Nicholas Herold

Antarctic glaciation caused ocean circulation changes at the Eocene–Oligocene transition

Two main hypotheses compete to explain global cooling and the abrupt growth of the Antarctic ice sheet across the Eocene–Oligocene transition about 34 million years ago: thermal isolation of Antarctica due to southern ocean gateway opening, and declining atmospheric CO2 (refs 5, 6). Increases in ocean thermal stratification and circulation in proxies across the Eocene–Oligocene transition have been interpreted as a unique signature of gateway opening, but at present both mechanisms remain possible. Here, using a coupled ocean–atmosphere model, we show that the rise of Antarctic glaciation, rather than altered palaeogeography, is best able to explain the observed oceanographic changes. We find that growth of the Antarctic ice sheet caused enhanced northward transport of Antarctic intermediate water and invigorated the formation of Antarctic bottom water, fundamentally reorganizing ocean circulation. Conversely, gateway openings had much less impact on ocean thermal stratification and circulation. Our results support available evidence that CO2 drawdown—not gateway opening—caused Antarctic ice sheet growth, and further show that these feedbacks in turn altered ocean circulation. The precise timing and rate of glaciation, and thus its impacts on ocean circulation, reflect the balance between potentially positive feedbacks (increases in sea ice extent and enhanced primary productivity) and negative feedbacks (stronger southward heat transport and localized high-latitude warming). The Antarctic ice sheet had a complex, dynamic role in ocean circulation and heat fluxes during its initiation, and these processes are likely to operate in the future.

Middle Miocene tectonic boundary conditions for use in climate models

Utilizing general circulation models (GCMs) for paleoclimate study requires the construction of appropriate model boundary conditions. We present a middle Miocene paleotopographic and paleobathymetric reconstruction geographically constrained at 15 Ma for use in GCMs. Paleotopography and paleogeography are reconstructed using a published global plate rotation model and published geological data. Paleobathymetry is reconstructed through application of an age-depth relationship to a middle Miocene global digital isochron map, followed by the overlay of reconstructed sediment thickness and large igneous provinces. Adjustments are subsequently made to ensure our reconstruction may be utilized in GCMs.

Early to Middle Miocene monsoon climate in Australia

The present-day Australian monsoon delivers substantial moisture to the northern regions of a predominantly arid continent. However, the pre-Quaternary history of the Australian monsoon is poorly constrained due to sparse and often poorly dated paleoclimate proxy evidence. Sedimentological and paleontological data suggest that warm, humid, and seasonal environments prevailed in central and north Australia during the Miocene, though it is unclear whether these were products of the Australian monsoon. We perform a series of sensitivity experiments using an atmospheric general circulation model, combined with an offl ine equilibrium vegetation model, to quantitatively constrain the areal extent of the Miocene monsoon. Our results suggest a weaker than modern monsoon climate during the Miocene. This result is insensitive to atmospheric CO 2 , although somewhat sensitive to vegetation interactions and the presumed distribution of inland water bodies. None of our Miocene experiments exhibit precipitation rates greater than modern over north Australia, in disagreement with paleoclimate record interpretations. Vegetation modeling indicates that inferred precipitation values from fossil fl ora and fauna could only support Miocene vegetation patterns if atmospheric CO 2 was twice the modern concentration. This suggests that elevated CO 2 was critical for sustaining Miocene vegetation.

How much does it rain over land

Despite the availability of several observationally constrained data sets of daily precipitation based on rain gauge measurements, remote sensing, and/or reanalyses, we demonstrate a large disparity in the quasi-global land mean of daily precipitation intensity. Surprisingly, the magnitude of this spread is similar to that found in the Coupled Model Intercomparison Project Phase 5 (CMIP5). A weakness of reanalyses and CMIP5 models is their tendency to over simulate wet days, consistent with previous studies. However, there is no clear agreement within and between rain gauge and remotely sensed data sets either. This large discrepancy highlights a shortcoming in our ability to characterize not only modeled daily precipitation intensities but even observed precipitation intensities. This shortcoming is partially reconciled by an appreciation of the different spatial scales represented in gridded data sets of in situ precipitation intensities and intensities calculated from gridded precipitation. Unfortunately, the spread in intensities remains large enough to prevent us from satisfactorily determining how much it rains over land.

Modeling the Miocene Climatic Optimum. Part I: Land and Atmosphere*

AbstractThis study presents results from the Community Climate System Model 3 (CCSM3) forced with early to middle Miocene (~20–14 Ma) vegetation, topography, bathymetry, and modern CO2. A decrease in the meridional temperature gradient of 6.5°C and an increase in global mean temperature of 1.5°C are modeled in comparison with a control simulation forced with modern boundary conditions. Seasonal poleward displacements of the subtropical jet streams and storm tracks compared to the control simulation are associated with changes in Hadley circulation and significant cooling of the polar stratosphere, consistent with previously predicted effects of global warming. Energy budget calculations indicate that reduced albedo and topography were responsible for Miocene warmth in the high-latitude Northern Hemisphere while a combination of increased ocean heat transport and reduced albedo was responsible for relative warmth in the high-latitude Southern Hemisphere, compared to the present. Model–data analysis suggest...

How Well Do Gridded Datasets of Observed Daily Precipitation Compare over Australia

Daily gridded precipitation data are needed for investigating spatiotemporal variability of precipitation, including extremes; however, uncertainties related to daily precipitation products are large. Here, we compare a range of precipitation grids for Australia. These datasets include products derived solely from in situ observations (interpolated datasets) and two products that combine both remote sensed data and in situ observations. We find that all precipitation grids have similar climatologies for annual aggregated precipitation totals and annual maximum precipitation. The temporal correlations of daily precipitation values are higher between the interpolated datasets, but the correlations between the most widely used interpolated product (AWAP) and the two remotely sensed products (TRMM and GPCP) are still reasonable. Our results, however, point to distinct structural uncertainties between those datasets gridding in situ observations and those datasets deriving precipitation estimates primarily from satellite measurements. All datasets analysed agree well for low to moderate daily precipitation amounts up to about 20 mm but diverge at upper quantiles, indicating that substantial uncertainty exists in gridded precipitation extremes over Australia.

The influence of soil moisture deficits on Australian heatwaves

Several regions of Australia are projected to experience an increase in the frequency, intensity and duration of heatwaves (HWs) under future climate change. The large-scale dynamics of HWs are well understood, however, the influence of soil moisture deficits—due for example to drought—remains largely unexplored in the region. Using the standardised precipitation evapotranspiration index, we show that the statistical responses of HW intensity and frequency to soil moisture deficits at the peak of the summer season are asymmetric and occur mostly in the lower and upper tails of the probability distribution, respectively. For aspects of HWs related to intensity, substantially greater increases are experienced at the 10th percentile when antecedent soil moisture is low (mild HWs get hotter). Conversely, HW aspects related to longevity increase much more strongly at the 90th percentile in response to low antecedent soil moisture (long HWs get longer). A corollary to this is that in the eastern and northern parts of the country where HW-soil moisture coupling is evident, high antecedent soil moisture effectively ensures few HW days and low HW temperatures, while low antecedent soil moisture ensures high HW temperatures but not necessarily more HW days.

Comparing early to middle Miocene terrestrial climate simulations with geological data

The early to middle Miocene was significantly warmer than present, particularly at high latitudes. Relatively few climatic details are known about this time period compared with earlier (e.g., Cretaceous/Eocene) and later (e.g., Quaternary) intervals. In this study terrestrial proxy data are quantitatively compared with three simulations of early to middle Miocene climate (20–14 Ma) carried out using the National Center for Atmospheric Research (NCAR) Community Atmosphere Model 3.1 (CAM) and Community Land Model 3.0 (CLM). Three different meridional sea-surface temperature gradients are prescribed in order to test a range of plausible climates. Our simulations yield generally cooler and more arid conditions than indicated by the proxy record. Mean model-data discrepancies for precipitation and temperature decrease from −320 to −170 mm/yr and −0.5 to −0.4 °C, respectively, when tropical sea-surface temperatures are increased by ∼4 °C from inferred Miocene values to near modern values. The poor agreement with respect to mean annual precipitation may be attributed to the preclusion of an interactive ocean model and/or model bias.

Climate model sensitivity to changes in Miocene paleotopography

The Middle Miocene Climatic Optimum (MMCO), which occurred between 17 and 15 Ma, was the last of a series of warming events to have punctuated the Cenozoic and is the warmest the planet has been since 35 Ma (∼6°C warmer than present at middle latitudes). Paradoxically, CO2 concentrations during the MMCO are reported at present-day values or less and an explanation for warming has yet to be established. We test the sensitivity of version three of the Community Atmosphere Model (CAM) and Community Land Model (CLM) to changes in Middle Miocene paleotopography using prescribed sea-surface temperatures. We create four separate global paleotopography datasets based on altered Andean and Tibetan Plateau elevations as well as changes to global sea-level. We find that the largest warming is achieved with a lowering of the Tibetan Plateau from 4700 to 2600 m, resulting in a local temperature increase of up to 9°C—consistent with the modern-day lapse rate measured in China—and a global mean ground temperature increase of 0.5°C. An additional effect of this lowering is to weaken the summer and winter monsoons, and we discuss implications such changes may have for coupled ocean–atmosphere simulations. We conclude that while changes to topography significantly affect local temperatures, they do little to reconcile the disagreement between temperature and CO2 measurements derived from the geological record.

Temperature and precipitation extremes in century‐long gridded observations, reanalyses, and atmospheric model simulations

Knowledge about long-term changes in climate extremes is vital to better understand multidecadal climate variability and long-term changes and to place todays extreme events in a historical context. While global changes in temperature and precipitation extremes since the midtwentieth century are well studied, knowledge about century-scale changes is limited. This paper analyses a range of largely independent observations-based data sets covering 1901–2010 for long-term changes and interannual variability in daily scale temperature and precipitation extremes. We compare across data sets for consistency to ascertain our confidence in century-scale changes in extremes. We find consistent warming trends in temperature extremes globally and in most land areas over the past century. For precipitation extremes we find global tendencies toward more intense rainfall throughout much of the twentieth century; however, local changes are spatially more variable. While global time series of the different data sets agree well after about 1950, they often show different changes during the first half of the twentieth century. In regions with good observational coverage, gridded observations and reanalyses agree well throughout the entire past century. Simulations with an atmospheric model suggest that ocean temperatures and sea ice may explain up to about 50% of interannual variability in the global average of temperature extremes, and about 15% in the global average of moderate precipitation extremes, but local correlations are mostly significant only in low latitudes.