


Note: A commentator below seems astounded by the well know fact that there is currently negative net desertification and wants to see additional references:

According to Desertification published by the International Panel on Climate Change (IPCC):

the drylands are greening on average

enter image description here

Fig 3.6 from the IPCC publication shows increased vegetation areas in shades of green and decreased vegetation areas in red, 1982-2015. They grey areas are areas to which the UN definition of desertification ("land degradation in arid, semi-arid and sub-humid areas") does not apply.

See also Impact of CO2 fertilization on maximum foliage cover across the globe's warm, arid environments Geophysical Research Letters (Volume 40, June 2013, pages 3031-3035):

The increase in water use efficiency of photosynthesis with rising [atmospheric CO2 level] has long been anticipated to lead to increased foliage cover in warm, arid environments ... and both satellite and ground observations from the world's rangelands reveal widespread changes toward more densely vegetated and woodier landscapes ... Our results suggest that [atmospheric CO2 level] has played an important role in this greening trend and that, where water is the dominant limit to growth, cover has increased in direct proportion to the CO2-driven rise in [water use efficiency of photosynthesis].

5
  • 1
    1. This reads contradictory to the common narrative (cf also the very title of the study you linked to; you should fully that here with its title) 2. This is 1 study you present as "the authoritative last word on this"? How good is this study: merely a modelling BS GIGO, some precise repeat measurement, what (in text: "estimates" )? And how does this paper compare to other studies, how does it relate to the BBC/UN claims? I can infer you prefer your linked one, but why? Jul 1 at 16:47
  • 1
    @LangLаngС the headline number in the article is about one component of contribution to desertification, not the actual desertification. This article is very recent, in a prominent journal, and acknowledges and references older studies.
    – DavePhD
    Jul 1 at 18:04
  • 1. Am less interested in getting answered in comments, but suggesting how to improve the A; your comment really needs integration in the post bc: 2. currently one needs to read the linked paper itself to reach any conclusion, which may be valid, but lacking the bemoaned context and evalution. 3. The criteria you mentioned are quite weak for an evaluation of its quality ('Why is your study good?' "It's in Nature Communications!" —& notice how 'climate sceptics' celebrate this paper just as "see CO2 is very good, a greening fertilizer/plant nutrient; all around, need more of it")? Jul 1 at 18:35
  • 1
    @LangLаngС If you want to dismiss the article because someone else likes it, it's on you to find something to back that up, not on OP who already found reasonable justification for its quality as a source. Dismissing a published and peer-reviewed paper is as much "original research" as simply writing your own theory without anything to back it up.
    – pipe
    Jul 1 at 19:44
  • @pipe Understand: Nobody "dismisses the article", but purely eminence-based reasoning is not "reasonable" (and ignores the weighing with other research/data), but a quite poor standard to judge any article's qualities. Require more.I do say this post might benefit from more context & evaluation, be it from other articles, more summarising from that very article or whatnot. How does the info-tidbid here compare to the original claim? Is that spelled out? Brevity is not always a strength and this topic widely & often researched, with varying results. One single paper is not god's word on it. Jul 1 at 20:06

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .