Data sources
============
* climatic zones: http://eusoils.jrc.ec.europa.eu/projects/RenewableEnergy
* population density: http://sedac.ciesin.columbia.edu/gpw 
* nighttime lights of the world: http://ngdc.noaa.gov/eog/dmsp/downloadV4composites.html 
* rice and wheat production:  http://gaez.fao.org/Main.html
* EDGAR HTAP V2 NOx emissions:  http://edgar.jrc.ec.europa.eu/htap_v2/index.php?SECURE=123
* OMI NO2 columns: Chris McLinden at Environment and Climate Change Canada
* Google maps elevation API: https://developers.google.com/maps/documentation/elevation/intro
* ETOPO1 digital elevation: https://www.ngdc.noaa.gov/mgg/global/global.html
* HTAP regions: http://iek8wikis.iek.fz-juelich.de/HTAPWiki/WP2.1?action=AttachFile&do=view&target=_region_definition_document_v4a.docx
* MODIS landcover classification: https://lpdaac.usgs.gov/dataset_discovery/modis/modis_products_table/mcd12c1