A very significant change everyone can make is to consume more plants and less animal products. Diet is one the most impactful changes one can make individually.

See here: https://www.nature.com/articles/s41558-024-02084-1

Second, if you live in a cold country, improve your house heating efficiency and/or switch to heat pumps.

Third, consider a more efficient method of transportation, like bikes, walking, eletric vehicles, public transit, etc.. (bikes, walking and transit have the benefit of also being very good for our health)

Fourth, vote for politicians aware of the issues and enacting changes.

There's a lot everyone can make individually and collectively :)

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Important to mention: if you're going to adopt a plant-based diet, there are a few things you need to supplement, for the most part vitamin B12, maybe Omega-3 and others, and you do need to make sure you're getting necessary nutrients from a variety of plants (for example, walnuts and chia seeds also contain Omega-3). There are many resources online with dietary guides.