The future in the past: A major cause of uncertainties in climate projections is our imprecise knowledge of how much warming should occur as a result of a given increase in the amount of carbon dioxide in the atmosphere. Paleoclimate records have the potential to help us sharpen that understanding because they record such a wide variety of environmental conditions. Tierney et al. review the recent advances in data collection, statistics, and modeling that might help us better understand how rising levels of atmospheric carbon dioxide will affect future climate. Science , this issue p. eaay3701

Coupled ocean-atmosphere processes in the Indian Ocean amplify changes in Indo-Pacific climate under glacial conditions.

With CO₂ concentrations similar to today (410 ppm), the Pliocene Epoch offers insights into climate changes under a moderately warmer world. Previous work suggested a low zonal sea surface temperature (SST) gradient in the tropical Pacific during the Pliocene, the so‐called “permanent El Niño.” Here, we recalculate SSTs using the alkenone proxy and find moderate reductions in both the zonal and meridional SST gradients during the mid‐Piacenzian warm period. These reductions are captured by coupled climate model simulations of the Pliocene, especially those that simulate weaker Walker circulation. We also produce a spatial reconstruction of mid‐Piacenzian warm period Pacific SSTs that closely resembles both Pliocene and future, low‐emissions simulations, a pattern that is, to a first order, diagnostic of weaker Walker circulation. Therefore, Pliocene warmth does not require drastic changes in the climate system—rather, it supports the expectation that the Walker circulation will weaken in the future under higher CO₂.

Despite tectonic conditions and atmospheric CO₂ levels (pCO₂) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO₂ radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO₂. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO₂ forcing.