Use the command-line option -C <offset>
:
Continue/Resume a previous file transfer at the given offset. (...) Use "-C -" to tell curl to automatically find out where/how to resume the transfer. It then uses the given output/input files to figure that out.
In your exact case:
curl \
-o ../data/ETOPO1/ETOPO1.zip \
-C - \
'http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/data/ice_surface/grid_registered/georeferenced_tiff/ETOPO1_Ice_g_geotiff.zip'