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

Date:
February 8, 2023
Source:
Woods Hole Oceanographic Institution
Summary:
Ongoing climate change driven by greenhouse gas emissions is
often discussed in terms of global average warming. For example,
the landmark Paris Agreement seeks to limit global warming to 1.5
degrees C, relative to pre-industrial levels. However, the extent
of future warming will not be the same throughout the planet. One
of the clearest regional differences in climate change is the
faster warming over land than sea.

This 'terrestrial amplification' of future warming has real-world
implications for understanding and dealing with climate change.


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FULL STORY ========================================================================== Ongoing climate change driven by greenhouse gas emissions is often
discussed in terms of global average warming. For example, the landmark
Paris Agreement seeks to limit global warming to 1.5 ?C, relative to pre-industrial levels.

However, the extent of future warming will not be the same throughout
the planet. One of the clearest regional differences in climate change is
the faster warming over land than sea. This "terrestrial amplification"
of future warming has real-world implications for understanding and
dealing with climate change

==========================================================================
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 20th 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.

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========================================================================== Journal Reference:
1. Alan M. Seltzer, Pierre-Henri Blard, Steven C. Sherwood, Masa
Kageyama.

Terrestrial amplification of past, present, and future climate
change.

Science Advances, 2023; 9 (6) DOI: 10.1126/sciadv.adf8119 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/02/230208155728.htm

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