Temperature

How anomalous is modern warming relative to periods of the past?


Evolution of global temperatures since the Last Glacial Maximum

The evolution of global temperatures since the Last Glacial Maximum (the most recent Ice Age) has long fascinated climate scientists and the public alike. During this period, the Earth rapidly transitioned out of a state where ice sheets covered vast portions of the Northern Hemisphere, sea levels were ~130 m lower than present, and greenhouse gases were below half that of modern values. Thus, constraining the evolution of global surface temperatures during this time interval is critical for understanding the mechanisms and timescales of large scale climate change. While climate models and proxy data have independently enabled study of this key period, they have yielded disparate conclusions on temperature trends, magnitudes, and global phasing. Such discrepancies hinder our ability to contextualize modern warming trends and constrain future global climate responses to human-caused greenhouse gas emissions.

Together with colleagues from the Universities of Arizona, Michigan, Washington, and the National Center for Atmospheric Research, we pioneered the use of a new technique, paleoclimate data assimilation, to formally combine state-of-the-art global climate models with a global compilation of marine geochemical ocean temperature proxies to reconstruct global surface temperatures spanning the past 24,000 years.

Our main findings are as follows:

  • We show that the main drivers of climate change since the last ice age are rising greenhouse gas concentrations and the retreat of the ice sheets.
  • We show that global average temperatures underwent a general warming trend over the last 10,000 years, helping reconcile a debate about whether this period trended warmer or cooler.
  • We show that the magnitude and rate warming over the last 100 years far surpasses the magnitude and rate of changes over the last 24,000 years.

Our study can be found here.

See also the following Nature News & Views feature written by Shaun Marcott‬ and Jeremy Shakun, here, and our press release, here.

Global mean surface temperature (GMST) evolution of the Last Glacial Maximum to present. See Figure 2 of Osman et al. (2021).

references

  • Osman, M.B., Tierney, J.E., Zhu, J. et al. Globally resolved surface temperatures since the Last Glacial Maximum. Nature 599, 239–244 (2021).