*This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.*

**Objection:**

Natural variability is the null hypothesis and an anthropogenic CO2 warming effect needs compelling evidence before there is any reason to take it seriously.

**Answer: **

The null hypothesis is a statistical test and might be a reasonable approach if we were looking only for statistical correlation between increasing CO2 and increasing temperature. But we're not, there are known mechanisms involved whose effects can be predicted and measured. These effects are the results of simple laws of physics even if their interactions are quite complex.

But putting the inappropriate application of the Null Hypothesis aside, we are indeed well outside the realms of natural global variability as seen over the last two thousand years and even over the last 12,000 years. We can go back several hundreds of thousands of years and we still see that the temperature swings of the glacial/interglacial cycles were an order of magnitude slower than the warming rate we are now experiencing. In fact, outside of catastrophic geological events like the Paleocene Eocene Thermal Maximum there are no known precedents for warming this fast on a global scale. I'd say the case for "it's all natural" is the one that needs explaining.

Oh, and by the way, we do in fact have compelling evidence.

*This is just one of dozens of responses to common climate change denial arguments, which can all be found at How to Talk to a Climate Sceptic.*

"The Null Hypothesis Says it Natural" was first published here, where you can still find the original comment thread. This updated version is also posted on the Grist website, where additional comments can be found, though the author, Coby Beck, does not monitor or respond there.

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