The freezing and thawing of our planet's soil can make a big impact on our climate. Check out the blog below to see just how important this is.
SMAP’s focus also includes measurements of freeze/thaw, which tells us whether the land surface is frozen or thawed in areas north of 45-degree north latitude. This is very important to know, because when the vegetation is frozen there is minimal exchange of gasses (CO2 primarily) between the vegetation and the atmosphere. It’s as if the vegetation were in a state of hibernation. This changes however, when spring begins and temperature rises above freezing, causing the snow to melt and the vegetation to thaw and draw liquid water from the ground, marking the beginning of the growing season. During this period of time (which lasts until the freeze-up) plants grow by taking up CO2 from the atmosphere, which leads to a decrease in global atmospheric CO2 concentrations (it’s as if the planet were inhaling). Figure 1 illustrates this clearly. It shows global atmospheric CO2 concentrations measured at the Mauna Loa Observatory in Hawaii from 2000-2014. The yearly fluctuations are primarily driven by plant photosynthetic activity in the northern latitudes.
Figure 1: Yearly atmospheric CO2 concentrations measured from the Mauna Loa Observatory in Hawaii from 2000-2014 (source: NASA GISS)
The SMAP freeze/thaw product allows us to determine the beginning and end of the growing season and hence the role that vegetation in those latitudes play in taking up CO2 from the atmosphere. Also, SMAP observations of freeze/thaw will allow us to determine if the growing season is getting longer or shorter due to climate change.
Finally, there is another very important process in the high latitudes that can contribute significantly to greenhouse gasses and that SMAP freeze/thaw can help us better understand. These are soils that have been permanently frozen for hundreds or thousands of years and are also known as permafrost. They can range from centimeters to hundreds of meters in thickness and they store a massive amount of carbon (about twice as much as contained in the atmosphere). The concern is that with increasing temperature and a longer growing season, these soils might thaw and very quickly release into the atmosphere the large amount of carbon (in the form of CO2 and methane [CH4]) that they store. Hence, knowing the period of time that the land surface is frozen or thawed and how it is changing can help us better understand the potential impacts on permafrost.
An example of the SMAP freeze/thaw product is shown in Figure 2. Note the large transition from frozen to thawed conditions within an almost two-week period.
Figure 2: An example of the SMAP freeze/thaw product for two different days. Blue is frozen and red is thawed. Note the large transition from frozen to thawed during this short timeframe.
This product was produced from mid-April through early July 2015 using the SMAP radar and is freely available on a data archive center called the National Snow and Ice Data Archive Center (NSIDC) https://nsidc.org/data/smap. After this period the radar ceased to work and the SMAP team is currently working on deriving a freeze/thaw product from SMAP’s second instrument (the radiometer), which will likely be available at the end of this year. Since we cannot obtain good soil moisture measurements when the soil is frozen GLOBE observations of whether the soil is frozen or thawed is therefore extremely useful for the SMAP team.