eli5: What exactly is the carbon budget, and what happens if we don’t reach it?

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What is the carbon budget, and what happens if we don’t reach it? Why is 1.5 degrees warming our target? What does 1.5C mean, is that how much hotter the globe is now compared to 200 years ago? If we reach the carbon budget, does that mean climate change will become 100% irreversible and the earth will heat up until humanity has to move off-planet?

In: Earth Science
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The school of thought is basically if we don’t stop and pass that point, by 2050, the planet will struggle in ways we’ve never seen before.

Droughts, more humidity, crazier weather, etc.

Global warming is already irreversible in human lifetime terms. Short of finding a way to remove CO2 and CH4 from the atmosphere, we are already on the path of continuing temperature rise for the foreseeable future. Of course on geological scales, eventually Earth will hit another ice age, but that’s not really relevant for humans alive today. Carbon budget is just a convenient way of saying “how much can you emit, without going over the target”; we sort of realized that US, China, India, Indonesia, Brazil, Russia etc will not reduce their emissions fast enough; and accepted that some optimistic targets are better than no targets. The atmosphere is about 1C warmer now than it was in 1880; enough to decrease arctic ice and snow cover in the north hemisphere, etc etc etc

Carbon budget is an arbitrary target for man-made CO2 emissions predicated on computer models which have been proven wrong repeatedly.

And the effects are unknown because the models have been proven wrong repeatedly.

There is no doubt humanity contributes to a changing climate but the extent of which is unclear and the impacts are also unclear.