May 14, 2021
Have you ever worn a dark t-shirt on a sunny day and felt the fabric warm in the rays of the sun? Most of us know that dark colors absorb sunlight and light colors reflect it – but did you know that it doesn’t work the same way with the sun’s invisible wavelengths?
The sun is the earth’s energy source and emits energy as visible sunlight, ultraviolet radiation (shorter wavelengths), and near infrared radiation that we perceive as heat (longer wavelengths). Visible light is reflected from light surfaces like snow and ice, while darker surfaces like forests or oceans absorb it. This reflectivity, called albedo, is an important way the earth regulates its temperature. When the earth absorbs more energy than it reflects, it becomes warmer, and when it reflects more than it absorbs, it becomes cooler.
The picture becomes more complicated when scientists bring the other wavelengths into the mix. In the near-infrared region of the spectrum, surfaces like ice and snow don’t reflect – in fact, they absorb near-infrared light in the same way that a dark t-shirt absorbs visible light.
“People think snow reflects. It’s so shiny, ”said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York City and acting senior climate advisor to NASA. “But it turns out that in the near infrared of the spectrum it’s almost black.”
In order for climatologists to get a comprehensive picture of how solar energy enters and exits the Earth system, they need to include other wavelengths in addition to visible light.
Earth’s energy budget is a metaphor for the delicate balance between the energy received from the sun and the energy radiated back into space. Researching the exact details of Earth’s energy budget is critical to understanding how the planet’s climate can change, as well as variations in solar energy output. Credits: NASA’s Goddard Space Flight Center
Download this video and other supporting visualizations from NASA Goddard’s Scientific Visualization Studio
This is where the NASA sensor for total and spectral solar radiation (TSIS-1) comes into play. From its position on board the International Space Station, TSIS-1 not only measures the total solar radiation (energy) that reaches the Earth’s atmosphere, but also how much energy comes in at each wavelength. This measurement is known as spectral solar radiation or SSI. TSIS-1’s Spectral Irradiance Monitor (SIM) instrument, developed by the University of Colorado’s Boulder Laboratory for Atmospheric and Space Physics, measures the SSI to greater than 0.2% or within 99.8% accuracy the actual SSI values.
“With TSIS-1, we have more confidence in measuring visible and near infrared light,” said Dr. Xianglei Huang, professor at the Institute of Climate and Space Science and Technology at the University of Michigan. “How you divide the amount of energy at each wavelength has an impact on the mean climate.”
The composition of this light that falls on Earth is important in understanding Earth’s energy budget. NASA’s Total Solar and Spectral Irradiance Sensor (TSIS-1) measures solar energy in 1,000 different wavelengths, including visible, ultraviolet, and infrared radiation known as solar spectral irradiance. Credits: NASA Goddard Space Flight Center Download this video and other supporting visualizations from NASA Goddard’s Scientific Visualization Studio
Huang and his colleagues at the University of Michigan, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Colorado Boulder recently used TSIS-1 SSI data in a global climate model for the first time. “Various SSI inputs have been used in several studies in the past to analyze the sensitivity of climate models.” However, this study was the first to investigate how the new data changed the modeled reflection and absorption of solar energy at the Earth’s poles, according to the Dong Wu Scientist project for TSIS-1 at Goddard.
They found that using the new data, the model had statistically significant differences in the absorption and reflection of energy ice and water when compared to using older solar data. The team ran the model, known as the Community Earth System Model or CESM2, twice: once with new TSIS-1 data averaged over an 18 month period, and once with an older, reconstructed average based on data the disused solar radiation NASA based climate experiment (SORCE).
The team found that the TSIS-1 data had more energy at wavelengths of visible light and less energy at wavelengths in the near infrared than the older SORCE reconstruction. These differences resulted in sea ice absorbing less and reflecting more energy in the TSIS-1 run, so polar temperatures were between 0.5 and 1.3 degrees Fahrenheit cooler and the amount of summer sea ice cover was about 2.5% higher.
“We wanted to know how the new observations compare with those of previous model studies and how this affects our view of the climate,” said lead author Dr. Xianwen Jing, who was doing this research as a postdoctoral fellow in the Department of Climate and Space Sciences and Engineering at the University of Michigan. “If there is more energy in the visible range and less energy in the near infrared, it affects how much energy is absorbed by the surface. This can affect how the sea ice grows or shrinks, and how cold it is at high latitudes. “
This tells us that in addition to monitoring total solar radiation, Huang said, we also need to keep an eye on the spectra. More precise SSI information, while not changing the overall picture of climate change, can help modelers better simulate how energy at different wavelengths affects climate processes such as ice behavior and atmospheric chemistry.
Although the polar climate looks different with the new data, more steps need to be taken before scientists can use it to predict future climate change, the authors warned. The team’s next steps include studying how TSIS data affects the model at lower latitudes, as well as making further observations into the future to see how the SSI changes over the solar cycle.
As you learn more about how solar energy, at all wavelengths, interacts with the earth’s surface and systems, you will get more and better information on modeling current and future climates. With the help of TSIS-1 and its successor TSIS-2, which will be launched on board its own spacecraft in 2023, NASA is shedding light on the earth’s energy balance and how it is changing.
Banner image: In this photo from the International Space Station, the rising sun casts long shadows over the Philippine Sea. Recognition: NASA