Many of the earlier epochs that occurred over the past 2000 years — were much smaller in scale than current human-induced global warming, according to two complementary studies published on 24 July in Nature and Nature Geoscience (1,2).
By threading together geological and biological archives, the authors have created a new perspective on climate change over the past 2000 years – the so-called Common Era. They obtained the data the community-sourced database from PAGES2k, which contains nearly 700 temperature-sensitive proxy records from 648 locations.
Measurements of the Earth’s surface temperature were not available prior to 1850. Therefore, scientists must instead rely on ‘palaeothermometers’ – data derived from trees, endoskeleton of massive coral systems, as well as lake sediments, glacier ice, and bivalve molluscs. These historical records contain detailed information on temperature change. For example, the rings of trees growing in colder climates like the Arctic depict the year-to-year variations in summer temperature.
Importantly, the records allow scientists to place the current scale and spread of industrial-era warming in the context of natural climatic variability. In other words, how much of the present-day climate variability is attributed to humans?
Global temperature changes are caused by both natural anthropogenic factors. In their Nature paper, Neukom et al. provide strong evidence that over the past 2,000 years, “anthropogenic global warming is unparalleled in terms of absolute temperatures” and “unprecedented in spatial consistency”.
During epochs like the Little Ice Age — the coldest epoch of the last millennium — the lowest temperatures varied regionally. This is in direct contrast to warming over the past 150 years, which shows similar levels of warming around the globe.
Furthermore, in the accompanying Nature Geoscience paper, the team of researchers show that major volcanic eruptions were the main factor leading to decades-long temperature swings that persisted for several decades. Whereas, historically, greenhouse-gas concentrations had a small impact and the sun’s radiation seemed to have little or no effect.
Based on reconstructions using seven different statistical methods, it seems the largest warming trends — at timescales of 20 years or more — occurred during the second half of the twentieth century, “highlighting the unusual character of the warming in recent decades.”
The climate is, indeed, always changing and we now know a bit more about how and why the Earth warms or cools. And knowing this will be critical in predicting future societal impacts and coming up with solutions to adapt to the extraordinary degree of climate variability.
The new proxy estimates are also consistent with physics-based climate models. We can, therefore, have at least some confidence in future projections of climate change, the authors write. One thing is certain though, the extent of current climate change is unprecedented in both its severity and scale.
(1) Neukom, R. et al. No evidence for globally coherent warm and cold periods over the preindustrial Common Era. Nature (2019). DOI: 10.1038/s41586-019-1401-2
(2) Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era. Nature Geoscience (2019). DOI: 10.1038/s41561-019-0400-0
(3) PAGES2k Consortium. A global multiproxy database for temperature reconstructions of the Common Era. DOI: 10.1038/sdata.2017.88