Arctic warming early in 20th century

The Arctic showed an abrupt increase in temperature around 1920, a cooling, and then a steadier climb beginning in the 1960s. Increased greenhouse gases are pretty well correlated with the second increase, but what caused the first?

20th century: Arctic temperature
20th century: Arctic temperature Hugo Ahlenius, UNEP/GRID-Arendal

Worldwide, the results aren’t quite the same.
Climate change mostly anthropogenic
Climate change mostly anthropogenic — variations depend partly on volcanoes, also El Nino/La Nina conditions. (El Nino years tend to be warmer — 1998 was hot because of El Ninl; this is now the temperature.)

Notice that the temperature change in the Arctic is much greater than elsewhere. Among other reasons, there is significant positive feedback — water absorbs most light, ice reflects most light.

The September 7 Science (here for research article, here for perspective, subscription needed) reports on a study of a Greenland ice core, to correlate the effect of aerosols (airborne particles) on temperature. Soot warms the area by increasing absorption (though most other aerosols cool the Earth).

Traces of vanillic acid and sulfates tell us where the soot comes from. The former indicates burning coniferous trees, from summer forest fires, the latter burning fossil fuels, then mostly in the winter.

The cores, covering 1788 – 2002, showed relatively stable black carbon for 60 years (neglecting large volcanos); soot and vanillic acid concentrations were highly correlated . From 1850 – 1951, soot levels were significantly higher, particularly in winter, up to 10 times as high, and correlated not with vanillic acid, but with sulfate. After, soot values declined, though they remain higher than before 1850.

Cleaner air in North America deposited less carbon at this location, even as Asia became a more important source of soot.

The change in absorbed sunlight is of minor importance in the Arctic winter, still,

McConnell et al. estimate an average Arctic warming effect from soot of more than 1 W/m2 between 1850 and 1951, peaking in 1906 to 1910 at more than 3 W/m2–eight times the natural forcing. For comparison, the globally and annually averaged forcing from the total anthropogenic CO2 increase in the year 2006 was ~1.7 W/m2

More records need to be examined, but it does appear as if a major contributor to early 20th century warming for the Arctic has been found.

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