Observing the Earth’s climate 1/2

We breathe and we eat. Oxygen and carbohydrates. Oxygenator and fuel. One key ingredient to life on Earth is photosynthesis. To keep an eye on the Earth’s health, we have to monitor the photosynthesis. And – Our planet is an ocean planet, so we start our journey at sea. The Baltic Sea.

7 August 2015. Algal bloom in the Baltic sea captured by Sentinel-2A. Credit: ESA
7 August 2015. Algal bloom in the Baltic Sea captured by Sentinel-2A. Credit: ESA

The picture shows an algal bloom west of the Gulf of Riga. The southern tip of the Estonian island Saaremaa and the coast of Latvia are visible to the right in the true color image to the left. The map gives us location and scale. The processed images to the right show a whirlpool, an “eye of the algal storm“. Follow the links and you can read more about these images.

The color of the ocean tells us the amount of phytoplankton in a certain area. Phytoplankton are microscopic organisms that, like terrestrial plants, use chlorophyll in the process of photosynthesis. Thousands of phytoplankton species in massive abundance in the surface layers provide a food source for creatures ranging from animal-like zooplankton to whales. During photosynthesis, carbon dioxide is absorbed, carbohydrates (the food mentioned above) is produced and oxygen released. Their distribution in different regions can change dramatically from one year to another. So are their bloomings.

The most effective way to monitor phytoplankton distribution is to use satellites. The microscopic phytoplankton themselves cannot be seen, but their abundances and chlorophyll concentration are visible through the variations in the color of the ocean water. The true color image above looks somewhat bluish due to light scattering in the atmosphere. In fact, 80% of the total signal reaching the sensors on a satellite imager has to be substracted to get a clean image of the sea surface color. 20 years of observations have increased the understanding of the life of phytoplankton. New satellites equipped with multispectral imagers also enable us to distinguish between the different species of phytoplankton.

Phytoplankton have been around almost since the dawn of life on Earth. There have surely been great variations in species and abundance before. That’s also true about algal bloomings. Now it is important to find out wich changes are natural, and wich are caused by humans. By looking at existing data, and collecting new data now, we soon may be able to tell the true story.

The first map of sea-ice thickness, January/February 2011, from ESA’s CryoSat mission. Credit: CPOM-UCL-ESA
The first map of sea-ice thickness from ESA’s CryoSat mission. Credit: CPOM-UCL-ESA

To see if our planet is getting warmer, it’s a good idea to look right in front of your feet. If you find species of grass, herbs and trees which used to be found closer to the equator there may be a reason to take a closer look. There may be several natural reasons for these changes, but it is important to collect data for the future.

Another indicator of global temperature is glaciers and sea ice. Ice sheets have been easily monitored by satellite images if we are interested in the area covered by ice. But how about the thickness of the ice?

ESA’s Cryosat satellite is placed in a near-polar orbit, reaching latitudes of 88° north and south. Its main instrument is the Synthetic Aperture Interferometric Radar Altimeter (SIRAL) which is specially designed for ice. SIRAL is able to detect small variations in the height of the ice. It also makes accurate measurements of the sea level.

There is a lot of work to be done before launching a satellite with such a specific mission. Instruments and software have to be designed and calibrated. Before launch, to know what to expect, and during the mission, to check if the calibrations are right, expeditions have been made both to the Arctic the Antarctic.

To understand the climate, there is a lot of research to be made. It’s by definition a long-term job. The standard averaging period to define a climate is 30 years. Looking back we know for sure that the climate has its natural changes. Once again, it is most important to learn about natural variations if we want to find human fingerprints on a crime scene. It’s only a brief period we have been able to collect specific data to compare our climate over time. Massive, accurate, global data is in fact something new. A great tool for the future.

 


JanJan teaches mathematics and interdisciplinary science to pupils 13-16 years of age at Sursik School, Pedersöre, Finland. Space-related science often gives some sort of answer to the question “Why?”, a question quite common in math class. It also triggers curiosity, one key component in progress.

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