climate change

climat change

Corrections below:

climate change

Today the weather is grey and the sky is cloudy, this is not a good day. Since sometimes the climate of the planet is really crazy, the seasons aren't more like before.

I understand what you mean, but the sentence is unnatural. I would say:

Because the earth's climate is changing, the seasons are different than they used to be.

The winter is hotter and the sun isn't here in summer. Same as above. I would say: The winter is hotter and the summer is cooler.
In some countries the drought kills lots of people and in others this the rain which give the dead so that before the climate was more regular and less dangerous for the population. 

In the blue part of the sentence above, I can't quite understand what you mean. I think you mean "there is lots of rain in other countries. Before, the climate was more regular and not so dangerous for people.
The people want all to be ecologist if this is the neighbours who doing the efforts. Alone we don't can changed the story of planet but if all people of world decided of doing some things we will can hope that our children will be happier more late.

Everyone thinks they are ecologists but they don't really know what they are talking about. Individually, we can't change the story of our planet, but if all people of the world decided to change their ways, we can hope that our children will inherit a happier planet.

Très bon! Votre Anglais est bonne, et je vous comprends très bien. Je souhaite qu'on pouvons être "amis" en Italki!