Uneven spatial sampling distorts reconstructions of Phanerozoic seawater temperature

Paleotemperature proxy records are widely used to reconstruct the global climate throughout the Phanerozoic and to test macroevolutionary hypotheses. However, the spatial distribution of these records varies through time. This is problematic because heat is unevenly distributed across Earth’s surface. Consequently, heterogeneous spatial sampling of proxy data has the potential to bias reconstructed temperature curves. We evaluated the spatiotemporal evolution of sampling using a compilation of Phanerozoic δ18O data. We tested the influence of variable spatial coverage on global estimates of paleotemperature by sampling a steep “modern-type” latitudinal temperature gradient and a flattened “Eocene-type” gradient, based on the spatial distribution of δ18O samples. We show that global paleotemperature is overestimated in ~70% of Phanerozoic stages. Perceived climatic trends for some intervals might be artifactually induced by shifts in paleolatitudinal sampling, with equatorward shifts in sampling concurring with warming trends, and poleward shifts concurring with cooling trends. Yet, the magnitude of some climatic perturbations might also be underestimated. For example, the observed Ordovician cooling trend may be underestimated due to an equatorward shift in sampling. Our findings suggest that while proxy records are vital for reconstructing Earth’s paleotemperature in deep time, consideration of the spatial nature of these data is crucial to improving these reconstructions.

The unidentified eruption of 1809: a climatic cold case

The “1809 eruption” is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the third largest since 1500 with a sulfur emission strength estimated to be 2 times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ∼ 1809 eruption that is similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSIs) and uncertainties from ice-core records. Three of the forcing reconstructions represent a tropical eruption with an approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12 and 19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble-mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.

Reconstruction of Temperature Distribution for a Turbulent Free Jet Using Background Oriented Schlieren

Background Oriented Schlieren (BOS) has been shown to be an excellent tool for qualitative flow visualization, and more recently, literature has shown that the technique can be expanded to yield quantitative measurements as well. In this study, a BOS setup was built to construct the temperature distribution of a heated turbulent free 12mm diameter jet near the nozzle. A 1080p DSLR camera was used to view a black and white speckled background plane through the heated free jet in question. Comparing images of the background with and without flow present using a cross correlation algorithm gave the apparent displacement of all points on the background viewed through the flow. Once this displacement field was obtained, a ray-tracing algorithm was implemented to reconstruct the refractive index of the center plane of the jet. Then, the Gladstone-Dale and ideal gas relations were combined and used to calculate the temperature of the center plane. Reynolds number, based on the jet diameter, was held constant at 6,000 for all cases, and steady state nozzle temperature was varied from 57°C to 135°C. Reconstructed temperature distributions were validated using K-type thermocouple measurements by allowing the system to reach steady state before acquiring data. Average agreement of 4–6% was observed between thermocouple and BOS measurements for axial locations of at least 30 mm downstream. Due to experimental error, accuracy decreases as axial location moves towards the nozzle, and as nozzle temperature increases. Improvements to the setup are being considered to improve the agreement in low accuracy regions. Further, this technique has the potential to be used to determine the temperatures in open and optically accessible closed reactive flows. Having information about near wall temperature in closed reactive flows will give insight into wall convective heat transfer characterization and will also help benchmark combustion based numerical models in applications such as gas turbines.

The unidentified volcanic eruption of ~1809: why it remains a climatic cold case

<p>The  "1809 eruption” is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the 3rd largest since 1500 with an eruption magnitude estimated to be two times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ~1809 eruption, similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous.  Here, we present the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSI) and uncertainties from ice core records. Three of the forcing reconstructions represent a tropical eruption with approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12 -19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure.  While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability. </p>

Influence of Southern Ocean dynamics on Antarctic temperatures and on the global carbon cycle over the past two millennia.

<p>Reconstructions of Antarctic surface temperature covering the past millennia display a large centennial variability that is not synchronous with fluctuations recorded on other continents and which is generally not well simulated by models. Many processes can be at the origin of these temperature variations such as teleconnections with tropical oceans and changes in the Southern Ocean. The focus here will be on the latter, in particular on the influence of westerly winds that have a large impact on the exchange of heat and carbon between the ocean and atmosphere. Changes in the Southern Ocean circulation and stratification also influence the carbon cycle at global scale. It is generally suggested that atmospheric CO<sub>2</sub> variations over the past two millennia were mainly controlled by land processes but the Southern Ocean might have also played a role. We will thus test whether the joint analysis of Antarctic temperature and atmospheric CO<sub>2</sub> concentration fluctuations can inform us on the origin of the observed changes over this period. In this purpose, we use the climate model LOVECLIM which includes a representation of the global carbon cycle. Experiments over the last two millennia will address the sensitivity to realistic perturbations of the wind stress. Finally, experiments with data assimilation will allow assessing what constraints are needed for model results to better reproduce the atmospheric CO<sub>2</sub> concentration and reconstructed temperature history.</p>

Timing and extent of Late Pleistocene glaciation in the Chugach Mountains, Alaska

Geomorphic mapping, landform and sediment analysis, and cosmogenic 10Be and 36Cl ages from erratics, moraine boulders, and glacially polished bedrock help define the timing of the Wisconsinan glaciations in the Chugach Mountains of south-central Alaska. The maximum extent of glaciation in the Chugach Mountains during the last glacial period (marine isotope stages [MIS] 5d through 2) occurred at ~50 ka during MIS 3. In the Williwaw Lakes valley and Thompson Pass areas of the Chugach Mountains, moraines date to ~26.7 ± 2.4, 25.4 ± 2.4, 18.8 ± 1.6, 19.3 ± 1.7, and 17.3 ± 1.5 ka, representing times of glacial retreat. These data suggest that glaciers retreated later in the Chugach Mountain than in other regions of Alaska. Reconstructed equilibrium-line altitude depressions range from 400 to 430 m for late Wisconsinan glacial advances in the Chugach Mountains, representing a possible temperature depression of 2.1–2.3°C. These reconstructed temperature depressions suggest that climate was warmer in this part of Alaska than in many other regions throughout Alaska and elsewhere in the world during the global last glacial maximum.

The unidentified volcanic eruption of 1809: why it remains a climatic cold case

The 1809 eruption is one of the most recent unidentified volcanic eruptions with a global climate impact. Even though the eruption ranks as the 3rd largest since 1500 with an eruption magnitude estimated to be two times that of the 1991 eruption of Pinatubo, not much is known of it from historic sources. Based on a compilation of instrumental and reconstructed temperature time series, we show here that tropical temperatures show a significant drop in response to the ~1809 eruption, similar to that produced by the Mt. Tambora eruption in 1815, while the response of Northern Hemisphere (NH) boreal summer temperature is spatially heterogeneous. We test the sensitivity of the climate response simulated by the MPI Earth system model to a range of volcanic forcing estimates constructed using estimated volcanic stratospheric sulfur injections (VSSI) and uncertainties from ice core records. Three of the forcing reconstructions represent a tropical eruption with approximately symmetric hemispheric aerosol spread but different forcing magnitudes, while a fourth reflects a hemispherically asymmetric scenario without volcanic forcing in the NH extratropics. Observed and reconstructed post-volcanic surface NH summer temperature anomalies lie within the range of all the scenario simulations. Therefore, assuming the model climate sensitivity is correct, the VSSI estimate is accurate within the uncertainty bounds. Comparison of observed and simulated tropical temperature anomalies suggests that the most likely VSSI for the 1809 eruption would be somewhere between 12–19 Tg of sulfur. Model results show that NH large-scale climate modes are sensitive to both volcanic forcing strength and its spatial structure. While spatial correlations between the N-TREND NH temperature reconstruction and the model simulations are weak in terms of the ensemble mean model results, individual model simulations show good correlation over North America and Europe, suggesting the spatial heterogeneity of the 1810 cooling could be due to internal climate variability.

A statistical–dynamical downscaling methodology for the urban heat island applied to the EURO-CORDEX ensemble

Regional Climate Models (RCMs) are the primary climate information available to public stakeholders and city-planners to support local adaptation policies. However, with resolution in the order of ten kilometres, RCMs do not explicitly represent cities and their influence on local climate (e.g. Urban Heat Island; UHI). Downscaling methods are required to bridge the gap between RCMs and city scale. A statistical–dynamical downscaling methodology is developed to quantify the UHI of the city of Paris (France), based on a Local Weather Types (LWTs) classification combined with short-term high-resolution (1-km) urban climate simulations. The daily near-surface temperature amplitude, specific humidity, precipitation, wind speed and direction simulated by the RCMs are used for the LWTs attribution. The LWTs time series is associated to randomly selected days simulated with the mesoscale atmospheric model Meso-NH coupled to the urban canopy model Town Energy Balance to calculate the UHI corresponding to the successive LWTs. The downscaling methodology is applied to the EURO-CORDEX ensemble driven by the ERA-Interim reanalysis, and evaluated for the 2000–2008 period against station observations and a 2.5-km reanalysis. The short-term dynamical simulations slightly underestimate and overestimate near-surface minimum and maximum air temperature respectively, but capture the UHI intensity with biases in the order of a tenth of a degree. RCMs show significant differences in the variables used for the LWTs attribution, but the seasonal LWT frequencies are captured. Consequently, the reconstructed temperature fields maintain the small biases of the Meso-NH simulations and the statistical–dynamical downscaling greatly improves the UHI compared to the raw data of RCMs.