「不毛の砂漠」に樹木１８億本、衛星画像とディープラーニングで発見　アフリカ

【東京ＡＦＰ＝時事】不毛の大地にしか見えないアフリカのサハラ砂漠やサヘル地域に、これまで知られていなかった豊かな緑が点在していることが、高精細衛星写真とディープラーニング（深層学習）技術を組み合わせた研究で明らかになった。英科学誌「ネイチャー」（電子版）に14日、論文が掲載された。（写真は資料写真）
　研究チームによると、西アフリカのサハラ砂漠やその南縁に広がるサヘル地域と乾燥半湿潤地域に、従来のイメージを覆す約１８億本もの樹木が生えているのが分かったという。
　論文の筆頭著者を務めたデンマーク・コペンハーゲン大学の地理学者、マルティン・ブラント氏は、「サハラ砂漠にこれだけ多くの木々が生えていることに、とても驚いた」とＡＦＰに語った。
　「もちろん、一本も木がない場所は広範囲に及ぶ。だが、木々が密集している場所もあり、砂丘の間にもあちこちに木が生えていた」
　米航空宇宙局（ＮＡＳＡ）のゴダード宇宙飛行センターが協力した今回の調査は、森林破壊を食い止める取り組みや土壌の二酸化炭素貯留量のより正確な測定の指針を示すデータを研究者や自然保護団体に提供するものだ。
　研究では、面積１３０万平方キロを対象に、１万１０００枚を超える衛星画像を分析した。ディープラーニング技術の活用にはプログラムを訓練する必要があり、ブラント氏は丸１年かけて９万本近くの木を自ら数えて分類した。
　論文を査読した米ニューメキシコ州立大学植物環境科学部のナイオール・ハナン氏とジュリアス・アンチャン氏は、今回用いられた技術について「一定の制限はあるものの、間もなく世界中の全ての木々の位置と大きさを地図化することが可能になるだろう」と述べている。
　ブラント氏は、正確な木の数を把握することが、気候変動とその加速に対する理解を変えるかどうかはまだ分からないと語った。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/10/16-10:43）

At first glance the apparently barren expanses of the Sahel and Sahara deserts feature little greenery, but detailed satellite imagery combined with computer deep learning has revealed a different picture.
In fact, some 1.8 billion trees dot parts of the West African Sahara and Sahel deserts and so-called sub-humid zone, a previously uncounted bounty that overturns previous assumptions about such habitats, researchers say.
We were very surprised that there are quite (so) many trees growing in the Sahara desert, lead author Martin Brandt told AFP.
Certainly there are vast areas without any trees, but there are still areas with a high tree density, and even between the sandy dunes there are here and there some trees growing, added Brandt, an assistant professor of geography at the University of Copenhagen.
The survey provides researchers and conservationists with data that could help guide efforts to fight deforestation and more accurately measure carbon storage on land.
For preservation, restoration, climate change and so on, data like this are very important to establish a baseline, said Jesse Meyer, a programmer at NASA's Goddard Space Flight Center who worked on the research.
In a year or two or ten, the study could be repeated... to see if efforts to revitalise and reduce deforestation are effective or not, he said in a NASA press release.
Finding and counting the trees was no simple task.
In areas with plenty of trees, thick clumps of growth appear relatively clearly in satellite images, even at low resolution, and are easily distinguishable from bare land.
But where they are more spread out, satellite imagery can be too low-resolution to pick out individual trees or even small groups.
Higher resolution imagery is now available, but even then problems remain: counting individual trees, particularly over vast areas of territory is an almost impossible task.
Brandt and his team came up with a solution, pairing satellite images at very high resolutions with deep learning -- essentially training a computer programme to do the work for them.
But that didn't mean they could just sit back and wait for the results.
Before the deep learning programme could get to work, it had to be trained, an onerous process that saw Brandt individually count and label nearly 90,000 trees himself. It took him a year.
The level of detail is very high and the model needs to know how all kind of different trees in different landscapes look, he said.
I did not accept misclassifications and further added training when I saw wrongly classified trees.
- Setting a conservation baseline -
It was worth the effort, he said, allowing what would have taken millions of people years of work to be computed in just hours.
Other studies are based on estimations and extrapolations, here we directly see and count each tree, it is the first wall-to-wall assessment.
The survey, published Wednesday in the journal Nature, covered an area spanning 1.3 million square kilometres (around 500,000 square miles) and involved analysis of more than 11,000 images.
The technique suggests it will soon be possible, with certain limitations, to map the location and size of every tree worldwide, wrote Niall P. Hanan and Julius Anchang of New Mexico State University's Plant and Environmental Sciences Department, in a review of the research.
And accurate information on vegetation in deserts and other arid zones is fundamental to our understanding of global-scale ecology, biogeography and the biogeochemical cycles of carbon, water and other nutrients, they wrote in the review commissioned by Nature.
Better information may help determine how much carbon is being stored in these sites, which are not usually included in climate models, Brandt said.
But it's too early to say whether having an accurate count of this tree life will affect how we understand climate change and its acceleration, he added.
He hopes now to use the technique elsewhere, to map more previously hidden trees in the 65 million square kilometres of arid regions in the world.