Excellent point. We have definite causation of warming with man, but we lack the same causation with natural events. So why would anyone assume that it would be the natural events driving things?
Actually, that's what you SHOULD assume, starting out with no other information.
Is it the sun? Analyze data and determine, no, solar output is stable or has declined over the past centuries.
Is it orbital/Milankovitch cycles? Analyze data and determine, no, we are just past the peak of the last orbital cycle, and are due for another Ice Age in maybe 50,000 or 100,000 more years.
Is it ocean cycles? Analyze data and determine, no, El Nino/La Nina cannot explain the consistent warming, and we are not 'cooling' during La Nina cycles anymore, we only 'hold serve' on global temperatures in the past century or more during long La Nina stretches.
Is it cosmic rays? Analyze data and determine, no, we cannot find any variation in cosmic ray influx vs. temperatures.
Is it volcanic emissions? Analyze data and determine, no, humans actually output >100x more CO2 than all terrestrial and submarine volcanoes combined.
Additionally, major volcanic events serve to
cool the climate, not warm it up (which is the actual cause of the Little Ice Age), and we have not had any of those since Pinatubo (which was relatively small on the 'major volcanic events scale). Paleoclimate data indicate prior major volcanic events, occurring over centuries, DID release lots of CO2, which DID cause major warming events over 1000 years or more millions of years ago. Thus, we have objective and paleo data which directly show CO2 can drive temperatures.
We are simply out of 'natural' phenomena which can explain the data and the recent warming.
The combination of rising temperatures AND ocean acidification can ONLY be explained by CO2 level increases. And we KNOW that those are presently being driven by human processes: deforestation and fossil fuel burning. Additionally, we KNOW that the CO2 runup in the atmosphere is coming from fossil fuels, because the isotope signature or ratio of the carbon is consistent with the sequestered carbon, not terrestrial carbon. That has also been proven -
we KNOW where that extra carbon and CO2 has originated from based on the isotope signatures.
(FWIW: this is also why 'planting a tree' to 'offset' your carbon footprint is really nonsense - a tree can sequester that carbon for MAYBE 100 years or so. Then, it dies and the carbon is re-released as the tree decays. We also have limits on how many tons of 'new trees' can be holding carbon at any given time, based on soil footprints and fresh water requirements. The ONLY thing that can reduce or limit a carbon footprint is to leave the fossil fuels in the ground, or sequester the tons of carbon/carbon dioxide underground for thousands, or millions, of years, which is where it was before we dug it up or pumped it out.)
Another side-note: one or more of the MAJOR extinction events which occurred millions of years ago has been attributed to massive CO2 releases from specific types of volcanic eruptions. Those extinctions didn't likely occur because of higher temperatures: they occurred because the oceans became significantly more acidic, killing off many of the microbic species at the bottom of the food chain which rely on creation carbonate shells to survive. We are on pace to ramp up CO2 levels and ocean acidity on par with those massive extinction events.
If we do disrupt the bottom of the food chains in the oceans, we will ultimately eliminate major food supplies for most of the world's populations. That may not happen for another century or so (but easily could be sooner), but if it does, the Earth will not be able to support 7 billion people anymore, let alone the 9 or more billion we might be at by then.
I.e.: we should KNOW with absolute certainty we will not/can not cause that to happen with our emissions, not 'guess' or 'hope' that our emissions won't cause it. That is simply Risk Management:101.
