Why should I use わけだ and not ものだ in this sentence?

Question

今日一日だけで、人類は新たに二十五万人をこの地球という惑星に加えている。そしてそれは毎日繰り返されている。現状では、毎年ドイツの全人口と同じだけの人間が増えている _______ 。

1.わけだ 2.ものだ

The answer is 1.わけだ

But I think the last phrase is a statement of fact, which fits ものだ's usage.

Answer 1

ものだ for "cold fact" is used like this:

• 人間はいつか必ず死ぬものだ。
• 親が知らない間に、子どもは大人になっていくものだ。
• パソコンは、5年も使えば壊れてしまうものだ。
• 好きな人と一緒にいると、時間があっと言う間にすぎるものだ。

(Some of these were taken from this page)

～ものだ is used to show an unavoidable, uncontrollable fact everyone should be aware of. Whether you like it or not, the speaker believes it's the truth of this world. That's why the same construction also expresses an obligation.

But "毎年ドイツの全人口と同じだけの人間が増えている" is not really a piece of information of which everyone should be aware. It's no more than his own catchy way of explaining the aforementioned information. In addition, by using 現状では and ～ている, he indicates it's a "temporary" fact. In such a case, ものだ is not appropriate.

Answer 2

Whether it is a statement of fact or not doesn't factor into it. What matters is the speaker's attitude about the facts, or "mood".

ものだ is used to express a mood of "common sense": naturally, it's the case that… of course, one would expect that… as everyone knows, it's the case that… (E.g. 学生は勉強するものだ → naturally/of course, to be a student means studying).

わけだ is used to express a mood of "this implies that", "to put it another way": from this, it follows that… as a consequence, it's the case that… to put in other words, this means… (E.g. 大学４年生です → それじゃあ、来年はもう社会人になるわけだ).

In your example, it's: The Earth's population increases by 250000 people every day. This means that it's / To put it another way, it's like (=わけだ) growing a Germany's worth of people per day.

(Source and examples: chiebukuro.)

Answer 3

今日一日だけで、人類は新たに二十五万人をこの地球という惑星に加えている。そしてそれは毎日繰り返されている。現状では、毎年ドイツの全人口と同じだけの人間が増えている ____ 。

At first, "現状では、毎年ドイツの全人口と同じだけの人間が増えているものだ。" is unnatural. The reason is answered in naruto's and Ieoboiko's answer. If you want to change the sentence into natural one using ものだ, you have to add some phrase to make the sentence an undoubted fact like: 現状では、毎年ドイツの全人口と同じだけの人間が増えているというのは理解できないものだ。

As for "わけだ", let's examine its suitability considering its meaning. If you extract the main meaning from each given sentence, they would become like:

A: 今日一日だけで、人類は新たに二十五万人をこの地球という惑星に加えている。--> 一日で、新たに二十五万人を加えている。--> 一日で二十五万人が増えている。 More than 250,000 people are increasing in a day.

B: 現状では、毎年ドイツの全人口と同じだけの人間が増えている --> 毎年ドイツの全人口と同じだけの人間が増えている Every year, the same number of people as Germany's total population is increasing.

250 thousand/day (A) x 365 = 91.25 million/year ≑ Germany's total population (81.41 million in 2015) (B)　

わけ (訳 in kanji) in わけだ means: conclusion from reasoning, judgement or calculation based on something read or heard; reason; cause; meaning; circumstances; situation​s

The context of the given sentences would be as a whole like:
3. A is equivalent to B.

わけだ could reasonably combine sentence A and B into one sentence compatible to sentence 3.

Combined possible expressions are as follows:

• ＡはＢの訳｛わけ｝だ。A is the reason of B.
• ＡはＢになる訳｛わけ｝だ。A is the reason to become B.

If you rewrite it perfectly, it will be like:

今日一日だけで、人類は新たに二十五万人をこの地球という惑星に加えている。そしてそれは毎日繰り返されている。結果｛けっか｝として、 現状では、毎年ドイツの全人口と同じだけの人間が増えている訳｛わけ｝だ or 理屈｛りくつ｝だ。

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