How Much of Climate Change is Caused by Humans?


Image: UCAR

It's a question that there's still (not-so-surprisingly, given our political and cultural climate) too much confusion about. And it's one that I get asked variants of relatively frequently, typically with connotations of disbelief, like: Well, how much are we really causing global warming? Or its more pointedly negative counterpart: It's just ridiculous to think that we little ol' humans could change the earth's climate, right? Well, there is a scientific answer to the question of how much of climate change is cased by humans: Almost all of it. That's the brief but generally accurate response. But perhaps you'd trust the verbatim response of an actual top-tier climate scientist over that of a slightly grumpy blogger? In that case, here's NASA's Gavin Schmidt, answering the question "What percentage of global warming is due to human causes vs. natural causes?":

Over the last 40 or so years, natural drivers would have caused cooling, and so the warming there has been ... is caused by a combination of human drivers and some degree of internal variability. I would judge the maximum amplitude of the internal variability to be roughly 0.1 deg C over that time period, and so given the warming of ~0.5 deg C, I'd say somewhere between 80 to 120% of the warming. Slightly larger range if you want a large range for the internal stuff.
Without the increasing greenhouse gas emissions from industrialized humans, the earth's global temperatures would most likely be cooler for three reasons, according to Climate Progress's Joe Romm:

1) The major volcanic eruptions of recent past would have contributed to a cooling effect, 2) we're coming off of the deepest solar minimum of the century, and 3) the planet had been experiencing a long-term cooling trend prior to humans kicking up greenhouse gas emissions en masse.

In effect, humans (and their tendency to industrially revolutionize using carbon-based fuels) are solely responsible for the planet being warmer today than it was a couple hundred years ago.

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