Claims about the costs of global warming

We quantify global and regional aggregate damages from global warming of 1.5 to 4 °C above pre-industrial levels using a well-established integrated assessment model, PAGE09. We find mean global aggregate damages in 2100 of 0.29% of GDP if global warming is limited to about 1.5 °C (90% confidence interval 0.09–0.60%) and 0.40% for 2 °C (range 0.12–0.91%). These are, respectively, 92% and 89% lower than mean losses of 3.67% of GDP (range 0.64–10.77%) associated with global warming of 4 °C. The net present value of global aggregate damages for the 2008–2200 period is estimated at $48.7 trillion for ~ 1.5 °C global warming (range $13–108 trillion) and $60.7 trillion for 2 °C (range $15–140 trillion). These are, respectively, 92% and 90% lower than the mean NPV of $591.7 trillion of GDP for 4 °C warming (range $70–1920 trillion). This leads to a mean social cost of CO2 emitted in 2020 of ~ $150 for 4 °C warming as compared to $30 at ~ 1.5 °C warming. The benefits of limiting warming to 1.5 °C rather than 2 °C might be underestimated since PAGE09 is not recalibrated to reflect the recent understanding of the full range of risks at 1.5 °C warming.

That is from a new paper by R. Warren, et.al.  The model does cover uncertainty, quadratic damages, and other features to steer it away from denialism.  At the end of the calculation, however, for a temperature rise of three degrees Centigrade they still find a mean damage of 2% of global gdp, and a range leading up to three percent of global gdp in terms of foregone consumption.  That is plausibly one year’s global growth.

If I understand them correctly, and I am not sure I do: “These give initial mean consumption discount rates of around 3% per year in developed regions and 48% [!] in developing ones.”  And what are the non-initial rates?  I just don’t follow the paper here, but probably I do not agree with it.  Perhaps at least for the developed nations this is a useful upper bound for costs?  And it is not insanely high.

Here is a piece by Johannes Ackva and John Halstead, “Good news on climate change.”  Excerpt:

However, for a variety of reasons, SSP5-RCP8.5 [a kind of worst case default path] now looks increasingly unlikely as a ‘business as usual’ emissions pathway. There are several reasons for this. Firstly, the costs of renewables and batteries have declined extremely quickly. Historically, models have been too pessimistic on cost declines for solar, wind and batteries: out of nearly 3,000 Integrated Assessment Models, none projected that solar investment costs (different to the levelised costs shown below) would decline by more than 6% per year between 2010 and 2020. In fact, they declined by 15% per year.

And:

Fundamentally, existing mainstream economic models of climate change consistently fail to model exponential cost declines, as shown on the chart below. The left pane below shows historical declines in solar costs compared to Integrated Assessment Model projections of costs. The pane on the right shows the cost of solar compared to Integrated Assessment Model assessments of ‘floor costs’ for solar – the lowest that solar could go. Real world solar prices have consistently smashed through these supposed floors.

…in order for us to follow SSP5-RCP8.5, there would have to be very fast economic growth and technological progress, but meagre progress on low carbon technologies. This does not seem very plausible. In order to reproduce SSP5-8.5 with newer models, the models had to assume that average global income per person will rise to $140,000 by 2100 and also that we would burn large amounts of coal.

And: “Global CO2 emissions have been flat for a decade, new data reveals.”  Again, better than previous projections.

As I said in the title of this post, these are “Claims.”  But overall I would say that the new results are slanting modestly in the less negative direction, though I am not sure that the headlines of the last two weeks are equally encouraging.

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