Please try this with r7873:
local ws = world.get_size()This code snippet tries to turn one line of the map into desert. It does not work on deep water and on non-empty tiles, but is a start.
local tool = command_x(tool_set_climate)
local err = tool.work(player_x(1), square_x( ws.x/2, 1 ).get_ground_tile(), square_x( ws.x/2, ws.y-1 ).get_ground_tile(), ""+cl_desert)