Animating the Aral Sea

Earlier this week Google released an update to the global mosaic that Google Earth shows when zoomed out. In our post where we were having a deeper look at the mosaic, we mentioned that it would be interesting to try and put a date on the imagery by looking at inland lakes or seas that are known to be growing or shrinking over time. So, we decided to have a go at doing this.

We chose what is possibly the largest and best known example of this – the Aral Sea. According to Wikipedia it is technically a lake and the ‘Sea’ in the name is a reference to the sea of islands that used to inhabit it. Formerly one of the four largest lakes in the world, it had shrunk to just 10% of its former size by 2007. So, it seemed like an excellent candidate for our project.

We had a look through historical imagery, and clearly we were not the first to think of this, as Google has kindly provided historical images of just this location starting in 1973. Below, you can see an animation made with Google Earth historical imagery and the final frame is the new global mosaic.

Speed in milliseconds per image:

Next, we had a look at Google Earth Engine’s ‘timelapse’ feature. Just go to this site, move the map to the location of the Aral Sea and start the animation. Again, it looked fairly encouraging, except for the fact that it seemed to experience a regrowth in 2010. Google Earth Engine’s timelapse feature is made in a similar way to the method used for creating the global mosaic. Many Landsat images from a particular year are combined to create an image that is as cloud free as possible. The difference with the global mosaic is that there is no need to restrict the search for cloud free imagery to a single year. Earth Engine’s ‘timelapse’ feature currently ends in 2012 – which is when Google first released a global mosaic for Google Earth. Let’s hope that with this new global mosaic, they will consider updating the ‘timelapse’ feature too.

So, the next step was to download Landsat imagery of the region for more recent dates. We stuck to Landsat 8 imagery, which begins in 2013. We also selected only images with less than 50% cloud cover. The result can be seen in the YouTube video below:

As you can see above, the Aral Sea is not consistently shrinking over time, but varies quite considerably over time. Whether this was also the case in previous years, we don’t know, as the Google Earth Engine timelapse only does one image per year and that image is a compound image from multiple images from the year. Clearly the Aral Sea has shrunk considerably and is still, on average, shrinking, but it is not so easy to judge the exact time the imagery in the global mosaic was captured. Compounding this is the fact that there is ice on the shores in the winter months, which changes the appearance of the shoreline. However, by carefully looking at the south western branch of the lake, which appears to be shrinking fairly consistently, we believe the Google Earth global mosaic of that part, most closely matches the size of the lake in early 2015. But, the area as a whole is a compound of multiple images and we are fairly sure we can even detect Landsat 7 stripes in the central region between the two lakes. Keep in mind that the Landsat 7 satellite is still operating, so this does not mean that the imagery used is old.

Finally, below is an animation combining both Google Earth historical imagery and the Landsat imagery. We are not sharing the Landsat imagery as a download because it is rather large (90Mb).

In the above video, the dates shown in the timeline are only approximate for the Landsat images, as there were often no corresponding historical imagery dates to match. The correct dates for the Landsat imagery are shown in the first video.