How did the temperature relate to the “recommended temperature limit” (solid horizontal red line)? Someone please just explain this question

The “recommended temperature limit” indicates limit of the temperature that is good for our world.

Explanation:

The temperature related to the “recommended temperature limit” means that in the graph there is a limit of temperature for our world's environment. if the temperature is between this limit, there is less or no adverse effect on our environment while on the hand, if the temperature exceeds from this limit, then it adversely affected our world because the concentration of carbondioxide gas in the atmosphere increases due to this increase temperature..

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