Scientists are increasingly finding that abrupt, decadal-scale temperature changes of multiple degrees Celsius can arise unforced and spontaneously.
The surface temperatures for the Greenland ice sheet have been known to warm 8 to 16°C “within decades or less” – yes, fewer than 10 years – due to “unforced oscillations.” In other words, no external forcing mechanism is required for these abrupt warming events (Li and Born, 2019).
Image Source: Li and Born, 2019
Scientists (Gomes et al., 2020) examined the sea surface temperatures (SSTs) off the coasts of Lisbon in a newly published study. In addition to finding the SSTs were 5°C warmer (19.7°C vs. 14.6°C) about 11,000 years ago compared to the last millennium, they record a “sudden warming episode (in <100 years), of ~8 °C” for this region. That’s a warming of nearly 1°C per decade.
Image Source: Gomes et al., 2020
Modern summer sea surface temperatures in the Western North Pacific average 16°C (15-17°C) (Davis et al., 2020). Proxy evidence indicates peak SSTs reached 20-23°C ~8000 to ~6000 yrs ago, and SSTs twice warmed 4-5°C in a century or two during the Early Holocene.
Image Source: Davis et al., 2020
Another new study finds Iceland was ~2°C warmer and the Arctic region had much less sea ice as recently as ~1400 to 1000 years ago (Miles et al., 2020).
What’s most interesting is that in the last 1400 years the Arctic experienced marked climate changes (~1.5°C in decades) that may have occurred “spontaneously,” or without requiring “an external trigger”. In other words, these warming events may arise due to unforced natural variability.
If climate changes of these magnitudes can and do occur without an external forcing mechanism, there is little reason to assume that the much more modest changes in SSTs in the last century fall outside the range of internal variability either.