Plant Cuttings

Blue + yellow = ?

What can satellites tell us about the earth's chlorophyll density?
Image: Wikimedia Commons.
Image: Wikimedia Commons.

Keen planet-watchers amongst you will surely know that seen from space the Earth is considered to be a blue planet, dominated as its surface is by water. However, when you combine the blue of life-giving water with the vital, energising quality of the yellow sun you get green plants. OK, a little simplistic, but I hope you get the idea. And one of the great things about plants is that their green pigment – chlorophyll – can be visualised using satellites well away from the Earth’s surface. Now, since chlorophyll is fundamental to photosynthesis, which in turn is the driving force behind primary productivity, and hence the ability of any area to sustain an ecosystem, being able to map its relative concentration across the globe is of major importance to understanding the ecology – and such global concerns as food production – of the oblate biospheroid we call home.

Although the technology may be complicated, the biology behind such mapping is relatively straightforward: various pigments in plant tissues – e.g. chlorophyll in leaves – absorb much of the sun’s visible light (VIS) that they intercept, and reflect much of the so-called near-infrared (NIR) wavelengths. In contrast, poorly vegetated regions – e.g. deserts – reflect both VIS and NIR wavelengths. Thus the different areas can be detected and distinguished. Largely out of sight – but not out of mind – the SUOMI National Polar-orbiting Partnership (NPP) satellite (a partnership between NASA and the National Oceanic and Atmospheric Administration, NOAA) measured ratios of VIS and NIR between April 2012 and 2013 from across the globe. The results are available to be viewed as a narrated animation that shows how the Earth’s verdant mantle changes from week to week, and features both terrestrial and marine habitats.

Extending the survey timescale a little – from 1889 to 2000 – but focusing on the marine realm, Marcel Wernand et al. have examined worldwide trends in ocean colour and chlorophyll concentration. Using the Forel-Ule scale record (‘a hardly explored database of ocean colourֹ’) instead of satellite data (well, there weren’t (m)any around for most of the survey period examined…), the group provide evidence that changes of ocean surface chlorophyll – and, by proxy, inferences about primary productivity – can be reconstructed with confidence from this record. This is a great boost to ‘old-fashioned’ – i.e. traditional – scientific approaches, and neatly extends such data far beyond the ‘satellite period’ giving an all-important historical dimension to those much more recent data sets. Interestingly, their analysis did not reveal a globe-wide trend of change in chlorophyll concentration during the past century; rather, their study suggests that explanations of chlorophyll changes over long periods should focus on hydrographical and biological characteristics typical of single ocean regions, not on those of ‘the’ ocean. Nice to see that both large-scale, hands-off, high-tech approaches AND small-scale studies are needed to give the big picture!


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