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A milestone for me and a milestone for remote sensing

April 13, 2012

This is my 100th post on my blog! To be honest I wasn’t sure what to do with it – science, sightseeing or sustenance? But my hand has been forced by recent developments in my scientific work, so I’ve decided to share the stage for this post with the satellite that forms the backbone of our remote sensing work in the Benguela.

Envisat was launched by the European Space Agency (ESA) in 2002, as the biggest environmental observing satellite in history. Despite a designed mission life time of only 5 years, this satellite celebrated its ten year anniversary in space this year!

Image from ESA.

Over the course of this amazing feat of mechanical endurance, Envisat and its instruments, have been watching as a number of major environmental events have unfolded. As the eruption of the Icelandic volcano brought air travel in Europe to a halt, data from the MERIS instrument on board Envisat was used to estimate the height of the ash cloud and track its movements. Volcanic eruptions in the Congo, earthquakes in Italy, oil spills in the Gulf of Mexico, Hurricane Katrina, the 2004 tsunami, flooding in China and humanitarian crises in Darfur are just a few of the natural and man made disasters that Envisat has allowed scientists and emergency teams to monitor and coordinate rescue efforts and management plans.

Beyond it’s impressive work providing data for disaster management, Envisat has been there to see a decade of changes in the Earth system – a vital long term record for helping scientists extract potential anthropogenic changes from a background of natural variability. A truly interdisciplinary satellite, Envisat provided the first space based measurements of the greenhouse gas methane , the first satellite estimates of the speed of surface ocean currents and has shed light on fluctuating levels of the protective gas ozone, in our atmosphere. During the last 10 years, Envisat has provided a birds eye view of the areas suggested to be most sensitive to predicted future climate changes – the frozen polar regions. From the dramatic collapse of the Larsen B ice shelf in 2002 to the fluctuations in sea ice formed in the Arctic – this sort of satellite data provides the only current means of conducting global scale assessments of seasonal, annual and interannual changes in many Earth systems.

Anyway, you can read all about the many ways in which Envisat data has been used through a simple google scholar search, so what about me and my work? What does this data mean for me and my research colleagues? Working in the Benguela – a highly productive patch of the ocean off the west coast of South Africa – comes with its own challenges. To give you an idea of why it’s so productive you’ll have to give me two minutes with your scientific brain to explain “upwelling”. Despite some fairly nasty physics being involved, upwelling is basically when deep, cold and most importantly, nutrient rich waters come to the surface. In coastal regions like the Benguela, this happens because winds blow along the coast and thanks to the spin of the Earth (the coriolis force) and some nifty physics (worked out by a guy called Ekman) waters move perpendicular to the wind – to the left in the Southern Hemisphere (us!) and to the right in the Northern Hemisphere. As this water moves offshore, something has to fill the gap, so up comes the deep, cold and nutrient rich waters which act as fertiliser for these regions. This causes huge growth of photosynthetic life – microscopic organisms called phytoplankton – they take sunlight and convert it, with the help of these nutrients, just like plants do on land. They contain a green pigment called chlorophyll. You can learn more about upwelling here.

In terms of ocean colour, what this means is that we have some of the most colourful waters on Earth! Most of the worlds surface oceans contain somewhere around 0.1-1 milligrams of chlorophyll per metre cubed of water. That’s a pretty awful amount to imagine if you don’t study the stuff. So imagine a box about the size of your washing machine – that’s about the amount of water. Now drop perhaps one tiny grain of sugar in that amount – that’s probably more than the amount of chlorophyll in most of the ocean – it’s not a lot – the oceans are mostly deserts as far as phytoplankton are concerned. The Benguela by comparison can contain something like up to half a teaspoon of sugar per washing machine! Comparatively it’s a forest.

If you look at the light reflected off the ocean – which you do, all the time when you look at the ocean, you’re almost seeing what a satellite does, albeit in a very condensed fashion – you see all the visible light together – the colour you perceive. Satellites look at particular wavelengths – roughly corresponding to colours. A lot of the time, if you measure this light in the Benguela, it looks something like this:

Or to the human eye, something like this:

The “Meris” sensor on board Envisat, allows us to see these colours from space! And from this we can work out how much chlorophyll is in the water.

Chlorophyll from space - some work from my PhD!

This is important to help us understand how productive we can expect the whole ecosystem to be and if this productivity is changing – very important to estimate how many fish there will be. We can also use it to detect “red tides” like those which you see in the photo above. These natural phenomena are caused when phytoplankton grow in huge blooms – chlorophyll values can reach up to 2000 milligrams per metre cubed of seawater! The species involved can be toxic – causing paralytic shell fish poisoning in unlucky people who eat contaminated seafood But the phytoplankton can also grow so much that when they die and decompose, a lot of the oxygen is taken out of the water – imagine a compost heap – it smells a bit like that! When this happens, lots of fish and other sea creatures can die. Understanding and predicting these things helps fishery and aquaculture managers prepare for them. Envisat and its onboard meris instrument have been particularly useful to us for this research because it is the only satellite which measures light around that big peak in the red/709nm region you see in the graph above.

If you’ve managed to understand all this scientific geekery, you’ll be able to understand how sad we were when we heard that the European Space Agency (ESA) had lost communications with Envisat this week. We’re all hoping that communications can be restored – although after doubling its predicted operational life, I think the satellite will be deserving of retirement – just hopefully not until it’s replacements – the Sentinels, are launched in 2013/14!


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