Big Questions for Earth
The Earth is the only known harbor for highly diversified life in the universe. In contrast to Mars and Venus, Earth’s atmosphere, vast quantities of surface liquid water, and internally generated magnetic field maintain an environment conducive to life and human civilization.
NASA, with international partners (Canada, France, Netherlands, Finland, United Kingdom, and Japan), has put into orbit a truly unprecedented suite of satellite as part of its Earth Observing Program. The A-Train will eventually consist of six satellites orbiting the Earth which are carefully choreographed by NASA ground controllers to observe the same portion of the Earth over the time span of twenty three minutes. The Earth’s land, oceans, ice packs, and atmosphere are now being observed by five of the six satellites using fourteen instruments which observe the Earth radiation from the ultraviolet to the millimeter wavelength region including polarization properties of clouds and aerosols.
The five satellites now in orbit include Aura, Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL), CALIPSO, Cloudsat, and Aqua. The instruments include spectrometers, radiometers, polarimeters, and lasers, which map or determine vertical distributions beneath the A-Train. A particular target is the composition of the Earth’s atmosphere, which is being studied by a full range of A-Train instruments, resulting in tremendous synergy by combining datasets. For example, simultaneously measured cloud and trace gas properties can be studied to better understand the formation of clouds and aerosols and their interactions with gases from near the ground into the stratosphere. This will be valuable in understanding the connections between atmospheric chemistry and climate. The launch of OCO, the sixth satellite, in 2008 will complete the constellation and make the first global measurements of CO2 sources and sinks.
Perhaps more than any other human activity, a half-century of progress in Earth observation from space has steadily changed our perception of the Earth as our home planet. Satellite measurements of essential characteristics have enabled human understanding of the Earth as a system of interconnected parts. It is now clear for example that the characteristics of Earth’s atmosphere so critical to human habitability are maintained by complex and tightly coupled circulation dynamics, chemistry, and interactions with the oceans, ice and land surface; all of which are driven by solar radiation and gravitational forces.
From the vantage point of space we see at continental and planetary scales the vast extent and complexity of human activities. Over the past 50 years, world population doubled, world grain supplies tripled, and total economic output grew sevenfold. From space, we see that expanding human activities now affect virtually the entire land surface and are altering world oceans and ice masses, as well. Over the next 50 years, the world population is likely to grow to 11 billion, exerting ever more demands for habitable land and natural resources and greater influences on climate. Natural variability in the Earth system occurs in many temporal and associated spatial scales, from very short-term weather (such as tornadoes), through the diurnal cycle, longer-period weather phenomena like frontal systems, then through the annual cycle (El Niño Southern Oscillation) and even longer oscillations (like the Pacific Decadal Oscillation). These are punctuated with episodic events such as volcanic eruptions and accompanied by longer-term change. Spatial scales vary from global processes such as changes in thermohaline circulation to regional such as melting of polar ice sheets to local-scale processes such as those manifested by floods and droughts. Understanding of these varying scale processes and their interaction enables predictive capability of the Earth system to inform resource management decisions and multiple level policies.
Thus, we live on a planet undergoing constant change due to natural phenomena and our own activities. To maintain and improve quality of life on Earth (e.g., support sustainable development), we need global information about the state of the environment and its future evolution. Continuous global observations of variability and change are required to reveal natural variability and the forces involved, the nature of the underlying processes, and how these are coupled within the Earth system.
- How is the global earth system changing?
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Earth is currently in a period of warming. Over the last century, Earth's average temperature rose about 1.1°F (0.6°C). In the last two decades, the rate of our world's warming accelerated and scientists predict that the globe will continue to warm over the course of the 21st century. Is this warming trend a reason for concern? After all, our world has witnessed extreme warm periods before, such as during the time of the dinosaurs. Earth has also seen numerous ice ages on roughly 11,000-year cycles for at least the last million years. So, change is perhaps the only constant in Earth's 4.5-billion-year history.
- What are the primary forcings of the Earth system?
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The Sun is the primary forcing of Earth's climate system. Sunlight warms our world. Sunlight drives atmospheric and oceanic circulation patterns. Sunlight powers the process of photosynthesis that plants need to grow. Sunlight causes convection which carries warmth and water vapor up into the sky where clouds form and bring rain. In short, the Sun drives almost every aspect of our world's climate system and makes possible life as we know it.
- How does the earth system respond to natural and human-induced changes?
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Climate scientists have been monitoring Earth's energy budget since the 1978 launch of NASA's Nimbus-7 satellite. That mission carried a new instrument into space called the Earth Radiation Budget Experiment (or ERBE), designed to measure all of the energy leaving through the top of Earth's atmosphere. All of the incoming sunlight minus all of the reflected sunlight and emitted heat is our world's energy budget. The second law of thermodynamics compels Earth's climate system to seek equilibrium so that, over the course of a year the amount of energy received equals the amount of energy lost to space. So typically the global energy budget is in balance.
- What are the consequences of change in the earth system for human civilization?
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Earth's climate system has been remarkably stable over the last 20,000 years or so. Human civilization developed in that time span, and our world's average temperature warmed by about 5°C to the temperature it is today. This fact points to one of climate scientists' main concerns about global warming: the temperature is rising faster than at any other time in the history of human civilization and such rapid climate change is likely to seriously stress some populations who cannot adapt quickly enough to the changes.
- How will the Earth system change in the future?
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As the world consumes ever more fossil fuel energy, greenhouse gas concentrations will continue to rise and Earth's average temperature will rise with them. The Intergovernmental Panel on Climate Change (or IPCC) estimates that Earth's average surface temperature could rise between 2°C and 6°C by the end of the 21st century.
- Earth Science Past Missions
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These missions explored the boundaries of our understanding of the complex, dynamic system we call the Earth. They have been retired, but are not forgotten.