lecture: Watching the changing Earth
warning: gravity ahead
For a few decades by now, satellites offer us the tools to observe the whole Earth with a wide variety of sensors. The vast amount of data these Earth observations systems collect enters the public discourse reduced to a few numbers, numbers like 3 or even 300. So, how do we know the amount of ice melting in the arctic or how much rain is falling in the Amazon? Are groundwater aquifers stable or are they are being depleted? Are these regular seasonal changes or is there a trend? How can we even measure these phenomena on a global scale?
This talk will provide one possible answer: gravity.
The melting of ice during the summer and the regrowth of ice shields in winter or any variation of mass on the surface of the Earth and inside the Earth, in general, are reflected in the change of its gravity field. By monitoring the gravity field from space, we can infer the mass variations necessary to result in the measured gravity changes. Satellite missions like GRACE (Gravity Recovery and Climate Experiment) offer us a monthly view of the Earth's changing gravity field since 2002. Providing a look into the mass redistribution driven geophysical processes, climate, and human civilisation. Furthermore, the combination of gravity with additional types of measurements allows us to get a better understanding of our planet.
The objective of this presentation is not to discuss the last significant decimal in some indicator of climate change. A look at the gravity field offers much more information, e. g., continental and global hydrology, changing ocean currents, mass flow in the mantle. This talk will give a brief introduction into space geodetic techniques used to monitor the gravity field of the Earth with a focus on the GRACE mission, its scientific results and applications. I will explain their working principle and the process which leads to a mathematical representation of the gravity field. We will look at a few selected examples, and try to answer the questions as mentioned above. Fortunately, the necessary data products are freely available. Additionally, there are services which spare us the math and offer tools to generate visualisations for a straightforward approach to this topic.