Hi Roger,

very interesting article! Did the one about the 1.5 C target also get published somewhere on your site?


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I will be interested to Roger's rationale for a target focused simply on a path to net zero. The advantage of such a target is that it’s easy to understand and progress towards the target can be reliably measured. The other advantage is that it can be tailored to what is reasonably possible. What's missing for me presently is what value for humanity will such a target achieve? This has to expressed quantitatively to ensure the costs of achieving the goal doesn't exceed the benefits.

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Do you have a similar breakdown on the move to 1.5?

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Two problems with the 2C or 1.5C, or any other threshold:

1) The kind of pseudo-precision implied by these numbers conveys the idea to policymakers that these round numbers represent actual thresholds in the climate system. In reality there's no inherent physical or biogeochemical significance to 1.5 or 2C. But their repetition is often interpreted that everything's hunky-dory at 1.49C of warming, and the world falls apart at 1.51C. In reality we can already see impacts emerging now at ~ 1.1 C warming and it's not clear exactly at what point some types of impacts would emerge (from variability). I often write that these types of thresholds are best understood as "milestones along a continuum of risk."

2) The use of global warming (and thus global average surface temperature) thresholds as policy targets at least two degrees of policy causation causation away from the one aspect of climate change we (humanity) can control: greenhouse gas (GHG) emissions. Because we still don't fully understand all the elements of the climate system's sensitivity to changes in GHG concentrations we don't know yet what constitutes a GHG concetration that will keep us in a "safe" climate. And because we still don't fully understand the carbon cycle and how it might respond to warming and other anthropogenic perturbations, we don't know how to "set" GHG concentrations at those "safe" levels. We can't control climate system sensitivity and we can't control carbon-cycle feedbacks, but we've set a policy target that depends on deeply-embedded assumptions about these variables. We have not built policy targets around what we *can* control: GHG emissions.

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Jul 15, 2022Liked by Roger Pielke Jr.

Thank You. This is very informative. Using computer generated global mean temperature changes as meaningful drivers of climate policy is bad enough but tethering it to some specious determination of a "tolerable temperature window" is just way over the top.

Sanity needs to prevail.

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Jul 15, 2022Liked by Roger Pielke Jr.

Very helpful commentary on how the official goals are determined.

One concern for me is that they are world-wide averages, making a complex, large set of local and regional climates into a single one.

Another is that the average temperature change does not distinguish between changing daily low temperatures and changing daily high temperatures.

Here in Kansas City there have been 69 record highs from 2000 to 2022. 34 of them have been in the winter months (December to March) and only 11 in the summer months (June to September).

And there have been 70 record lows for KC from 2000 to 2022. Only 12 of these have been in the winter months and 32 have been set in the summer months.

This indicates that changes in the average temperature for this geographically representative area are complex. If the average temperature has gone up by say 0.3 C in this period that could be an entirely beneficial or an entirely dangerous temperature change or somewhere in between.

Likewise, an increase of 2 C over the period 2000 to 2100 could be beneficial or dangerous or in between. The average numbers do not tell us what is correct.

[data at: https://www.weather.gov/eax/kcrecnorm]

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