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New series: #ECS in the real world

18 Oct 2023 | Science Notes

We’ve had plenty to say here at CDN about the elusive number known as Equilibrium Climate Sensitivity or ECS. If it’s a new topic for you please look at our Backgrounder Video from 2020 because ECS is a vital concept in climate science, referring to the absolute amount of warming caused by doubling the relative amount of CO2 in the atmosphere (once the climate system has fully adjusted to the change). For many years the IPCC argued that the Earth’s ECS is between 1.5C and 4.5C, with a best estimate around 3C. Even so, leading economic models have shown that if the ECS is 3C then even though emissions eventually cause some harm, trying to stop warming would be far costlier than just living with it. Meanwhile if ECS is closer to 2C economists have shown that CO2 doesn’t cause any net economic harm and may even be a net benefit, so all climate policies are a deadweight loss not just bad on balance. Modern climate models have ECS values that range from a low of 1.8C to a high of 5.6C. So the “settled science” doesn’t even know if CO2 is a problem or not. But what if we ignore models and estimate ECS based on the past 150 years of real-world data? As we will show over the coming weeks the numbers keep coming back around 2C or less. This week we start with a study from 2000 that placed it at 1.6C.

The paper is: Stern, D and R. Kaufmann (2000) “Detecting a global warming signal in hemispheric temperature series: A structural time series analysis.” Climatic Change 47:411-438.

It’s a rather dense statistical analysis by a pair of economists who took tools usually used in macroeconomic forecasting and applied them to the Earth’s climate. The first thing they found was that global average temperatures behave differently than radiative forcing from greenhouse gases, and even though both are trending upwards a closer look showed that neither one apparently acts as a control knob for the other. This would seem to put the kibosh on there being any connection between CO2 and climate, but the authors kept digging. They broke the data up into the Northern and Southern Hemispheres and looked at whether maybe the two halves of the planet behave slightly differently, and they found that when each one is examined in isolation the connection to greenhouse gases began to show up.

They were able to fit a model that yielded evidence that greenhouse gases do in fact drive warming, but only a little. Their model provided an estimate of ECS for each hemisphere, and for the world as a whole. For the Southern Hemisphere they estimated it to be 1.2C and for the northern hemisphere it was 2.0 C, for a global average of 1.6C. Which is lower than the lowest ECS value in modern climate models, and is low enough that we can confidently say climate change is not a crisis or an emergency.

Stay tuned: there are lots more where that one came from.

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