How much of the Earth’s surface is covered by water? – 2/3 elementary school knowledge. But how to confirm? And what fraction of the southern hemisphere mid-latitudes is covered by ocean?
We need some data first. For a quick shot, the MODIS land cover type classification should be sufficient. MCD12C1 provides global coverage at 0.05° in the HDF4 format. It is a resampled and stitched together version of the 500m MCD12Q1 version. The user guide provides a nice overview.
Now we need to do some calculations. The python jupyter notebook is on github. The recipe is rather simple: Load the HDF4 dataset, select the IGBP[1] classification, quick plot for visualization, get the projection and the pixel sizes right and finally do some conditional sums.
The projections and the pixel sizes are a crucial point. The dataset is in the MODIS climate modeling grid, which is a geographic lat-lon projection. The pixel area gets smaller towards the poles, which we have to keep in mind, when calculating the size of the per pixel.
Looking at the final numbers, the fraction of water is 71.6%, which is sufficiently close to available estimates. Some discrepancy is expected, as we do not include ice shelfs, sea ice, tides, etc and the underlying resolution is ‘only’ 5km. Barren surfaces cover around 4.0% of the Earth (13.9% of the land) and ice sheets, including permanent snow cover 2.9% (10.2% of the land).
When splitting up the hemispheres, in the Northern Hemisphere water covers 61.4%, barren 7.5% and ice less than 1%. Of all the land, barren surfaces cover 19.4%. In the Southern Hemisphere, water makes up 81.8% of the surfaces, with barren less than 0.5% and ice 4.8%.
Finally the mid-latitudes. As a rather crude definition, the latitudes between 30° and 70° are used. In the northern mid-latitudes water covers 48.0% (5.7% barren and 0.7% ice). Barren surfaces make up 11.0% of the land. The southern mid-latitudes are overwhelmingly covered with water (93.9%, barren <0.2%, ice 1.4%).
[1] International Geosphere-Biosphere Programme: http://www.igbp.net/
