Climate sensitivity | EPFL Graph Search

Climate sensitivity is a measure of how much Earth's surface will cool or warm after a specified factor causes a change in its climate system, such as how much it will warm for a doubling in the atmospheric carbon dioxide () concentration. In technical terms, climate sensitivity is the average change in global mean surface temperature in response to a radiative forcing, which drives a difference between Earth's incoming and outgoing energy. Climate sensitivity is a key measure in climate science, and a focus area for climate scientists, who want to understand the ultimate consequences of anthropogenic global warming. The Earth's surface warms as a direct consequence of increased atmospheric , as well as increased concentrations of other greenhouse gases such as nitrous oxide and methane. The increasing temperatures have secondary effects on the climate system, such as an increase in atmospheric water vapour, which is itself also a greenhouse gas. Scientists do not know exactly how strong the climate feedbacks are and it is difficult to predict the precise amount of warming that will result from a given increase in greenhouse gas concentrations. If climate sensitivity turns out to be on the high side of scientific estimates, the Paris Agreement goal of limiting global warming to below will be difficult to achieve. The two primary types of climate sensitivity are the shorter-term "transient climate response", the increase in global average temperature that is expected to have occurred at a time when the atmospheric concentration has doubled, and "equilibrium climate sensitivity", the higher long-term increase in global average temperature expected to occur after the effects of a doubled concentration have had time to reach a steady state. Climate sensitivity is typically estimated in three ways: using direct observations of temperature and levels of greenhouse gases taken during the industrial age, using indirectly-estimated temperature and other measurements from the Earth's more distant past, and computer modelling the various aspects of the climate system with computers.

High sensitivity, incoherent differential imaging

Chiara Bonati

Microscopic visualisation of optically transparent samples has been a topic of interest for several decades. Features such as density or chemical composition can influence the optical phase of transmitted light, and phase contrast can reveal these structures. Several methods of phase contrast have been developed, which can be categorised as either interferometric or non-interferometric, based on the type of coherence properties of the light used. In this work, I focus on incoherent based phase contrast, in particular Differential Phase Contrast (DPC). The choice of incoherent light brings benefits such as the absence of distortions like speckle patterns and ringing patterns, increased spatial resolution, and a simpler setup that can be used for in-vivo applications. Moreover, DPC allows reconstruction of quantitative phase maps of the samples. On the other hand, coherent-based techniques demonstrate superior performance in terms of phase sensitivity. The first part of this thesis offers a quantitative analysis of the phase sensitivity of DPC and investigates the influence of optical parameters and sample characteristics. With simulations and experiments, a relation between numerical aperture and phase sensitivity is demonstrated, and the concept of spectral matching is introduced to enhance the contrast. The methods can be generalised to any DPC setup, and allow a-priori investigation of the sensitivity of a DPC microscope at the design stage rather than through testing. Comparison between the best sensitivity that can be achieved in DPC and state-of-the-art interferometric techniques, shows that it is not possible to reach comparable single-shot performances. In this thesis, it is shown that the reason for this limitation is the strong background in DPC images, which degrades the dynamic range. DPC images are obtained with mirrored illuminations, for which the background is identical and the phase term switches in sign. The difference between these pairs of images is computed digitally, which does not improve the limited dynamic range. Lock-in DPC is proposed as a solution: instead of sampling different illumination states, lock-in DPC demodulates the phase signal when the illumination is switched, and the background is never encoded. This is enabled by periodical switching of sources coupled with a smart pixel detector, the so-called "lock-in camera". Part of this work is dedicated to the theoretical description of this method, and the analysis of the expected benefit. Experiments are also described, that demonstrate a factor of 8 improvement in single-shot sensitivity compared to standard DPC. DPC is not the only imaging technique to suffer from high intensity background: it is easy to see how the use of a lock-in camera for differential imaging can be generalised to any situation where weak modulations can be induced over a strong background. Here, an example is presented with lock-in Shifted Excitation Raman Difference Spectroscopy (SERDS). SERDS is an established Raman spectroscopy technique that takes advantage of the Raman emission spectrum being relative to the excitation wavelength to remove unwanted fluorescence emission. Two spectra with shifted excitation wavelengths are measured, their difference is computed, and the fluorescence is thus digitally removed. The parallel with DPC is immediately apparent. Both simulations and experiments are used to demonstrate the advantage of analog demodulation.

EPFL2022

Point-of-care lung ultrasonography for early identification of mild COVID-19: a prospective cohort of outpatients in a Swiss screening center

Mary-Anne Hartley, Jean-Baptiste Francis Marie Juliette Cordonnier, Jean-Yves Meuwly, Luca Bosso, Tanguy Espejo, Siméon Schaad

Objectives Early identification of SARS-CoV-2 infection is important to guide quarantine and reduce transmission. This study evaluates the diagnostic performance of lung ultrasound (LUS), an affordable, consumable-free point-of-care tool, for COVID-19 screening. Design, setting and participants This prospective observational cohort included adults presenting with cough and/or dyspnoea at a SARS-CoV-2 screening centre of Lausanne University Hospital between 31 March and 8 May 2020. Interventions Investigators recorded standardised LUS images and videos in 10 lung zones per patient. Two blinded independent experts reviewed LUS recording and classified abnormal findings according to prespecified criteria to investigate their predictive value to diagnose SARS-CoV-2 infection according to PCR on nasopharyngeal swabs (COVID-19 positive vs COVID-19 negative). Primary and secondary outcome measures We finally combined LUS and clinical findings to derive a multivariate logistic regression diagnostic score. Results Of 134 included patients, 23% (n=30/134) were COVID-19 positive and 77% (n=103/134) were COVID-19 negative; 85%, (n=114/134) cases were previously healthy healthcare workers presenting within 2-5 days of symptom onset (IQR). Abnormal LUS findings were significantly more frequent in COVID-19 positive compared with COVID-19 negative (45% vs 26%, p=0.045) and mostly consisted of focal pathologic B lines. Combining clinical findings in a multivariate logistic regression score had an area under the receiver operating curve of 80.3% to detect COVID-19, and slightly improved to 84.5% with the addition of LUS features. Conclusions COVID-19-positive patients are significantly more likely to have lung pathology by LUS. However, LUS has an insufficient sensitivity and is not an appropriate screening tool in outpatients. LUS only adds little value to clinical features alone.

BMJ PUBLISHING GROUP2022

Methods to Enhance Nuclear Magnetic Resonance Sensitivity at High Magnetic Field

Claudia Christina Zanella

MR is a well-established technique routinely used in preclinical and clinical studies. This thesis focussed on the improvement of sensitivity of different methods in MR. Hyperpolarized 129Xe gas is used for studying pulmonary disease or cerebral perfusion. When hyperpolarization is conducted inside a polarizer, the achieved solid state polarization is currently limited to a few percent. One aim of this thesis was to counteract this bottleneck by investigating the effect of microwave frequency modulation for different spin systems. An increase in nuclear 129Xe spin polarization up to 14.5% was achieved compared to the polarization obtained without frequency modulation. Electron spin-lattice relaxation times T1S, as short as 5 ms, correlated directly with the gain in 129Xe nuclear polarization due to microwave frequency modulation, irrespective of the solvent or its deuteration. Simulations of the electron spin system confirmed that microwave frequency modulation could be an efficient method to considerably enhance nuclear spin polarization in short T1S matrices or in the presence of large linewidth radicals by promoting spectral diffusion. DNP matrices can also be hyperpolarized through non-persistent radicals generated by UV-irradiation. They have the advantage of quenching upon dissolution, yielding a hyperpolarized solution with no need for radical filtration. Radical precursors researched up to date either yielded low 13C polarization, contained toxic matrix components or interfered with metabolic processes. Therefore, the two novel endogenously-occurring precursors, alpha-ketovalerate (akV) and alpha-ketobutyrate (akB) were investigated. Liquid state polarization of the metabolic substrate glucose increased to 16.3% using akB compared with 13.3% obtained with pyruvate. [1-13C]butyric acid was polarized to 12.1% for akV and 12.9% for akB and used for in vivo hyperpolarized cardiac MRS. Hyperpolarized [1-13C]butyrate metabolism in the heart revealed label incorporation into a wide range of metabolites. This study demonstrated the potential of using UV-induced radicals generated in the endogenously-occurring metabolites akV and akB as polarizing agents, enabling high polarization without requiring radical filtration for radical-free hyperpolarized MRI. While MRI requires RF coils that generate high B1 homogeneity, MRS strongly depends on high sensitivity and potentially high B1 efficiency to allow for short-TE acquisitions. Two custom-made 1H coils resonating at 600 MHz, a single-channel saddle coil and an 8-leg quadrature birdcage coil, were investigated with respect to those criteria for applications at ultra-high field (14.1 T). The saddle coil yielded a 54% higher transmit field efficiency with a 20% higher SNR compared to the birdcage coil after full-FOV corrections. Using the saddle coil in glycoCEST imaging of a homogeneous phantom resulted in an MTR asymmetry coefficient of variation as small as 5.2% over an 11 mm diameter ROI. Overall, the saddle coil provided a good compromise between globally homogeneous excitation and high sensitivity. Finally, it was successfully used to map skeletal muscle glycogen content in vivo. This thesis focused on improving MR sensitivity by means of adequate custom-made RF coils as well as DNP through the implementation of a method capitalizing on electronic spin properties and the development of novel endogenous precursors for in vivo hyperpolarized MRI.

EPFL2021