New research – past climate (August 4, 2016)

Some of the latest papers on past climate are shown below. First a few highlighted papers with abstracts and then a list of some other papers. If this subject interests you, be sure to check also the other papers – they are by no means less interesting than the highlighted ones.

Highlights

Reconstructing Earth’s surface temperature over the past 2000 years: the science behind the headlines (Smerdon & Pollack, 2016) http://onlinelibrary.wiley.com/doi/10.1002/wcc.418/abstract

Abstract: The last quarter century spans the publication of the first assessment report of the Intergovernmental Panel on Climate Change in 1990 and the latest report published in 2013–2014. The five assessment reports appearing over that interval reveal a marked increase in the number of paleoclimate studies addressing the climate of the last 2000 years (the Common Era). An important focus of this work has been on reconstruction of hemispheric and global temperatures. Several early studies in this area generated considerable scientific and public interest, and were followed by high-profile and sometimes vitriolic debates about the magnitude of temperature changes over all or part of the Common Era and their comparison to 20th- and 21st-century global temperature increases due to increasing levels of atmospheric greenhouse gases. Behind the more public debates, however, several consistent themes of scientific inquiry have developed to better characterize climate variability and change over the Common Era. These include attempts to collect more climate proxy archives and understand the signals they contain, improve the statistical methods used to estimate past temperature variability from proxies and their associated uncertainties, and to compare reconstructed temperature variability and change with climate model simulations. All of these efforts are driving a new age of research on the climate of the Common Era that is developing more cohesive and collaborative investigations into the dynamics of climate on time scales of decades to centuries, and an understanding of the implications for modeled climate projections of the future.

Flat meridional temperature gradient in the early Eocene in the subsurface rather than surface ocean (Ho & Laepple, 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2763.html

Abstract: The early Eocene (49–55 million years ago) is a time interval characterized by elevated surface temperatures and atmospheric CO₂, and a flatter-than-present latitudinal surface temperature gradient. The multi-proxy-derived flat temperature gradient has been a challenge to reproduce in model simulations, especially the subtropical warmth at the high-latitude surface oceans, inferred from the archaeal lipid-based palaeothermometry, TEX^H₈₆. Here we revisit the TEX^H₈₆ interpretation by analysing a global collection of multi-proxy temperature estimates from sediment cores spanning millennia to millions of years. Comparing the variability between proxy types, we demonstrate that the present TEX^H₈₆ interpretation overestimates the magnitude of past climate changes on all timescales. We attribute this to an inappropriate calibration, which reflects subsurface ocean but is calibrated to the sea surface, where the latitudinal temperature gradient is steeper. Recalibrating the proxy to the temperatures of subsurface ocean, where the signal is probably formed, yields colder TEX^H₈₆ -temperatures and latitudinal gradient consistent with standard climate model simulations of the Eocene climate, invalidating the apparent, extremely warm polar sea surface temperatures. We conclude that there is a need to reinterpret TEX^H₈₆ -inferred marine temperature records in the literature, especially for reconstructions of past warm climates that rely heavily on this proxy as reflecting subsurface ocean.

The improbable but unexceptional occurrence of megadrought clustering in the American West during the Medieval Climate Anomaly (Coats et al. 2016) http://iopscience.iop.org/article/10.1088/1748-9326/11/7/074025/meta

Abstract: The five most severe and persistent droughts in the American West (AW) during the Common Era occurred during a 450 year period known as the Medieval Climate Anomaly (MCA—850–1299 C.E.). Herein we use timeseries modeling to estimate the probability of such a period of hydroclimate change occurring. Clustering of severe and persistent drought during an MCA-length period occurs in approximately 10% of surrogate timeseries that were constructed to have the same characteristics as a tree-ring derived estimate of AW hydroclimate variability between 850 and 2005 C.E. Periods of hydroclimate change like the MCA are thus expected to occur in the AW, although not frequently, with a recurrence interval of approximately 11 000 years. Importantly, a shift in mean hydroclimate conditions during the MCA is found to be necessary for drought to reach the severity and persistence of the actual MCA megadroughts. This result has consequences for our understanding of the atmosphere-ocean dynamics underlying the MCA and a persistently warm Atlantic Multidecadal Oscillation is suggested to have played an important role in causing megadrought clustering during this period.

Reconstruction of early Holocene Thermal Maximum temperatures using present vertical distribution of conifers in the Pannon region (SE Central Europe) (Molnár & Végvári, 2016) http://hol.sagepub.com/content/early/2016/07/19/0959683616658528.abstract

Abstract: Palaeoclimatic reconstruction is a main subject of palaeoecology, clarifying fossil palaeoenvironmental patterns. Our study provides a macroecological approach to reconstruct the mean annual temperature (MAT) of the Pannon region at the early Holocene Thermal Maximum (HTM, warmest period of the Holocene), based on the absence of forest-dwelling conifers in the North Hungarian Mountains and their presence in the surrounding Carpathians on the same altitude. We suppose that the HTM was enough warm to drive conifers to extinction from elevations between 900 and 1100 m a.s.l. in the relatively isolated N-Hungarian Mts. Conversely, HTM still allowed the survival of residual dwarf pine (Pinus mugo) stands on the isolated peaks of the West Transylvanian Mountains between 1600 and 1800 m a.s.l. Our study provides an estimate for the value of MAT of HTM of Pannon region with an interval of 0.4°C, relying on macroecological considerations. We calculate the temperature of the HTM 1.3–1.7°C warmer than the present temperature. This method can be used in a general sense, if conditions meet the requirements of the method even in horizontal cases, with area isolates of climate-sensitive species.

Low atmospheric CO2 levels during the Little Ice Age due to cooling-induced terrestrial uptake (Rubino et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2769.html

Abstract: Low atmospheric carbon dioxide (CO₂) concentration during the Little Ice Age has been used to derive the global carbon cycle sensitivity to temperature. Recent evidence confirms earlier indications that the low CO₂ was caused by increased terrestrial carbon storage. It remains unknown whether the terrestrial biosphere responded to temperature variations, or there was vegetation re-growth on abandoned farmland. Here we present a global numerical simulation of atmospheric carbonyl sulfide concentrations in the pre-industrial period. Carbonyl sulfide concentration is linked to changes in gross primary production and shows a positive anomaly during the Little Ice Age. We show that a decrease in gross primary production and a larger decrease in ecosystem respiration is the most likely explanation for the decrease in atmospheric CO₂ and increase in atmospheric carbonyl sulfide concentrations. Therefore, temperature change, not vegetation re-growth, was the main cause of the increased terrestrial carbon storage. We address the inconsistency between ice-core CO₂ records from different sites measuring CO₂ and δ¹³CO₂ in ice from Dronning Maud Land (Antarctica). Our interpretation allows us to derive the temperature sensitivity of pre-industrial CO₂ fluxes for the terrestrial biosphere (γ_L = −10 to −90 Pg C K⁻¹), implying a positive climate feedback and providing a benchmark to reduce model uncertainties.

Other papers

Destabilization of glacial climate by the radiative impact of Atlantic Meridional Overturning Circulation disruptions (Galbraith et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL069846/abstract

The role of El Niño in the global energy redistribution: a case study in the mid-Holocene (Saint-Lu et al. 2016) http://link.springer.com/article/10.1007%2Fs00382-016-3266-5

The last interglacial climate: comparing direct and indirect impacts of insolation changes (Pedersen et al. 2016) http://rd.springer.com/article/10.1007%2Fs00382-016-3274-5

Co-evolution of oceans, climate, and the biosphere during the ‘Ordovician Revolution’: A review (Algeo et al. 2016) http://www.sciencedirect.com/science/article/pii/S0031018216301559

Are Karakoram temperatures out of phase compared to hemispheric trends? (Asad et al. 2016) http://link.springer.com/article/10.1007%2Fs00382-016-3273-6

Correlation between climate and grain harvest fluctuations and the dynastic transitions and prosperity in China over the past two millennia (Yin et al. 2016) http://hol.sagepub.com/content/early/2016/07/20/0959683616646186.abstract

Influence of external forcings on abrupt millennial-scale climate changes: a statistical modelling study (Mitsui & Crucifix, 2016) http://link.springer.com/article/10.1007%2Fs00382-016-3235-z

The influence of Southern Ocean surface buoyancy forcing on glacial-interglacial changes in the global deep ocean stratification (Sun et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL070058/abstract

Strong effects of tropical ice-sheet coverage and thickness on the hard snowball Earth bifurcation point (Liu et al. 2016) http://link.springer.com/article/10.1007%2Fs00382-016-3278-1

Reorganization of the North Atlantic Oscillation during early Holocene deglaciation (Wassenburg et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2767.html

An extended and higher-resolution record of climate and land use from stalagmite MC01 from Macal Chasm, Belize, revealing connections between major dry events, overall climate variability, and Maya sociopolitical changes (Akers et al. 2016) http://www.sciencedirect.com/science/article/pii/S0031018216302425

Coexisting responses in tree-ring δ13C to high-latitude climate variability under elevated CO2: A critical examination of climatic effects and systematic discrimination rate changes (Helama et al. 2016) http://www.sciencedirect.com/science/article/pii/S0168192316303021

Strong mid-depth warming and weak radiocarbon imprints in the equatorial Atlantic during Heinrich 1 and Younger Dryas (Weldeab et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016PA002957/abstract

April–August temperatures in the Czech Lands, 1499–2015, reconstructed from grape-harvest dates (Možný et al. 2016) http://www.clim-past.net/12/1421/2016/

North Atlantic summer storm tracks over Europe dominated by internal variability over the past millennium (Gagen et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2752.html

Extreme storms during the last 6500 years from lagoonal sedimentary archives in the Mar Menor (SE Spain) (Dezileau et al. 2016) http://www.clim-past.net/12/1389/2016/

Climatic effects and impacts of the 1815 eruption of Mount Tambora in the Czech Lands (Brázdil et al. 2016) http://www.clim-past.net/12/1361/2016/