Use Google Earth Engine to Map CH4 Concentration

The Global Methane Pledge signed in COP26 has raised much attention on methane (CH4) concentration issue (Yiu, 2021a). People are interested in knowing about the distribution of CH4 concentration around the world in the past few years, and is there an upward trend, etc. This article shows how to use Google Earth Engine (GEE) to access the offline methane data (OFFL CH4) collected by the Sentinel 5P satellite of the EU/ESA/Copernicus Program and to plot the global CH4 concentration map (measured by the column average ppbV of the air mixing ratio) on different dates.

Copernicus Project

The EU/ESA/Copernicus Program was originally called GMES (Global Monitoring for Environment and Security). It collects data of the following five categories: atmosphere monitoring, marine environment monitoring, land monitoring, climate change, emergency management, and security. (Figure 1). a Youtube of the Copernicus Program is available at https://youtu.be/MGJss4lDaBo

Google Earth Engine GEE

The CH4 data may be too big to be run in our own computer, now the Google Earth Engine provides an online platform with google cloud to process the big data, the CH4 concentration can be mapped on GEE.

The GEE platform requires an online registration at (https://earthengine.google.com/), GEE “is a computing platform that allows users to run geospatial analysis on Google’s infrastructure. The Earth Engine Code Editor is a web-based IDE for the Earth Engine JavaScript API. It requires log in with a Google Account that’s been enabled for Earth Engine access. Code Editor features are designed to make developing complex geospatial workflows fast and easy. The Code Editor has the following elements (illustrated in the figure):” (GEE, 2021)

As shown in Figure 2, the code editor layout is divided into the upper and the lower parts: the lower part shows a map, and the upper part can be divided into left, middle and right sub-parts. The left one shows the files and directories, the middle shows the JavaScripts, and the right shows the task manager and the console, etc.

Programming — Map CH4 Concentration

Let’s start programming. The JavaScripts is simple and easy to understand. GEE also provides a sample code for copy and paste to RUN. The program can be divided into three sections. codes after // are remarks:

Section 1 is to get access to the CH4 data from COPERNICUS/S5P/OFFL/L3_CH4:

//import CH4 value

var collection= ee.ImageCollection(‘COPERNICUS/S5P/OFFL/L3_CH4’)
.select(‘CH4_column_volume_mixing_ratio_dry_air’)
.filterDate(‘2020–10–01’, ‘2020–10–31’);
//1. Define the variable collection =ee.ImageCollection (file name)
//2. Select the data about CH4 (the first column of the data file)
//3. Define the date as October 2020

Further information of the data definitions and parameters can be found at Earth Engine Data Catalog: Sentinel-5P OFFL CH4: Offline Methane (https://developers.google.com/earth-engine/datasets/catalog/COPERNICUS_S5P_OFFL_L3_CH4), for example, what is the earliest date of CH4 data available? It shows that it is from February 8, 2019 till now (Figure 3).

Then, we have to define color codes and map the mean values of CH4 concentrations of the period. As shown in Figure 4, “black”, “blue”, “purple”, “cyan”, “green”, “yellow”, and “red” represent the CH4 level of 1750 to 1900 ppbV, Blue and black areas indicate lower CH4 levels, yellow and red indicate higher CH4 levels. Then map the Mean() value of CH4 concentration level on a add layer to the map according to the color codes, and define the center coordinates of the map at 0.0, 0.0, 2:

var band_viz = {
min: 1750,
max: 1900,
palette: [‘black’,’blue’,’purple’,’cyan’,’green’,’yellow’,’red’]
};

Map.addLayer(collection.mean(), band_viz,’S5P CH4');
Map.setCenter(0.0, 0.0, 2);

//define band_viz from 1750 to 1900, and color palette: [‘color name’, …]

//Map.addLayer(collection name.mean(), …);

//Map.setCenter(coordinates)

RUN the programme will show Figure 4. You may try changing the dates to compare the changes in CH4 levels in different years. So far, only a youtube in Cantonese is prepared (Yiu, 2021b).

References

Yiu, C.Y. (2021a) Global Methane Pledge of COP26, Medium, Nov. 6. Global Methane Pledge of COP26

Yiu, C.Y. (2021b) uses Google Earth Engine to draw a global CH4 map, Youtube, November 7th. https://youtu.be/WrrAxk

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