Arctic sea ice

Yes, "The" is optional. As a headline, it means the same thing with or without it.

In a headline, it would be most unusual to see "The" used.

Thank you, but

1. I mean as the starting sentence, complete sentence in a news report, not the headline; would THE still be optional?

2.Without having talked/mentioned Brazilian cars; is THE optional?:

-50%/67%/half etc of (the) new Brazilian cars are good vehicles.

Yes, it is still optional. In your example about Brazilian cars, you would not use "the."

OT but I might have writ "…has thinned to 10%-40%…"

THank you~

Is this THE optional?:

-(The) wind is created by the movement of atmospheric air mass as a result of variation of atmospheric pressure, which results from the difference in solar heating in different parts of the earth surface.

as a definition; without referring to any wind before.

One could argue that it is optional, but i , personally, would not use it. That's for the same reason you should not use "the" with Brazilian cars. It is not a reference to a subset of a group having family resemblance.

Thank you~
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Without context, is THE optional?

eg The more (the) torque an engine has, the bigger the load it can pull in the same gear.

Yes, in that the second definite article is not needed.

Thank you~

do you mean we can also simply say?:

-the bigger (the) load it can pull in the same gear/
- it can pull in the same gear bigger load.

THE being optional in the first sentence above?

You misunderstood me. Here is your sentence, with the second definite article highlighted:

The more (the) torque an engine has, the bigger the load it can pull in the same gear.

The first definite article begins the sentence. The second definite article is optional, and i don't think most natives speakers would use it. The third and fourth definite articles are not optional, although the fourth definite article might be replaced by the indefinite article:

The more (the) torque an engine has, the bigger a load it can pull in the same gear.

So, were i to write that sentence, this is how i would write it:

The more torque an engine has, the bigger the load it can pull in the same gear.