Climate models underlie a lot of the catastrophism about climate change that's extremely common these days. But even as that catastrophism has settled into conventional wisdom, disquiet has been rising in scientific circles about the climate models people have been relying on, particularly those driven by the so-called business as usual scenario, RCP (Representation Concentration Pathway) 8.5. They're just too damn hot. From the Science magazine website:
"Next month, after a yearlong delay because of the pandemic, the U.N. Intergovernmental Panel on Climate Change (IPCC) will begin to release its first major assessment of human-caused global warming since 2013. The report, the first part of which will appear on 9 August, will drop on a world that has starkly changed in 8 years, warming by more than 0.3°C to nearly 1.3°C above preindustrial levels. Weather has grown more severe, seas are measurably higher, and mountain glaciers and polar ice have shrunk sharply. And after years of limited action, many countries, pushed by a concerned public and corporations, seem willing to curb their carbon emissions.
But as climate scientists face this alarming reality, the climate models that help them project the future have grown a little too alarmist. Many of the world’s leading models are now projecting warming rates that most scientists, including the modelmakers themselves, believe are implausibly fast. In advance of the U.N. report, scientists have scrambled to understand what went wrong and how to turn the models, which in other respects are more powerful and trustworthy than their predecessors, into useful guidance for policymakers. “It’s become clear over the last year or so that we can’t avoid this,” says Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies.
Ahead of each major IPCC report, the world’s climate modeling centers run a set of scenarios for the future, calculating how different global emissions paths will alter the climate. These raw results, compiled in the Coupled Model Intercomparison Project (CMIP), then feed directly into the IPCC report.....
In the past, most models projected a “climate sensitivity”—the warming expected when atmospheric carbon dioxide (CO2) is doubled over preindustrial times—of between 2°C and 4.5°C. Last year, a landmark paper that largely eschewed models and instead used documented factors including ongoing warming trends calculated a likely climate sensitivity of between 2.6°C and 3.9°C. But many of the new models from leading centers showed warming of more than 5°C—uncomfortably outside these bounds.
The models were also out of step with records of past climate. For example, scientists used the new model from NCAR to simulate the coldest point of the most recent ice age, 20,000 years ago. Extensive paleoclimate records suggest Earth cooled nearly 6°C compared with preindustrial times, but the model, fed with low ice age CO2 levels, had temperatures plummeting by nearly twice that much, suggesting it was far too sensitive to the ups and downs of CO2. “That is clearly outside the range of what the geological data indicate,” says Jessica Tierney, a paleoclimatologist at the University of Arizona and a co-author of the work, which appeared in Geophysical Research Letters. “It’s totally out there.”...
[T[he IPCC team will probably use reality—the actual warming of the world over the past few decades—to constrain the CMIP projections....For now, policymakers and other researchers need to avoid putting too much stock in the unconstrained extreme warming the latest models predict, says Claudia Tebaldi, a climate scientist at Pacific Northwest National Laboratory and one of the leaders of CMIP’s climate projections. Getting that message out will be a challenge. “These issues don’t translate very well in practice,” she says. “It’s going to be hard for people looking to make some projection of a water basin in the West to make sense of it.”
Already scientific papers are appearing using CMIP’s unconstrained worst-case scenarios for 2100, adding fire to what are already well-justified fears. But that practice needs to change, Schmidt says. “You end up with numbers for even the near-term that are insanely scary—and wrong.”
As the saying goes, garbage in, garbage out. Climate change is too important an issue for alarmist modeling. Wise policy needs the best and most plausible models, not the scariest.
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