Historical records missed one fifth of global warming

Nasa data has highlighted problems in historical climate records over the past 150 years

Almost a fifth of the global warming that has happened during the past 150 years has been missed by historical records because of "quirks" in how temperatures have been recorded.

That's according to a new Nasa-led study which applied these quirks to climate models. The agency then performed the same calculations on both the models and the observations to make the first true "apples-to-apples comparison of warming rates".

The models and observations were found to largely agree on expected near-term global warming and may explain why projections of future climate, based solely on historical records, have a tendency to lower the rates of warming compared to similar predictions made using climate models.

The study, published in the journal Nature Climate Change, was led by Mark Richardson of Nasa's Jet Propulsion Laboratory, California. Although scientists have known about these quirks for some time, this is the first study to calculate their impact.

"They're quite small on their own, but they add up in the same direction," Richardson said. "We were surprised that they added up to such a big effect."

The Arctic is warming faster than the rest of Earth, but there are fewer historic temperature readings from there than from lower latitudes. A data set with fewer Arctic temperature measurements naturally shows less warming than a climate model that fully represents the Arctic.

Consequently, because it isn't possible to add more measurements from the past, the researchers instead set up the climate models to mimic the limited coverage in the historical records.

The new study also highlighted two other issues. Firstly, that historical data mixed air and water temperatures, whereas model results refer to air temperatures only. This quirk means data is skewed toward the cool side, because water warms less than air.

The other issue is that there was considerably more Arctic sea ice when temperature records began in the 1860s, and early observers recorded air temperatures over nearby land areas for the sea-ice-covered regions. As the ice melted, later observers switched to water temperatures instead. That also pushed down the reported temperature change.

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The combination of these quirks has hidden around 19 per cent of global air-temperature warming since the 1860s, the scientists said. This is enough to suggest that calculations generated from historical records alone were cooler than about 90 per cent of the results from the climate models used by the Intergovernmental Panel on Climate Change (IPCC).

In the apples-to-apples comparison, the historical temperature calculation was close to the middle of the range of calculations from the IPCC's suite of models. As a result, Richardson conceded that any research comparing modelled and observed long-term temperature records could suffer from the same problems.

"Researchers should be clear about how they use temperature records, to make sure that comparisons are fair," he said. "It had seemed like real-world data hinted that future global warming would be a bit less than models said. This mostly disappears in a fair comparison."

This article was originally published by WIRED UK