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The cause of global warming

September 12, 2017

It does not take complex modeling to point the finger. Direct measurements and basic physics implicate the CO2 we generate as the most likely suspect. 1) We measure its increase year by year. It has increased by half in recent decades, reaching a point not seen in human existence. Click on the graph, left. The Keeling curve — that part of the graph since 1958 — is as hard a fact about the atmosphere as any. 2) We know from the timing, from how much fossil fuel we burn, and from how much concrete we produce that we are responsible for that increase. That fact is separately corroborated by isotopic analysis. 3) We know CO2 is a weak but long-lived greenhouse gas whose increase captures more heat than our atmosphere otherwise would. The greenhouse effect is straightforward physics, measured both in the lab and in situ. 4) We know that extra heat once captured has to go some place. That is the first law of thermodynamics.

Of course, the atmosphere and oceans are complex systems, and all sorts of things affect global temperature. If the globe weren’t warming, we would be scratching our heads and looking for where that extra heat went and what unknown mechanism somehow undid the effect of a significant increase in one greenhouse gas.

Alas, 5) the globe is warming. So the question is quite different from the question yesterday, whether global warming will make hurricanes more frequent or worse. There, we are wondering if a putative cause, which we observe, will lead to a result, which we don’t yet observe. In the current case, we observe both the putative cause, man significantly increasing CO2 in the atmosphere, and the putative consequence, global warming of the atmosphere and seas. Those are connected by some basic, well-tested physics.

More, the other known causes of temperature anomaly — such as changes in solar irradiance or aerosols — in recent decades have had neutral or cooling effect. That is why estimates of how much warming is caused by the additional CO2 typically are more than 100%. It first is making up for the cooling that we otherwise would be experiencing.

One can imagine that there is some mechanism that undoes the effects of CO2 increase, while some second mechanism is causing the warming. It’s not an easy defence: “Yes, my client shot the victim in the head. But he already was dead. The real murderer, yet unknown, had already killed him by means yet unknown.”

Still, the atmosphere and oceans are complex. There have been such alternate explanations proposed and published in the scientific literature over the years. So it is interesting when reviewers go back and look at those. What they find is that those alternate proposals don’t stand up well. (Cite.)

Every year, the evidence grows for the theory with a measured cause, a variety of measured effects, and physics connecting those. That’s without any discussion of energy-flow models.

Or of hurricanes. It’s not Harvey or Irma that should drive someone to be concerned with global warming. It’s climate science and the data it generates that should drive that concern.

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