## 4 Results  
### Extracted Spatiotemporal Continuum Extreme High XCO\({}_{2}\)  
Here, we show the 10 largest extreme high XCO\({}_{2}\) units (referred to as 'Ex-1' through 'Ex-10') in Figure 2. The selected high XCO\({}_{2}\) units have grid sums numbering more than 2000 in time and space, and they are identified by their rank number. The spatial distribution of these 10 largest units is separately shown in Figure 2b. There are three (Ex-1, Ex-7, Ex-8), one (Ex-9) and six units (Ex-2, Ex-3, Ex-4, Ex-5, Ex-6, Ex-10) that occurred over the period of 2009-2010, 2013, and 2015-2016, respectively, as shown in Figure 2a. The largest event, Ex-1, occurred in Eurasia, with mixed land cover of forest, cropland and other vegetation, spread from July to October in 2010. It has a large special extent but is relatively concentrated in time. That is consistent with the influence of high temperatures and wildfire in the region ([PERSON] et al., 2016). The Ex-7 was distributed over north-eastern North America covered mostly in forest, occurred in the similar period of Ex-1. Ex-8 happened over eastern Australia shrub-land in late of 2009. Ex-9 distributed in the similar space of Ex-7, but happened in mid-2013. The five units (Ex-2, Ex-3, Ex-4, Ex-6, and Ex-10) happened in the tropical continents (Australia sparse shrub-land, southern Africa savanna and shrub-land, eastern and southern South America savanna, grassland and  
Figure 1: Flowchart showing the basic steps for the end-to-end extreme XCO\({}_{2}\) detection and evaluation.  
shrub-land, south-eastern Asia forest and cropland, respectively). They occurred from January to May of 2016, although with different duration within that window. These events correspond to the large CO\({}_{2}\) release related to the 2015-2016 EI Nino [22]. Ex-5 occurred over Central Asia bare land in early 2016. In addition, some units (Ex-2 and Ex-8, or Ex-7 and Ex-9) are overlapped in space.  
Time series of mean dXCO\({}_{2}\) and XCO\({}_{2}\) over each detected unit are shown in Figure 3. Extremes could happen in more than one area during one time period, and more than one time period in one area. Mean dXCO\({}_{2}\) and corresponding Z scores within detected units both in time and space are summarized in Table 2. The mean dXCO\({}_{2}\) within detected grids was approximately 1.4-1.8 ppm over the Northern Hemisphere (NH) and 1.2-1.4 ppm over the Southern Hemisphere (SH). Units over forest and cropland (Ex-1, Ex-7, Ex-10) are larger than 1.6 ppm, and those over savanna and shrub-land (Ex-2, Ex-3, Ex-4, Ex-6, Ex-8) are around 1.28-1.4 ppm. The mean Z score for all the 10 extremes are around 2.3-2.5 (high values indicate a high probability of extreme high XCO\({}_{2}\)).  
Figure 3: _Cont._  
Figure 2: The duration (**a**) and spatial distribution (**b**) of the ten largest extreme events (Ex). Those occurring in 2009–2010 are outlined in black (Ex-1, Ex-7, Ex-8), 2013 is in blue (Ex-9) and 2015–2016 are in red (Ex-2, Ex-3, Ex-4, Ex-5, Ex-6, Ex-10). The main land-cover type over the ten selected extreme units was based on MCD12C1 in 2009 from the Moderate Resolution Imaging Spectroradiometer (MODIS). (**a**) Influenced duration of largest 10 extreme high XCO\({}_{2}\) units; (**b**) Largest 10 extreme high XCO\({}_{2}\) units with influenced grids in space.