Are Glacials Dry? Consequences for Paleoclimatology and for Greenhouse Warming

Abstract

Past cold climates are often thought to have been drier than today on land, which appears to conflict with certain recent studies projecting widespread terrestrial drying with near-future warming. However, other work has found that, over large portions of the continents, the conclusion of future drying versus wetting strongly depends on the physical property of interest. Here, it is shown that this also holds in simulations of the Last Glacial Maximum (LGM): the continents have generally wetter topsoils and higher values of common climate wetness metrics than in the preindustrial, as well as generally lower precipitation and ubiquitously lower photosynthesis (likely driven by the low CO2), with streamflow responses falling in between. Using a large existing global pollen and plant fossil compilation, it is also confirmed that LGM grasslands and open woodlands grew at many sites of present-day forest, seasonal forests at many sites of present-day rain forest, and so forth (116–144 sites out of 302), while changes in the opposite sense were very few (9–17 sites out of 302) and spatially confined. These vegetation changes resemble the model photosynthesis responses but not the hydroclimate responses, while published lake-level changes resemble the latter but not the former. Thus, confidence in both the model hydrologic and photosynthesis projections is increased, and there is no significant conflict. Instead, paleo- and modern climate researchers must carefully define “wetting” and “drying” and, in particular, should not assume hydrologic drying on the basis of vegetation decline alone or assume vegetation stress on the basis of declines in hydroclimatic indicators.

Vegetation and Climate Change in Southwestern Australia During the Last Glacial Maximum

“…With the descent into full glacial temperatures at~28 ka, eucalypt-dominated forest cover appears to have been largely eliminated, replaced by shrubland and herbaceous vegetation. At the same time, potential evapotranspiration was reduced under low glacial temperatures, contributing to greater retention of moisture in landscapes (Scheff et al, 2017). Glacial treelessness in southern Australia, as in many other regions, has been widely interpreted as indicating climates more arid than today (Hope, 1994;Kershaw et al, 1991).…”

Whither the 100th Meridian? The Once and Future Physical and Human Geography of America’s Arid–Humid Divide. Part II: The Meridian Moves East

“…In response we would say that the model error that leads to an incorrect mean state aridity arises from the precipitation simulation, while the projected aridity change arises most strongly from the temperature and vapor pressure deficit change, which we suspect is more faithfully simulated. Second, while current Earth system models (the subset of all climate models that simulate, in varying degrees of complexity, vegetation and carbon dynamics) predict widespread declines in soil moisture and increases in continental aridity, they also simulate increases in net primary productivity (Scheff et al 2017;Mankin et al 2017). This is because, within the models, the beneficial effects on photosynthesis and water-use efficiency of increased CO 2 overwhelm the effects of increased temperature and vapor pressure deficit.…”

“…Hence, perhaps, the aridity gradient as expressed in vegetation and crops will not move east as suggested here on the basis of a simple metric like AI in which the computation of PET does not account for biophysical changes. In response, we would say that CO 2 effects thus far appear to be highly geographically variable (Zhu et al 2016) and that models quite conceivably overestimate the biophysical response to CO 2 [see review by Cook et al (2016) and discussions in Scheff et al (2017), Mankin et al (2017), and Allen et al (2015)]. However, it is also likely that enhanced CO 2 is increasing crop water-use efficiency to some degree and may alter the relationship between the farm economy and AI.…”

Bayesian Analysis of the Glacial‐Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling

“…During the LGM, the lower CO 2 significantly reduced plant water use efficiency and is thought to be responsible for around 15% of the total change in 13 C between the two time periods (Kaplan, Prentice, Knorr, et al, 2002). The lower temperatures and hence potential evaporation and generally drier environment will have competing impacts on the plant available moisture (e.g., Scheff et al, 2017).…”