Past records help to predict different effects of future climate change on land and sea

A new paper studying terrestrial amplification focuses on how geochemical records of past climate on land and at the sea surface allow scientists to better predict the extent to which land will warm more than oceans -- and will also get drier -- due to current and future greenhouse gas emissions. "The core idea of our study was to look to the past to better predict how future warming will unfold differently over land and sea," says Alan Seltzer, an assistant scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution (WHOI) and the lead author of the paper.

"One reason why understanding terrestrial amplification matters is that under future global warming, the magnitude of warming that the planet will experience is not going to be the same everywhere," says Seltzer. "Adding a firm basis to climate model simulations, that is rooted in observations of past climate and basic physics, can tell us about how the regional differences in ongoing and future warming." Seltzer notes that terrestrial amplification (TA) is analogous to "polar amplification," a prediction of climate models that higher latitudes will experience more warming than low latitudes.

Although modern observational records are noisy due to big year-to-year variations driven by other parts of the climate system, the prediction of greater warming over land surfaces is now apparent in climate data since the 1980s. The drivers of this terrestrial amplification have been linked to changes in moisture over land and sea, through a theory developed by climate scientists over the past decade. This new study, published Wednesday in the journal Science Advances, "uses paleoclimate data for the first time to evaluate the theory for how land and sea surfaces will be impacted by future warming," Seltzer says. "The research gives us more certainty in the way models predict regional changes in future warming."

The paper investigates terrestrial amplification during the Last Glacial Maximum (LGM) -- which occurred about 20,000 years ago -- in the low latitudes, which they define as 30?S-30?N. It is in those latitudes, the authors say, where the theoretical basis for TA is most applicable. The authors drew on new compilations of paleoclimate records on land and from the sea surface to estimate the magnitude of TA in the LGM, to compare with climate model simulations and theoretical expectations. Efforts to better understand how cold the continents were in the LGM are an ongoing focus of Seltzer's research at WHOI, and this new paper builds upon a recent study that used insights from dissolved gases trapped in ancient groundwater as a thermometer for the past land surface.

The authors extended a thermodynamic theory for terrestrial amplification that is based on coupled changes in moist static energy (the potential energy represented by the temperature, moisture content, and elevation of a parcel of air) between land and sea. In the LGM, when sea level was 120 meters lower than today due to the growth of large ice sheets on land, the sea surface was slightly warmer and more humid than it would have been without a change in sea level. By taking this effect into account and drawing on paleoclimate records, the authors were able to directly compare past terrestrial amplification to future predictions. The paper notes that while the mechanisms underlying TA are well understood to arise from fundamental thermodynamic differences between humid air over the ocean and drier air over land, a number of factors -- natural variability, observational limitations, thermal lags, and non-CO₂ forcings -- have previously precluded a precise estimate of TA from 20^th century warming. "Narrowing the range of terrestrial amplification will aid in future predictions of low latitude climate change, with relevance to both heat stress and water availability," the paper states.

Co-author Pierre-Henri Blard says the paper is a "step forward for climate science," and it will be significant for other scientific fields and the general public. "We show that a simple model, involving humidity and sea level changes, robustly describes the amplification of temperature changes over the continent -- at low to mid-latitudes at any time scale -- as being 40% larger than over the ocean. This result is important because, while most paleoclimatic archives are located in the ocean, the present and future of humanity crucially rely on our knowledge of continental climates," says Blard, a Director of Research at the National Center for Scientific Research (CNRS) at the Center for Petrographic and Geochemical Research (CRPG) in Nancy, France.

The research is important "because it helps us make sense of Earth's past climate record and how to relate it to our models and expectations for the future," co-author Steven Sherwood says. The paper "should clear up any misconceptions that land and ocean warm or cool at the same rate in different climates -- we know otherwise and should use that knowledge. The implications for the future are that Earth's continents will continue to warm faster than the oceans as global warming continues, until hopefully we reach net zero and bring this to a stop," says Sherwood, a professor in the ARC Centre of Excellence for Climate Extremes in the University of New South Wales's Climate Change Research Center, Sydney, Australia.

Co-author Masa Kageyama says she considers the paper important "because it touches on a feature which is ubiquitous in climate change projections, produced by complex climate models: continents warm more than oceans. In this paper, we analyze this feature for a climate change, from the last glacial maximum to present, the amplitude of which is of the same order of magnitude as the expected warming in the next centuries," says Kageyama, director of research at CNRS' Climate and Environment Sciences Laboratory (LSCE) at the Pierre Simon Laplace Institute at the University of Paris-Saclay, France.

"It is remarkable that tropical temperature reconstructions, state-of-the-art climate models, and a simple theory relying on the coupled changes of moisture and heat over continents and oceans all converge to provide a robust estimate of terrestrial amplification," says Kageyama. "In my view, this strengthens the projections for future climate change, and at the same time brings new understanding of past climate changes."

Funding for this research was provided by a National Science Foundation Division of Earth Sciences award and by the French National Centre for Scientific Research.