M. Siddall

Bio: M. Siddall is an academic researcher from Lamont–Doherty Earth Observatory. The author has contributed to research in topics: Sea level & Ice sheet. The author has an hindex of 5, co-authored 10 publications receiving 741 citations.

High rates of sea-level rise during the last interglacial period

Abstract: Sea level during the last interglacial stood at least 4 m higher than at present, with evidence of short-term fluctuations of up to 10 m. A new continuous sea level record from the Red Sea and coral ages suggest that during these fluctuations, sea level changes were on the order of 1.6 m per century. The last interglacial period, Marine Isotope Stage (MIS) 5e, was characterized by global mean surface temperatures that were at least 2 ∘C warmer than present1. Mean sea level stood 4–6 m higher than modern sea level2,3,4,5,6,7,8,9,10,11,12,13, with an important contribution from a reduction of the Greenland ice sheet1,14. Although some fossil reef data indicate sea-level fluctuations of up to 10 m around the mean3,4,5,6,7,8,9,11, so far it has not been possible to constrain the duration and rates of change of these shorter-term variations. Here, we use a combination of a continuous high-resolution sea-level record, based on the stable oxygen isotopes of planktonic foraminifera from the central Red Sea15,16,17,18, and age constraints from coral data to estimate rates of sea-level change during MIS-5e. We find average rates of sea-level rise of 1.6 m per century. As global mean temperatures during MIS-5e were comparable to projections for future climate change under the influence of anthropogenic greenhouse-gas emissions19,20, these observed rates of sea-level change inform the ongoing debate about high versus low rates of sea-level rise in the coming century21,22.

Making sense of palaeoclimate sensitivity

Abstract: Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.

EDITORIAL Absence of post-Miocene Red Sea land bridges: biogeographic implications

Abstract: In a large number of studies concerned with species movements between Africa and Eurasia, including the migrations of hominids out of Africa, a frequentlycited dispersal route is across a hypothetical land bridge in the southern Red Sea, which is suggested to have emerged during glacial sea-level lowstands. This paper, however, unequivocally demonstrates that palaeoceanographic and palaeoecological data are incompatible with the existence of Red Sea land bridges since the Miocene. The case is made by presenting the first quantitative history of water depth above the Red Sea sill for the last 470,000 years, a time period that includes the four most recent glacial–interglacial cycles, and by discussing the predictable consequences of any land bridge formation on the Red Sea sedimentary and microfossil records. The absence of post-Miocene Red Sea land bridges has extensive implications for biogeographic models in the Afro-Arabian region. Genetic, morphometric and palaeontological patterns reported in the literature cannot be related to dispersals over a land bridge, or in the case of marine organisms, separation of the Red Sea from the Indian Ocean by a land bridge. If such patterns in terrestrial species are only congruent with a southern Red Sea dispersal route, then they need to be considered in terms of sweepstake rafting, anthropogenic introduction, or in the particular case of the Out-of-Africa migration by modern humans, seafaring. The constraints imposed by our palaeoenvironmental record on biogeographic reconstructions within and around the Red Sea will hopefully encourage both the review of previous works and the preference for multidisciplinary approaches in future studies.

New constraints on the timing of sea level fluctuations during early to middle marine isotope stage 3

Abstract: [1] To settle debate on the timing of sea level fluctuations during marine isotope stage (MIS) 3, we present records of δ18Oruber (sea level proxy) and magnetic susceptibility from the same samples within the single sediment archive (i.e., “coregistered”) of central Red Sea core GeoTu-KL11. Core-scanning X-ray fluorescence and environmental magnetic data establish the suitability of magnetic susceptibility as a proxy for eolian dust content in Red Sea sediments. The eolian dust data record similar variability as Greenland δ18Oice during early to middle MIS 3, in agreement with previous observations that regional Arabian Sea climate fluctuated with a timing similar to that of Greenland climate variations. In contrast, the sea level record fluctuates with a timing similar to that of Antarctic-style climate variations. The coregistered nature of the two records in core KL11 unambiguously reveals a distinct offset in the phase relationship between sea level and eolian dust fluctuations. The results confirm that sea level rises, indicated by shifts in Red Sea δ18Oruber to lighter values, occurred during cold episodes in Greenland during early to middle MIS 3. This indicates that the amplitudes of the reconstructed MIS 3 sea level fluctuations would not be reduced by inclusion of regional climate fluctuations in the Red Sea sea level method. Our analysis comprehensively supports our earlier conclusions of large-amplitude sea level variations during MIS 3 with a timing that is strongly similar to Antarctic-style climate variations.

Convincing evidence for rapid ice sheet growth during the last glacial period

Abstract: PAGES News, Vol.16 • No 1 • January 2008 Sp ec ia l S ec tio n: P al eo ce an og ra ph y Convincing evidence for rapid ice sheet growth during the last glacial period Mark siddaLL1*, e.J. rohLing2 and h.W. arz3 Department of Earth Sciences, University of Bristol, UK; siddall@ldeo.columbia.edu. National Oceanography Centre, Southampton, UK. Reserach Center for Geosciences, Potsdam, Germany. *currently at Lamont-Doherty Earth Observatory, Pallisades, USA.

Chapter 12 - Long-term climate change: Projections, commitments and irreversibility

Abstract: This chapter assesses long-term projections of climate change for the end of the 21st century and beyond, where the forced signal depends on the scenario and is typically larger than the internal variability of the climate system. Changes are expressed with respect to a baseline period of 1986-2005, unless otherwise stated.

Target atmospheric CO2: Where should humanity aim?

Abstract: Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Target atmospheric CO2: Where should humanity aim?

Abstract: Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Implications of climate change for agricultural productivity in the early twenty-first century

Abstract: This paper reviews recent literature concerning a wide range of processes through which climate change could potentially impact global-scale agricultural productivity, and presents projections of changes in relevant meteorological, hydrological and plant physiological quantities from a climate model ensemble to illustrate key areas of uncertainty. Few global-scale assessments have been carried out, and these are limited in their ability to capture the uncertainty in climate projections, and omit potentially important aspects such as extreme events and changes in pests and diseases. There is a lack of clarity on how climate change impacts on drought are best quantified from an agricultural perspective, with different metrics giving very different impressions of future risk. The dependence of some regional agriculture on remote rainfall, snowmelt and glaciers adds to the complexity. Indirect impacts via sea-level rise, storms and diseases have not been quantified. Perhaps most seriously, there is high uncertainty in the extent to which the direct effects of CO2 rise on plant physiology will interact with climate change in affecting productivity. At present, the aggregate impacts of climate change on global-scale agricultural productivity cannot be reliably quantified.

IPCC fourth assessment report

Abstract: The Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report (AR4) in 2007. Since the IPCC considers only published, peer-reviewed science in its assessments, the AR4 did not examine any research published after July 2006. In the years since then, a significant body of new peer-reviewed science has been published, much of which is relevant to policy decisions that will be made before the next IPCC assessment, due in 2014. A brief overview of some key new findings is provided here.