我觉得可以将这个句子看作伪判断句（false judgematic sentence）。抱歉，我在谷歌没搜到这个术语。该术语出自杨元兴编写的《新编英语句型句法全解》第 289 页（照本宣科走起）：

这种主系表句子，除了要和截图中的隐喻句区别，还要和转喻句相区别。

这类句子，可以通过上下文把握句意。就题主这句，试译：如果整个国家都浪费食物，则这个国家就是世界第三大温室气体制造者。当然这种情况是不可能的，所以原句用了虚拟语气。

以下例句均来自外媒：

伪判断句：

In this case the problem is Italy.

Ukraine is a refugee crisis in the making for Poland.

转喻句：

The venue was the Conservative Political Action Conference (CPAC), an annual gathering of activists that once sat at the far right of Republican thought, but now appears to be its centre.

用 venue 地点指召开的会议

In America a firm called World Energy employs a former oil refinery in Paramount, California, to do something similar. Paramount was, indeed, the first plant to make saf commercially, having been converted to do so in 2016 by Bryan Sherbacow, a green entrepreneur.

用地点 Paramount 指 World Energy 公司。类似的转喻，白宫指美国政府，北京指中国政府，不一而足。

是不是可以考虑将原句视为省略句，即：

If food waste were a country, it would be the third largest producer of greenhouse gases in the world.

=If what (=that which) caused food waste were a country, it would be the third largest producer of greenhouse gases in the world.