Snyder and the global temperature graph

A paper in this week’s Nature by Caroline Snyder at Stanford provides a fresh attempt at estimating global average surface temperatures across the Pleistocene (her estimates cover the last 2 million years).

Unfortunately there appear to be major issues with this work — the estimate of ‘earth system sensitivity’ to atmospheric carbon dioxide change appears based on an unsound premise, and consequently the ‘committed temperature change’ conclusion drawn from it is invalid (it’s way too pessimistic). But the global average temperature reconstruction looks impressive, though it’s sure now to attract some pretty heavy duty scrutiny. It compiles a diverse set of sea surface (not deep water) temperature proxies to estimate a spatially-weighted global average temperature anomaly. That seems a way more satisfactory approach than the crude estimate in Hansen et al (2013) that I’ve been using, which relies on an interpretive leap from a deep water temperature proxy (oxygen isotope ratios from the microscopic shells of bottom-dwelling marine critters).

So how does the new attempt look? Here’s my global temperature graph with Snyder (2016) added (median estimate):

Global average temperatures -- adding Snyder (2016)

Global average temperatures — adding Snyder (2016)

Her glacial-interglacial range is a bit larger, for example the last glacial maximum (LGM, ~20 ky) is nearer -6°C than -5°C, and the temperatures prior to the ‘mid Pleistocene transition’ (circa 1 My, some say 1.2 My) are rather different. She has the early Pleistocene warmer, with a much steeper descent into a colder mid Pleistocene:

Snyder (2016) vs Zachos (2008) + Hansen (2013)

Snyder (2016) vs Zachos (2008) + Hansen (2013)

That’s all just ‘details-details’ of course, but details matter if we want to understand the Earth’s climate system properly … and we sure as hell need to. I shan’t be presenting the new plot as the standard version until all this dust settles a bit.



  1. C.W. Snyder, “Evolution of global temperature over the past two million years”, Nature, 2016.
  2. Hansen, J, Sato, M, Russell, G & Kharecha, P (2013). Climate sensitivity, sea level, and atmospheric carbon dioxide. Phil. Trans. R. Soc. A, 371, 20120294. doi:10.1098/rsta.2012.0294