It’s been ten years since I addressed this issue in a specific blog post, so I thought it would be useful to revisit it. I mention it from time to time, but it is so important, it bears repeating and remembering.

Over and over again.

I continue to strive to simply these concepts, so here goes another try. What follows is as concise as I can make it.

1. The temperature change in anything, including the climate system, is the result of an imbalance between the rates of energy gain and energy loss. This comes from the First Law of Thermodynamics. Basic stuff.
2. Global warming is assumed to be due to the small (~1%) imbalance between absorbed sunlight and infrared energy lost to outer space averaged over the Earth caused by increasing atmospheric CO2 from fossil fuel burning.
3. But we don’t know whether the climate system, without human influence, is in a natural state of energy balance anyway. We do not know the quantitative average amounts of absorbed sunlight and emitted infrared energy across the Earth, either observationally or from first physical principles, to the accuracy necessary to blame most recent warming on humans rather than nature. Current best estimates, based upon a variety of datasets, is around 239-240 Watts per sq. meter for these energy flows. But we really don’t know.

When computer climate models are first constructed, these global-average energy flows in and out of the climate system do not balance. So, modelers adjust any number of uncertain processes in the models (for example, cloud parameterizations) until they do balance. They run the model for, say, 100 years and make sure there is little or no long-term temperature trend to verify balance exists.

Then, they add the infrared radiative effect of increasing CO2, which does cause an energy imbalance. Warming occurs. They then say something like, “See? The model proves that CO2 is responsible for warming we’ve seen since the 1950s.”

But they have only demonstrated what they assumed from the outset. It is circular reasoning. A tautology. Evidence that nature also causes global energy imbalances is abundant: e.g., the strong warming before the 1940s; the Little Ice Age; the Medieval Warm Period. This is why many climate scientists try to purge these events from the historical record, to make it look like only humans can cause climate change.

I’m not saying that increasing CO2 doesn’t cause warming. I’m saying we have no idea how much warming it causes because we have no idea what natural energy imbalances exist in the climate system over, say, the last 50 years. Those are simply assumed to not exist.

(And, no, there is no fingerprint of human-caused warming. All global warming, whether natural or human-caused, looks about the same. If a natural decrease in marine cloudiness was responsible, or a decrease in ocean overturning [either possible in a chaotic system], warming would still be larger over land than ocean, greater in the upper ocean than deep ocean, and greatest at high northern latitudes and least at high southern latitudes).

Thus, global warming projections have a large element of faith programmed into them.