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The very small crisis

27 Nov 2019 | Science Notes

We often hear that anthropogenic climate change is an emergency, and that greenhouse gases are the main drivers of everything that happens in the natural world. So let’s step back for a little perspective on scale, including on "radiative forcing", which measures how many Watts of energy are absorbed and radiated per square meter of the Earth’s surface. The IPCC says incoming solar energy averages 342 Watts/m2, with about a third bouncing off clouds or the ground leaving 235 to be absorbed. If there were no GHGs it would all then radiate back to space leaving Earth a lifeless snowball. If too much is trapped, the Earth becomes a sauna. But how much extra energy is being trapped as a result of all human greenhouse gas emissions around the world, assuming they stay in the atmosphere? About two-tenths of one Watt per square meter per decade, or 0.09% per decade. Since that figure is 50 times smaller than the average annual measurement error in radiative forcing it’s far too early to say we’ve measured a crisis. We haven’t really measured anything.

You may have heard that all it takes is extra CO2 and temperature shoots through the stratosphere. But “the science” makes no such claim, not even the most alarmist science. Rather, as our simple physics slogan video noted, it’s widely agreed that doubling atmospheric CO2 wouldn't do much to temperatures unless some big hypothetical feedbacks kick in. And some people are unwilling to believe in those feedbacks until they see some evidence.

So let’s look at the radiative forcing numbers in more detail. The IPCC (see figure 1.2) says 342 Watts per square meter come in from the sun, of which 107 bounces back off the cloud tops and the Earth's surface. That leaves 235 Watts per square meter to reach the Earth's surface, which if “energy balance” and hence a stable temperature is to be maintained must equal the radiation leaving the Earth's surface for space. And when extra CO2 is added to the atmosphere, everyone agrees, it slows down the process of escape of the surface energy. But by how much? Since 1750 the total effect has been equivalent to 1.82 Watts per square meter, which is about 0.8 percent of the total. And in recent times the effect has been growing at about 0.2 Watts per square meter per decade, which is 0.09 percent every 10 years.

It's a very small number on any reckoning. But especially when you consider that these numbers are all very approximate. The annual average error in measurement of outgoing energy at the top of the atmosphere of even the best satellite systems is about 11 Watts per square meter. So we are talking about a theoretical change in radiative forcing over ten years on the scale of 1/50th of the annual range of measurement error.

Science is too often presented as a matter of hard numbers and irrefutable logic. But real science, in addition to testing hypotheses with relentless skepticism, is also very careful to note uncertainties of measurement and include “error bars” in charts to make clear that in some cases an apparent finding is solidly supported but in others lies well inside the fog of uncertainty.

With radiative forcing we’re in the latter situation. It would take 550 years at the current rate of human emissions for the increase in radiative forcing to be larger than the range of measurement error.

So you can see why we have difficulty believing there’s an emergency.

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