Our heading here is the title of a peer-reviewed paper published in 2014 by a group of scientists from around the world led by Zhengyu Liu of the University of Wisconsin-Madison who pointed out a problem that, once they explained it, makes us wonder why, first, no one had mentioned it before, including the IPCC, and second, why the media ignored the paper at the time… and ever since. The problem is that paleoclimate reconstructions show that after the end of the last ice age the world warmed up for a few thousand years until the mid-Holocene about 6,00 years ago, then spent the next six millennia gradually cooling until 20th century warming happened. But over the past 8,000 years CO2 levels in the atmosphere were rising and climate models say CO2 is the control knob on the global thermostat so the world should have been steadily warming the whole time, not cooling. So the models predict one thing and the data show the opposite happened, not just over a decade or two, but over 6,000 years. Why then has it been ignored? Because in climate science, unlike the normal kind, if the data doesn’t fit the theory you ignore the data. Nothing to see here folks. Move along. Sky on fire. Film at eleven.

How bad is the problem? Well, here’s what the mismatch looks like, according to Liu et al, in a chart where the blue line is a paleoclimate temperature reconstruction, one of several in the literature that all show the same pattern of warming then cooling over the past 12,000 years, the black line is the average model reconstruction over the same period of what the theory says should have happened, and the red line is another reconstruction where they tweaked the climate models to focus only on the times of year best represented by the proxy record:

Either way the models don’t reproduce the cooling. Which means that they are obviously wrong, unless someone can come up with a very clever explanation of why they are secretly right.

There has been a little bit of discussion of this issue in the scientific literature. Including a new paper in Quaternary Science Reviews in November 2023 that looked at whether Sahara greening and other land surface changes could explain the difference.

From 15,000 to 5,000 years ago the Sahara desert was much greener than today (yes, blame global warming for that catastrophe and thank subsequent cooling for the desertification), with shrubs and grasses covering much of it. Maybe, the theory goes, that reduced the amount of sand and dust blowing from Africa onto the North Atlantic, which warmed up the ocean which warmed up the climate, but then the desert returned and the dust came back and zzzzzz sure whatever. Unfortunately for alarmists, if you use this theory to fiddle the knobs on a climate model you can get a slightly better fit in some places, but at the expense of it getting worse in others. And in general land use reconstructions over the past 10,000 years are too uncertain to resolve the problem.

Here’s an interesting chart from that paper, showing the major external forcings that affect climate or at least are hypothesized to. Notice something?

Well, the top row shows, on the left, declining solar insolation in the Northern Hemisphere and on the right, rising CO2. According to modern climate models, the sun is irrelevant and CO2 is the control knob yet as the sun wanes and CO2 waxes, the planet cools.

The next row shows methane and Nitrous Oxide, which don’t change much over the period in question. And the final row shows sea levels, which don’t drive temperature but respond to it, at least while the planet is warming out of a glaciation, and possible land use change reconstructions, which are very uncertain and indicate the changes are mainly very recent so again of no real relevance to a long-standing phenomenon.

So you can see why climate models show warming over the Holocene: all that extra CO2 gets added to the system, and the models prove that CO2 drives warming by assuming that CO2 drives warming. But the paleo data shows cooling took place. Kind of like the sun was running the show.

Which apparently must not be. Unless climate scientists would like to return to a data-driven approach, and policy-makers to that fabled evidence-driven decision-making.