AOC said the world is going to end if nothing is done, but she's very misinformed. Basically, Greta and the climate scientists are saying that if we don't cut our carbon emissions, humanity will suffer serious consequences, and the longer we wait, the worse it will be. Many of the scientists' predictions are becoming apparent now. Global surface temps are increasing exponentially, oceans are warming, the Arctic, Antarctic, and Greenland ice are melting, and the sixth mass extinction is well under way. The climate crisis is not like an asteroid strike. It's a slow motion train wreck that's been happening for decades, and if we had acted decades ago, we would not be in this mess. Fossil fuel interests have played a big part in delaying action by confusing the public and policy makers with disinformation and propaganda.
Several studies I've seen show China (27.2%) , US (14.6%), and India (6.8%) at one, two, and three in annual CO2 emissions. Though the US population is much smaller than China's and India's, many people in the US enjoy a higher standard of living. We drive cars, travel by air, and in general, consume a lot of everything, all of which leads to more CO2 emissions. The average American contributes more than twice the carbon emissions of the average Indian and Chinese person combined. And China and India are running massive programs to deal with pollution, whereas Trump is doing everything he can to roll back US environmental regulations. If you want to know which country has been contributing the most to climate change, look at this link. https://www.weforum.org/agenda/2019/01/how-each-country-s-share-of-global-co2-emissions-changes-over-time/
Dealing with the climate crisis will cost big money and will require new legislation, so we will need the governments' cooperation. That doesn't mean socialism, though. But the politicians won't act until enough people wake up and push for action. That's where the activists like Greta come in - they are waking up people. Without their efforts, nothing will happen, so they are performing an immense service to all of humanity. The climate scientists define what needs to be done - basically we need to reduce our carbon emissions. Engineers and technicians will develop the actual solutions like fusion nuclear power, renewable energy, carbon capture and storage, and so on.