[求助]Barron test reading 5-1

第二段的最后二句,是在讲什么啊? The loss of these ice shelves does not significantly raise sea level, for they already displace seawater. The concern is for the possible surge of grounded continental ice that ice shelves hold back from the sea.

我不懂displace是当什么,指取代或是排水呢? Hold back 又是指什么呢?
                

请MM用中文帮我,翻译一下 ,thank you

displace 是取代,替换. 这里是说,那些在海里面的冰架化了后,就变成海水,添补原来的地方,所以sea level并没有升高.

而在路上的那些冰架,化了后,会使sea level上升.

Thank you so much, Alexwithibt